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CHAPTER 4

OPEN-HEARTH FUELSi CQMBUSTION, AND INSTRUMENTATION

T HE developments in construction and operation of the basic open-hearth furnace that have taken place in recent years

have placed new emphasis on the vital influence played by regulation and control of the combustion system in both furnace production and furnace life. The several factors influencing flame development, such a s regulation of fuel supply, a i r supply, a i r preheat, furnace draft, and furnace reversal, must be controlled closely if successful furnace performance is to be accomplished. Rugged instrumentation of distinctive design is required to supply dependable information concerning operating conditions, which is requisite to proper regulation of combustion. This chapter attempts to give a practical discussion of the various features of open-hearth practice and instrumentation related to the efficient heating of the furnace, control of temperature, and protection of the furnace structure from accidental overheating. The appended references will be helpful to readers wanting a more detailed treatment of individual topics.

OPEN-HEARTH FUELS

The most important open-hearth fuels are listed in ~ a b l e 4 - 1 together with their compositions and other data useful for combustion calculations.

A detailed discussion of open-hearth fuels is not attempted here, but certain factors peculiar to their application to this type of process are briefly indicated on the following pages. Very often a n open-hearth shop is forced by economic factors to use a fuel to which its furnaces are not very well adapted, because the general fuel-allocation scheme for the works as a whole seems to demand it. Often a variable marginal supply of coke-oven gas or t a r must be consumed by the open hearth, together with another fuel such a s producer gas, natural gas, or oil, to fill out total requirements. Most furnaces employ more than one kind of fuel during their useful life; however, the partial redesign

8 1

82 Chapter 4--FUELS, COMBUSTION, AND INSTRLIMENTATION

and adaptation of any furnace to different fuels is usually not very difficult because of the inevitable constant rebuilding and repair that accompany steelmaking .operations.

General Types of Fuels. Among the general types of fuel are those which are preheated in regenerators separate from the a i r checkers. These preheated fuels include producer gas, mixtures of 65 to 70 per cent blast-furnace gas and 35 to 30 per cent colie- oven gas, and producer gas with 10 to 15 per cent coke-oven gas added. Proper burning of these gases requires relatively complex port-end and downtake design. Usually the gas ports need water cooling to preserve their contours for proper flame direction, and this entails some additional heat loss. There is also a 2 to 4 per cent loss of these fuels in reversals because gas trapped in regenerative zones is wasted directly to the stack.

Oil, tar, and powdered coal may be warmed up somewhat for convenience in handling but cannot be preheated. These fuels are always injected a t rather high pressure, and the energy im- parted to the flame helps to carry along the preheated a i r a s well as to make the flame sweep down over the bath surface a t the proper angle. This means not only that the separate gas port is eliminated but also that the requirements of port-end roof slopes and wing walls for directing the flame a re not a t all critical. End zones can therefore be relatively open to give plenty of room for passing outgoing gases.

Coke-oven gas and natural gas are not preheated. They are usually injected a t the ports a t low or moderate pressures, so that flame direction depends to a n important extent upon the kinetic energy of the preheated air. This means that port-roof arches and sides must have steep slopes, and the port openings must be constricted to cause the flame to sweep over the bath surface properly; consequently, there is none too much flow area a t this bottleneck for outgoing gases.

Coke-oven gas and natural gas could be preheated to not more than 1500 to 1600 F (815 to 870 C). Higher temperatures would cause decomposition of hydrocarbons and probably excessive carbon deposition in gas regenerators and ports. However, the net gain would probably be negligible because of the moderate preheat temperatures possible and because of the relatively small heat capacity of these fuels. Likg'oil and tar , these gases a r e

I I Typical composition. volume percentage Typical values of 1 Products of combustion with 10% excesa air.

' I Other

Gaseous fuels C0z CO CHI hydrocarbon

I

Producer gas. . . . , , 4 . 6 24 .5 3 . 1 1 . 2 I l l 1 Coke-oven gas. . . . . 1.81 5 . 3 28 .1 4 . 0 Natural gas. . . . . . . ' 0 . 2 8 5 . 0 11.0 Mixed coke-oven

a n d blaa t - fur-

Liquid fuels

Fuel oilb.. . . . . . Raw tarc. . . . . . Rcsidual tard. .

Density relative

Typical composition. . weight percentage

. .

Mean specific Heat of Composition. vol- heat, 1000 to

combustion, ume percentage 2600 F (540 tc net Btu -- , , , , 1425 C), Btu

per per cubic foot CO2 Oz Hz0 N2 cubic foot

- - -- - - ---

1 5 6 + s e n s i - 1 5 . 5 1 . 2 1 0 . 4 7 2 . 9 0.0254 ble heat

(about 24) 510 7 . 3 1 . 6 2 1 . 7 6 9 . 4 0.0248

1,010 8 . 9 1 . 7 1 7 . 9 7 1 . 5 0:0250

240 12 .5 1 . 4 1 5 . 9 7 0 . 2 0.0252

Volume per 1 0 0 c u f t of fuel, cu f t

Products of combustion with 10% excess air Typical values of and including 3 Ib of steam per gal for atomizinl

relative

ture I water

Deg. F 1 Heat of Composition, vol- Mean specific Weight temp. combustion, ume percentage heat, 1000 to per a t 100 net Btu - 2600 F (540 to pound of

1425 C), Btu fuel. Ib

Theoretical amount of

air required for combustion of 100

cu f t of fuel gas.

cu f t

k 211 g

i! n C

Theoretical amount of V)

air required per pound of

fuel. Ib

a These figures represent approximately the actual waste-gas analyses only in the melting period, i .e. , before hot metal, sinee the gas composition is altered by subsequent bath reactions.

b Usually No. 6. Averages 0.08 to 0.10 per cent aah by weight. d Averages 0.03 to 0.05 per cent ash by weight.

84 Chapter 4--FUELS, COMBUSTION, AND INSTRUMENTATION

concentrated fuels. All such fuels require about 14 to 15 lb of air per pound of fuel for complete combustion, compared with air requirements for the preheated fuels that are diluted with nitrogen of about 1.5 lb for producer gas and 2.5 to 3.0 lb for mixed blast-furnace and coke-oven gas.

Powdered Coal. Powdered coal may be derived from many different types of lignite, sub-bituminous, and bituminous coals ground to particles mostly around 0.002 to 0.001 in. in diameter. Powdered coal has been used in relatively few open hearths. It is a concentrated fuel possessing some of the advantages of oil and tar. I t gives a highly luminous flame and allows a simple port-end design. About 20 to 30 per cent of the total air must be injected with the coal stream to give proper flame velocity and direction, and a moderate percentage of excess a i r is usually required for complete combustion; both of these factors reduce thermal efficiency. The ash particles in the fuel are mainly carried out in the gas stream but tend to deposit in regenerative zones, increasing end-zone corrosion of brickwork, filling of slag pockets, and clogging of checker openings and passages under the checkerwork. These natural disadvantages make powdered coal inferior to many other fuels, and ordinarily a rather large cost differential in favor of coal is needed to make i t the most economical fuel in a given shop.

The greatest drawback to successful use of powdered coal for open-hearth melting probably has been the lack of flexibility of flame regulation. The usual methods of handling and introducing powdered coal into the furnace do not lend themselves readily to the manipulation of flame shape and length so essential to good open-hearth operation, with the result that powdered-coal fui-naces have not made outstanding production records and their refractory costs have been high. The mixing of powdered coal and fuel oil has been attempted a t various times in recent years in efforts to stretch available supplies of fuel oil and to lower fuel cost. This expedient furnishes a means of making use of the desirable characteristics of powdered coal and a t the same time retaining the favorable flame-regu!ation features-of fuel oil. This mixture of powdered coal and fuel oil has been called "colloidal fuel," although the name is a misnomer since the coal particles are carried in the oil in suspension. The stability of the suspen-

OPEN-HEARTH FUELS 85

sion depends on the fineness and uniformity of coal grinding and the thoroughness of mixing of coal particles with oil, so that careful engineering is required to construct a grinding and mixing plant that will produce satisfactory fuel.

Some development work%n colloidal fuel was carried out in 1944-1945 a t the instance of the Government in an effort to con- serve the critical supplies of fuel oil. The results of this test on colloidal fuel indicated that faster melting and lower fuel consumption could be obtained and that checker deposits were not appreciably greater than from straight oil. The colloidal-fuel practice was discontinued because of plugging of coal in oil lines, valves, heaters, and meters, as grinding facilities did not produce coal ground finely enough to remain in suspension. The experimental work established a limit of about 25 per cent coal in 11 API fuel oil for a circulating temperature of 140 F (60 C ) .

The use of natural or coke-oven gas as a carrier for powdered coal should have interesting possibilities in the open-hearth furnace, since the higher carbon content of the coal would ensure flame luminosity, and the higher hydrocarbon content of the gas would tend to retard combustion slightly so that desirable flame coverage over the bath could be maintained. The rich gases, either natural or coke-oven gas, when used as the sole fuel, have the disadvantage that flame coverage needed for the final stages of the heat is difficult to establish if the installation is designed for rapid meltdown, or, vice versa, if the flame is adjusted for good finishing practice, meltdown is likely to be slow. Powdered coal, on the other hand, when used alone, tends to produce an intensely hot, localized flame, which does not cover the bath well and is destructive to furnace refractories. The two fuels in combination should produce a quite desirable open-hearth fuel, the possibilities of which should be exploited whenever fuel oil is in short supply.

Fuel Oil and Tar. Fuel oil and tar are perhaps the most satisfactory fuels to use in open-hearth operation. They give a bril- liantly luminous flame that radiates its heat effectively in the melting chamber. With adjustable burners, the flame direction can be controlled to sweep as desired over the charge or the slag surface. Burner design, atomization, and the position of the point of exit of the fuel from the burner should be correct to


develop maximum flame intensity about opposite the first door on the incoming end with combustion completed as the flame reaches approximately 60 to 70 per cent of the length of the bath.

Both oil and tar must be atomized by some hot gas under pressure. Steam is most common!y employed for this purpose, preferably supplied a t 150 to 200 psig pressure and delivered to the burner a t 80 to 120 psig or more, and preferably superheated somewhat. The usual ratio of steam to oil is about 3 to 4 lb per gallon. Compressed air a t around 100 to 150 psig pressure heated to 200 to 400 F (95 to 205 C) may also be used, requiring about 2.5 lb of air per gallon of oil. Atomization by oxygen-enriched air and blast-furnace gas has been tried experimentally with some success. Mechanical atomization has not yet been successful in open-hearth applications, in which a large stream of fuel must be broken up and burned very rapidly to obtain the necessary heat dissipation in the short period of travel through the melting chamber. The hot, compressed liquid stream tends to vaporize and expand as it is released in the burner, and this effect is enhanced by the atomizing agent. Also, the atomizing fluids used are all oxidizing to the fuel, tending to initiate the first stages of combustion throughout the inner flame zone.

The main difference between oil and t a r as open-hearth fuels is the slightly higher carbon content and more complex molecular structure in tar, which tends to make the t a r flame even more luminous than that of oil but also produces less water vapor and lower selective gas-radiating capacity in the combustion gases. Tar may also require somewhat more steam (4 to 5 lb per gallon) for atomization than oil.

In many steel plants, the raw tar, a by-product of coke, is frac- tionated to remove light oils that are much in demand and corn: mand a good price outside the steel industry. The residual tar, which contains about 72 per cent or more of the original raw tar, is then used as an open-hearth fuel. This heavy tar , usually termed residual tar , topped tar, or pitch, contains a high amount of free carbon and is much more viscous than the original raw tar. The temperature a t which the t a r has a viscosity of 100 SSU usually ranges from 260 to 390 F (125 to 200 C) , depending upon the amount of fractionating that is done.

Most open-hearth shops using fuel oil will have more than one

OPEN-HEARTH FUELS

source of supply, and the physical characteristics and composition of oils vary among suppliers and even 'among different shipments from the same supplier. Under such conditions i t will be worth while to maintain a t least a nominal amount of quality control on the open-hearth oil supply. Such control may range between the extremes of spot checking periodically, which has doubtful value, to analyzing cumulative samples gathered for a turn or a day. Cumulative samples collected weekly and checked for specific gravity, heating value, and sulfur will usually serve a s satisfactory contro!~ on fuel supply if the source is reasonably dependable. Spot sampling of individual ' shipments may ,be necessary if oil is secured from jobbers on a spot-market.

Natural Gas. Like t a r and oil, natural gas is high in carbon content ( t a r 90 to 92 per cent carbon, oil 87 to 89 per cent carbon, natural gas 75 per cent carbon) and tends to give a moder- ately luminous flame. It is usually supplied to the furnace a t only a few pounds of pressure. Compared with coke-oven gas, natural gas is much denser, and i t is not so difficult to make its flame sweep down over the bath surface and avoid the licking of flame zones upward over the main roof surface.

.The use of natural gas is of course very largely a matter of local supply conditions and cost in comparison with other' possible fuels, but where i t is economically available i t makes an excellent fuel comparable in performance w'ith tar and oil. It has the additional advantage of. almost zero sulfur content, so that no sulfur will be added to the bath by the flame gases. There is some evidence, in fact, that a net loss of sulfur from the bath to outgoing gases may result from use of this fuel.

Coke-oven Gas. The use of coke-oven gas, both alone and in combination with oil or tar , is favored in some plants where an excess of this fuel is available as a by-product from nearby coke plants. Coke-oven gas is burned in the open hearth usually for one or the other of two reasons : (1) to consume coke-oven gas to balance between coke-plant production and mill heating load, and (2) to pad out short supply of fuel oil or reduce over-all fuel cost. The practice of disposal of coke-oven gas to the open hearth should be studied carefully; many plants have experienced adverse effects on fuel consumption and furnace produc-


tion resulting from its use. If it is necessary to use the open I

hearth as an outlet for marginal coke-oven gas, effort should be made to use it in small amounts on several furnaces so as to 1 minimize the disturbance to furnace operation.

Especially when used alone, coke-oven gas suffers from two handicaps :

1. The high hydrogen content of this gas (around 50 to 55 per

I cent) seems to have a tendency to break up the chain reactions in the flame that lead to the formation of carbon particles that lend luminosity or extra radiating power to the flame. I ts flame thus tends to be nonluminous, and the heat-transfer rate is somewhat decreased.

2. This same hydrogen content decreases the specific gravity of the gas mixture. Regardless of the fuel used in the open hearth, the very high temperature of the gases in the outer zones of the flame makes them lighter than the surrounding gases and they tend to rise upward toward the roof. This tendency must be offset by a sufficient downward component of kinetic energy or velocity so that the flame gases sweep low over the bath until they are caught by the draft in the outgoing end and pulled into the downtakes. With coke-oven gas, this tendency is increased by the low specific gravity, which makes it more difficult to im- part sufficient kinetic energy and gives the combustion gases more rising force. The resultant tendency to burn the roof and heat the bath surface less effectively can be overcome partly by a higher inlet gas pressure and proper direction, coupled with port slopes similar to those in natural-gas furnaces to help direct the flame. Oil or tar are frequently burned in combination with coke-oven gas to give the flame more luminosity.

I

Coke-oven gas is fairly high in sulfur, unless i t has been specially treated, and, contrary to natural gas, i t has a tendency to introduce sulfur into t3e charge during the melting down period. Despite its disadvantages, coke-oven gas may be economical, and with good port-end design to give proper flame direction and rapid combustion it can be made to perform successfully alone or in combination with other fuels.

Producer Gas. Producer gas was the fuel used originally in the open hearth; in fact, the gas producer.~was developed by the Siemens brothers in England in the same period (around 1850

OPEN-HEARTH FUELS 89

to 1860) in which they were first applying the regeneration of heat to high-temperature furnaces. I t was almost the only open- hearth fuel used for several decades, and the application of other fuels has become general only within the last 20 to 30 years. A large percentage of the present American furnaces were initially designed for the use of producer gas, most of these having since been changed to use other fuels for one or another of a number of reasons. Melting practice with fuel oil has developed faster melting with less down-time for cleanouts and rebuilds, with the result that a premium for fuel oil could be well justified. Furnace construction for fuel oil is much simpler, costs less, and requires less time. Increased demand for furnace coke has led to expansion of by-product coke-oven capacity, which has also produced a large quantity of t a r and coke-oven gas that could be used for open-hearth melting.

In modern producer-gas practice, the tendency is toward installations with individual producers for each furnace, their operation being controlled in part by the open-hearth operator. Efficient producer operation is mainly dependent on the following factors:

1. With or without steam as the motive power to force air through the producer, i t is important to have a design that will give a reasonably accurate control of the proportions of steam and air a t all rates of operation.

2. As the ratio of steam to air is decreased, the hydrogen content of the gas decreases, the peak temperature near the bottom of the fuel bed increases, and, for any given coal, a t a certain ratio of steam to a i r , fusion and "clinkering" of the ash will begin, becoming continuously more pronounced until operation becomes very difficult with respect to ash removal. It is important, however, to keep this peak temperature up as near the "trouble point" as possible by control of steam-air ratio, as, for example, by measuring the saturation temperature or dew point of this mixture, which will commonly be held a t around 125 to 135 F (52 to 57 C).

3. Other factors contributing to efficient operation are the maintenance of a deep bed of coal, reasonably uniform gas-flow resistance through this bed, and not too much ash accumulation. These factors plus the high peak temperature will favor a low


COz and Hz0 content and a high t a r and hydrocarbon content in the gases. On the other hand, the bed must not be so deep as to prevent the attainment of desired rates of gas flow, and the gas temperature leaving the producer should preferably be high enough, i.e., around 1150 to 1250 F (620 to 675 C ) , so that t a r precipitation in the mains does not become troublesome. There is inevitably some dust accumulation in mains, which should be removed about once a week. Mains should be insulated and short enough to retain most of the sensible heat in the gas so that it will enter the furnace system a t a temperature of 1100 to 1200 F (595 to 650 C).

The suspended dust and ta r and hydrocarbon content in producer gas is essential to the formation of a luminous flame in the melting chamber, so that it is important not only to have these contents reasonably high (such as t a r contents of 8 to 12 grains per cubic foot) in the gas leaving the producer but also to retain them as fa r as possible in the gas arriving at the furnace ports. This apparently demands that the producer gas be not preheated to a temperature higher than around 1850 to 1950 F (1010 to 1065 C). The reactions in the coal bed of the producer are not given sufficient time to reach equilibrium in the very short period of gas passage, so that even under best operating conditions enough excess H20 and COz will remain normally in the gas to react with all the heavier hydrocarbons and suspended carbon particles usually present. Up to the limits in preheat temperature mentioned, the hydrocarbons present will tend to be largely broken down to form hydrogen and suspended carbon particles, but these will act to produce increased luminosity or radiating power in the flame in some measure, in the same manner as the more numerous carbon particles formed in the flame zone with tar and oil fuels. But if the gas is preheated to temperatures approaching 2000 F (1095 C) or higher, the heat-absorbing reactions between carbon or methane and the CO, and H,O present become fairly rapid, and the resulting loss of flame luminosity is likely to decrease the rate of heat transfer in the melting chamber enough to more than offset the small advantage of a little larger heat storage by regeneration.

Producer gas should have a heat of combustion of around 150 to 160 Btu per cubic foot with a sensible heat content of 20 to 25

OPEN-HEARTH FUELS 9 I

Btu per cubic foot. With most of the energy loss in the producer retained as sensible heat in the gas to the furnace, such gas made from cheap coals might be considered as a very efficient fuel. This is only partly true, however, because there is a corresponding loss in ability to regenerate the waste heat leaving the melting zone. With the gas supplied a t 1100 to 1200 F (595 to 650 C) from the producers and preheated only to about 1850 to 1900 F (1010 to 1040 C) in a gas checker of limited capacity, most of the outgoing gases (about 60 to 75 per cent) must be diverted to the a i r checkers (which tend to run too hot), and a somewhat larger percentage of waste heat is dumped into the stack zone. With efficient waste-heat boilers, much of this stack loss is recovered in steam credits.

Producer gas is sometimes enriched by the admixture of around 10 to 15 per cent of coke-oven gas. Such a mixture more nearly resembles the mixed-gas fuel discussed in the following section and probably can advantageously be preheated to higher temperatures of around 2000 to 2050 F (1095 to 1120 C) . I t has been used successfully in practice and should in theory give a higher efficiency than straight producer gas. When made from high-sulfur coals, producer gas may introduce some sulfur into the bath, or, a t the very least, handicap the desulfurization process.

Mixed-gas Fuels. In theory, straight blast-furnace gas could -, be used to make steel, but it is not a practical fuel because it

would require extremely large regenerators and, with the .high rate of fuel input required and the lack of luminosity in its flame limiting the heat-transfer rates, the furnace end zones would probably be melted down before a sufficiently high melting-zone temperature cou!d be obtained. However, by mixing this fuel with around 12 to 15 per cent of natural gas or 30 to 35 per cent of coke-oven gas, preheating the mixture, and burning it in a properly designed furnace with reasonably large regenerators, it may be used successfully in the open hearth.

A mixture of coke-oven and blast-furnace gases is about the most efficient of all open-hearth fuels from the standpoint of utilization of all by-product energy within the steel-producing plant with a minimum of outside purchased fuel. The gases need not be cleaned very thoroughly, but they will tend to be saturated


with moisture, and it is important to cool them sufficiently- maximum temperature around 75 to 80 F (24 to 27 C)-to ensure a reasonably low water content. Also, the mixing arrangements should be under good control. The essential principle in the use of mixed gas is to add the coke-oven gas in proportions such that its hydrocarbon content will suffice to reduce practically all the CO, and H,O in the mixture to CO and H, with enough excess so that its decomposition during preheat will form enough suspended carbon particles to lend a fair degree of luminosity to the resultant flame in the melting chamber.

The relative size of gas checkers may be made larger for this fuel as compared with producer gas, and both regenerators may be as deep as possible. Since around 50 per cent or more of the outgoing gases can be diverted to the gas checkers, it becomes desirable to design the furnace with such dampers and gas-port by-pass arrangements as to force such a gas-flow diversion away from the air checkers. Few data are available on such factors as relative luminosities of flame and maximum obtainable efficiency with mixed-gas fuel as compared with producer gas or oil,.but several shops in England and Germany have used this fuel with marked success. I t is used to a very limited extent in American practice.

OPEN-HEARTH COMBUSTION SYSTEM

Most efficient heating of the open hearth requires that the fuel, whether liquid or gaseous, must be delivered to the furnace a t as nearly constant conditions as are reasonably obtainable. The fuel must then be injected by a suitable burner i n to the stream of preheated air in such a way as to'form a flame having the correct volume, d'irection, and luminosity for the heating conditions peculiar to the open-hearth process. The fuel-handling system, burner requirements, and. port-end design of the furnace depend upon the type of fuel to be burned.

Delivery of Fuel to Furnace. With gaseous fuels such as natural gas and coke-oven gas, pressure variation is practically the only condition that must be coped with; temperature. variations usually are not large enough to affect operations. The main gas line in the open hearth should have sufficient capacity to supply all furnaces without an appreciable pressure drop. Each furnace

OPEN-HEARTH COMBUSI'ION SYSTEM

should have an individual pressure-regulating valve to assure constant pressure across the metering orifice and ahead of the flow-regulating valve. Main-line gas pressure must be high enough to permit sufficient pressure drop across the pressure- regulating valve a t maximum flow to ensure good regulation. Piping from reversing stand to furnace should be of ample size so that disturbing line-pressure drop is avoided.

Liquid fuels, oil and tar , require adequate storage facilities inasmuch as usually they are received in batch lots by tank car or truck. Storage capacity should be ample to assure open-hearth operation in the face of a temporary interruption of fuel-oil supply caused by weather, strikes, or equipment failures. Piping to storage tanks should be laid out to insure that some mixing will take place in the storage tank. If the filling line is run to the same side as the suction line, as is the usual practice, a load of off-specification oil will find its way immediately to the open- hearth furnaces, which may upset operation for hours.

Liquid fuels must be heated to be pumped, t a r of course requiring higher temperatures than fuel oil. The storage tanks are usually equipped with steam coils that will hold the liquid a t a high enough temperature so that i t will flow from the tank. Be- tween the tank and the pump is a heater that further raises the temperature of the fuel to facilitate pumping. The fuel lines should be insulated, of course, and i t is good practice to include small tracer steam lines wrapped together with the fuel :lines for added heat. Each furnace usually has an individual liquid- fuel reheater with temperature controller to assure constant temperature of the fuel going into the burner. Different grades of oil or t a r require different temperatures, so this controller should be adjustable. A good controller a t higher initial cost will be more economical in the long run than a poor one. The oil heater also should be chosen carefully, keeping in mind that occasional cleaning may be necessary and that pressure-vessel specifications must be met.

Careful attention to details in laying out a liquid-fuel system will pay handsome dividends in dependability of operation and fuel economy. Normally, the main line is in the form of a loop with return line to the pump house so that a small flow of oil or t a r can be recirculated to the storage tanks or the suction line of

94 Chapter &FUELS, COMBUSTION, AND INSTRLIMENTATION

the pump and thus insure that lines will be kept open even if the last furnace in the shop is down for repairs. Fuel-oil viscosity varies widely with different oils and with temperature fluctuation, so the friction factor must be given close attention in deciding upon the most economical size of fuel lines. A motor- driven, self-cleaning filter is a worth-while investment to insure against stoppages from foreign matter such as rags, pebbles, or wood splinters; and for handling residual tar , a self-cleaning filter is indispensable for the removal of the larger particles of free carbon (coke breeze) suspended in the tar.

The need for surge tanks and pressure regulators or relief valves depends largely upon the type of pump installed. The steam-driven or motor-driven duplex or triplex pump has the advantages of low initial and operating cost and trouble-free service, but the pulsating discharge may have disturbing influence on burner operation. Positive rotary or screw pumps deliver a t uniform pressure but have the disadvantages of nonvariable discharge with consequent high energy loss in the recirculating system and disturbing line-pressure fluctuations if the pressure- relief valve is not well engineered. Although they are satisfactory from a maintenance standpoint for handling fuel oil, rotary and screw pumps are not suitable for pumping residual t a r because of the abrasive particles carried by this fuel. Pump discharge pressures range from 100 to more than 200 psig, depending upon the particular installation.

The fuel-oil system must be designed to minimize pulsations and to give a steady flow of fuel to the burners of each furnace a t any desired setting. In addition to the pressure-regulating devices appropriate to the pumping equipment, flow controllers are widely used. Some controllers are self-contained and others require auxiliary equipment, usually operated by a i r or electricity, which meters the oil or tar. If there is any variation from the pre-set fuel rate, the meter detects this and sends an impulse to a flow-regulating valve that adjusts as necessary to restore the predetermined flow. Without this type of flow controller, any slight variation in line pressure will cause a corresponding change in the fuel rate to the furnace. The meter should be equipped with indicator, recorder, and integrator-the indicator for use of the furnace operator, the recorder to supply a record

OPEN-HEARTH COMBUSTION SYSTEM 95

for the information of supervision, and the integrator to give cumulative fuel consumption. The controller must be sensitive and a t the same time be free from tendency to over-control.

Atomization. The first and most important step in the development of flame from liquid fuel is to change the physical state of the fuel from liquid to a finely divided suspension in a carrier gas. The general term atomization is applied to this process. Atomization seems to be necessary to develop rapid and intense combustion.

Atomization is influenced mainly by viscosity. The viscosity of a liquid fuel is a n inverse function of its temperature. The viscosity-temperature relation is not linear, as is shown by the typical curve in Fig 4-1. A small temperature change in the range between atmospheric temperature and 140 F will result in a relatively large change in viscosity, while in the range above

Fig 4-1. Viscosity-temperature relation for a typical fuel oil (9.6 API sp gr a t 60 F].

240 F, a large increase in temperature is required to bring about a small drop in viscosity. The most practical temperature range for fuel-oil circulating systems is in the neighborhood of 140 F

96 Chapter &FUELS, COMBUSTION, AND INSTRUMENTATION

Sfearn

' 017 udjusfmenf.

. . , -I cl Fig 4-2. Types of atomizers in general use.

a , pipe-tee atomizer; b, Krause oil atomizer; c, Crowe atomizer.


(60 C ) , while oil should be supplied to the atomizer a t about 220 F (105 C) . The cost of maintaining oil temperatures in excess of this is likely to be out of proportion to the benefits gained. The viscosity of most of the fuel oils in use will be below 200 sec Saybolt Universal a t a temperature of 220 F as supplied to the atomizer, and many of the oils available will have a viscosity as low as 100 SSU a t this temperature. By way of comparison, 100 SSU is roughly equivalent to 20 centipoises, which is more than 20 times the viscosity of water a t room temperature. Where lighter fuel oils must be used, lower oil temperatures, say between 150 and 200 F (65 and 95 C) , will produce better flame characteristics.

The atomizing process usually employed for the heavy, viscous fuel oils used in open-hearth melting depends on high-pressure steam or a i r to break up the oil into a high-velocity stream of fine mist. The degree of atomization determines how readily the fuel stream can mix with the enveloping blanket of highly preheated a i r and thus establishes a limit on the speed a t which combustion can be developed. The atomizing mixer must be constructed so that oil and atomizing agent are mixed intimately and uniformly. Many different types of atomizers are available. The most familiar, perhaps, is the old homemade tee-mixer (Fig 4-2a) made from an ordinary pipe-tee, with the oil supply piped into the side of the tee, the atomizing agent piped through a nozzle bushed into one end of the tee, and the burner pipe threaded into the opposite end. The turn-down range on such a crude atomizer must be exceedingly limited, and burners should be used that will mix fuel oil and atomizing agent evenly and uniformly over a fairly wide range of oil flows. Another type of atomizer (Fig 4-2b) that has been widely used provides for passage of the atomizing agent through a narrow annular ring orifice and with a sheet of oil or a number of jets directed a t right angles to the stream of atomizing agent. An expansion chamber follows the atomizing throat, which provides for thorough mixing before the atomized oil is delivered into the burner pipe. Still another successful atomizer (Fig 4-2c) uses a mixing chamber into which the oil is admitted a t the end with the atomizing agent entering through the side of the mixer a t a tangent, so that a spiral swirl is developed.


Atomizer performance is affected by the pressure needed to force the oil mixture through the burner pipe and to produce the jet energy required for the development of flame across the hearth. This pressure obviously must act as back pressure against the atomizer, which must be exceeded by the pressures of oil and atomizing agent as delivered into the atomizer. Some nozzling of each must be provided in the construction of the atomizer in order to establish a stable pressure balance and to reduce the chance, as f a r a s possible, of either one of the fluids backing up .on the other and limiting flow. Working out an atomizer that will function well for a stated fuel input is a fairly simple problem, but any departure from this fuel flow will upset the sensitive balance in the atomizer with resultant adverse influence on atomization. Reduction of oil input will have the effect of lowering the back pressure against the atomizer, which will permit more atomizing agent to pass its constriction and therefore the flame will be shortened. Increasing oil flow will have the opposite effect of lengthening the flame and reducing direction control. A double-burner installation has been used in recent years which has partially solved the problem of turn-down but has in turn introduced additional problems in installation and operation.

Emphasis has been placed on the necessity of delivering fuel oil to the atomizer a t high temperature, and i t is equally important that the atomizing agent be maintained hot and dry. Furthermore, it is important that no cooling influence be introduced that may act on the fuel mixture beyond the atomizer. The practice sometimes found of preheating oil to 200 to 220 F and atomizing steam to 550 to 600 F and then passing the atoniized mixture through 8 to 10 f t of water-cooled burner pipe, smacks of inconsistency. The necessity for water cooling on an oil burner in a simple box-end open-hearth furnace must be recognized, but some provision should be made to separate the cooling jacket from the burner pipe, either by substantial insulation or by use of a steam jacket, to prevent cooling of the atomized-oil stream.

The atomizing agents most commonly used are steam and compressed air. There are heat-content differences between the various atomizing agents. In two actual practices, in which 2.5 lb of

OPEN-HEARTH COMBUSTION SYSTEM

air or 4 lb of steam, respectively, was used per gallon of fuel oil and where total excess air (10 per cent) and leakage (15 per cent) were assumed to be the same, the heat content in products of combustion was found to have increased from 23.1 million Btu per hour for a i r atomization to 24.4 million Btu for the steam. This indicates that almost 6 per cent more heat was available above 2860 F (1570 C) with air as the atomizing agent. Flame radiation thus should be more intense with either air or blast-furnace gas as the atomizing agent than with steam. Actual experience has in some cases shown higher flame intensity, faster melting, and some fuel economy with air. In many shops, the benefits have appeared small or negligible, and with steam available a t high pressures and often a t lower cost, such shops have reverted to the use of steam for atomization.

One expedient that has proved to be helpful in plants using steam, especially in shops where the steam supply is on the wet side, is to preheat the atomizing steam to 550 or 600 F (290 or 315 C) by means of hairpin heaters placed in the flue gas ducts after the checkers. Such heaters must be designed carefully, since temperatures much in excess of 600 F (315 C) introduce complications with valves and piping and involve a safety hazard from possible failure of metals ordinarily used.

Compressed a i r finds wide use as an atomizing agent, especially in small melting shops where an adequate supply of dry steam ' : a t high pressure is not available. Compressed blast-furnace gas has been tried, even in the face of the necessity of providing a compressor and special cleaning facilities. High-pressure natural gas has been used extensively in a t least one large shop.

Burners. Considerable difficulty arises from the fact that no burner has yet been designed that incorporates means for adjustment of oil and steam jets. Usual burner assemblies are built with fixed relationships for steam and oil entrance, and experi- mentation involves removal of the burners from the furnace and machining to make desired changes. ,Another difficulty lies in the fact that a great deal of time must be devoted to the study by qualified test engineers, and these men must have the interested cooperation of intelligent furnacemen in order to work out dependable results. Fig 4-3 illustrates an open-hearth burner that is in use in several open-hearth shops and that has the feature


of steam-jet adjustment (and provision for oxygen enrichment). Conventional all-purpose oil and gas burners are built to mix

fuel and air rapidly and completely so that maximum heat libera- tion can be accomplished quickly in limited space, but such pre- mixing is quite impractical in an open-hearth furnace. The open- hearth burner must spread a flame over most of the length of the hearth so that melting will s tar t immediately a t the burner end and temperature will be maintained a t the offtake end. Relatively

Fig 4-3. Combination oil, gas, and oxygen burner. (Courtesy Crowe Engineering Company.]

small changes in burner design or manipulation may have the effect of changing a furnace from running hot on the burner end and cold on the offtake end to the opposite condition. Manipula- tion of atomizing steam is a useful aid to the furnaceman for making adjustment of flame coverage.

The part played by steam in the combustion of fuel oil is not well understood. Steam has a definitely oxidizing influence in the open-hearth coinbustion system and a t the same time involves a heat-consuming reaction, so it is possible that steam may have an effect in dragging out the flame development over the hearth in the same way that water injection smoothes out the combustion in internal-combustion engines. This may weli have a n influence on heat transfer, resulting in benefits that may outweigh the


sensible heat loss involved in raising the steam from the incoming temperature of 550 F (290 C) to the combustion-chamber temperature of approximately 2900 F (1600 C) (a fraction of the added sensible heat in the steam is recovered in the checkers).

It should be evident from the preceding discussion that an open-hearth oil burner comprises an atomizer, a burner pipe, and some means of protection for the burner pipe against the heat of the outgoing gases (refractory cover such as a "doghouse" or water cooling or both). The length of the burner pipe is established by the dimensions of the furnace and usually is worked out so that the'flame approaches the bath between the first and second doors of a five-door furnace. The burner should be adjustable so that the burner slope can be varied up or down to suit charge conditions. Many first helpers will learn to make use of the burner-slope adjustment to good advantage on every heat. One ob- jection to burner-slope adjustment has been raised, namely, that burner position is critical and a slight misadjustment may result in undue punishment of furnace refractories. Full advantage of the burner-slope adjustment cannot be realized unless care is taken to keep the aprons low enough so that the flame is unob- structed between the burner nozzle and the bath. Build-up from the outgoing gases and from overgenerous bottom repair can quickly build up the aprons to a height that will obstruct the flame and have a critical effect on heat transfer. Correction of such a condition, after i t has been allowed to develop to a critical point, is difficult because of lack of access and oftentimes must be de- ferred until the furnace is shut down for repairs.

Considerable experimental work has been carried on both by plant engineers and by burner manufacturers in an effort to develop burner nozzles that would result in improved bath coverage. Types tried have included constricted nozzles with bell- shaped contours, elliptical nozzles with major axis both horizontal and vertical, multiport nozzles, and round nozzles of various diameters. None of the special nozzle arrangements have shown any significant improvement in performance and all have proved impractical from the standpoint of maintenance, so that common practice is to use a straight burner pipe free from constrictions.

The diameter of the burner pipe should be worked out in accordance with length of hearth and fuel input to develop proper


flame coverage. If the burner pipe is too large, the flame will tend to be long and wavy with slow melting characteristics, while a scant-sized burner pipe will produce a short, sharp flame, which will melt fast locally but will have inadequate bath coverage. An undersized burner pipe also will have the effect of building up high back pressure against the atomizer, which will cause the atomizer to be sensitive to slight changes in either steam or oil pressure. For an oil flow of 500 gal per hour, 50-ft hearth length, and 200-ton heats, a burner pipe diameter of 0.75 in. will be on the scant side, 1.5-in. diameter will be too large, and good results should be obtained with pipe of about 0.9-in. diameter, which corresponds very nearly to 1%-in. double extra heavy pipe.

Port-end Design. Port-end design is dependent upon the type of fuel to be burned. Liquid fuels and combinations of gaseous and liquid fuels are normally fired through a single burner a t either end of the furnace. The velocity and weight of the flame produced by these fuels is sufficient to give proper flame direction without the aid of special port-end design. When any straight gas fuel is burned alone, on the other hand, a relatively complex port-end construction is necessary to direct the incoming air in such a manner that i t will mix with the gas to form a flame that will carry through the center of the furnace in contact with the charge. Details of construction are described in Chapters 1 and 3.

The port-end design for furnaces burning natural or coke-oven gas is similar to furnaces burning producer gas except for the method of introducing the gas. Coke-oven or natural gas is introduced into the furnace through burners installed through the side walls of the furnace and just above the apron, except that when these gases are burned in combination with oil or tar , the gas-burner pipe may be integral with the liquid-fuel burner inserted through the port end. Producer gas is introduced through a burner the axis of which is parallel to the center line of the furnace. These burners slope downward toward the bath. Proper maintenance of this burner port and the apron or slope in front of it, along with the bridge walls, is a vital factor in the operation of a furnace fired with producer gas.

Consideration of port-end design presumes an understanding of gas flow through the furnace system. Checker and uptake draft provides the force that draws combl~stion air into the fur-


nace, so that the air stream rises to the top of the uptake under considerable velocity directed vertically. If an opening were left in the furnace roof above the uptake, the air stream would pass directly upward into the atmosphere and no air would enter the furnace chamber. Actually, the air stream strikes the furnace roof and is turned into the furnace chamber. If the roof should be built flat, the air stream would largely hug the roof, with the result that fuel and air would mix slowly and combustion would be delayed. Obviously then, the contours of the port ends must be shaped so as to give such direction to the incoming air as is necessary to produce the flame development required. The port contours for liquid fuels such as oil and tar can be relatively simple, while for producer gas, carefully worked out contours are vital to successful performance of the furnace. Steep roof slopes, as well as pronounced constriction a t the throat, are necessary to give the direction and velocity needed to burn producer gas properly. Such port-end construction not only increases furnace building cost but also introduces an operating problem because these same areas must serve as exit ports for flue gases when fuel is on the opposite end of the furnace, and the constriction desirable for the incoming end will be prohibitive for outgoing gases. For this reason, all port designs must be compromises to some extent between the generously sized ports desired for outgoing gases and the constriction needed on the incoming end for control of the air stream. Recent trends in construction for furnaces designed for liquid fuels have been toward less and less constriction of both roof and side walls, and this has had a favorable influence on both rebuild cost and furnace life, but some reservation must be made as to the extent to which opening up port slopes and uptakes can be carried.

A certain amount, of constriction must be introduced into the incoming air passages between the checker bridge wall and the furnace bridge wall to insure that the incoming combustion air stream is distributed evenly over the furnace bridge wall. If the constriction is insufficient, the air stream will be concentrated on the side of the furnace farthest from the checker, with the result that uneven flame development will punish refractories on that side of the furnace whi!e leaving a slow-melting spot on the opposite side. Furthermore, under such conditions, the air stream


may enter the furnace hugging the roof, so that flame development may be retarded and a considerable portion of the air supply may travel across and out of the furnace without becoming involved in the combustion process. The necessary constriction may be introduced either by sizing the uptake area closely, which puts the constriction in the horizontal plane, or by raising the furnace bridge wall, which places the constriction in the vertical plane. The latter is preferable because the aprons will quickly build up to the top of the bridge wall and the resulting slope will have some influence, in conjunction with the slope of the furnace roof, in directing the air supply downward to and around the incoming fuel stream.

Combustion Air Supply. The function of combustion air s u p ply has received only perfunctory attention even though i t has a critical influence on furnace performance. There are still many open-hearth furnaces in operation on which combustion air regulation is limited to the movement of a saucer damper over an opening in the waste-gas flue beyond the checker. It should be obvious that such elementary means of regulation cannot lend itself to either close adjustment or adequate flow measurement. If consideration is given to the fact that more than 20 tons of air must be brought into an open-hearth furnace every hour, of which 77 weight per cent, or better than 15 tons, is inert gas that must be heated to furnace temperature, i t becomes evident that lack of close control of air supply may have a critical effect on heat-transfer potentials.

Regulation of a i r input is complicated by several factors peculiar to the cross-regenerative combustion system characteristic of open-hearth furnaces. Air is moved into the furnace by forces developed entirely by the chimney effect of the checkers and uptakes. Such forces are small in magnitude and quite sensitive to variations in furnace balance. Careless reversing practice, for instance, in which fuel is allowed to remain on one end of the furnace for an unreasonably long period of time, will unbalance the air system, since the incoming air checker will be cooled unduly and the outgoing checker will be overheated. The resulting disturbance to the balance between the checker drafts on the two ends of the furnace may persist for some time and may, indeed, become cumulatively worse unless some action is


taken to compensate for the unbalance. The advantage of balanced checker reversal, which will be discussed a t length later, is usually cited as a gain in air preheat; but it is probable that the influence of this device in promoting balanced checker draft and, as a result, balanced a i r supply, furnishes still greater justification for such instrumentation.

The open-hearth system from the furnace uptakes to the stack can neither be built nor maintained absolutely gastight. I t is inevitable, therefore, under the prevailing condition of draft, that a certain amount of unmeasured air will be drawn in either to increase the combustion a i r supply and lower a i r preheat on the incoming end, or to cool down the furnace gases on the outgoing end. The influence of such infiltrated a i r can in time build up to a point where furnace performance may be seriously affected. A systematic program must be carried on throughout the furnace campaign to keep the exposed surfaces of flues, checkers, fantails, and uptakes carefully sealed against cracks. Open-hearth plants can be laid out so that access is provided to ,-

all exposed wall surfaces, which will simplify the job of sealing. The double checker arrangement still in use in many plants com- plicates the sealing problem because much of the wall surface is inaccessible ; but ingenious and persistent operators can usually find ways, with the aid of the spray-gun equipment now available, of reaching practically all of the exposed surfaces. h he amount of a i r infiltrated through masonry cracks must be 'held to the absolute minimum if effective regulation of combustion a i r supply is to be established.

Metering of combustion a i r is not a simple problem. All experience with the problems of measuring fluid flow, irrespective of whether the fluid be a liquid or a gas, demonstrates that the first requisite must be a condition of stable flow. Consideration of the flow conditions in an elementary open-hearth system where the a i r source is the atmosphere, with its characteristic fluctuations of pressure and temperature, and where the driving force is the draft set up by the chimney effect of the checkers, amounting to perhaps 1/4 or 1/3 in. water gage, will make it evident that the stability of flow requisite to adequate metering simply cannot exist. The function of "forced air," a term heard frequently among open-hearth engineers, is to set up conditions


favoring adequate metering and flow regulation. The elements involved in the arrangement for good metering conditions will include an enclosure to box in the existing air inlet, an air duct from a fan to the air inlet, a constriction in the duct to build up resistance to flow, a regulating damper that can also be made to serve as a shutoff damper, a fan with drive motor, and a measuring orifice or venturi throat. The duct should be amply sized so that velocities need not exceed 30 f t per second.

Compensation should be made for ambient temperature fluctuation. The weight of oxygen delivered to the furnace will vary roughly 2 per cent for every fluctuation of 10 Fahrenheit degrees in ambient temperature if the volumetric flow rate is held constant. Fluctuations of 80 deg within 24-hr periods are not a t all uncommon, which means that oxygen delivery to the furnace may vary as much as 6 per cent for the same air and fuel settings. Such variations are bound to have an adverse influence on furnace performance and fuel economy. Many modern blast furnaces and cupolas are provided with control equipment built into the blower system, which compensates automatically for temperature fluctuations. The open-hearth system is certainly no less critical considering the dominating influence of the combustion air on furnace performance. Incorporation of air-temperature compensation and air-flow control into a measuring installation such as has been suggested in the preceding para- graph is neither difficult nor unduly expensive.

Oxygen for Flame Enrichment. The use of oxygen as a tool for increasing open-hearth production has made great strides since 1946. Interest was first directed to the use of oxygen as an agent to intensify combustion to speed up melting and increase production, but as facilities for oxygen were made available, several other uses for oxygen were found that have proved advantageous. The use of oxygen instead of ore for working down carbon, or in addition to ore, has grown to a point where this usage overshadows that of flame enrichment (see Chapter 8) .

The effect of oxygen used for flame enrichment is to reduce the quantity of combustion gases so that flame temperature is increased and the amount of heat lost in flue gases is reduced. More fuel can be burned in the same space and time, with the result that melting speed will be increased. Flame-enrichment

PRACTICAL FUEL-ENGINEERING CONTROL 107

oxygen is used normally during the meltdown period up to the time of hot-metal addition because the greatest benefit comes from faster meltdown of loosely charged scrap and also because the furnace is relatively cold during this period and there is little danger of damage to furnace refractories. The benefits of oxygen used for flame enrichment are likewise greatest for high-scrap charges where long meltdown. time establishes the limit on furnace production. Flame enrichment has been used to very good advantage to help maintain open-hearth production during periods when the hot-metal supply was restricted because of blast- furnace rebuilds.

The detail of burner design seems to be determined mainly by individual preference or plant conditions, and quite significant changes in burner design seem to produce small or negligible changes in flame condition. There may be some advantage in placing the oxygen jet below the oil stream.

The amount of oxygen used has varied from 260 cfm to a maximum of 700 cfm. With the present high cost of oxygen, there seems to be a tendency for plants to use less than 500 cfm. The general practice is to add about 10 to 20 per cent of the combustion requirement of the fuel as oxygen. This decreases the volume of flame zone in the furnace so that, by maintaining the normal air flow, an additional 10 to 20 per cent of fuel may be burned and the melting rate increased. The top limit seems to be set by the amount of fuel that can be used without combustion occurring in or close to the downtakes.

PRACTICAL FUEL-ENGINEERING CONTROL IN THE OPEN-HEARTH SHOP

Over the past 30 years, figures representing total heat consumed per ton of steel produced have dropped from values frequently as large as 7.0 to 7.5 million Btu down to those as low as 2.5 to 3.0 million Btu in a few shops. The significance of this obviously large improvement in fuel-engineering control can best be appreciated only in comparison with a reasonably accurate value for the minimum possible heat requirement for making a ton of steel. Such a minimum value cannot be accurately estimated ; the amount of net heat input to the bath for melting and reactions is around 0.8 million Btu per ton for a typical favorable

108 Chapter &FUELS, COMBUSTION, AND lNSTR(JMENTATl0N

charge with 50 per cent hot metal, but the accompanying high- temperature heat required to offset the inevitable heat losses from this zone of the furnace is a much less definite value. Com- bustion calculations place the maximum practical value of efficiency in high-temperature heat production a t about 60 per cent, indicating an absolute minimum of fuel heat for the bath requirements alone of around 1.4 million Btu per ton. The best present figure of 2.5 million Btu is thus apparently rather close to the practical minimum, especially when i t is considered that in duplexing furnaces about 1.5 million Btu per ton. is used merely to keep the furnace hot with negligible contribution of useful heat to the charge.

The factors making it possible to cut fuel consumption by more than half (7.0 to 2.5 million Btu) may be summarized as follows: In early furnace practice, very little attention was given to either draft control or a i r leakage; the latter was oftensas high as 50 per cent. Too much excess a i r was often present, and regeneration efficiency was on the low side a t around 30 to 35 per cent. With these three factors of leakage, air-fuel proportioning, and regeneration efficiency so inadequately regulated, the earlier furnaces had an approximate high-temperature heat efficiency (see Chapter 21) of about 30 per cent as compared with a maximum of roughly 60 per cent. These efficiency factors thus explain a t most a reduction of about 2 to 1 ; the larger improvement already obtained is due mostly to the combined effects of insulation above floor level and higher production rates caus- ing a decreased heat loss from the high-temperature zone of the system. (Insulation below floor level exerts its effect through reduction of leakage and improved regeneration efficiency.) Also, the more efficient furnaces require a lower average rate of fuel input per ton of furnace capacity, and, as a result, the smaller volume of flame gases is probably able to transmit its available heat more efficiently to the bath during its short period of 2 to 3 sec in the melting chamber. The point to be emphasized is that a given improvement in the inherent thermal efficiency tends to have a cumulative effect larger than would be expected from the percentage of additional useful heat made available.

The fuel engineer may use many other practical methods in the open hearth, but in general his object is to put the control

PRACTICAL FUEL-ENGINEERING CONTROL

on a rational basis and to obtain the control data in terms of the various fundamental elements affecting thermal efflciency. Such control is more important than the mere saving of fuel costs would indicate. Furnaces that are kept tight, efficient, and "sharp-working" normally give higher production rates; also, heats come along a t a more regular schedule, labor costs for repairs are'lowered, checkers are easier to keep clean, and many problems of production control are made easier to solve.

A program having for its objective such rational control of open-hearth fuel economy must be built on the basis of adequate

I I Y I I I I I I I I I. I I U I I l l ,#c?*r L P ' ~ ? ' ~ 7 6 9 ~ ,,, M.,,vd~ ~,.k/+!&-I.Jr-/76;#~

Fig 4-4. Daily 'open-hearth fuel report. . .

performance data. Emphasis is placed on the importance of this factor because a great deal of detailed work is involved in the collection of such data. While flue-gas analyses, infiltration checks, draft differentials, and temperatnre surveys through the system will be useful tools in a program to maintain furnace per-

1 10 Chapter &-FUELS; COMBUSTION, AND INSTRUMENTATION

formance a t high level, that objective will not be reached unless detailed, comprehensive records of performance are maintained.

A daily report of heats made by irldividual furnaces together with fuel consumption data might be similar to the one shown in Fig 4-4. This report will be particularly useful in a shop or- ganized to operate against standards. Variables such as hot- metal addition, grade specifications, charging delays, p i t delays, bottom making, and others all affect furnace performance. Some of these variables, notably hot-metal charge and grade specifications, are beyond control by open-hearth supervision while others are subject to their influence. Standards can be set up that will make allowance for conditions beyond the control of the operator. Comparison of actual shop performance against such standards will prove useful in working toward and maintaining a good performance.

INTRODUCTION TO CONTROL AND MEASURING EQUIPMENT FOR THE OPEN HEARTH

The difficulties of instrumentation and automatic controls in the open hearth reside mainly in the complexity of functions involved, which include: (1) control of fuel combustion, (2) protection of refractories, and (3) aid to close metallurgical control,

- as well as saving of manual labor. Probably no other large-scale batch process has such a combination of extreme and variable service conditions. Iron oxide is all-pervasive in the furnace system, first as a liquid that acts as a nearly universal solvent for all available refractory materials in the *melting chamber and end zones, then as a finely divided solid that deposits to- obstruct flow of gases and heat in the checkers, flues, and waste- heat boilers. The raw materials of scrap and iron are always variable enough to cause changes from heat to heat; for example, timing and intensity of ore boils, rates of charging and melting of scrap, and carbon content a t melt are always sufficiently variable and unpredictable that every heat sets up a new problem in pattern of air and fuel input and metallurgical working. Erosion and dust deposition keep changing the furnace lines,.and gas-flow areas throughout each furnace campaign. Shop conditions of temperature fluctuation, vibration, and dirt are severe on sensitive instruments and add to the difficulty of maintenance.

CONTROL AND MEASURING EQUIPMENT I l l

Much progress toward nearly complete automatic control has been made in spite of these handicaps,'though a great deal remains to be done to perfect instruments and to adapt controls to the inherent complexity and variability of the process. Auto- matic draft control has been found superior to manual control in view of the changes in fuel demand, gas evolution from bath, and gas-flow resistances, and yet the ultimate in simplicity and rug- gedness of these instruments may still be far from attained. Various automatic reversal systems are available but are used only in a moderate percentage of all shops ; the best compromise here between simplicity and completeness of-control is not yet decided. Temperature measurements in flues, checkers, and melting chamber need more development toward greater accuracy and easier maintenance but have already helped to limit roof temperatures and guide fuel input and furnace reversal. Methods of fuel-rate and air-flow measurement and control are well developed and are used to a greater or less extent in nearly all shops. Automatic proportioning of fuel and air is used in a few shops but is handicapped by the variable pattern of a i r demand in various stages of differing heats as well a s a lack of knowledge of the optimum excess air for each set of conditions. In- volved here is the fact that even with much progress already made, a fully satisfactory methpd for routine measurement of excess oxygen in outgoing gases has yet to be developed, so that use of this obviously desirable criterion of combustion regulation has been limited. Measurement of air or gas temperature in regenerative zones is still a practically unsolved problem. Bath- temperature measurement has made great progress but is well developed in only a few shops.

When controls are more nearly perfected, maintenance troubles should be minimized, and the more perfect training of both maintenance men and furnace operators to cooperate toward smooth performance of automatic controls should ulti- mately be a big factor in making the job of steelmaking both more pleasant and more efficient. In the meantime, however, the expenditures of extra maintenance labor and the experimenta- tion with new methods and apparatus is an unavoidable cost of developmentJ'in this field.

Space here permits only a general description of instruments

1 12 Chapter 4--FUELS, COMBUSTION, AND INSTRUMENTATION

and control methods plus some of the difficulties peculiar to this process. For 'details of instrumentation, the reader may consult the references a t the end of this chapter.

BATH-TEMPERATURE MEASUREMENTS

The practical aspect of bath temperature from the operator's viewpoint lies in the degree of superheat above the liquidus or start-of-freezing temperature. This factor has some importance even early in the refining period, to indicate the time and size of ore feed possible without too much chilling of the bath, and also, in conjunction with the bath carbon, to give the first helper a

Carbon, per cent

Fig 4-5. Temperatures i n steelma,king.

better idea of how much more heat input will be needed before the steel is in shape for tapping. This degree of superheat is of more importance as the heat app'roaches its desired composition

BATH-TEMPERATURE MEASUREMENTS 113

a t tap. Some 100 to 130 Fahrenheit degrees is inevitably lost in tapping and ladle filling (more if basket pouring or reladling is intended). At the pouring platform, another 50 to 80 Fahren- heit degrees of superheat is normally needed to insure reasonably clean pouring, so that tapping temperatures are usually from 150 to 200 Fahrenheit degrees above the initial freezing temperature.

The chart of Fig 4-5 is given here in an attempt to help toward a better concept of temperature in general in open-hearth steelmaking. I t is not possible to make such a chart really accurate; it can apply to an average of plant conditions fairly well but may not be true for any special conditions. I t may serve, however, as a frame of reference to give a more graphic concept of the general trends. The shaded area between "freezing begins" and "freezing complete" shows the freezing range for iron- carbon al!oys between 0 and 2.0 per cent carbon, and shows why the tapping and pouring ranges can drop in temperature as tap carbon increases, provided other conditions remain constant.

The limiting temperature range for a silica roof a t 3010 to 3040 F (1655 to 1670 C) is only slightly below the true temperatures above which most commercial silica brick will start to melt after having become slag-saturated in service. (If alumina plus alkalies are higher than about 1.1 to 1.4 per cent, this limit is probably even lowe;:) I t is rarely economical to operate the roof surface above this temperature level for even short periods. With flame on, however, the apparent surface temperature of a well- glazed silica roof will usually be some 50 to 80 Fahrenheit degrees higher than true temperature because of reflection of flame radiation, so that a roof pyrometer limit can usually be set a t 3060 to 3090 F (1680 to 1700 C) except when the roof is very new or the flame very short (see following section).

As fa r as is known a t this writing, all other values on this chart correspond c!osely to the true or thermodynamic temperature scale, but any given method of measuring bath or pouring- stream temperatures may not give values in accordance with this scale. F'or example, the emissivity value of 0.40 that is usually assumed for optical pyrometer readings on steel streams in the open should apparently be more nearly 0.45 to 0.48 for the range 2800 to 3000 F (1540 to 1650 C). This explains why tapping

1 14 Chapter &FUELS, COMBUSTION, AND INSTRUMENTAI-ION

temperatures of 3000 to 3030 F (1650 to 1665 C) are,often re- ported for very low-carbon steels and certain other special heats, indicating that the silica roof should have been melting down near tap time on many such heats. On the t rue temperature scale, tapping temperatures from furnaces with? silica roofs probably never exceed about 2970 to 2980 F (1630 to 1640 C) . The reader is urged to remember that the tapping range show11 here is typical, but not true for all conditions. :Heats from furnaces of 30 to 50 tons or less, heats for small castings, heats handled by reladling or basket pouring, etc., will usually require highel: tapping temperature.

Qualitative Methods of Estimating Bath Temperature. 'AS evolved by many steelmelters by long years of operating practice, the commonest methods of estimating superheat a re spoon pouring and rod melting. The former is perhaps the more common. The helper carefully "slags" a sampling spoon, dips out a full spoon of metal, and pours i t slowly on the 'floor, often with a slight oscillating motion of the spoon while pouring. Observa- tion of the distance of flow of the metal on the floor, or the amount of skull left in the spoon, to estimate superheat' is obviously difficult to.describe and must simply be learned by practice, but i t is fairly reliable with a consistent technique. Usually, a metal that begins to freeze in the spoon early in the period of pouring is "too cold," and one that pours completely, with no skull, is usually "hot enough" for all purposes, or may even be "too hot." In this method, as also in observations of slag and roof-surface temperatures in the furnace, the first heloer is as- sisted by his blue~glasses, .which act a s a crude color pyrometer;

In the rod-melting method, a typical practice is to take a low- carbon rod a t least 10 to 12 f t long and about 1% to 1% in. round or square, preheat a 3 to 4-ft length to red heat, and bend it in the door to a 20 to 30-deg angle. This length is immersed in the steel and moved steadily with a weaving motion until the decreasing resistance tells the helper that i t is melted off close to the slag surface. The rod is now removed carefully and the end contour left after melting is observed. A "cold" heat will normally show a long (4 to 8 in.) smooth taper ("rat tail!') a t the melted end; a heat "hot enough" should usually be cut off nearly sharp with only some 1/2 to 1 in. of irregular taper ; and a

BATH-TEMPERATURE MEASUREMENTS

"very hot" heat may be cut off sharply with little or no convexity or even a slight concavity of the end. This procedure is again a matter of practice, reproducible technique, and availability of stirring rods of constant size and composition.

These subjective methods of estimating.superheat are still the essential basis of operation in most shops. They are simple and direct and, in the hands of a crew of'melters and first helpers who are careful and well trained, they are perhaps accurate enough for most purposes. The reasons for efforts to supplant them by instrumental methods, a t least for the final temperature adjustment near tap, are obviously as follows:

1. Partial or complete elimination of human fallibility. 2. Increased accuracy, in values that may be more precisely

specified in terms of temperature figures for each individual grade or practice.

3. The securing of figures that may be recorded whenever desired, giving data of potential usefulness for various control purposes.

The "Quick-immersion" Thermocouple Method. Early efforts to use high-temperature thermocouples (available materials are confined largely to platinum alloys, tungsten, ,molybdenum, graphite, and carborundum) were impractical largely because of the weakness or lag present with nearly all protection-tube materials plus the absence of practical, quick-acting recording instruments. The difficulties involved have now been largely circumvented by: (1) the development of practical, accurate "electronic" type voltage recorders, which give a record within 3 to 5 sec, and (2) the use of small-wire thermocouples (of platinum to platinum-rhodium alloy) protected for a few inches back from the junction by only a thin, replaceable protection tube of fused silica.

The "quick-immersion" thermocouple technique can be illustrated best by the diagram of Fig 4-6, showing a comparatively light-weight assembly. The essentials in this design are: (1) a replaceable tip assembly of graphite plug and thin silica protection tube, (2) a section of refractory-coated tube, which passes through the slag, also replaceable but a t longer intervals, and (3) a movable connection to extension leads that is always within the insulated portion of the pipe handle.

. . . . 4'2 92 'd//en set screw I" couphirq~

G 0

,. Redu~ercou~//hgs-14; 1': '! ,,, %VWhnq ~. , , 8

: Y

- - - - - - - - . - - %'pibe /OU'?OU~ with sfd pipe fhd, bofh ends - -- - - -- -

Fig 4-6. -Immersion thermocouple, weight 34 pounds. z - .

BATH-'TEMPERATURE MEASUREMENTS 117

The hot junction dips about 4 to 8 in. into the steel and is heated through the thin silica tube to very nearly the temperature of the steel within some 10 to 15 sec, and (if connected to a fast-acting recorder that will follow this rapid rate of heating) the pyrometer may then be immediately removed from the furnace. The silica tube must be replaced after each reading (or a t most two or three). Most shops have found that the thermocouple wire does not suffer appreciable contamination in this short period of immersion provided no water or other source of gas is present in the design. Usually accidental breakage or melting takes place before couple contamination. Short platinum lengths may be used and replaced, or a longer couple may be used with provision for pulling out a reserve length as needed and moving the cold-junction position.

This method has an advantage in direct standardization of the temperature scale with closely reproducible thermocouple alloys. It is somewhat expensive in replacement and maintenance items, but a t this writing i t is in use in several electric-furnace and open-hearth shops in America, England, Sweden, and other coun- tries. First-class maintenance and some little care and judgment in measurement are essential. The pyrometer measures the temperature of only one small zone a t one point in the bath, and although the end-to-end differences in temperature in the metal in a normally boiling bath are fortunately rather small, the bottom layers are normally cooler than the upper layers of metal near the slag. The difference may be only some 10 to 20 Fahren- heit degrees in a bath on a good boil that is not being rapidly heated, but with more rapid heating near tap, the vertical gradient may be a t least 30 to 60 Fahrenheit degrees according to limited data now available. The slag is usually hotter than the metal, so that depth of immersion is critical.

The "Blowing-tube" Method. With the variable and indefinite temperature gradients present between liquid metal and upper slag surfaces, plus the corrosive effect of the slag and the limitations in refractory materials that might be inserted into the bath, no radiation o r brightness measure of bath temperature was successful until the development of the so-called blowing tube. In essence this is simply a heavy-walled pipe 6 or 7 f t long with a side tube for compressed air near the closed rear end

1 18 Chapter 4--FUELS, COMBUSTION, AND INSTRUMENTATION

and with the front end stopped down by a steel plug to about a %-in. opening. The open end is inserted through a wicket hole some 10 to 15 in. into the bath with the a i r flowing, and the bubbles forming a t this opening under the metal radiate back through the plug opening a t an intensity closely approximating that of a black-body radiator a t the corresponding steel temperature. This brightness value was read originally with an optical pyrometer and good temperature values were so obtained. The human element involved and the extra man required, however, soon caused the incorporation of a radiation receiver within the blowing tube so that the radiation intensity value is automatically recorded on a high-speed instrument. The essential point in this technique is again the factor of speed, as with the quick-immersion couple. The tube need be inserted only some 8 to 10 sec in the bath; the slag-coated steel tube can be only partially heated up in this short period, which largely eliminates

Fig 4-7. Blowing-tube pyrometer in steel bath.

melting, oxidation, or distortion. With care, a tube may be used for several hundred measurements over about six months or more and is simple to replace or repair. The diagram of Fig 4-7 illustrates the position of the blowing tube during a temperature measurement.

BATH-TEMPERATURE MEASUREMENTS 119

In one development of this tube, a photocell receiver (commonly the blocking-layer type, which converts, part of the radiation energy into its own electric current output, a few micro- amperes a t a potential of a few millivolts) is housed in the rear end of the tube behind a sealing window of glass. An inner tube with diaphragms (to eliminate reflected radiation) extends from the cell to the heavy steel tip, the cone of radiant energy being defined by the first diaphragm near the photocell and the tip opening through which the air flows into the liquid steel. The cell is permanently connected through electrically shielded con- ductors to an electronic-type recorder having a speed of response of about 3 to 4 sec. A heavy push-button switch in the end of the tube may be used to close the recording circuit for some 5 to 8 sec during the actual immersion of the tube in the liquid metal. Air pressure as delivered to the blowing tube must always be held up to a minimum level, usually around 10 to 20 psi. The flow of air is both inside and outside of the inner diaphragm tube.

In use, the air pressure is turned on and the tube is inserted through a wicket hole, slagged, and then plunged as deeply as possible down into the bath to a depth of a t least a few inches below the slag-metal level. The push-button switch is immediately closed and held for 6 to 8 sec, then opened. The tube is immediately removed from the furnace and laid on a floor rack. Adher- ing slag and steel are knocked off and the tip opening is inspected for steel fins or ring skulls before another'reading is taken. The tube should not be in the furnace for a total of more than 12 to 15 sec, and the whole operation requires only about a minute. Successive readings with the same tube should not usually be closer than 6 to 8 min apart, to permit cooling of the tube.

The photocell commonly used has an output that increases with about the twelfth power of the absolute temperature, giving high sensitivity over the upper temperature range of 2850 to 3050 F (1565 to 1675 C) in which it is usually- most needed. Cell response is nearly instantaneous so that time for measurement in the bath is limited by the speed of the recorder and of the operator. The main requisites of proper installation and maintenance are as follows:

1. Clean a i r a t proper pressure is essential ; normally this in-


volves a pressure-control valve and an oversize trap to remove oil and water, dirty air being fatal to accurate measurement.

2. Photocells should not be heated above about 120 to 140 F (50 to 60 C). Checking the output with a small checking box (standard brightness lamp, milliammeter, etc.) may be done on the open-hearth floor and need not be oftener than about once a week unless an overheating of the cell or erroneous reading is suspected.

3. About the only routine maintenance items a re frequent checking of the diameter of the tip opening with occasional ream- ing out, inspection of tube for excessive warping and of the inside of the tube for obstructions in the optical path, and occasional cleaning of the surfaces of the sealing glass window in the cell holder.

Maintenance requirements in this method are not really bur- densome but must be followed meticulously to insure sufficiently accurate and reliable measurements. A skilled first helper or melter can estimate superheat by the simple procedures described earlier to a relative accuracy of perhaps 30 to 50 Fahrenheit degrees, and the instrumental measurements a re not good enough to be worth the trouble unless their accuracy is nearly a!ways kept up to about plus or minus 10 to 15 Fahrenheit degrees. With the cooperation and interest of the furnacemen and some one in the shop really interested in maintaining accuracy, this appears quite feasible. A fairly simple method of checking absolute accuracy of this whole blowing-tube-photocell unit is to sight it through a wicket hole a t the roof surface with flame off near the end of a heat, checking against an accurate optical or radiation pyrometer, black-body conditions being closely approached under these conditions.

Another modification of the blowing-tube pyrometer involves the use of a small total-radiation pyrometer receiver unit (about 1 in. in diameter by 4 in. long), which is housed in a radiation- shielding housing and set down inside the tube not very fa r from the tip end which enters the bath, the receiver being corrected more or less for ambient temperature variations. Except for details, the design and the method of measurement are otherwise very similar to those for the photocell unit just described, an electronic-type recorder with about a 3 to 4 see speed again be-

ROOF-SURFACE TEMPERATURE 121

ing employed. The checking device for this pyrometer is a "standard" total radiation receiver which may be attached parallel to the blowing tube, the two being connected in opposition through a galvanometer. Sighting the two parallel units through a wicket hole gives a galvanometer deflection only when the immersion unit is out of adjustment, and this is corrected to zero to place the whole immersion unit again in proper calibration. At this writing, it appears that further practical experience will be required to show whether there are appreciable differences in usefulness between this unit and the photocell type. Both are now in active use in a few American open-hearth shops.

The electrical circuits of blowing-tube pyrometers can easily be adjusted to bring the temperature scale in close agreement with the immersion-thermocouple scale, which is accepted as being reasonably close to true temperature. The older "steel-plant temperature scale" based upon optical pyrometer readings de- parts from the thermocoup!e scale by an amount which increases regularly with increasing temperature but varies in absolute magnitude according to optical pyrometer practice from plant to plant. For example, optical readings may be 40 to 70 Fahrenheit degrees too high a t a temperature of 2900 F (1595 C) determined by thermocouple.

Both the quick-immersion and blowing-tube bath pyrometers appear promising, and i t seems likely that the use of one or.,both of them in open-hearth shops will increase in the future. Bath temperature is of some importance to control of steel the degree of importance varying with the type of steel and product. Tapping temperature is also of practical concern in relation to ladle skulls, stool stickers, ladle life, casting defects, and similar factors of cost or productivity as well as quality.

ROOF-SURFACE 'TEMPERATURE MEASUREMENT AND CONTROL

The primary purpose of a continuous indication of inner surface temperature on the main roof arch21 lies in the protection of the refractories. As discussed in more detail in Chapters 3 and 18, even the best quality silica brick have a definite temperature limit (Fig 4-5), which is below that of the flame and not much above maximum operating temperature in the bath. Above this limit, the skeleton of silica-rich crystals comprising the


framework of load-bearing refractory substance . in ..the arch melts and flows downward slowly in stalactites of a viscous liquid. Besides the loss of an appreciable layer: of brick thickness in even 5 to 10 min of overheating, the result is a roughen- ing of the arch surface. This surface, when'smooth and even, is a very good reflector of radiant heat down on the bath. When roughened by even a short period of overheating, it not only radiates and reflects heat less effectively to the bath but con- versely it absorbs heat better and overheats more easily. This effect of even 1/2 hr or less of overheating may last for many days thereafter, until the arch surface is gradually smoothed out again by the normal fluxing erosion.

Thus it is important not only for maximum roof life but also for maximum driving rate to avoid even shor t -periods of overheating of the roof above its limiting temperature. This limit is somewhat variable and not precisely known, but probably varies from around 3010 to 3020 F (1655 to 1680 C) for poorer brick in a basic furnace to perhaps 3050 to 3060 F (1675 to 1680 C ) for best brick in an acid open hearth, with 3030 to 3040 F (1665 to 1670 C ) a good average for a silica arch in a basic furnace; thus the limit is known .accurately enough for practical purposes. At this writing, very little definite information is available about the optimum or safe maximum temperature range for the basic arch brick now under development. Pre- sumably, a roof of basic brick will withstand higher operating temperatures than a silica arch, but there is very likely some optimum limit; and it also appears best to avoid cooling or heating the roof too rapidly or below a minimum level. Thus, even with all-basic furnace construction, roof-temperature recordings are likely to be valuable.

A continuous record of roof-surface temperature is also useful as a general guide to furnace driving conditions. Knowledge of the rate of temperature rise during melting and the closeness of average approach to the maximum safe temperature is a very appreciable help to the first helper as a guide in working toward maximum production rate. At certain times, a rapid rise may warn of the advent of foamy or heavy .slags, and a t other times the roof-temperature chart may help to give an approximate idea of bath temperature.

.' ROOF-SURFACE TEMPERATURE - 123

An experienced first helper can detect excessive roof temperatures very well by eye, which would seem to make any measuring instrument more or less unnecessary. However, this only ob- tains when the first helper (1) has the requisite experience to be a good judge of such temperatures and (2) has the time and opportunity to keep a reasonably close watch on the roof. Thus there is no doubt that a method of recording by instrument that is accurate, reliable, continuous, and involves little trouble or maintenance will be not only superior to the first helper's eye but a definite help to him in a frequently harassing job. But the comparison is also complicated by the facts that (1) no method so f a r developed gives an entirely accurate measure of the inner arch-surface temperature and (2) ,the instruments all involve a certain amount of trouble and maintenance in operation. Thus, the practice is still somewhat in dispute; nevertheless, devices for either simple indicating and recording or actual automatic control of roof temperature are now in use on a large percentage of open-hearth furnaces in American shops.

The uniformity of surface temperature over the whole large area of the melting-chamber roof is normally good enough that a continuous record of the temperature in one small fixed area is also rather closely representative of the whole area. Any appreciable departure from this condition is nearly always caused by the too close approach of a flame zone to a certain roof area. Thus, too long a flame may overheat the outgoing roof slope or knuckle, or too rapid flame development may overheat the roof on the incoming end. Other causes of local overheating are deflection of flame by high-piled heavy scrap or localized eddying in the flame from irregularities in port slope or apron surfaces. Such conditions may be compensated to only a limited extent by lowering the roof-control limit below the true limiting value, but this is not a good solution; every effort should be exerted to avoid irregularities in arch-temperature distribution.

Two practical methods of measurement of roof temperature have so fa r been developed : (1) sighting directly through a hole in a side wall on the inner roof surface; (2) sighting down into a hollow roof block in the arch on a surface kept within about one inch or less from the inner arch surface.

Side-wall Direct-sight .Method.. In this scheme, a permanent


hole of 3 to 5 in. diameter, sloped upward toward the roof, is maintained in either back or front wall of the melting chamber. This hole may be formed by: (1) a water-cooled port fixture in the wall, (2) a block or pair of blocks, usually of chrome-magnesite refractory of full wall thickness, shaped to include an upward-slanting hole, or (3) an iron pipe cemented in with chrome-magnesite mortar. A holding fixture, usually water- cooled, points into this opening and contains the radiation receiver to supply the impulse. related to the intensity of radiation, and this impulse is transmitted to a recording instrument on the furnace control board. An air jet in this fixture supplies a small, continuous air flow through the hole into the furnace chamber, which serves to prevent settling of dust on the receiver protection glass. A side-wall installation is shown diagrammatically in Fig 4-8.

This direct-sight method has the advantage of immediate reaction to roof-temperature change as well as direct observation on the inner arch surface ; also, the hole and instrument are acces- sible for cleaning or examination. On the other hand, the opti-

~ i d ~ e o n rece) ver

Fig 4-8. Roof pyrometer in front-wall or back-wall installation.

cal path may be obstructed by : (1) dolomite, etc., in the wall opening, (2) drips of slag across the inner.end, and (3) "shots" of liquid metal, which move like a rocket and occasionally line up with the axis of the hole. Mainly on account of the latter effect, an easily removable slide or ring of some sort is usually


included in the fixture to hold a thin, replaceable glass window in the line of sight in front of the sensitive radiation receiver. Clearance of the hole or replacement of the window may occur a t in terva!~ varying from once or twice every heat to as long as a week; thus a check by the first helper about twice during each shift will nearly always take care of these difficulties. Any serious obstruction in optical path will usually show up as an obviously low reading on the recorder. Fig 4-9 illustrates a fixture designed to protect the receiver.

The chief difficulty involved in the direct-sight method is the high error caused by direct or reflected flame radiation. To minimize the incidence of direct flame in the field, the fixture and wall opening are arranged about as shown in the diagram of Fig 4-8 just beneath the skewback in either back or front wall and sighted across the upper corner a t an adjacent roof area.

Fig 4-9. Design of fixture for radiation receivers for open-hearth applications.

The a i r jet through the opening is not only additional protection for the receiver element but also prevents errors caused by flame o r combustion gases blowing outward through the sight hole. Re- flected flame radiation from the glazed surface of a silica roof cannot be avoided, however. For the common total-radiation receivers, the resultant high error may vary from as much as 100 to 150 Fahrenheit degrees above true roof temperature during early periods of the heat a t 2400 to 2700 F (1315 to 1480 C) to as little as 30 to 40 degrees a t higher temperatures. Fortunately, the error becomes smaller as the roof approaches its safe limit;

126 Chapter 4--FUELS, 'COMBUSTION, AND .INSTRUMENTATION

on the other hand, the reflection effect varies somewhat with type of fuel and combustion conditions and is definitely smaller under the following conditions :

1. With a new silica roof during the first few days of operation before the brick surfaces have become glazed, the reflectivity may be so low as to give very little error.

2. With a very foamy or heavy slag under which the bath absorbs heat so slowly that both slag and roof surfaces may approach' the safe limit with only a very low fuel-input rate, the flame may be so short a s to give only a small error from reflected radiation even from a well-glazed arch surface.

In spite of these difficulties, experience shows the direct-sight method to be a good practical tool for roof protection and for partial guidance in driving for maximum furnace production, wherever the furnace operators will cooperate in maintenance and in setting of temperature limits. The simplest scheme with the silica roof is to set the cutoff point a t around 3000 to 3020 F (1650 to 1660 C) , occasionally raising i t temporarily to 3060 to 3080 F (1680 to 1695 C) whenever the furnace requires extra hard driving for some reason. This practice normally limits true roof temperature to below about 2950 to 2980 F (1620 to 1640 C) , however, and limits the furnace somewhat below the maximum and probably the optimum driving rate. I t is preferable to set the normal high limit a t about 3060 to 3080 F (1680 to 1695 C) apparent temperature, with the first helper cutting i t to around 3000 to 3020 F (1650 to 1660 C) a t times, as with a new roof or a very small flame, a t which time there is a small difference between true and apparent temperature. A front- wall setting has some advantage in convenience over the back- wall arrangement. With the fixture designed about as in Fig 4-9, the receiving unit and fixture may be confined usually within a length of about 6 to 10 in. so that i t may be housed within the furnace buckstays and protected by a heavy guard plate. There is some advantage in placing the unit near the middle door to avoid end-to-end variation of flame effect on reversal.

Roof-block Method. In the roof-block method, the sighting tube is focused into a closed-end silicon carbide refractory tube that is fitted into the roof arch in such location as to sight approximately to the center of the hearth (see Fig 4-10). This


method has some definite advantages and a t 'the same time introduces serious problems. Advantages can- be listed as follows.:

1. Since the tube is sealed off,.less opportunity exists for the lens to become coated from dust and vapors.

2. The closed tube eliminates interference withreadings from . . . flames or fumes. , . .

3. Slagging over the tube is eliminated, since the tube is. fitted into the roof. . .

4.- The location of the tube reduces the 'likelihood of damage or tampering.

5. No compressed a i r is needed. . .

. .

. .

I .

. .

. .

, .

, , . . ...

. . . .

Fig 4-10. Roof-block pyrorneter in open-hearth furnace.

The following disadvantages can be cited : 1. Servicing involves a safety hazard because of the roof loca-

tion. Mounting t h e roof must be limited' to 'the time after charging or after the refining period and must be.prohibited dlir-

. . - .. ing furnace additions or lime boils.

2: Improper installation of the silicon carbide block may . . . . . . weaken the roof: . - . . , , .

3. Roof failure may destroy the 'costly measuring-element. 4. A leaky water-cooled jacket may cause damage to.'refrac-

tories. , . . ... : . .


5. The measuring element must be removed prior to the end of roof life, to prevent damage.

6. Supporting structure is costly. 7. A temperature differential and a time lag exist between the

roof-face temperature and the instrument reading because the sighting block must be set back from the inside roof surface. There is question whether this is important, since the relation appears to reflect change in roof-face temperature quickly enough. However, the need for frequent calibration checks is probably greater than with the direct-sight method.

Instruments for Measurement of Roof Temperature. In theory, any kind of sensitive radiation receiver that gives a reproducible response in units of voltage, current, pressure, etc., can be used for roof-temperature measurement. Actual instruments in American practice have been mainly total-radiation pyrometers, with blocking-layer type photocells also used in a few cases.lg Compensation for ambient temperature variation is an advantage that is important only when the receiver is not enclosed in a water jacket, but water cooling is always necessary under the severe conditions next to a furnace wall. There is little to choose between total-radiation and photocell receivers. The photocells require somewhat more effective cooling and have a narrower effective temperature span than total-radiation receivers, but the photocell has the advantage of greater sensitivity in the range near maximum roof temperature. The errors caused by flame reflection may be somewhat smaller for photocells than for total-radiation receivers, but comparative data are somewhat limited. In view of the small size and the relatively low cost of the photocell, i t may be used eventually to a greater extent for this application.

Several of the standard commercial recording potentiometers, either round or strip chart, are quite satisfactory for roof temperatures, though there is a practical advantage in having an en- larged indicating scale and pointer that may be read from a distance of 30 to 50 ft. The newer electronic-circuit type recorders have only a slight advantage a s related to this application, but their insensitivity to vibration or shock and their simplicity a s regards maintenance will probably enable them to replace the older mechanical types. The speed of pen movement in the

AUTOMATIC REVERSAL 129

electronic type gives it a small advantage in one respect, namely, that the pen movement follows the temperature change very closely and smoothly after the flame is cut off; it drops within a fraction of a second to very nearly the true roof temperature as the flame disappears, and if flame is held off for 15 to 30 sec the pen drops quickly to the slag surface temperature and remains close to i t until the fuel goes on again. If the slag is stirring vigorously on a good boil with little or no foam, its surface temperature tends to halt appreciably in cooling when it reaches the temperature of the steel below. Thus, a first helper with good observation and judgment can often use this pen movement on a delayed reversal a s an indication not only of true roof-surface temperature without flame but also, under favorable conditions, as an indication of bath temperature toward the end of a heat.

The simplest arrangement for control of roof temperature is to use the temperature indication simply as a guide to manual fuel adjustment by the first helper, and this has been found helpful in practice. For automatic limitation of roof-surface temperature near a set maximum during a large fraction of operating time, the recorder must include hydraulic or electric "governing means" operating on automatic flow-control valves in the fuel lines. The reader who is interested in details of such control is referred to source^^^^ l7v20 in the literature. In brief, experience has shown that the automatic limiting of roof temperature involves: (1) progressive throttling of fuel flow over an adjustable range, plus (2) an automatic reset arrangement that shifts the position of this throttling range to compensate for the inevitably wide fluctuations in heat absorption and fuel rates in the furnace. On furnaces fired with almost any single fuel, this arrangement, under most operating conditions, will make fuel adjustment nearly completely automatic except for an occasional adjustment of control point by the first helper. With double-fuel firing of a gas and a liquid fuel, there have often been difficulties in maintaining flame direction, and the control problem has not yet been entirely worked out in all cases.

AUTOMATIC REVERSAL

With an average critical temperature level of around 2750 to 2900 F (1510 to 1590 C ) , combustion of any fuel with ordinary

130 Chapter 4--FUELS, COMBUSTION, AND INSTRllMENTATlON

a i r . i n the -open hearth requires a high temperature preheat (usually 2100 to 2400 F, 1150 to 1315 C) of the.air for combustion to obtain any reasonable efficiency for the furnace. The furnace is arranged for firing from either end, so that while the furnace gases are giving up heat to the regenerator on one end, combustion air is.coming to the furnace through the checker chamber on the opposite end and picking up heat stored up from the previous reversal. . .

For each change in;direction of firing, some 10 sec'or 'more is required for the flow of air and combustion gases to reverse and build up close to the normal rate in the opposite direction through the long furnace system, and fuel should not be turnedon to the new' end untit ' the- previous combusfion'gases have been swept out andnormal preheated air has reached the new burner and port.' This delay,' plus the total time Yor the various motions of reversal mechanisms, results in a ' time loss ' a t each reversal of from' 15 to 50 sec. . .

. As t o thk'thebietical influence of the time cycle of reversal bn

regenerator efficiencjr, there is little be said except that for a n idealized furnace with practically'iristantaneous reversal and an ideal 'regeherator of ' thih heat-absorbing 'layers, enormous surface area and narrow openings, reversal eGery few'seconds wduld give the greatest efficiency: For any real furnace with theinevit- able' time loss a t each reversal, minimum brick thickness of about 2.0 to 2.5 in. required forstability, and openings large enough to allow for dust accumulation and: cleaning, regeneration efficiency tends to drop off rathkr rapidly' below a minimum reversal tiine bf a few minutes. w i t h h a i f - ~ ~ ~ i e peridcis of mote than around 20 to 30 min, however,' efficiency also tends' to decrease, more gradually, because t h e heating checker surfac&s, as they approach' the temperature'levels'of the combustion begin to absorb heat more s lody , than the cooling checker is giving up heat to the incoming airi'and the flame intensity begins to lower appreciably. There is thus a n intermediate range, not very critical, of between 'something like 6 to 10 m,in and 15 to 25 min for the period between consecutive reversals which gives a n approximate maximum' of' heat .interchange efficiency, a s indicated .both-by calculations and by ,practical experience. The time periods used in .practice run usually between 10 and 20 min.


Also, nearly all automatic reversal methods are correctly based, a t least in part, on this time period as the fundamental criterion.

Until about 20 years ago, all furnaces were reversed manually, so that this simple but monotonnus and never-ending chore be- came an accepted part of the first helper's job. In most of the many shops still using manual reversal, it has been found profit- able to operate air dampers and slide damper or other flue valves in synchronism by a motor drive with limit switches so that a push-button station operates the whole gas-flow reversal; also, manual shutoff valves are usually placed within reach of this station, and where this is not feasible, automatic valves are usually employed for convenience and time saving.

General Requirements of Automatic Reversal Systems. In principle, conversion from manual to automatic reversal should merely involve the application of a prime mover to each of the regulating valves or dampers in the reversing system and a cycle timer that will actuate these prime movers in the elementary sequence of: (1) shut off fuel, (2) reverse stack dampers, -(3) reverse air supply, (4) purge system, and (5) turn on fuel..:The actual installation usually will be complicated by a number of considerations. The valves in use may not lend themselves to application of usual prime movers, so that replacement may be necessary. Care must be taken to choose the type of prime-mover equipment that will be best adaptable to local conditions. .The choice of motive power, whether electric, pneumatic, or oil hydraulic, must be made with an eye to dependability, continuity of operation, and cost of installation. Conditions of line pressure drop, speed of response, and tightness of shutoff, must be considered. Provision must be made for manual operation in the event of power shutoff or equipment failure. The cycle timer will consist of a motor-driven shaft fitted with the necessary number of electric contacts so arranged that the sequence time of the various contacts can be adjusted to suit the conditions.

Care must be taken that the sequence of operations can be completed in the shortest practical length of time. For instance, a trial automatic reversal installation was actually made on one furnace that could be reversed manually in 12 sec where, because of unusual furnace conditions, the automatic installation required 27 sec. On the basis of 4 reversals per hour and 23 h r of opera-


tion per day, the loss of furnace time per day amounted to 23 niin. I t is a reasonable assumption that an additional equivalent period of furnace time is required to return to the furnace the heat that was lost to the stack during the off-period, which means that the actual loss of furnace time amounted to practically x4 h r per day, or 3.1 per cent.

Space does not permit a complete discussion of valves and prime movers. The conditions of frequent action, severe punishment, and the tendency of some fuels (notably coke-oven gas) to form gummy deposits, require that valves be of the best possible design and materials of construction. As a rule, reversal valves should be arranged so that spring action opens the valve in order to permit manual operation if the motive power fails. Diaphragm motors are suitable for small valves or those requiring low lift, such as oil and steam valves and butterfly valves smaller than about 30 in. Pistons or electric motors are required for long- throw dampers such as stack or flue dampers. Compressed air is a popular choice for motive power because in most steel plants it is less subject to interruption than is electric service, and its disadvantages can be eliminated effectively by providing an adequate filter and dehydrator for the air supply to control instruments. A reversal system comprising suitable valves and motive power can readily be adapted to actuation from a timer or from any of the "temperature-difference" reversal controls, or to simple manual reversing by means of push-button switches.

Measurement of Temperature in Flue or Top Checker Zones. The first logical refinement in manual-reversal and firing-rate control came with developments in temperature measurement in furnace zones below floor level where direct observation by the first helper is difficult, including top checker brick or the space above them and the flues a t the exit from regenerator chambers and near the entrance to the waste-heat boiler or stack. I t was soon found that thermocouple maintenance in the space above the checkers a t 2200 to 2500 F (1200 to 1370 C) was very difficult. (Platinum thermocouples are expensive and eventually tend to become embrittled and contaminated, while heavy base- metal couples burn out, and protection tubes are difficult to maintain.)

With thermocouples extending down into flues near the checker


chambers, or in the common flue leading to the stack or waste- heat boiler, the temperature ranges are low enough so that couple maintenance is relatively easy and inexpensive. In some shops, 4-point or 6-point recorders have been installed, giving records of temperature in 2 or 4 flues after checkers and before and after waste-heat boiler, which may provide useful information as to the distribution of gases o r air between the various checkers.

In the common flue near the waste-heat boiler or stack, gas flow is always unidirectional, and a thermocouple gives a close approximation to the temperature of the gas stream. In all parts of the regenerative zones, however, the constant reversal of flow makes brick surfaces much hotter than gases during cooling periods and cooler during heating periods. The thermocouple receives heat from the gases by contact mainly and from all wall surfaces visible to i t by radiation; its temperature thus fluctu- ates in a range intermediate between gas stream and various nearby brick surfaces but the true temperature of neither is obtained. Thus a couple extending through the regenerator roof arch into the gas space over the checkerwork might oscillate between a low of about 2150 to 2250 F and high of about 2400 to 2500 F , yet top checker surfaces may be around 2100 to 2200 F, and combustion gas stream around 2500 to 2650 F.

With the development in the past . l 5 years of more reliable

L

Y //////////////////////////A Fig 4-1 1 . Sighting arrangement of radiation pyrometer on regenerator checkerwork

near bridge wall.


radiation receivers (photocells and total-radiation pyrometers) , such units have been placed just outside a regenerator wall and sighted through a wall opening or tube onto the surfaces of the top checker brick. Although they may require some a i r or water cooling and occasional inspection and realignment, such receivers have a long service life and they give a close approximation to the true temperature of checker-brick surfaces. Such temperature records from two or four chambers a t both ends of the system can provide a highly sensitive measure of unbalance between furnace ends and a measure of the rate of driving of the furnace ,with respect to fuel input and overheating of furnace end zones or top checker brick. The diagram of Fig 4-11 gives a typical position for such a radiation receiver.

Automatic Reversal Systems. The simplest method of automatic reversal is perhaps that given by timing relays that close c;ne or more contacts a t the end of a time period determined by the manual setting of a dial, which usually permits a selection within a range from 0 to 30 min. The motion is obtained by a small synchronous motor like that in an electric clock. Either a single reversing timer o r two timers may be used; in any case, the timing circuit usually is tied in with limit switches on the reversing motor or on the damper movement and with the position of the automatic valves so that (1) the damper movement can only be in the proper direction in all circumstances, and (2) timing periods will. always s tar t with the opening of the fuel valves on the corresponding firing end of the system. The timer lisually resets automatically to zero a t some point in the reversal operation, ready to start the appropriate timing motion when the fuel goes on a t the new end. A sequence timer may be used in the circuit that closes various contacts in proper timing and sequence. For example, if the fuels were oil and coke-oven gas, the sequence might be as follows a t the end of, say, 15 min set on a timing relay:

1, Oil valve closes on old end. 2. Gas valve closes on old end. 3. Reversing motor starts and damper motion closes limit

switch that opens atomizing steam valve to burner on new end. 4. Damper motion complete, with motor stopped by its own

AUTOMA'I'IC REVERSAL . , . : . . . . 135

limit switch and damper limit switch opened, which closes atomizing steam valve to old end.

5. Time delay of 5 to 10 sec to permit gas-flow reversal to push air to new furnace end.

6. Gas valve opens on new end. 7. Oil valve opens on new end. Various other circuit arrangements are used in addition to the

one just indicated. An automatic-manual switch should always be provided in any kind of reversal installation to set the circuit for manual reversal by push-button only, as might be desired occasionally by the first helper during heating of a new furnace, bottom repairs, or various emergency conditions. A switch for changing to complete manual operation of valves and reversing motor contacts may be provided for emergency; i t is perhaps unnecessary if a maintenance man is always available, with spare parts for quick repairs.

Although such'reversal circuits are not very complex, it is important that only the most reliable timing and other relays are used, which are closed against dust infiltration and are insensitive to vibration. The time-setting dials should be available to the first helper so that he may vary them as' desired and may set

- different periods for the two ends to help balance the system. Many furnaces tend naturally to approach temperature balance on equal reversal periods, but this is not truesat all times nor in all furnaces. Obviously, a furnace can be best pushed toward maximum speed with minimum damage to refractories when it is kept close to temperature balance between ends. The more regular timing of reversals usually obtained with this control method as against manual reversal will help in this direction but i t will not insure automatic and close balancing, nor does i t give any protection o r warning against overheating of end or checker- brick zones. However, i t has the great advantage of relative simplicity.

With the application of thermocouples in flue exits below the regenerators on both ends of the system came the development of the "temperature-difference" method22 of automatic reversal, which has found rather wide application. By using thermocouples with nearly linear curves for electromotive force vs temperature and connecting the two couples (a t opposite symmetrical posi-

136 Chapter 4-FUELS, COMBUSTION, AND INSTRLIMENTA'I'ION

tions in the flues) in opposition to an indicating or recording potentiometer, a continuous record of temperature difference between ends is obtained. A regenerator averaging a t a low temperature level will cool faster to reach any given difference value, and if the reversing mechanism is designed to be actuated by a set temperature difference, the furnace tends to be fired for shorter periods on the cold end, giving an automatic tendency to bring the two ends into balance. The difference value can be adjusted to keep the normal reversal periods mainly within the optimum time range. I t has usually been found advisable to add timing relays in this system to provide a secondary cause of reversal if the thermocouples or recorder should fail or if the set difference value would be reached much too slowly (which sometimes occurs), with a possibility of overheating a furnace end. Thus the system tends to be more complex than a straight time-reversal control. Of itself the temperature-difference method of automatic reversal supplies no temperature record of any zone below floor level nor does i t necessarily protect against overheating of end or top checker zones, but i t does provide a stable and easily maintained relationship that can be used to keep balance between the checkers.

Even with the addition of a recorder to provide a chart of actual flue temperatures, the temperature-difference system of reversal does not offer certain probable advantages which may be obtained from specific top checker temperatures measured by radiation receivers. In view of the great difficulty of utilizing such receivers (with their nonlinear temperature-emf relationship) in the temperature-difference system, another system, which might be termed a "time-temperature" meth0d,~3 has been developed. A radiation receiver is sighted into a t least one regenerator chamber a t each end of the furnace, .with a 2-point or 4- point recorder giving a close approximation to the average temperature of top checker-brick surfaces near the bridge wall. Two automatic-reset timers, set for a minimum and a maximum (such as 10 and 15 min) reversal period, respectively, are combined with a floating-temperature limit switch in the recorder in such manner that : (1) automatic correction of end-to-end unbalance in temperature between the respective top checker levels is obtained and (2) reversal always occurs within an optimum time

FURNACE-PRESSURE CONTROL 137

range as determined by the two time-relay settings. An additional maximum-temperature limit switch in the recorder may be used to cause earlier reversal a s a warning and protection against excessive fuel input and possible overheating of end or top checker zones. With temporary failure of temperature-measuring or recording units, automatic reversal on a fixed time cycle will be continued.

Reference No. 23 gives details of this time-temperature reversal control, which is now used in a few shops. It is more complex than the straight time reversal method, perhaps slightly more so than the temperature-difference method. If one assumes a certain usefulness in having a continuous record of top checker temperature plus some protection against excessively high temperatures below floor level, certain definite advantages would seem to be inherent in this system. It is claimed to be more nearly completely automatic as a reversal control than any of the other methods described.

The essential requirement for success in practice with any reversal control method is naturally the availability of maintenance personnel familiar with all details of automatic valves as well as the electrical and temperature-measuring circuits and cquipment, with certain spare parts available for quick replacement. Besides e!iminating a routine chore and thus giving the first helper more opportunity to devote his time to the other more important aspects of steelmaking, automatic reversal normally tends to ba!ance the furnace temperatures and cut off occasional abnormal temperature peaks. Any direct fuel saving is small in amount, but sometimes is measurable; the most important effect is usually the addition of a few extra heats to the average furnace campaign, which alone.may be sufficient to compensate for the investment and maintenance costs of such controls.

The open-hearth combustion system can be considered as having two separate and distinct furictions. The first function is to deliver the fuel and air supply to the furnace and to combine them in such manner as to develop an intense flame over the bath; this has been discussed a t length in connection with burners and air supply. The second function is that of removing the


gases formed by combustion and bath reaction from the furnace and discharging them into the atmosphere.

I t was pointed out in the discussion of combustion air supply that the force responsible for bringing the combustion a i r into the furnace is the chimney effect of the checkers and uptakes. This same force, which is a one-way force tending to move gases toward the furnace, must be overcome by the counteracting force required to remove the flue gases from the furnace. (See Chapter 20, Figs 20-4 and 20-5.) Furthermore, the frictional drag of the flues required to carry the gases to the stack, the loss of head due to right-angled turns in the flues, and the pressure drop through dampers and other auxiliary equipment such as waste-heat boilers, all tend to increase the load that must be carried by the stack, forced-draft fan, or the forced-air ejector, as the case may be, that is employed to provide the force needed to carry the gases away from the furnace. These opposing and continually varying conditions, coupled with the fact that immense weights of gases are involved, amounting to as much as 25 tons per hour for a fuel input of 400 gal of oil, combine to make the function of flue- gas removal vitally important.

The foregoing statement that the incoming and ' outgoing functions are separate and distinct must not be interpreted as meaning that they are not interdependent. Actually, flue and bath gases must be removed exactly as produced. If too much draft is used, the furnace pressure is negative and cold air is drawn in through wicket holes and around doors, which has the effect of distorting the flame pattern over the bath, of increasing the weight of flue gases to be handled, and of reducing the heat transfer to the checkers that will eventually have the effect of reducing air preheat. If insufficient draft is provided, pressure will be built up within the furnace that will cause undue punishment of refractories and will back up against the incoming current of combustion air with adverse effect on flame development.

The balance point between the incoming and the outgoing functions must of necessity be a t the middle of the furnace in order that one point may serve as the criterioil for either direction of firing. The ideal pressure condition would be balanced a t atmospheric or zero pressure a t the sill-line elevation over the entire length of the furnace, which would entirely eliminate

FURNACE-PRESSURE CONTROL' 139

infiltration of air from around the doors. Attainment of such a pressure condition in a conventional open-hearth furnace is impossible because the nature of the flame development requires a gradient of pressure over the length of the hearth that varies from negative a t the firing.end to positive a t the outgoing end. Furthermore, the measurement of pressure a t the sill line in the middle of the furnace is impractical because of location and the small magnitude of pressure changes a t this point, so the measurement of furnace pressure is usually taken through the roof arch, where it is less subject to interference from bath action and furnace working and where the pressure is appreciably higher

Fig 4-12. Pressure grodients through o 180-ton tilting furnace ot the wicket-hole level.

because of the chimney effect of the hot furnace chamber. Typical pressure gradients in an open-hearth furnace a t the wicket-hole level for a roof-arch pressure of'+0.07 in. of water are shown diagrammatically in Fig 4-12. An approximation of the temperature gradients through the furnace under the ideal condition of no infiltration is shown in Fig 4-13a while the effect of excess draft and consequent air infiltration is illustrated in Fig 4-13b.

The usual method of regulating draft is by means of a damper in the flue leading to the stack. In its simplest form, the damper is actuated by a cable and winch located near the reversing stand. Furnace pressure is judged by the intensity of the lick of flame out through the wickets in the charging


doors, and the first helper adjusts the damper manually according to the appearance of this lick of flame. Conscientious and intelligent first helpers can develop good open-hearth performance with even such e!ementary means of draf t regulation.

.Manual draft regulation has one definite advantage in that draf t conditions will be stable a t the particular setting in use.

Direction o f firing (gas)

L ;inif o f e ffec f o f air /eakage,

7

(b) Fig 4-13. Field of temperature (degrees ~ahrenhe i t ] ' in an open-hearth furnace.

(After S~hwiedessen.~') a, with no air infiltration; b, with air infiltration.

Experience has demonstrated that significant fluctuations of furnace pressure can occur that will not be reflected in the appearance of the sting of the flame from the door, so there is need for a more positive measurement. A sensitive pressure recorder can be provided, which will not only show slight changes in furnace pressure clearly but also will provide a record that will prove useful. Use of such a pressure instrument introduces the problem of installing and maintaining a pressure connection through the roof of the furnace. This installation must be made carefully and with close attention to detail, since its location

FURNACE-PRESSLIRE CONTROL 141

must of necessity be difficult of access for maintenance. Piping should be arranged so that the pressure line can be blown back periodically with compressed air. Generously sized pipe should be used and an atmospheric-pressure line should be run beside the furnace-pressure line to eliminate the influence of difference of elevation between the pressure point and the pressure instrument. Several makes of practical, sensitive pressure instruments are available, which, coupled with carefully constructed pressure taps and piping, will give satisfactory operation over an entire furnace campaign.

The greatest drawback to the use of manual control for regulation of furnace pressure lies in the fact that the first helper must first be sufficiently alert to notice that draft adjustment is needed and then must be interested enough to make the necessary change. Extreme pressure conditions will attract attention so that necessary correction will be made, but many conditions will occur that are not so readily noticed but that may still have a n appreciable effect on melting performance. The first helper has many duties requiring close attention, so that he cannot be expected to give his undivided attention to regulation of furnace draft, however important i t may be. Automatic regulation of furnace draft has been developed to take this duty off the first helper's shoulders, and some authorities maintain that, of all the controls applied in recent years to improve open-hearth performance, draft control has the greatest value.

Automatic draft control is accomplished by adding a sensitive pressure controller to the pressure recorder discussed in the foregoing paragraphs, together with a prime mover to actuate the draft damper in response to impulses from the controller. The prime mover may be a n air-operated or oil-operated cylinder, or a n electric motor, according to the type of controller chosen. The draft controller must be specially designed in view of the peculiar conditions of extremely sensitive pressure balance in the furnace, of the long distances from the furnace to the controller and from the controller to the stack, and of the opposing forces of stack draf t and chimney effect of the checkers. The controller must be sensitive enough to respond instantly to minute changes of pressure and still be stable enough to hold settings without overshooting or undershooting, which is an


exacting order since sensitive controls usually are subject to hunting, and stable controls are likely to have sluggish response characteristics.

Gases evolved from the bath have been estimated to amount to roughly 20 per cent of the amount of the combustion flue gases, but the rate of evolution will vary from a nominal amount at certain periods of the heat to very large amounts a t others, so the volume of gases leaving the furnace will be subject to con- tinual and sometimes radical change. The draft controller, in order to fulfill its function properly, must be designed so that the draft can be regulated evenly to meet such changing demands.

Open-hearth furnaces usually are built and rebuilt carefully according to definite dimensions and lines in order that balance between the flow of a i r and flue gases to and from the two ends of the furnace can be established and maintained, but in spite of meticulous attention to construction, some difference in flow resistance is usually found to exist. Such differences as a rule are small when the furnace is new, but a s the campaign pro- gresses they tend to be accentuated as a result of change in lines and areas due to refractory erosion and of change in resistance to flow through the checkers in consequence of the build-up of checker deposits. An example of such unbalance is illustrated by the chart from a pressure recorder shown in Fig 4-14a which was taken from a furnace operated with manual draft control. Due to a difference in flow resistance between the two ends of the furnace, the flow of air into the furnace is greater on the end having the lower resistance while the draft on the other end is less because of the greater resistance. As a result, the level of pressure is approximately 0.03 in. water gage higher for firing from one end, even though fuel, air, and draft settings remain the same. Such unbalance is bound to be reflected in a difference in melting rate between the two ends of the furnace, which will have an adverse influence on furnace performance.

Well-designed fuel and air-flow controllers, together with signal reversing based on checker temperature difference, will help greatly to prevent the development of such unba!ances, and automatic draft control provides an additional valuable tool to help keep the furnace in balance. The effect of automatic draft

FURNACE-PRESSURE CONTROL

Fig 4-14. Charts from a pressure recorder. a. Furnace with unbalance between ends and without automatic draft control.. b. From same furnace after installation of automatic draft control.


control on the same furnace is shown by the chart of Fig 4-14b, which indicates that considerable improvement has been accomplished. The open-hearth shop from which these charts were taken credits automatic draft control with a fuel saving of 5 per cent and a production increase of 2 per cent. There is a possibility that still further benefits could be realized if the controller could be stabilized to eliminate the frequent over- swings evidenced by the charts.

FUEL AND AIR MEASUREMENT AND CONTROL

The necessity for close measurement of fuel and combustion air was emphasized in the section on Combustion Air Supply, but widespread interest in air-fuel ratio controls makes further discussion of the subject advisable. Assuming that sufficient money has been spent and sufficient pains have been taken to work out adequate flow controllers for fuel and combustion air, a combination of the two with a ratio controller, which will maintain a constant relation between fuel and air, would be a fairly simple matter. Such an installation would include a means of readily adjusting the ratio between fuel and air to suit the atmosphere requirements of the process. The advantage gained by this control would be that fuel rate would be adjusted up or down as required, and the air rate would be adjusted automatically in ratio with the fuel-rate change. In the absence of such control, the first helper's procedure must be to adjust fuel rate as desired and then adjust air rate to match according to experience, or according to an established chart for fuel and a i r settings. Such procedure should not put any undue burden on either the first helper's time or his intelligence, but i t is true nevertheless that elimination of the need for matching fuel and a i r settings would reduce the probability of having this important relationship get out of hand.

There are other conditions, however, which must be given consideration in connection with fuel-air ratio control. Such controls are developed by the instrument companies on the premise that open-hearth combustion conforms to conventional combustion principles, which is not true. An open-hearth flame is developed in such a manner a s to transfer heat evenly and intensely over the entire bath area as f a r as is possible. Great

MISCELLANEOUS MEASUREMENTS 145

pains are taken to inject the fuel so that it will entrain air from a surrounding envelope as needed to produce a flame that will cover the bath. Because of this singular combustion situation and also because the nature of the open-hearth process requires that an oxidizing atmosphere should be maintained in spite of the evolution of reducing gases from the bath, the extent of which may vary widely a t different stages of the heat, i t is necessary that the supply of air must often be considerably in excess of that required for complete combustion of the fuel. It is essential that the right combination of volumes, velocities, and flow directions be worked out to develop the flame coverage required for fast melting. The relationships thus established are sensitive and easily upset by any disturbing influence. The hunting action of a ratio controller may have the effect of upsetting these relations, with the result that the benefits expected from combustion control may be lost in the adverse influence on flame coverage. This line of thought is offered as a possible explana- tion of the fact that many fuel-air ratio-control installations have been discontinued after a period of use.

In addition to the requirement of oxygen to burn the com- bustible gases (mainly carbon monoxide) evolved from the bath, there is also a variable absorption of oxygen into the charge and bath. Oxygen absorption rate is probably highest in early melting periods, and gas evolution is highest during boil periods after the hot-metal addition, but the pattern of variation-, in oxygen demand is not constant even from one heat to another. A reliable and practical method for continuously recording the oxygen content in the outgoing gases is needed to supply a basis for autblnatic control of fuel-air ratio or to serve a s a criterion for eval;&ting its usefulness.

MISCELLANEOUS MEASUREMENTS A very considerable amount of work has been done in recent

years by various open-hearth shops in efforts to throw light on aspects of furnace operation that are not easily measurable but have a vital influence on performance.

Flame-radiation Pyrometer. A notable contribution has been the development of instrumentation that records a measurement of the total heat radiation frorn an open-hearth flame and the


application of this device to throw light on the influence on flame development of various factors such as furnace temperature, combustion air supply, air preheat, furnace pressure, fuel flow, and steam consumption. The use of a thin mica window in front of a conventional radiation pyrometer has made possible the measurement of most of the radiation from the flame, since mica transmits energy wave lengths up to 8 microns, which covers very nearly the entire range of radiation transfer. (The range of wave lengths covered by visible light extends from 0.4 micron to about 0.8 micron, and only a fraction of the total energy trans- ferred is covered by this range.) With lens-type receivers, if a fused-silica lens is used,.about 80 to 85 per cent of the energy in the spectrum will be absorbed, so that these may also be used to measure relative rates of radiation flow. Flame-radiation measurements supply a very useful guide in working out burner designs, atomizing steam ratios, or fuel-air settings, since they provide an immediate indication of the effect of changes and help to shorten the time required to arrive a t dependable con- clusions by cut-and-try methods. If sufficient care is given to the coordination of control of fuel, combustion air, atomizing agent, and furnace pressure so that good melting conditions can be established and maintained, there should be little need of flame-radiation measurement as a n operating tool, in which case its function would be to serve as a check against settings much the same as that of flue-gas analysis.

Flue-gas Analysis. The idea of using flue-gas analysis a s an operating tool for control of open-hearth combustion has been suggested many times, and extensive and expensive installations have been made in attempts to apply this idea. The oxygen content of the flue gases has usually been used as the basis for control, since this constituent is least affected by bath reactions and serves as the best criterion of combustion conditions. Several methods are available for the determination of oxygen, the most practicable being based on burning out the oxygen by means of an added fuel supply and measuring the heat so developed. An analyzer installation based on this principle can be worked out without difficulty, but the problem of collecting a sample of flue gas continuously has never been solved satisfactorily. The sample must be taken from the flue gases immediately after they

MISCELLANEOUS MEASLIREMENTS 147

leave the hearth in order to avoid dilution by infiltrated air, and the conditions of heat and fume a t this location are so intense that i t is very difficult either to construct a sampling tube that will last for any length of time or to work out a technique that will keep the sampling tube open. Many worth-while open-hearth installations have died a slow death for no other reason than that periodic attention was required to keep them operating, and this has tended to be the ultimate fate of all of the flue-gas continuous analyzers of knowledge.

Air Preheat Temperature. The influence of air preheat on furnace performance is recognized universally as being critical, but very little information is available as to exact air temperatures for lack of a practical method of obtaining them. The reading from a thermocouple inserted through the end wall of the furnace is influenced more by the refractory temperatures than by air temperature, so that use of the temperature thus obtained a s an index of air preheat temperature will be misleading. At- tempts have been made to obtain true air preheat temperature by means of an aspirating pyrometer in which the gas is drawn a t high velocity past a thermocouple that is shielded against radiation exchange with the brickwork. Means of developing draft must be provided, working space is usually limited, and, in spite of careful technique, errors are liable to occur, so that there is question whether the results gained will justify the work and expense involved. Once again, if primary regulation is or- ganized so as to make the best use possible of the available heat- exchange capacity of the checkers, there,ig perhaps little more to be gained by knowledge of the true pre$bat temperature. From a n academic standpoint, such knowledge would be useful and informative. Calculations of the heat balance of an open- hearth system, for instance, certainly must be open to question unless a reasonably accurate estimate of the heat content of the combustion a i r is available, and this requires that the true temperature of the air supply be known.

Roof-brick Heating Rate. Thermocouples placed in the hearth, the roof arch, checker brick and other locations have been used to measure temperature gradients, permissible heating rates, heat losses, etc., during initial days of a campaign, but they are


mainly useful only for special studies. The one application of routine usefulness here is perhaps that of a single thermocouple imbedded about 1 in. from the inside face of the silica-brick roof arch during its initial heating up period. This one couple will serve well enough to indicate the proper heating rate for the who!e structure; the temperature should rise no faster than about 60 to 100 Fahrenheit degrees per hour in any silica-brick zone while it is below 575 F (300 C ) .

The hot junction of the thermocouple may be cemented with silica cement into a hole drilled from one edge face to the center

Fig 4-15. Time-temperature curves a t various distances from the hot . face in a new silica roof during initial heating up.

of a roof block, one inch back from and parallel to what will be the hot end. This provides an isothermal zone for the junction and 2 or 3 in. of the leads. The leads are carried to the upper surface of the roof through a groove cut in the edge face of the block. A chromel-alumel couple may be used, since it will have served its purpose long before i t burns out. Several such blocks

FUNCTIONS OF CONTROL EQUIPMENT 149

may be kept on hand so that one is always ready to be placed near the center of each newly built roof.

Such an inexpensive control thermocouple makes it much simpler to heat a new roof a t the maximum safe rate with a probable saving of 10 to 15 h r of operating time. A new arch may be brought up to operating temperature in 20 to 30 hr, depending somewhat upon the physical properties of the silica brick used. Fig 4-15 shows time-temperature curves measured by thermocouples a t various distances back from the hot face of a new silica roof as it was being heated for the first time. The heating rate for the 1-in. position was held to about 80 to 100 Fahrenheit degrees per hour for the first 10 hr, and the fuel was turned on full after the hot face was safely past the critical zone and above 600 F (315 C) . The roof surface could then be heated much more rapidly without subjecting the inner layers of the brick to a faster heating rate than 100 Fahrenheit degrees per hour in the sensitive range. Steelmaking was begun within 25 to 28 h r after light-up of this particular furnace, yet no observable spall- ing of roof brick was encountered. A single couple a t the 1-in. depth therefore serves a s a good guide, because the brick itself is sufficiently insulating to prevent too rapid heating of the inner layers if the fuel is not turned on full until after the 1-in. depth has been heated above 600 to. 700 F. Such controlled heating is equally important after any major repairs or cleanout whenever a partially worn-out silica roof is left on to finish out a campaign. The various zones in the used brick expand a t different rates; the inner cristobalite zone undergoes an especially sharp expansion a t around 450 to 600 F (230 to 315 C) , and the heating rate up to 600 F should be even slower than for a new roof.

SUMMARY OF FUNCTIONS OF CONTROL EQUIPMENT

There is a wide variety of furnace control equipment available for the open-hearth shop, both as to intended function and specific design. Shop conditions under which the controls may be applied are equally varied. I t is not surprising that no unanimity of opinion exists among open-hearth operators on the relative merits of the several kinds of control, and even within this chapter there may be reflected some differences in point of view among the contributors. No blunt statements can be made that

150 Chapter 4--FUELS, COMBUSTION AND INSTRUMENTATION

good steelmaking is impossible without such-and-such a control device, but the following summary is an attempt to state the principles upon which consideration of control equipment should be based.

Melting steel in the open-hearth furnace is inherently a cooking job and a homely analogy can be made. Time-tested recipes with the best materials available and with the most modern kitchen equipment provided with the last word in automatic control, in the hands of an inexperienced cook may turn out un- savory messes, whereas a good cook with nothing but a table- spoon, a teacup, and an old coal range may concoct dishes fit for a king. However, if the good cook is supplied with the modern equipment, she will do her cooking in less time with less effort, and, more important, her failures will disappear and her performance level be raised. In the open hearth, the first helpers and the melters are the cooks. Without good control equipment, they are in the same position as the old-time cook with the primi- tive measuring tools and the coal range. Their level of performance might be acceptable, but too many heats will be an hour ,or more late for no explainable reason. The margin of operation is limited under the best of conditions, and variations not readily detected may have the effect of retarding the heat and lowering production.

It follows then that any instrumentation that will serve to help the furnace operators hold their furnaces up to top performance level should be well worth while. Pressure gages, pyrometers of various sorts, flowmeters, and controllers a r e needed to help the operator regulate the furnace. Some type of reversal control is needed to assist in keeping the two ends of the furnace in balance and to reduce the loss of furnace time due to reversals. A dependable indication and record of furnace pressure is much more desirable for regulation of this important condition than the tongue of flame from the wicket hole through the furnace door that always has been used a s a rough indicator of furnace pressure. A roof-temperature recorder should give warning to the first helper when destructive temperatures are approached and should be amply justified by the longer roof life made possible.

If these various functions can be controlled automatically with-

REFERENCES 151

out a t the same time introducing influences that may have an adverse effect on furnace performance, the value of the installations will be increased further, since mechanical operation tends to eliminate delays and failures chargeable to the human element under manual operation. I t should be recognized that control equipment cannot be depended upon to run the furnace, but that its sole justification lies in its usefulness as a tool to help the furnace operator do a better job. Instrument and control manufacturers would do well to study thoroughly the complexity of the flow of gases through an open-hearth system and the pecu- liarities of its combustion system in connection with design of control equipment in order that process limitations may be recognized and provided for. Conditions in open-hearth plants are rugged with respect to heat, diit, and vibration, so that it is necessary that instrumentation be especially designed and constructed to operate under such conditions. The record of ad- vance made in the development of control instrumentation in the quarter century since the middle 1920's, from the crude and flimsy equipment then available, makes an interesting story, and the manufacturers of instruments and controls are entitled to a tribute of appreciation for the contribution of research and service they have made.

Open-hearth Fuels

1. CAMP, J. M. and C. B. FRANCIS: The Making, Shaping and Treating o f Steel , 5th Ed., 1940, pp. 105-156. Carnegie-Illinois Steel Carp., Pittsburgh.

2. FLAGG, H. V.: Colloidal Fuel a t Armco, Middletown, Ohio. Open Hearth Proc., AIME, 1944, v. 27, pp. 47-55.

3. JAICKS, FRED G.: Effect of High Coke-oven Gas Firing on Open-hearth Operation. Open Hear th Proc., AIME, 1949, v 32, pp. 35-41.

4. LARSEN, B. M. and C. SIDDALL: heo ore tical Limiting ~ f f i c i e n c ~ of Various Fuels in the Open-Hearth. lrogc. Steel Eng., Dec. 1945, v. 22, pp. 76-96, 115.

5. SCHWARTZ, E. H. and G. E. ROSE: Open-hearth Furnace Operation- Four Million Btu per Ton. ,I,i.on Steel Bng., 1936, v. 13, pp. 1-8.

152 Chapter 4--FUELS, COMBUSTION AND INSTRUMENTATION

6. WINDETT, VICTOR, and Associates: The Open Hearth, pp. 209-251 (gas producers). Wellman-Seaver-Morgan Co. -New York, 1920.

Open-hearth Combustion System

7. BUELL, W. C., JR.: The Open Hearth Furnace, Its Design, Construc- tion, and Practice. Penton Publishing Co., Cleveland. Vol. 11, 1937, 260 pp.

8. FLAGG, H. V.: Flow and Velocities of Air and Waste Gases. Open Hearthi Proc., AIME, 1950, v. 33, pp. 253-269.

9. MARSH, J. S. : Operation of Oxygen-enriched Open-hearth Furnaces. Trans. AIME, 1948, v. 176, pp. 78-89.

10. STEINER, KALMAN: Fuels and Fuel Burners. McGraw-Hill Book CO. New York, 1946. 394 pp.

Bath-temperature Measurement

11. BAKER, R. C.: Installation and Use of Instruments on Open-Hearth Melting Furnaces. J . Iron Steel Inst. (London), 1947, v. 157. p t I, pp. 85-86.

12. CLARK, H. T. and S. FEIGENBAUM: A Radiation Pyrometer fo r Open- hearth Bath-temperature Measurement. Open Hearth Proc., AIME, 1946, v. 29, pp. 229-243.

13. SCHOFIELD, F. H.: Seventh Eeport on the Heterogeneity of Steel ingot^, pp. 222-238. Iron and Steel Institute. . .

14. SORDAI~L, L. 0. and R. B. SOSMAN: The Measurement of Open-Hearth Bath Temperature, in Temperature, Its Measure,ment and Control in Science and Industrg, pp. 927-936. Reinhold Publishing Corp. New York, 1941.

15. WEIYLENKORN, L. F.: A Method for the Determination of Bath Tem- perature. Elec. Furnace Steel Proc., AIME, 1944, v. 2, pp. 143-149.

Roof-surface Temperature Measurement and Control

16. BRISTOL, E. S., and J. C. PETERS: Some Fundamental Considerations ill the Application of Automatic Control to Continuous Processes. Trans. ASME, 1938, u. 60, pp. 641-650.

17. ECKMANN, DONALD P.: Principles of Indz~strial Pq-ocess Control. J. Wiley and Sons, Inc., 'New York, 1945. 237. pp.

18. LAND, T.: A Photo-electric Roof Pyrometer for Open Hearth Furnaces. J . Iron Steel Inst. (London), 1947, v. 165, pp. 568-576.

19. LARSEN, B. M., and W. E. SHENK: Temperature Measurement with Blocking-Layer Photocells, in Temperahre, i ts Measurement and Control in Science and Indztst,vy, pp. 1150-1158. Reinhold Publishing Corp. New York, 1941.

REFERENCES

20. SMITH, E. S.: At~tomatic Control. Engineering. McGraw-Hill Book Co. New Yolk, 1944, 367 pp.

21. SOSMAN, ROBERT B.: The Pyrometry of Solids and Surfaces, pp. 70-81. American Society for Metals. Cleveland, 1940.

Automatic Reversal

22. BRADLEY, M. J., and J. W. KINNEAR, JR.: The Automatic Reversing of Open Hearth Furnaces by the Temperature Difference Method. Iron Steel Engr., 1930, v. 7, pp. 481-484.

23. LARSEN, R. M., and W. E. SHENK: A Completely Automatic Control of Open-hearth Reversal. T?-ans. AIME, 1945, v. 162, pp. 37-48.

24. SCHWIEDESSEN, HELLMUTH: Falschluft im Ofenbetrieb (Infiltrated Air in Furnace Operation). Arch Eisenhiittenzv., 1936, v. 9, pp. 319-326.

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