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F U R N A C E S
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FURNACES
1. INTRODUCTION
A furnace is a device in which the chemical energy of fuel or electrical energy is
converted into heat which is then used to raise the temperature of material., called
the burden or stock, placed within it for that purpose. Furnaces operating at low
temperatures are often called ovens depending on their purpose. There are also
others used for higher temperatures for different purposes. When associated with
the ceramic industries they are called kilns. Glass is melted in tanks and other
special terms often occur, suggest the purpose, such as sintering machine used in
metallurgical ore dressing etc.
Classification of furnaces:
The classification of furnace can be done as:
- Batch type. - Continuous type.
- Direct heating type. - Indirect heating type.
Different metallurgical furnaces are:
- Crucible Furnace. - Hearth furnace - open hearth.
- Shaft furnace - blast furnace.
- Converters – Bessemer, side blown or top blown (LD).
- Muffle Furnace. - Soaking pits.
- Reheating furnaces. -Annealing furnaces.
Other types of furnaces:
- Rotary Kiln – Cement Kiln. -Tunnel Kiln.
- Boiler Furnace.
- Electric Furnaces – Three Phase or Single phase:
a) Heating by metallic resistors, or b) Heating by induction.
Chapter
1
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Based on type of fuel used:
- Solid fuel fired furnace (coal fired boiler).
- Liquid fuel fired furnace (rotary kiln).
- Gaseous fuel fired furnace (Coke oven, Sinter Furnace, Soaking pit, Re-heating
furnace etc.).
- Multi fuel fired furnace (Captive power plant boiler).
Based on use of heat saving appliances;
- Regenerative furnace (e.g. coke oven).
- Recuperative furnace (e.g. soaking pit, Re-heating furnace).
- Waste heat type furnace (e.g. Rotary lime kiln).
Based on draft control system:
- Natural draft furnaces (e.g. B.F. stoves, coke ovens etc.).
- Forced draft furnaces.
- Induced draft furnaces.
- Balanced draft furnaces (most large combustion furnaces come under this
category).
Some of the above classified furnaces e.g. batch furnace continuous furnaces;
Recuperative furnace and regenerative furnace have been shown in Fig.1. (a), 1(b),
1 (c), and 1(d).
In a batch furnace: The stock to be heated is charged in the furnace and is heated
for a predetermined period to achieve the required temperature. The heated stock is
then taken out of the furnace and the fresh cold stock is again charged. This
furnace can be put off as and when needed.
In a continuous furnace: The stock enters or is pushed in from one end (i.e.
charging end) of the furnace and it travels countercurrent to the hot flue gases to
the other end (i.e. discharge end). Its speed is so controlled that it gets heated up to
the required temperature by the time it reaches the discharge end. It is then
discharged out of the furnaces and another cold stock may be charged. This
process of stock charging and discharging goes on continuously.
In a recuperative furnace: In a recuperative furnace, waste heat recovery is done
from the flue gas in a metallic or ceramic before it exhausted out through the
chimney.
In a regenerative furnace: Waste heat recovery from flue gas is done by a matrix of
refractory checker work/bricks which gets heated up during the passage of hot flue gas
through it for a predetermined time (usually 20-30 minutes). In the next reversal cycle,
cold combustion air and/or cold fuel gas (provided its preheating is permissible) is routed
through the hot checker work/bricks of the regenerator which preheats them. A gain when
the hot regenerators bricks loose their heat and become cool, the passage of the hot flue
gas and cold air and cold fuel gas is reversed through the reversal damper/valve.
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That is why two sets of regenerators are provided for each furnace so that while
one set of regenerator is being heated by hot flue gas, the other set of already
heated hot regenerator is loosing its heat to the cold combustion air and/or cold
fuel gas for preheating them. And after a fixed interval (called reversal cycle
usually of 20-30 minutes), regenerators duty is interchanged/ reversed.
Furnace Auxiliaries:
The important furnace auxiliaries are:
- Damper. - Fan (forced draft and induced draft).
- Control valve. - Burner.
2. FURNACE EFFICIENCY
The furnace efficiency depends greatly upon:
- Flame temperature. -Air/ gas ratio.
- Furnaces structure. -Insulation levels of furnace.
- Radiation losses from furnace. -Method of furnaces operation.
Factors affecting fuel economy in the furnace:
Various factors which affect fuel economy in industrial furnaces are:
- Complete combustion with minimum excess air.
- Proper heat distribution. -Operating at the desired temperature.
- Reducing heat losses form furnaces openings.
- Minimizing wall losses. -Control of furnace draught.
- Optimum capacity utilization.
- Waste heat recovery from furnace flue gases.
- High emissivity ceramic coating of furnace refractors.
Complete combustion with minimum excess air:
To achieve complete combustion of fuel with minimum excess air, a number of
factors (such as proper selection and maintenance of control, excess air
monitoring, air infiltration, pressure of combustion air) are to be considered. The
magnitude of stack losses with different quantities of excess air in the case of a
furnace carrying away flue gases at 900oC is tabulated below.
Besides abnormal increase in stack losses, with the increase in excess air, the
ingress of too much excess air lowers the flame temperature and consequently
reduces furnace temperatures and the heating rate. If too little excess air were used,
combustion would be incomplete and chimney gases will a way unused fuel
potential in the form of unburned combustible gases such as carbon monoxide,
hydrogen and unburned hydrocarbons which should have otherwise been usefully
burnt in the combustion chamber.
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Excess air percentage
Percentage of total heat in the flue
carried away by waste gases (flue
gas temperature 900oC)
25
50
75
100
48
55
63
71
Waste heat recovery from furnace flue gases:
In any industrial furnace the products of combustion leave the furnace at a
temperature higher than the stock temperature. Sensible heat losses in the flue
gases, while leaving the chimney, carry 35-55% of the heat input to the furnace.
The higher the quantum of excess air and flue gas temperature, the high would be
the waste heat availability.
The sensible heat in flue gases can be recovered by:
- Charge preheating.
- Preheating of combustion air.
Besides, there are other waste heat recovery techniques such as steam generation
through waste heat boilers, gas turbine systems, vapor absorption, refrigeration
system etc.
The direct recovery of energy from waste gases can be achieved in metallurgical
furnaces by preheating the incoming raw materials. The effectiveness of charge
preheating depend upon the physical and dimensional properties of the material
being heated along with the handing methods employed.
A continuous furnace of pusher type can be so operated that it utilizes the waste
heat by preheating the stock. If the rate of heating is low, most of the heat is
imparted to stock at the hot end, and the produces of combustion can preheat the
slowly moving ingots.
Considerable fuel savings can be done by preheating the combustion air, by
recuperator and the regenerator.
In a recuperator, Fig. 1 heat exchange takes place between the flue gases and the
air through metallic or ceramic walls. On the other hand, in a regenerator, the flue
gases and the air to be heated are passed alternatively through a heat storing
medium, thereby resulting in transfer of heat.
Recuperates. They are of three types namely counter-flow parallel flow and cross
– flow, according to the flow of the hot flue gases and of the cold combustion air.
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Counter flow type allows the highest temperature of preheat to be reached whereas
the parallel flow type gives the lowest maximum temperature of the recuperator
wall.
The disadvantage of the recuperator is that it involves a convection process on
each side of the separating wall and conduction through the wall. Consequently,
the overall heat transfer co-efficient is low and large surfaces are required for high
heat recovery.
Recuperators for relatively low temperature processes are made of steel and cast
iron but for high temperature ceramic refractory material (e.g. SiC) are used. The
high temperature gives a high heat transfer rates but they thick walled and bulky
with a lower thermal conductivity than metal.
Regenerator: This equipment employs a discontinuous process. Two independent
masses of chequor bricks are provided. At any moment the hot flue gases are
passing through one mass of brickwork in one direction and heating it up while the
cold air is passing through the second mass in the opposite direction, being heated
in the process. After a fixed interval of time (usually 30 minutes) the process is
reversed.
Advantages of Regenerator over Recuperator:
- One convection surface and the separating wall are eliminated.
- Brickwork can be used over a wide range of temperatures.
- It is a cheap from of heating surface and can be used in standard brickwork
shape.
- It can store larger quantity of heat because its refractory can be heated up to say
900oC and can be cooled upto say 600
oC at the end of the cycle.
Regenerators are commonly used where large quantities of heat are invovled e.g.
in open hearth furnaces, coke ovens, etc.
Disadvantages of Regenerator Compared to The Recuperator:
- Chequer brickwork is heavy, bulky and massive unlike recuperator whose thin
metallic walls facilitates rapid heat transfer.
- It cannot be employed usefully on very small furnaces. Moreover it is
expensive in initial cost.
- Preheat temperature fluctuates with the cycle which necessities the use of
several chambers for smoothening the fluctuations.
- Changeover valves need automatic control and constant supervision for its
smooth operation.
Advantages of hot combustion air are:
- Saving in fuel consumption. -Increase in flame temperature.
- Improvement in combustion. -Reduction in initial heating time.
- Reduction in scale losses.
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In a recuperator, heat exchange takes place between the flue gases and the air
through metallic or ceramic walls. On the other hand in a regenerator, the flue
gases and the air to be heated are passed alternatively through a heat storing
medium, thereby resulting in transfer of heat.
Distribution of heat in a furnace. Most of the heat is released in the combustion
zone at the left and travels from there to the right. The heat thus liberated should
be absorbed (1) by the charge, which is most desirable. But heat also goes
elsewhere, some of it is passed into the furnaces walls and some into the hearth (3)
increasing the temperature of those parts. Another portion of the heat is lost to the
surroundings by radiation and convection from the outer surface of the walls or by
convection into the ground (2). Through cracks or other openings heat radiates
away (4), and furnace gases pass out around the door, (5), frequently burning in
the open and carrying off heat. Heat is lost overtime a door is opened. Heat is lost
a long pieces of the metal from the part which is outside the furnace (6). Finally,
heat passes out with the products of combustion and (7) either in the form of
sensible heat or as potential heat due to incomplete combustion.
Comparative features of parallel and counter – flow recuperator are tabulated in
Table 1.
Table 1. Comparative features of parallel and counter – flow recuperators
Parallel flow Recuperates Counter – flow recuperates
Less heat recovery achieved.
Minimum temperature difference
between hot and cold end.
Suitable for high temperature
applications.
Increase life of recuperator.
As metal temperatures at hot end is
considerably less, cheaper material
can be sued.
More heat recover achieved.
Too hot and cold ends are
encountered, hence higher risk of
thermal stress failure exists.
Suitable for all temperature levels.
The materials of construction should
be carefully selected for high
temperature.
Life depends on material of
construction.
Expensive materials are required to be
used at high temperatures.
Fuel Economy in Furnaces:
Means employed for heat conservation i.e. fuel economy on furnaces ensure its
economic operation. Furnace processes having higher temperature levels have the
greater thermal loss hence heat recovery must be made in this case. The largest
losses are contained in the waste flue gases and in radiation from furnaces walls.
Waste flue gases contain both sensible heat and the latent heat of vaporization of
water and sometimes potential heat (unburnt fuel gases).
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The recovery of heat of vaporization of water is not practicable and the
elimination of potential heat in waste flue gas is controlled by providing sufficient
air for combustion and adequate mixing of fuel and air.
The total heat loss in waste flue gas is minimized by providing only sufficient air
for combustion, and by preventing air infiltration. Waste flue gas temperature is
reduced and heat is recovered by heat exchanger such as regenerators, recuperator
and waste heat boilers.
Fuel – air proportioning. Combustion air supply should be little above the
theoretical requirement. In modern furnaces, all possible points of leakage in
furnaces system is sealed to prevent air infiltration and controlled quantity of air is
blown through the regenerators. Accurate and automatic proportioning of fuel and
air improves fuel economy, furnace efficiency and process control. The use of
oxygen analyzer on the products of combustion leads to improved fuel economy
and efficiency.
One of the most important factors influencing the efficient use of the fuels is their
stack loss i.e. that proportion of the fuel energy which is carried out of the
operating furnace as the sensible heat of the flue gases. The important factors
governing this parameter are normally the temperature of the flue gases leaving the
furnace and their oxygen content (which is related to the production of air used for
combustion above strict stoichiometric requirements i.e. % excess air). The general
relationship between the % oxygen content and % excess air for both low C.V. fuel
(e.g. blast furnace gas and rich fuel (e.g. coke oven gas, heavy fuel oil etc.) is
shown in fig.2 the factors that contribute to stack losses via the enthalpy values of
the contributing flue gases (comprising normally of CO, CO2, N2, O2) are given in
Fig. 3 whose significant feature is the much higher heat content of CO2 (at any
particular temperature) than any of the other constituents of flue gas. Apart from
water vapor, other gases behave similarly.
These date along with the appropriate combustion data are used to calculate stack
losses fig. 4 shows the stack losses for blast furnaces gas (the leanest gas i.e. low
C.V. fuel gas) and for coke oven gas (a rich gas i.e. high C.V. fuel gas) over a
range of flue gas temperature and % excess air. It clearly shows:
- The influence of exhaust flue gas temperature e.g. even with a high C.V. fuel
gas, some 50% of he energy supplied ends up in the waste gases at temperature
around 1000oC.
- That in the case of a low C.V. fuel gas, even with good combustion controls
most of the supplied energy leaves via the stack at temperatures as low as 600oC.
- How poor fuel can B.F. gas be is demonstrated by the fact that B.F. gas with
good combustion control is no better than C.O gas at 100% excess air (i.e. an
extremely poor level of combustion control).
The stack loss relationship of some common fuel gases taken at 10% excess air (a
normally accepted good level of combustion control) is shows in Fig.5 which
reveals that higher C.V. fuels behave very similarly.
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For low stack loss and therefore efficient use of fuel care must be exercised to
ensure that proper combustion takes place. Fig.6 shows the effect of using
insufficient (i.e. substoichiometric) air for combustion of natural gas it indicates
that:
- 10% deficiency in combustion air can result in the production of unblunts
equivalent to a loss of energy release equivalent to 6% of original gas.
- Even where a sufficient supply of combustion air is present, 1% reduction in
oxygen content (say for natural gas exhausting flue gas at 700oC) will lead to a
lowering of stack loss by1%.
Oxygen enrichment of combustion air. Air is mainly composed of 21% (by
volume oxygen and rest (i.e. 79%) inert nitrogen. When combustion takes place,
the oxygen combines with the carbon and hydrogen of he fuel and liberates heat.
The inert gases of the air absorb heat from the combustion and carry it out of the
furnace resulting in loss. It reduces flame temperature by absorbing heat and thus
reducing the rate of heat transfer to the stock.
If inert content of air could be reduced, much more efficient combustion would be
attained. This can be done by addition of oxygen to combustion air. Increasing the
oxygen content lowers the inert content of the air, consequently when a given
amount of fuel is burned with oxygen enriched air; the volume of waste gas is less
than if ordinary air were used. If the temperature of the waste flue gas is not
increased, the sensible heat loss in the flue gas will be decreased due to the smaller
heat capacity of the smaller volume. In furnaces operated at higher thermal loads,
reduction of inert results in decrease of waste gas temperature. With the same fuel
input, enriched air for combustion raises flame temperature of a given fuel thereby
increasing heat transfer rate and increasing production rate. The fuel input may be
decreased when enriched air is used, to maintain the same production rate as
obtained with more fuel using ordinary air.
The use of pure oxygen reduces the stack losses of fuels as shown in fig.7. The
really outstanding point is the transformation of low C.V. B.F. gas to be able to
emulate the high C.V. fuels when combusted with cold air–this without the high
flame temperature with limit the application where straight pure oxygen/ high C.V
flue can be used.
Use of oxygen enriched combustion air i.e. oxygen enrichment of combustion air
offers a method of achieving a measure of stack loss reduction when using oxygen
to combust the high C.V. fuels with the technical disadvantages associated with the
use of pure straight oxygen.
Fig.8 shows the effects of combustion of fuel oil with oxygen enrichment up to
25% oxygen, which indicates that a 1% increase in oxygen enrichment reduces the
stack loss at 1000o by 3%.
When steam is used for fuel oil atomizing, it influences fuel consumption
adversely in to way:
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Viz. (i) by the need to heat the steam used from supply temperate to exhaust
condition along with (ii) the stack loss associated with fuel needed to supply this
additional heat.
The adverse effect of using atomizing steam, at a rate of 0.5 kg steam/ kg oil is
shown in Fig. 9. This has a small but positive influence on effective stack loss,
typically 1% at flue gas temperature of around 1000oC.
Fuel saving through oxygen enrichment:
If air is enriched with oxygen, combustion of the fuel is faster and more complete.
The fig.10 bellow shows he relationship between oxygen content in the air and the
fuel saving rate. From the figure, it would be seen that in1300oC fuel fired furnace,
air of 30 percent oxygen concentration gives a fuel economization rate of 35% Viz.
– a – viz. normal air (21% oxygen) while in 900oC furnace, the fuel saving rate is
around 17%.
Furnace pressure control. If the pressure of the gases in the heating chamber of a
furnace is below atmospheric, cold outside air will be drawn into the furnace
through any openings that exist. If the interior pressure is above that of the outside
air, the hot gases would be forced out of the furnace through these openings and if
too much higher, will in addition, tend to penetrate refractories and overheat the
furnace causing damage.
Furnace temperature control. The use of excessive amount of fuel is wasteful and
results in high furnace exit gas temperature and damage to refractors. High fuel
rates hasten the heat transfer rate to the material and my cause damage to it in
some cases. The use of insufficient fuel reduces the rate of heat transfer and
prolongs process time. The optimum fuel rate for protection either of the material
being heated or the furnace refractories and often for control of production rate or
heating, is maintained by automatic temperature measurement.
Insulation. Installing materials are valuable to conserve fuel, to afford safer and
more comfortable working conditions, to protect materials susceptible to damage
from heat/ thermal strain and to speed up furnace operations.
3. HEAT TRANSFER IN FURNACES
In any transfer process, the net flow of a property is due to the influences of a
driving force. The rate of transfer is the flux and the intensity of the driving force
is the potential difference. The basis of the transfer process is that a liner relation
exists between the flux and the potential gradient. In all cases, the transfer process
or flux movement takes place in the direction of decreasing potential.
In case of heat transfer, difference in temperature is the potential. In general, the rate of
flow is directly proportional to the potential difference and inversely proportional to the
resistance and they can be represented by a generalized equation.
Resistance
Potential Current
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Heat Transfer:
It deals with the rate of exchange of heat between hot and cold bodies called the
source and receiver respectively. Heat flow towards the decreasing temperature,
the rate will depend on the difference of temperature between the source and
receiver. In case of evaporation or condensation heat transfer occurs and heat is
absorbed or given off at constant temperature on one side, as the case may be with
the gain or loss in internal energy respectively.
Process Heat Transfer:
Process heat transfer deals with the rates of heat exchange as they occur in the heat
transfer equipment of the engineering processes. A typical problem of process heat
transfer is concerned with the quantities of heats to be transferred, the rates at
which they may be transferred because of the natural of the bodes, the driving
potential, the extent and arrangement of the surface separating the source &
receiver and the amount of mechanical energy which may be expended to
facilitate the transfer. Since heat transfer involves an exchange in a system, the
loss of heat by one body will be equal to heat absorbed by another within the
confines of the same system.
Mechanism of Heat Transfer:
There are there modes in which heat may pass from source to a receiver namely
conduction, convection and radiation. In most of engineering applications more
than one simultaneously occurs.
Conduction:
Conduction is the transfer of heat through a fixed material such as a stationary
wall. The direction of heat flow is at right angle to the wall if the surface is
isothermal and the body is homogenous. The flux i.e. Amount of heat flowing per
unit time “dq” is directly proportion to the area A and temperature difference “dt”
and inversely to the thickness “dx” at right angles to the direction of flow.
Heat conduction in industrial furnaces is very common as the temperature
differences of the inside and outside surfaces are quite high. The furnaces walls are
made of refractory layers to withstand the high temperature of the combustion gas
products followed by insulating materials to minimize the heat loss to the
surroundings. The layers may be of different shapes from flat walls to cylindrical
layers. The heat transfer rates through these layers are calculated by determining
the combined resultant resistance and the overall temperature differences between
the source and he surroundings.
Convection. Convection is the transfer of heat between relatively hot and cold
portions of a fluid by mixing. The transfer may be through natural of free
convection wen a natural current is produced by the movement of hotter region
towards the colder region due to change in density or through forced convection
where mechanical agitation by stirrer or pump gives an artificial convection
current.
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The forced convection is much faster than natural convection. The heat transfer
rate is proportional to the area available and the temperature difference between
the hot and cold fluids. The rate equation is:
dq = hAdt,
Where h is called heat transfer coefficient.
Convection heat transfer occurs in process equipment‟s like heat exchangers,
condensers, vaporizers, superheaters; air preheater's etc. convection of heat also
takes place in steam generation in boilers and preheating crude tar or crude
petroleum for distillation.
Heat equation, dq = hA.dt in integrated form is
q = hA (test-tm) = hAt
Where only one connective resistance is considered (h is called also as the
individual film coefficient on a single side of the heat flow).
Radiation Heat Transmission. The relative importance of several mechanisms of
transfer of heat from one body to another differs greatly with temperature. The
phenomena of conduction and convection are affected primarily by temperature
difference and very little dependence on temperature level, whereas radiation
energy exchange increases rapidly with increase in temperature level. At low
temperatures, conduction and convection are the major contributory items to the
total heat transfer but at higher temperatures, radiation is the main controlling
factor.
4. CONTROL OF FURNACES ATMOSPHERE
During heating of solids in furnaces, chemical reaction between the charge/ stock
and the surrounding medium (which may be solid, liquid or gaseous) at high
furnaces temperature does occur. The gaseous medium in contact with charge
inside the furnaces is called furnace atmosphere. The chemical reaction between
the charge and atmosphere depends on the furnace temperature and the
composition of the stock and the atmosphere inside the furnace.
Two different kinds of furnace atmosphere are in existence.
- Combustion or natural atmosphere containing the products of complete or
incomplete e.g., CO, CO2, N2, H2O, H2, O2, SO2, etc.
- Prepared/ artificial atmosphere comprising mainly, of N2, CO and H2.
Heating and heat treatment of metals and alloys require special types of
atmosphere to improve the physical properties. Atmosphere inside the furnaces can
be of the following types (depending upon the specific duties furnaces are
supposed to perform).
- Oxidizing atmosphere (e.g. in boiler furnace).
- Reducing atmosphere (e.g. in blast furnace).
- Carburizing/ decarburizing atmosphere (e.g. L.D. converter).
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- Nit riding atmosphere (e.g. in metal surface hardening process).
- Neutral/ inert atmosphere (e.g. in steel annealing furnace).
In many processes in which materials are heated, it is important to maintain a
special atmosphere inside the furnace in contact with materials. In a furnace which
is heated internally by gas, the combustion of gas can be controlled to give an
oxidizing or reducing atmosphere.
In some cases, the heat treatment of metals involves chemical treatment of surface
layers (e.g. Carburising or intruding) which is carried out in carefully controlled
atmosphere. In other cases, the furnace atmosphere is controlled to prevent surface
attack of metals (e.g. oxidation, scaling, decarburisation or sulphur penetration).
At times, atmosphere is produced externally to the furnace (e.g. neutral atmosphere
for steel annealing is created by nitrogen) and fed to it. For many applications,
CO2, and H2O vapor are considered to be oxidizing agent (irrespective of any free
O2 present) and CO & H2 are reducing agents. Nitrogen gives inert atmosphere.
Oxidizing Atmosphere. In the low temperature heating furnace ( 700oC), an
oxidizing atmosphere does not cause much of scaling of material due to presence
of free oxygen in flue gas. If an oxidizing atmosphere is desired. Then use of as
little excess air is desired as will complete combustion and give the desired
atmosphere. In a furnace fired with coal gas, it is usual to adjust to 10.5% CO2 and
1.5% O2 in flue gas which gives mild oxidizing atmosphere.
During heating of steel / common metals, flue gas component like O2, CO2,SO2,
and H2O vapor oxidizes the metal to form scales at furnace temperature. Oxidation
/ scaling increases with the time of exposure, but at a diminishing rate and the
effect of temperature is very high when the melting temperature of oxides is
exceeded (indicated by shiny surface of steel). Scaling by flue gas components is
greatly reduced if the charge lies in a quiet motionless part of the furnace
atmosphere and is heated almost entirely by radiation heat transfer. At high
furnace temperature, oxidation and decarburisation of stele takes place
simultaneously in combustion atmosphere. The higher the temperature, the greater
is the affinity of carbon for oxygen in comparison with the affinity of iron for
oxygen.
Reducing Atmosphere. Presence of CO & H2 and absence of O2 in flue gas from a
furnace is the indication of reducing atmosphere in the furnace. When the furnace
condition is to be adjusted to reducing conditions (as for steel hardening), the
primary and secondary air controls are regulated in conjunction with flue damper
to give a faint curtain of flame as the furnaces door is opened. A piece of paper
thrown into furnace should char but not flame. This practical test indicates the
absence of oxygen but gives no indication of amount of CO present. Gas analysis
will show about 1-2% CO in flue gases depending on the process.
Reducing atmosphere in the furnace are obtained by adjusting the secondary air
quantity such that a small quantity of CO is present in furnace gases.
Carbonaceous particles such as smoke does not create reducing atmosphere in
furnaces.
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Reducing atmosphere (i.e. excess of fuel) during heating of steel forms less scale
than is formed by oxidizing atmosphere (e.g. excess of air) but the scale is tight
and adheres firmly. During heating of steel at about 1200oC, scale formation does
not take place if CO/CO2 equals 3 and H2/H2O exceeds 1.2 in the flue gas. Since,
combustion to carbon monoxide does produce some heat but little and unburned
hydrogen produces no heat, a furnace temperature of 1200oC cannot be attained
with these ratios without resorting to combustion with oxygen enriched air or
preheating of combustion air. During steel slab heating, manganese is not oxidized
if the furnace atmosphere has H2, CO and N2 in the flue gas which is possible in
reducing atmosphere.
Carburising atmosphere. Hydrocarbon gases carburise the surface of steel at
annealing temperature.
Carburisation which means increase of carbon content, is practiced for the purpose
of producing hard surface on mild steel.
Decarburising atmosphere. It involves removal of carbon from iron carbide
(Fe3C) thus affecting the crystalline structure of the steel. CO2 decarburizes steel
while CO carburises.
Fe3C + CO2 3Fe + 2CO
Hydrogen also decarburises at high temperatures (when methane is formed)
Fe3C + 2H2 = 3Fe + CH4
Hence, addition of CH4 to the atmosphere reduce decarburisng by hydrogen.
Nitrogen in presence of steam decarburises high carbon steels. In the absence of
steam, it is passive.
Decarburisation is usually considered to be harmful during steel heating because
the surface of steel becomes soft.
Nitriding atmosphere. Active nitrogen formed by cracking ammonia at the metal
surface causes nitride formation with increases in surface hardness.
5. FURNACE CONSTRUCTION
It may appear from earlier sections that the ideal furnace is a refractory box in
which the maximum amount of fuel can be burned as nearly as possible to the
theoretical flame temperature and the products of combustion removed with the
lowest possible residual heat content.
In fact, the metallurgical furnace has a job to do and getting the stock in and out is
as important as handling fuel and waste gas. In many cases furnaces are machines
in which materials are processed at high temperature and in the cases compromise
is usually necessary between what is thermodynamically desirable and what is
possible under prevailing circumstances.
Furnaces usually have a cast iron or steel frame or case and a refractory brick wall
and lining.
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The choice of refractory is outside the scope of this section but should be adequate
to the job in hand and should be as good an insulator as it is practicable to use.
Thermal expansion in brickwork must be accommodated, usually by leaving
spaces between bricks during building otherwise the frames will become distorted
because the expansion of the refractories is likely to be three or four times the
maximum elastic strain in steel. In steel. In the case of arched roofs the shapes of
the tapered bricks should be so calculated that a good tight roof is obtained at the
working temperature and the distance between skew backs must be adjusted go
give a perfect arch with the correct rise. This is done with adjustable tie-bars
across the top of the furnace between the vertical columns at the sides and ends
(Fig.7) in other designs the rise of the roof is restricted by controlled down ward
pressure, while in a third type the bricks making the roof are suspended
individually to give a roof of any contour, including flat.
Apart from the steel or cast iron skeleton of a furnace, metallic components are
used in parts such as doors and door frames, grates, burners, skids, and mechanical
rabbles and some of these have to work at quite high temperatures. When the
temperature is very high, water cooled copper components are need blast furnace
tuyeres for example. At more moderate temperatures are cooled cast iron can be
used as in most graters. Heat resisting cast iron is also available at a price which is
undoubtedly worth paying for some purposes such as in sintering machine grates.
A furnace should have stable foundations appropriate to its size and weight and the
main framework should be grouted into this and should carry the major part of the
weight of the brick work, as uniformly as possible. A sprung arch for example,
would sit on the skewback channels fixed to the frame and not on the walls. The
stacks of a blast furnace sites in a mantel ring which distributes the load through a
ring of heavy columns, and not on the walls. The stacks of a blast furnace sit in a
mantel ring which distributes the load through a ring of heavy columns, and not
via the bosh wall, to the foundation. Small tiling furnaces rest on trunnions through
which their weight is distributed to the foundation. Larger furnaces either rest on
roller bearings or on rockers which would be simpler to maintain. In all cases the
furnace case itself must be rigidly built using heavy steel plate, w ell supported and
with the brickwork packed into it carefully to avoid the possibility of break outs.
The use of steel sheet round brickwork minimizes gas leakage and allows low
temperature insulation to be packed in if desirable. It can be painted with
aluminum and can have good appearance but restricts access to brickwork.
Water cooled sections should be readily repairable in case of failure and
precautions should be taken against cooling units becoming silted up. Cooling
blocks should be of high purity copper where possible and the flow arranged so
that the coldest water flows over the hottest surfaces with very high velocity.
Means for charging and discharging the furnace and if appropriate for arranging
the flow of stock through it are usually straightforward engineering jobs.
Special tools are used for lifting and laying ingots at soaking pits, carrying pig and
scrap horizontally through open hearth furnace doors, and tipping it out of its box,
or pushing billets through rehearing furnaces; blast furnaces are charged with skips
F U R N A C E S
15
or buckets filled automatically and delivered through a system of bells designed to
minimize segregation by sizes (Fig. 11). Suitable ladles must be available for
receiving molten products and adequate crane capacity to handle the ladles and
also service the furnace itself. The stock may pass through the furnace by gravity if
the traverse is vertical or if sloping down an inclined plane (e.g. in rotating kilns)
or it may be pushed along skids or carried on a moving hearth or on rollers if
horizontal. Molten charges flow under gravity the hearth being so shaped as to be
self emptying once the tap hole has been opened. Some furnaces tilt for pouring
and occasionally liquids are discharged by syphon or suction arrangements (e.g.
lead or aluminum).
Stirring or mixing the stock in roasting or melting furnaces may be by rabbles of
rakes (Wedge roaster) or by rotation of the furnace (rotating kiln, crucible and
hearth furnaces). Molten stock is often mixed by reactions involving gas evolution.
Electromagnetic stirring has been applied to large electric arc furnaces and of
course, occurs in induction melting.
Water cooled sections should be readily repairable in case of failure and precaution
should be taken against cooling units becoming silted up. Cooling blocks should
be of high purity copper where possible and the flow arranged so that the coldest
water flows over the hottest surfaces with very high velocity.
Means for charging and discharging the furnace and if appropriate for arranging
the flow of stock through it are usually straight forward engineering jobs. Special
tools are used for lifting and laying ingots at soaking pits, carrying pig and scarp
horizontally through open hearth furnace doors, and tipping it out of its box, or
pushing billets through rehearing furnaces; blast furnaces are charged with skips or
buckets filled automatically and delivered through a system of bells designed to
minimize segregation by sizes (Fig. ). Suitable ladles must be available for
receiving molten products and adequate crane capacity to handle the ladles and
also service the furnace itself. The stock may pass through the furnace by gravity if
the traverse is vertical or if sloping down an inclined plane (e.g., in rotating kilns)
or it may be pushed along skids or carried on a moving hearth or on rollers if
horizontal. Molten charges flow under gravity the hearth being so shaped as to be
self emptying once the tap hole has been opened. Some furnaces tilt for pouring
and occasionally liquids are discharged by syphon or suction arrangement (e.g.,
lead or aluminum).
Stirring or mixing the stock in roasting or melting furnaces may be by rabbles of
rakes (Wedge roaster) or by rotation of the furnace (rotating kiln, crucible and
hearth furnaces). Molten stock is often mixed by reactions involving gas evolution.
Electromagnetic stirring has been applied to large electric arc furnaces and of
course, occurs in induction melting.
While refractories are expendable they are costly and it s essential that they should
be well chosen and well laid with suitable refractory cements.
The advantages from laying bricks well so that the roughness of the inner walls is
low are apparent in the last chapter. Slogging and spelling may be inevitable but
slag or saplings should not be allowed to accumulate to impede gas flow. The
F U R N A C E S
16
limitations of the bricks should be recognized and nay special precautions
necessary during firing or cooling rigidly observed.
The present day tendency is towards large and more economic units mechanically
rather than manually served, and fully instrumented. The success of a furnace
design is probably best measured (1) by the rate at which the furnace can
consumes fuel, (2) by the efficiency with which it transfer its energy to the stock,
and (3) by the amount of fuel it can consume before it is worn out. Fuel costs must
be related to capital charges as well as to costs of materials and labor. The efficient
use of refractories can be as important as the efficient use of fuel and design
should permit rapid repairs to be effected using castables or prefabricated section
where possible in order to minimize loss production time.
6. CLASSIFICATION OF FURNACES
Furnaces may be grouped in various ways, none of which forms a basis for a
comprehensive classification system. They may be grouped by purpose (melting,
roasting), by mode of firing (gas, coke, electricity), by shape (crucible, shaft) or by
the method of charging (in- out-, straight through etc...). No matter how it is done,
there is division and sub-division of each class, much over lapping and many
miscellaneous misfits left over. The present discussions recognize three major
classes of metallurgical furnace, the crucible furnace the hearth furnace and the
shaft furnace. Although these are distinguished mainly be shape even with this
very simple classification it will be seen that there are furnaces which could be
included in more than one group. To these must be added the minor classes, retorts
distinguished by their volatile products, converters which are high temperature
reaction vessels in which gas is blown through liquid metal; sintering machines in
which air gas is blown an incandescent bed of mixed ore and fuel, and a small
residue of types which will not fit into any of these classes. Finally, probably the
most important class of all, there is the boiler. Brief descriptions of various types
of furnaces in each of these classes are given and there modes of operation
considered in the light of the theory discussed in earlier chapters.
7. CLASS I – CRUCIBLE FURNACES:
combustion (Fig. 13, 14), Distribution of heat before melting is mainly by natural
convection within the crucible rotates on an inclined axis a gas or oil flame either
swirls round the outside or plays directly on the charge (Fig. 15). These are more
efficient, and useful for rapid melting of swarfe scrap and even in some easy
reduction processes. Induction melting furnaces (Fig. 16, 17) are of course, in this
class and are the historical replacement of the old multiple crucible furnaces a
regenerative fired chamber in which tool steel was traditionally made. The stock
may, if necessary, be isolated from the furnace gases in most designs of crucible
furnace. With induction heating any atmosphere can be imposed and the whole
melting and casting operation can be carried out in vacuum on a moderate scale.
F U R N A C E S
17
Crucible furnaces are never large a maximum of 200lb charge for fixed furnaces
and about five tons for tilting furnaces (except induction furnaces which may go
much bigger – seep. 147).
8. CLASS 2- HEARTH FURNACES
This is the widest class of all and includes a wide range of fixed, tilting, and
rotating furnaces and kilns fired with all fuels and used for roasting, melting,
reheating and many other purposes. In most cases the charge, on the floor or hearth
of the furnaces chamber is heated by convection and radiation form the flame (or
electrical sources) above it. In a minority of cases the fuel is mingled with the
stock and is burned by air passing over the hearth or blown down on to it as in the
case of the traditional blacksmith‟s hearth.
Hearth furnaces evolved from primitive concepts. The simple „air furnace‟ (Fig.
14) is the prototype for the complex modern giants.
Furnaces of this shape were used in the “old Welsh‟ copper smelting process,
bigger hearths being used for roasting furnaces, smaller hearth for melting. The
same type is still used for melting copper alloys, but the firebox is usually replaced
by gas or oil burners (Fig. 14).
Further development leads to the use of regenerator for preheating the gas and air
with heat from the waste gases, and probably reaches it s most complex form in the
venture type regenerative open hearth steel making furnace in which both gas and
air are preheated (Fig. 18). The traditional glass tank is a similar design. Modern
tendencies are towards the use of rich gas or oil or both together, only the air begin
preheated. These large steel making furnaces may be fixed or tilting (through
about 30o for slogging and pouring) and may have capacities up to about 400 tons
and even 1000 tons for inactive mixers. They are fully instrumented and recent
designs incorporate means of enriching the air with oxygen or injecting heat
energy through gas- oxygen lances inserted through the roof or end wall.
Nevertheless the over-all design is not theoretically very satisfactory. The
regenerative system imposes the condition that the furnace must be able to be fired
from either end so that the ports must also act as flues. Obviously the same ducts
cannot do both jobs perfectly. The development of a practicable high temperature
recuperator would make unidirectional firing possible. It would probably also lead
to other source.
At the same time smaller reverberatories have been developed for melting at the
lower temperatures needed by, for example, alloy cast irons and copper base
alloys. There are many types of barrel shaped furnaces (Fig. 19) fired with gas, oil
or pulverized fuel burned with recuperator preheated air. These are usually tilting,
rocking or rotating types in which the unmelted stock can be moved to aid heat
transfers and when molten the metal is made to wash round the lining. This
permits very high heat transfers rates by convection without endangering the
brickwork.
F U R N A C E S
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Billet heating furnaces have essentially the same form but the solid stock is placed
on a flat hearth and either removed through the only door when heated or pushed
through the furnace from one end to the other on skids in the time required for
heating usually counter to the gas flow, the burners being situated near the
discharge end. Some furnaces in which the burden passes through continuously are
built in two or three zones as indicated in Fig. (20). The first zone might preheat
the charge to about say 900oC in a billet heating furnaces, the second bring it up to
1100C and the third be designed to hold it at that temperature. All three burners
would discharge to a common flue and the counter flow principle would be
observed. The height of the roof would have to be such as would accommodate the
different volumes of gas in each zone. Flat products may be progressed through
such a furnaces by means of a walking beam hearth. The work is periodically lifted
from a grid on which it rests and advanced a short distance by a secondary support
system whose movement is controlled by cams or eccentrics. This device allows
some of the heating gases to the supplied beneath the work so increasing the
heating rate.
Many kilns are specialized hearth furnaces. Even the beehive kiln (Fig. 21) has the
same characteristics as the air furnace but its shape has changes. Firing across the
furnace instead of along it should give more uniform temperature distribution with
short hot flames. Some preheating of the solid charge might also become possible
(prior to the use of the hot gases in the recuperators) and the process could become
continuous. There are some refractors problems outstanding and it is not clear that
the capital cost of the changes would be covered by the saving in fuel at present.
The further development of this furnace is unlikely ever to be carried out,
however, since the march of progress in the steel industry has rendered it
obsolescent. The regenerative principle casting techniques is reducing the number
being built.
Outside steel making the best example of a regeneratively fired hearth furnaces is
the glass tank furnace (Fig. 22).
These are cross fired with producer gas (preheated) but are now commonly fired
with, oil, only the air being heated. The charge of sand and metallic oxides is fed
in at one end of the hearth where they melt together and flow along the hearth
where the entrained air bubbles separate out and homogeneity is achieved.
Different fringe rates can be employed along the furnace to provide the appropriate
temperature distribution. The glass at the holding end may be raised to 1500oC.
From there it flows under a skimming bridge into fore-hearth which may be
separately fired to maintain a lower temperature of only about 900C.
the glass is withdrawn at various points from this section into molding machines or for
manual fabrication. The whole process involves a nice blanching of energy and materials
flow rates. The choice of refractories is important because the glass must not become
contaminated from that developed to accommodate a large pile of brick or coal (for
cooking) and more connective heat transfer was accommodated. The beehive itself
developed from the isolated batch furnace into battery kilns (Fig. 22) in which the air for
combustion is preheated by using it to cool the previous charges while the waste
gases go to preheat the following charges.
F U R N A C E S
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Finally this system evolved into the tunnel kiln (Fig. 21) in which the stock is
loaded on to bogeys and passed through the waste gases, the firing section and
then against the cold secondary combustion air the stock moving through the fire
instead of the fire progressing through the stock. Tunnel kilns are used for drying
and firing bricks and also, in one process for reducing iron ore. They are
thermodynamically very efficient counter flow heat exchanges.
Figure (22) shows how a tunnel kiln design can be curled round into a circle. The
stock would be passed through either on pallets or on a moving hearth. It might be
discharged cold as indicated for forging in which case discharge is from the
highest temperature zone and the heat in the gases must be recovered in some other
way.
Rotating kilns (Fig. 23) also, follow the same pattern. They are long narrow
cylinders 200 ft x 10 ft for example rotating on a slightly inclined axis, so that the
charge is transported with the aid of gravity counter to the flow of the burning fuel.
Such kilns are used in cement and refractory manufacture and also in one method
of making sinter and one method for the direct reduction of iron ore.
Two special forms of hearth furnace are used in extraction metallurgy. One is the
multiple hearth „Wedge‟ type roasting furnace (Fig. 24) in which sulphide ore is
transported across a series of hearths by means of rotating rakes, being passed
from one hearth to the next lower one through holes alternately at the center and
periphery. The ore flows counter to a stream of hot air which reacts with its
sulphur content and removes it as sulphur dioxide. The sulphur is the fuel and the
process is cooled pyretic roasting. The ore hearth is a smelting furnace in which
ore and coke or coal are mixed one the hearth and hearth by air blast which reacts
with some of the coke forming carbon monoxide which creates sufficiently
reducing conditions to permit reduction of the ore. This is used for lead reduction
the special Neumann hearth being fitted with elaborate rabbling equipment to
ensure complete the same arrangement, the job being buried in a bed of coke
burning in a stream of air injected through a tuyere.
There are several kinds of electric hearth furnaces of which the most important is
the 3-phases Heroult furnace (Fig. 25) in which the 3- phase current passes
between three large carbine electrodes through the slage and via the metallic part
of the burden forming three large arcs of plasma with temperature of several
thousands degree Celsius. This is the standard from used in steel making in sizes
form a few tons up to about 200 tons capacity.
This type of runace was reserved for the production of the high alloy steel until about
1960 except where electricity was particularly cheap. In that application there were some
metallurgical advantages over alternatives, associated with the reducing conditions
prevailing within which afforded some protection against loss of valuable alloying
elements like chromium by oxidation. The cost of electricity made it unattractive for
tonnage steel production but by 1960 it became apparent that relative costs of electrical
energy, labor and capital had changed, and that high price of electricity could be more
that offset by savings in over heads gained by the faster melting rate under the electric arc
and by the lower labor costs involved in charging operations which are simply a
single drop from a basket while the roof is drawn aside.
F U R N A C E S
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This applies, however, only to steel making from an all-scrap charge as the
economies mentioned cannot be realized when steel is made from a molten iron
charge.
There are also metallurgical advantages with respect to the quality which can be
achieved under the reducing conditions available. The power consumption is high.
A 100ton capacity furnace would require a supply at the rate of 40.000 KVA at
voltage tapping between 100 and 500 depending on the state of the charge. The
current is controlled by automatic movement of the electrodes to position which
gives the optimum arc length and hence arc resistance. During melting there are
inevitably large surges of current which must be damped out by inductive reactors
in order to safeguard the supply system. A major difficulty encountered in running
the large sizes of these furnaces is that of attaining a uniform distribution of
temperature and composition in the bath. Electro- magnetic stirring of the bath and
rotating hearths are used to this end.
A second type is single phase with the arc struck between two carbon electrodes
(Fig. 26) the energy being radiated to the stock in the hearth directly and by
reflection off the roof. This is a clear cases for rotating the furnace after melt-
down in order to improve heat transfer and safeguard refractories. A third design
also operates on single phase, the current being passed through the charge from a
single electrode into a conducting hearth.
A fourth type of arc furnace has recently been developed for the matte smelting of
copper concentrates (Fig. 27). In this the electrodes are effectively buried in finely
ground calcine whish is charged through the roof. The electrodes reach down to
the slag level and current is passed through the slag and matte and back through
the slag again. In this design no visible radiation escapes the furnace and process
gases escaping through end flues are at a temperature of only about 200c. the
electrical arrangements may differ between plants. Primary supply would normally
be 3- phase a.c. but in one design each phase is rectified separately and three pairs
of electrodes operate separately (but simultaneously) providing d.c. arcs. Where
costs are favorable this type of furnace has replaced traditional oil-fired
reverberatory matte smelting furnaces.
Heat transfer in arc furnaces is primarily by radiation from the arc except with
respect to that part of the heat produced by the currents it passes through the stock.
In the matte smelting furnace heat transfer.
Is mainly by radiation which is totally absorbed. In the open hearth furnaces which burn
fuel the heat transfer is also primarily by radiation as provided the flame is luminous. This
is supplemented by convection if the flame is suitably directed on the stock. Radiation is
reflected off the roof which should be able to withstand temperature in excess of the
maximum stock temperature. As long as the stock is cold the net heat transfer is from the
roof downwards but when the charge melts its absorptive power falls and the temperature
of the roof may rise dangerously because the bath is behaving like a mirror, reflecting all
the heat of the flame back up to the roof. In steel making at temperature near to the limit
of the refractories it is necessary to keep the bath on the boil with bubbles of carbon
monoxide continually bursting on the surface and creating as they do so transient
but frequently recurring and quite effective black body conditions.
F U R N A C E S
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9. CLASS 3- SHAFT FURNACES
These are used for melting smelting and calcining or roasting. The are either
cylindrical or rectangular shafts through which the solid charge flows down under
gravity into the space vacated by discharge of solids or liquids or gasification of
part of the charge. Usually the fuel (coke) is included in the charge and is
preheated in descent by the ascending stream of hot gas (CO+N2). The combustion
air which may be preheated, is introduced via tuyeres placed all round the lower
end of the shaft a short distance above the hearth or crucible in melting furnaces.
The iron blast furnace (Fig. 27) is the most highly developed shaft furnace and can
operate very efficiently not only as a heating unit but as a high temperature
reaction chamber. Modern furnaces are nearly 30 m high (plus a considerable
superstructure) and may be over 15m in diameter at the hearth. Hot blast temperate
up to 1000oC are used and blast pressure may be as high as 3bar with positive
pressure throughout in some cases the so called high top pressure furnaces. The
largest furnaces now smelt up to 10.000 tons of iron in a day requiring air at a rate
up to 20.000mm/min. Recent developments include part-firing with oil or natural
gas introduced at the tuyeres and adjustment of air blast composition by adding
oxygen and/or steam. These steps are designed to give the highest practicable
combustion temperature combined with good over energy input than is available
through the coke charging rate alone. The charging system is important in
determining the distribution of the material size and hence the permeability of the
burden to the combustion gases. Resistance to gas flow should be low and uniform
across the furnace. The refractors are water cooled to give them greater resistance
to wear, in the upper reaches, and to slogging particularly in the bosh. Thecrucible
wall also is protected against breakouts by collers not shown in Fig. ( 11 ) and a
circular palisade of „stave‟ coolers safeguards the hearth. Cooling water losses a
mount to about 2 percent of total throughout. Blast furnaces are tapped for slag and
iron at regular intervals of about 4 hrs. furnace campaigns last for several year,
during which something like 2 million tons of iron may be made.
Heat transfer in this counter flow process is potentially very efficient but the
efficiency does depend up on the available heat in the ascending gases „matching‟
the demand for heat by the stock at all levels.
This can be controlled to a limited extent by those factors which determine
combustion zone temperature. The top gas temperatures is about 200oC. It cannot
be allowed to fall much lower because the gas must come past the dust catcher
above its dew point. The chemical heat in the gas by virtue of some 25 percent of
carbon monoxide is used for preheating the blast in the cowper stove.
Heat transfer in the blast furnace is mainly by forced convection in the stack where
the wind speeds are generally believed to be extremely high perhaps of the order of
150km/h. in the hearth and bosh the temperature are so high that a large part of the
transfer must take place by radiation from one particle of coke to another.
F U R N A C E S
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The cupola (Fig. 28) is a simple melting furnace for cast iron usually working on
cold, low pressure blast though hot blast is now sometimes used and even oxygen
enrichment, usually as a control on temperature.
Pig iron, scarp and coke are charged at the top over a large coke charge for
preheating the furnace and providing a continuous reducing bed through which the
iron passes to the hearth. These furnaces run for a few hours only at a time. On
continuous operation their thermal efficiency would be much improved, but their
design would have to be elaborated.
There are many designs of simple kilns (e.g. Gjer‟s, Fig. 29) based on the same
principle, for burning limestone or calcining ore. In these cases the solid product
must be withdrawn from the hearth mechanically. These kilns may operate on
natural draught one started up.
Low shaft furnaces have evolved from high shaft furnaces already described, for
use where a high degree of preheat and pre-reduction of ore is not necessary or not
possible. One case is the experimental low shaft iron smelting furnace al Liege
(Belgium) which was to produce iron from briquettes of ore and coal (which cold
not survive passage through a high furnace). A second case is the electric low shaft
iron smelting furnace use din Switzerland, Italy and Norway using cheap hydro-
electric power to supply the energy for the process and coke only as a reducing
agent. At this point the distinction between a hearth furnace and a shaft furnace
becomes historical. Ferro alloys are produced in a similar type of furnace. In each
of these cases a very large demand for high temperature heat is met by a high
temperature source with low capacity so that its temperature falls very rapidly and
there is no heat available for adequate preheating of the stock. Thermal efficiency
suffers because the burden is not preheated and designs have been put forward for
performing this preheat in a rotating kiln fried with the rich gas which comes off
the furnace.
One other important shaft furnace is the gas producer, which is still used in
steel making plants, and glass works, for example, through producer gas
firing is rapidly giving way to oil firing.
10. CLASS 4- RETORTS
In the production of gas from coal and in a few extraction processes the reaction
leading to a gaseous product are carried out in a retort. The traditional retort was a
narrow vessel open only at one end, and the process was always a batch process.
Retorts were operated in batteries with a common firing system. To a large extent
these have disappeared in favor of a more continuous system in which the charge
is passed down through a narrow chamber in which the reactions take place and
from which the volatile product can be drawn without contamination by air. This
applies in the coal gas and zinc industries but not so far in the coke industry which
retains batch processing. The most recent developments in zinc production
combine the blast furnace and the retort, lead being tapped from the hearth while
zinc is collected from the top gas.
F U R N A C E S
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11. CLASS 5- CONVERTERS
Converters (Fig 30-32) are used in steel making, in copper production and to a
limited extent in other non ferrous processes. Air is blown through or over the top
of the molten charge, reacting with part of it to produce sufficient heat to maintain
the process. In steel making the heat is derived from the oxidation of the elements
silicon, manganese, carbon and phosphorus which it is desired to eliminate and the
final temperature is several hundred degrees higher than the original. In copper
smelting the elements oxidized are iron and sulphur. Heat transfer is excellent as it
is produced within the charge but the losses in the waste gases are vast. Utilization
of this waste heat for steam raising in snow being adopted in some of the most
recently built plant, but as it is only intermittently available elaborate provision has
to be made to supply heat to the boilers from conventional fuel while the
converters are not blowing. It may appear that these processes must be very
economical on fuel but this is only partly true as carbon must be used in the blast
furnace to reduce the silicon, manages and phosphorus into the iron and
compressed air and oxygen also cost energy and money.
The heat available for the conversion of copper matte is however virtually without
cost, beyond blowing costs.
Particularly in the bottom blown converters the wear on refractories is very severe.
A new bottom may have to be fitted after every twenty or thirty heats and a
complete lining that can be done with water cooling and the best bricks are just not
good enough. The top blown or “L-D” converter is now used increasingly for steel making by the so-
called basic oxygen or BOS process. This has replaced not only the bottom blown
Bessemer process but also the open hearth process to the extent that all new plant for
refining blast furnaces iron to steel is now of this type. The first stage in this development
was the recognition that bottom blowing with air was introducing too much nitrogen into
the steel. Bottom blowing with pure oxygen was too hard on the refractories around the
tuyeres. The use of O2- CO2, and O2- H2O mixtures was technically successful but top
blowing by oxygen alone proved to be most economical. The process has now been
developed to deal with most qualities of iron and vessels are now being built with a
capacity of 400 tons which can be processed in under an hour. This high production rate
reduces the capital cost per ton to a figure far below that in open hearth steel making. Fuel
and refractories costs are also lower, the steel quality is not impaired and the heat balance,
without nitrogen in the reaction gases, is such that a high proportion of scrap can be
incorporated into the charge. The oxygen is delivered through a water cooled,
multinozzled lance and great care is taken with lance design and with the geometry of
impingement of the oxygen on the metal. A recent further development is the use of oxy-
fuel lances in which some propane can be introduced as supplementary fuel or to exercise
some control over the effective oxygen potential of the system. Another development is a
reversion to bottom blowing but with a sophisticated arrangement by means of which jets
of oxygen are introduced through tuyeres within an annular shroud of an inert gas
which delays the formation of iron oxide until the oxygen is well away from the
refractory bottom.
F U R N A C E S
24
12. CLASS 6- SINTERING MACHINES
Blast roasting or sintering is carried out on certain ores either to improve their
chemical composition with respect to carbonate or sulphide or to modify their
physical condition. Ore and coke are charged on to a grate and ignited. Air is
drawn down through the bed, burning out the coke, heating the ore, reducing it
with carbon monoxide and finally deoxidizing it after all the carbon is gone. This
is carried out on a sintering “pan” or more often, on a Dwight-Lioyd sintering
machine (Fig. 33) in which the process is continuos. Heat transfer condition are
good, but contact time is short and thermal losses in the waste and in the sinter
itself are very great.
13. CLASS 7- SOME MISCELLANEOUS FURNACES
a- Suspension Roasting is applied to the conversion of sulphides to oxides or
sulphates when the mineral has been very finely ground in the concentration
process. The sulphide particles pass down through a heated chamber against a
current of air, burning as they fall and providing the heat necessary for the
continuance of the process.
This type of process suggests fluidized bed techniques in which gases and fine
powders react while the latter are being carried along supported in the turbulent
stream of the former. Heat transfer in these beds is extremely rapid and
temperature control can be very accurate. This can be very important when
chemical reactions are involved in the process. The technique is well established in
chemical engineering and petroleum technology (where catalysts are entrained
with the reactants) and a very considerable extension of its use in extraction
metallurgy in the next few years seems likely. The limitation is that fine particles
of many minerals tend to sinter together at quite moderate operating temperature
and the critical particle size distribution necessary for fluidization cannot always
be maintained.
b- Soaking pits have developed from simple pits in the ground in which steel
ingots were allowed to cool slowly, smothered in ashes. Modern soaking pits are
regeneratively gas fried chambers each holding a number of ingots and still
charged and discharged through the top. Heating is mainly by connective transfer
from the hot gases whose flow pattern must be such as to give the most uniform
heating possible.
c- Forced air circulation furnaces (Fig. 34) are rather like soaking pits in so far as many
of them are charged and discharged through the top and heating is by convection.
A powerful fan keeps the air circulating first over the electric heating elements, and then
through the burden, which should be loosely packed so that the hot air can be well
distributed over its surface for the greatest effect. The assembly of nichrome heating coils
should also be arranged so that they lose heat to the air stream at the greatest possible
rate. These furnaces do not normally go beyond about 700oC but the elements
F U R N A C E S
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must of course run much higher than that. The efficiency of a furnace of this type
is determined largely by its insulation.
d- Salt baths are used for heating machined metallic parts out of contact with air
which might affect the surface properties. They are commonly use din the heat
treatment of steels. The salts used are commonly mixtures of nitrates, chlorides or
sometimes cyanides. Nitrates cannot be used above about 500oC as they become
unstable. Chlorides become volatile beyond about 800oC and can not be used
above about 900oC, except barium chloride which is used for heat treating high
speed tool steels at 135oC. cyanides are used as a component to effect case
carbonization. Glasses or slag might be used at still higher temperatures but are
often rather viscous, and more fluid compositions are usually rather reactive
toward either metal or containing vessels.
These baths may be in the form of simple crucible furnaces fried with gas or oil,
the salts being contained permanently in the crucible which would usually be an
alloy cast iron pot. Alternatively the salt may be heated by passing an electric
current through it between two electrodes.
Heat transfer to the stock placed into the salt is by natural convection. This is very
rapid much faster than from a gaseous atmosphere at the same temperature, mainly
because of the higher specific heat of the fluid.
e- Electrolytic cells as used for extraction of aluminum (Fig. 35) are also heated
by the passage of an electric current through a molten solution of salt. In this case
alumina is dissolved in cryolite and the bath operates at 900oC on the current used
for electrolysis. There is of course no heat transfer problem and efficiency depends
on the insulation provided and on other design features of the cell.
14. CLASS 8- BOILER PALNT
Economically, steam rising is one of the important uses to which heat is put.
Technologically it is often one of most inefficient of processes although large
modern plant leaves little to be desired in this respect. A furnace for boiling water
is usually called a boiler and this term covers everything from a simple water
jacketed firebox to the complex giants of the modern power stations.
There are three types of boiler. Small units for purposes such as central heating of
moderately sized buildings are quite simple “sheet” boilers in which the
combustion space above the grate and the flue to the chimney are surrounded by a
water jacket in the top of which the wet steam is collected. These are cheap, easily
operated and maintained but not very efficient. At the expense of a little
complication of design the heat exchange area can readily be extended either by
fitting water tubes across the firebox or flue, or by passing the combustion gases
through a number of parallel tubes running through the water space. Here
then are the beginnings of “water tube” and “fire tube” boilers respectively.
Fire tube boilers are used when larger quantities of steam are required at
moderately high pressures. The construction of these is also reasonably
simple and a number of designs have been in vogue for many decades. The
F U R N A C E S
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Lancashire boiler (Fig. 36) consists of two wide flues passing right through the
length of a cylindrical boiler drum back along the bottom of the drum and then in
two side flues to the rear again and to the chimney. The scotch Marine boiler and
the Economic boiler had narrower fire tubes carrying the hot gases through the
water filled drum and the locomotive boiler was similar in principle. The
construction of the drum of these designs was not suitable for use at higher
pressures than about 20 bar. Heat transfer was mainly by convection. Superheaters
could fitted into most of them.
For higher pressure operation the water tube boiler I necessary primarily of reasons
of strength. The boiler drums are aside from the path of the hot gases which are
made to pass through one or more banks of tubes in which water is circulated by a
pump from the drums. Another similar banks or coil of tubing constitutes the super
heater in which wet steam is raised to its working temperature. The hot gas is also
passed through an economizer in which feed water is preheated before entering the
boiler drum, and finally through an air heater to preheat the combustion air. Heat
transfer is almost entirely by convection. The heat transfer rate can be controlled
on the water side by designing the tube diameter and water flow rate for any
required Reynolds number.
One the outside, high gas velocity would be attained by setting the tubes closer
together. There is a limit to this however especially where there is the possibility
of depositing ash or soot which would restrict the passage of the gases and impede
the flow of heat.
The largest boilers used in central generating stations are water tube boilers with
some differences (Fig. 34). The fuel is pulverized coal burned with preheated air to
such a temperature that the ash is to a large extent slagged and collected at the
bottom of the large vertical combustion chamber.
This combustion chamber is lined entrierly with long vertical water tubes so that
its refractory walls are completely screened from the flame and its ash. The flame
temperature being very high, heat transfer in this section is by radiation to these
tubes in which the steaming is carried out. The gas must pass on the superheaters
sufficiently hot to raise the temperature of the steam over 500oC and for this reason
convective heating of a final bank of steaming tubes may have to be omitted.
There may be two banks of superheated tubes, primary and secondary the latter
being encountered by the hot gas first, of course.
The gases then exchange their heat with feed water in an economizer and finally in
the air heater in the usual way. In the most successfully designed units thermal
efficiencies approaching 90 per cent are now being approached. Under these
conditions fouling of the tube surface must be kept to a minimum in order to keep
the over all heat transfer coefficient high. If deposits are allowed to form on the
water side hot spots develop and failure of the tube is imminent either by corrosion
or by creep. Boiler waters are usually treated with chemical additions to minimize
corrosion. In the largest generating plant the feed water is practically distilled
quality and is also d-aerate so that corrosion and deposition are both very unlikely
to occur.
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Most type of boiler were originally designed for coal firing but have been found
adaptable to oil or pulverized fuel.
15. LABORATORY FURNACES
Laboratory furnaces are of all types and are designed on the same general principles as
industrial furnaces. They differ from these only in scale and in the degree of temperature
control which may be necessary. Special requirements in respect of the control of
atmosphere, the means of charging or discharging, and the siting of probes may have to
be met, and in certain cases these conditions, coupled with a very high operating
temperature may make design very difficulty indeed. Laboratory furnaces are usually
electric as these are cleanest and easiest to control. Gas is used where flexibility in needed
and especially where rather large spaces are to be heated to fairly high temperatures.
Surprisingly good results can be obtained with gas or oil, but they create a lot of noise and
do not admit much control of atmosphere unless completely muffled.Most electric
laboratory furnace are resistance heated, using whatever resistor elements are most
appropriate. The more expensive materials like platinum, rhodium, tungsten, tantalum and
silicon carbide are used for the highest temperature work. It is easier to find a resistor at
2000oC than to find a refractory on which to mount it. Indeed a refractory mount is best
avoided if possible. At moderate and low temperatures the nichrome and kanthal type
resistors are widely used.
A recent innovation for use at temperatures up to 1800oC is a composite element
consisting of a molybdenum conductor enclosed in an impermeable alumina
sheath. Electrical connections are made at one end of the sheath where there are
also connections for hydrogen, a slow stream of which must be passed through the
sheath while the element is hot. A number of these elements could be used
together in a furnace. They would have to be protected from chemical attack (by
almost everything) and from thermal and mechanical shock but this applies to
anything that might be used at such high temperatures. The alumina sheath will be
recognized as a muffle and to transfer heat through it to a furnace at 1800oC the
molybdenum wire inside will have to be at a very much higher temperature but
nevertheless the limitation is probably set by the refractoriness of the alumina
rather than that of the molybdenum.
Induction furnaces are very useful for very high temperature work, being rapid in
actin, clean, and ideal for vacuum or controlled atmosphere operation.
Temperature control to fine limits is, however, difficult. These furnaces are good
for making small melts of steel and other alloys. Laboratory units are usually
supplied by spark-gap or valve oscillatory frequency converters, and use
frequencies up to 100,000 Hz. The induced secondary current may be produced in
the metallic charge or in suitable screen of carbon or molybdenum (in vacuo) from
which the heat can be transferred by radiation to the stock which then need not be
electrically conducting.
Refractories in laboratory furnaces are again of all types but vitreous silica and
aluminous porcelain meet the needs of a high proportion of cases. Beyond 1350oC
refractory ware must be carefully chosen from sillimanite, mullite and alumina or
special refractories like thoria and zirconia. These materials can be obtained as
F U R N A C E S
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crucibles or tubes (with or without a closed end) which may be impermeable to
gases if required. Impermeable tubes have poor resistance to thermal shock. It is
obvious that resistance to slagging at very high temperatures is nil and furnaces
must be kept very clean. Castable alumina cement (alundum), alumina mixed with
a little clay as a bond, is commonly used for making refractory artifacts in the
laboratory but these are not impermeable. The alundum is made plastic with water,
molded to shape, dried and fired.
The designing of a laboratory furnace requires some experience, not the least in
guessing what power input depends on the size of space to be heated and the
working temperature and also on the degree to which the furnace can be insulated.
Thermal efficiency is sometimes depressed by the necessity of having one end
open for working or even occupied by probes which conduct heat away from the
hot zone. As a guide it may be said that under favorable conditions a 25 mm
diameter tube furnace at 1000oC would require about ½ KW while a 75 mm
diameter furnace at 1500oC would need 3 or 4 KW.
To achieve uniform temperature over a length of tube much greater than its
diameter some care has to be taken the furnace is usually wound differentially
(closer toward the ends) radiation baffles are mounted along the axis, and a
conducing insert used if practicable.
High temperature insulation using ordinary insulating materials is not very
successful as most of these are either not very refractory or are weak at high
temperatures. A series or radiation baffles or muffles will cut down the
transmission of heat and sets of concentric cylinders of molybdenum or platinum
can be used mounted outside the source of heat. These have to advantage of low
heat capacity and therefore do not lead to low heating of the furnace. They are
particularly appropriate under vacuum where there is no convection problem. A
simpler high temperature insulator is fine powder of any refractory material
magnesia, alumina, carbon, etc. in effect, this constitutes a complex multiple
radiation baffle, but it would also smother convection currents in a gaseous
atmosphere. It is desirable in all cases that baffles should be mounted at the end as
well as round the sides. Convection losses within the working space should also b
avoided, probably best by arranging that the hot chamber be fairly well filled by
stock. Vertical tube are particularly liable to losses by convection especially if the
top has to be left open.
Control the principle expounded there apply to laboratory equipment. Usually the
aim is to operate off mains voltage if possible. Gauge and length of resistor would
be chosen so that the wattage on full voltage was a little more than estimated
necessary to attain the required temperature. Manual control could then be effected
by series resistance, energy regulator or auto transformer. In a well designed
furnace there is little to choose between these provided it is working at design
temperature.
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For flexibility, however, the series resistance is wasteful of energy and the energy
regulator involves switching current on and off and may be hard on windings.
Some form of automatic control operated by a thermometer in the furnace is
usually desirable, preferably altering the energy input only fractionally. The
efficiency of any such control system obviously depends on the thermal inertia of
the system and this should be taken into account when the furnace is being built,
and kept as low as practicable.
16. PLASMA ARC FURNACE
plasma arc melting is the utilization of a direct current, transferred arc which is
directionally controllable and stable. This arc, when operated in an inert gas
atmosphere is a clean, uncontaminated source of heat.
The plasma arc furnace hearth design is similar in shape to conventional graphite
electrode furnaces, and utilizes the same refractory materials. Plasma arc furnaces
are equipped with an electrical connection to the charge through a water – cooled
bottom electrode mounted flush with the inner suffice of the hearth. To prevent
contamination of the atmosphere over the metal bath, the furnace roof side wall
joint is sealed with a labyrinth sand seal. When operating, the spout is covered, a
gas – tight cap is clamped over it, and chevron rings are used to seal against the
Plasma arc torch mounted in the center of the roof. (see fig. 38).
In order to insure chemical and temperature homogeneity of the bath as well as to
aid rapid melt in the furnace is equipped with two conductive stirring coils
connected directly to the bottom electrode. The magnetic field from these coils
interacting with the magnetic field of the plasma causes the molten metal bath to
be stirred gently. The coils operate on the direct current of the are and do not
require auxiliary controls or equipment of any kind.
17. THE HEAT BALANCE
The heat balance is an extremely useful calculation in nearly all kinds of furnace
operations. It has been widely employed in the design of furnaces and processes
for the purpose of estimating fuel requirements and also in the determination of
furnace efficiencies and general thermal study of processes.
A heat balance consists of a listing and evaluation of all items of heat supplied or
available and a similar listing and evaluation of heat used, lost, or distriubted, the
sum of the first set of items being equal to the sum of the second set. In practice it
is usually not possible to evaluate all items. In most cases the heat lost by
radiation, convection and conduction to the ground or supports cannot be
determined directly, but all other items of important magnitude may be
determined, and the difference between the total heat supplied and the determined
amount of heat distributed is the called heat lost by radiation, convection, and
conduction, the two sides being thus caused to balance. In other cases it may be
possible from a prior knowledge of similar furnace or processes to estimate fairly
closely the amount of heat that would be lost by radiation, convection, and
conduction, and this estimate may be entered in the balance sheet to afford a
determination by difference of some other unknown item. This procedure is
F U R N A C E S
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followed in estimating the quantity of heat to be supplied by fuel when the fuel
requirements are sought. In such cases it must be borne in mind that the accuracy
of the determination of the fuel requirement depends in large measure on the
accuracy of the estimate of heat loss by radiation, etc. There is danger of
overconfidence in the results of such a calculation an overconfidence which may
be enhanced very closely. In reality, when quantities are be determined by the fact
that some of the items of the balance sheet may be determined by difference it is
the old question of the chain being no stronger than its weakest link. The author
knows of one published determination of this kind in which the quantity of heat
calculated by difference was less than 1 percent of the total quantity of heat
involved in the balance, wherease the probable error in some of the items
amounted to several per cent of the total.
Items of heat supply: The sources of heat supply and the points of distribution
vary in different processes. In general the sources of heat entering may include the
following:
1- Heat of combustion of the fuel (or in electric furances the electrical energy
input).
2- Sensible heat in the fuel (usually small enough to negligible except in
preheated gas).
3- Sensible heat in the air supplied (often negligible).
4- Sensible heat in the furnace charge (usually negligible).
5- Heat developed by exothermic reactions in the furnace.
Items of heat distribution: In general the items of heat leaving may include the
following:
1- Heat consumed in endothermic reactions in the furnace.
2- Heat used in evaporation of moisture.
3- Sensible heat in the hot flue gases.
4- Sensible heat in the principal furnace product, e.g, metal or matte.
5- Sensible heat in the slag.
6- Heat absorbed by cooling water.
7- Heat lost by radiation to surroundings.
Heat lost by convection to the ground or furnace supports.