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REFRIGERATION IN THE DAIRY INDUSTRY

by P. B. H. BROWN, (Paper read at Meeting of N . Ireland Section, 19th March, 1952)

In addressing a society such as this it is not necessary to stress the importance of refrigeration to the dairy industry. The vital necessity for refrigeration in the processing and handling of milk, and other dairy products may be taken as an accepted fact. It is my intention in this address to review first of all the principles upon which the mechanical refrigeration plant works and then to discuss the equipment available to the dairyman for the various refrigeration services which he requires and the advantages and dis- advantages of the various types.

I feel that I ought perhaps to make some apology for going back to the basic principles upon which a mechanical refrigeration plant works and for describing the simple cycle. Although I suppose most of you have learned about this a t one time or another I expect that to many of you it is not very familiar, and so much of what I shall say depends upon an understanding of basic principles that I feel that I should start a t the beginning.

All the mechanical refrigerating plants used in dairies work on the “vapour compression cycle.” Some chemical such as anhydrous ammonia which can be made to evaporate and condense at suitable temperatures is used as the working fluid, or refrigerant.

The evaporator contains the refrigerant in liquid form. The compressor draws vapour from the evaporator thus lowering the pressure and causing the liquid refrigerant to evaporate or boil at a low temperature. In boiling the refrigerant must obtain its latent heat of evaporation and takes up this heat from the air, water or brine surrounding the evaporator tubes, thus producing the cooling effect. The refrigerant vapour is dis- charged into the condenser at a higher pressure where it is cooled and condensed back into a liquid, giving up its latent heat of condensation to the air or water with which the condenser is cooled. Finally the liquid refrigerant is allowed to flow back into the evaporator under the control of some form of regulating valve.

In theory more or less any chemical element or compound which can be made to evaporate and condense could be used as the refrigerant. In practice, however, it is necessary to find a sub- stance which evaporates and condenses a t appro- priate temperatures and at reasonable pressures, and also does not attack the metals that one wants

to use in the compressor and other parts of the plant.

REFRIGERANTS Anhydrous ammonia is usually considered to

be one of the most suitable for medium and large size plants. Its working pressures are reasonable. It evaporates a t -28°F. at atmospheric pressure and at 6°F. at 20 lbs. per sq. in., and condenses a t 100 to 180 lbs. per sq. in. over the temperature range of 64” to 95°F. In the presence of moisture it attacks copper and its alloys which consequently are not used, but this, whilst a nuisance, is not a serious disadvantage. It has a strong pungent smell, which, whilst it may be an objection, is often an advantage by drawing attention to leaks. I t is poisonous, but owing to its smell there is no risk of anyone breathing or swallowing inadvertently enough to do himself harm.

Dichlorodifluoromethane, (CC12F2) commonly called Freon 12 or Arcton 6 is a compound specially developed for use as a refrigerant. Its working pressures are somewhat lower than for ammonia and the volume to be handled by the compressor somewhat greater for a given capacity. I t has practically no effect upon metals, it has no smell and is non-poisonous. It is however, expensive compared with ammonia and owing to its lack of smell a leak is liable to go undetected. In fact for detecting small leaks a special detector lamp must be used, or a complicated piece of electronic apparatus.

It is proposed in this paper to use, when referring to Dichlorodifluoromethane, the British trade name of Arcton as opposed to the perhaps more generally used American trade name of Freon.

Methyl Chloride, another commonly used refrigerant, has much the same properties as Arcton 6 but has the disadvantage that it attacks aluminium and also other metals if any moisture is present. The chief objection to its use however, is that it is poisonous, and, whilst it has a slight smell, it is quite possible to breathe without discomfort sufficient to cause illness or even death. I t also forms an explosive mixture with air. To relieve the anxieties of those who may have methyl chloride plant it should be noted that a normal escape of gas into a reasonably ventilated room is very unlikely to cause harm, but care must be exercised if methyl chloride plant is

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installed in an enclosed space, or if there is a leak inside a cold room. Most, but not all, of the accidents which have occurred have been on board ship where the plant may have been installed in a compartment to which the only access is through a small hatch.

Carbon dioxide, C 0 2 , a t one time commonly used, is seldom if ever used nowadays. The one real objection to its use is the very high pressure which is needed, ranging up to 1300 lbs. per sq. in., in the condenser. This makes necessary specially heavy construction of the compressor and other parts. A C 0 2 plant also needs more power for a given capacity.

A number of other substances have been used as refrigerants such as Ether, Sulphur Dioxide, Carbon Bisulphide, but those mentioned above are practically the only substances now commonly used, although there are some othex specially manufactured chemicals such as Arcton 4 or Freon 22 (CHClF,) and Arcton 9 or Freon 11 (CC1,F) which are used for special purposes, such as for producing unusually low temperatures, or for centrifugal compressors.

At the present time *or the dairy the choice of refrigerant really lies between ammonia and Arcton 6. Methyl Chloride is a possible alter- native to Arcton if the latter should be unobtain- able. Within limits a plant designed for Arcton can be charged if necessary with methyl chloride without alteration. Ammonia still holds the field almost without challenge for the larger industrial type plants, where the possibility of leakage of ammonia is not regarded as too great a hazard. AIcton is used for most of the smaller plants of about 10 h.p. rating or less. For such sizes the lower working pressures and larger volumes to be handled are an advantage rather than otherwise and the possibility of using copper pipes and copper bellows in control devices is a convenience. Larger refrigerating plants using Arcton are in general installed only where risk of a leak of ammonia cannot be tolerated, such as for the air conditioning of public buildings or for use on passenger ships. In sizes of much over 10 h.p. rating ammonia plant is usually cheaper in first cost and more robust and reliable, but probably the most important consideration is the risk of leakage. The value of the refrigerant charge in a large Arcton plant may be consider- able, perhaps Ll,OOO or more, and it can all, or most of it, leak away without anyone realising it, till it is gone.

COOLER DESKS In order to cause heat to flow from the fluid to

be cooled to the evaporating refrigerant it is obviously necessary that the refrigerant should be evaporated at some temperature below that to which the fluid is to be cooled. Now, the lower

the temperature a t which the refrigerant is evaporated the lower must be the pressure in the evaporator and therefore the greater the volume which must be handled to produce a given amount of cooling. The higher, therefore, the tempera- ture a t which the refrigerant can be evaporated, the greater is the capacity which can be obtained from a given size of plant. From this point of view therefore i t is important to design the system so that the fluid to be cooled does not have to be cooled to an unnecessarily low temperature and also to design the evaporator so that there is as small a temperature difference as is economi- cally possible between the fluid and the evapora- ting refrigerant. Another important point how- ever is that the evaporator must be designed and controlled so that particles of liquid refrigerant are not carried over to the compressor. The presence of even a small amount of liquid refriger- ant in the form of fine spray or mist in the vapour entering the compressor reduces very greatly the capacity and efficiency of the compressor.

Evaporators used in dairies are employed for cooling brine or water which in turn is used for cooling milk or air, or for the direct cooling of air in cold rooms. In some cases also the evaporator cools the milk directly using a so called direct expansion milk cooler. It is not proposed to discuss in detail the relative merits of the use of brine cooled, water cooled or direct expansion milk coolers but a few comments from the point of view of the refrigerating engineer are offered.

The direct expansion system, by eliminating the necessity for an intermediate carrier of refrig- eration, makes possible the use of a higher evap- orating temperature of the refrigerant and there- fore greater capacity from a given size of machine and lower consumption of power. In actual practice, however, the difference is not great. The design of the direct expansion milk cooler, however, is restricted by the fact that it must also serve as an efficient evaporator and must be strong enough to withstand a high refrigerant pressure when being sterilised.

The use of chilled water as opposed to brine for cooling milk has the advantage of eliminating the messiness of brine and the cost and trouble of maintenance of the brine. Corrosion troubles with brine should be no more serious than with water, but are liable to occur if the brine is not properly maintained and treated. I t is not practicable to cool water to much less than 35°F. and this temperature is barely low enough to cool milk to 40". With water, however, it is possible to build up a reserve of cooling capacity by freez- ing a layer of ice on the cooling coils. In fact in some cases a special arrangement of evaporator coils is used, and the plant run overnight t o build up a considerable quantity of ice to carry over peak load periods during the day.

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The advantage of using brine is that it can be cooled down to a lower temperature and can therefore be used for purposes such as the cooling of cold rooms. This may make it unnecessary to have separate plant for the cooling of cold rooms. From the point of view of reliability and generally satisfactory working, simplicity is of great importance, and it is very desirable to avoid connecting a number of separate evaporators on different duties to one compressor or range of compressors. By having one tank of brine which can be cooled to the required temperature and circulating this brine to the various points where cooling is required this simplicity of the refrig- erant circuit is achieved.

The type of evaporator which was used on most refrigerating plants until some twenty years ago consisted of a coil or coils of comparatively small bore piping. The liquid refrigerant was admitted to one end of the coil under the control of a hand adjusted regulating valve and the vapour was drawn off the other end direct to the compressor. With this arrangement it is almost impossible to adjust the regulating valve so that particles of liquid are not swept over with the vapour and also a very large proportion of the inside of the coil is filled with vapour as opposed to liquid. As it is the evaporation of liquid which produces the cooling effect it is necessary for efficiency to have as much of the cooling surface as possible in contact with liquid refrigerant. This type of evaporator therefore tended to be inefficient and it was necessary to provide a large amount of cooling surface for a given cooling effect.

On most medium or large size refrigerating plants whether using ammonia or other refrigerant it is usual in order to overcome these defects to arrange the evaporator to work on what is called the “ flooded system.” Such evaporators con- sist of a number of comparatively short coils or grids of tubing which are connected to a vessel called an accumulator or surge drum in such a way that the liquid refrigerant recirculates naturally from the accumulator through the pipe coils. The liquid from the condenser is fed into the accumulator vessel from which the cold liquid runs down into the coils. These coils become more or less flooded with liquid. The vapour which forms in the coils rises, sweeping some of the liquid along with it over into the accumulator. If this is large enough the liquid separates com- pletely from the vapour. The vapour is drawn off to the compressor and the liquid runs back to the coils. If a system of this sort is suitably designed a rapid circulation of the liqkid reffig- erant is set up and if the brine or water on the outside of the coils is also circulated rapidly a very high rate of heat transfer can be obtained. A cooling surface of the order of one-fifth of that

required with the old fashioned type of pipe coil will give the same result.

The other form of evaporator which is much used for brine or water cooling is the shell and tube type. Here the brine or water passes through the tubes and the liquid refrigerant surrounds the tubes. Owing to the high velocity of the brine in the tubes and the fact that they are fully surrounded by liquid refrigerant a good rate of heat transfer is achieved. To take advantage of the whole of the cooling surface available however it is necessary to keep the shell more or less full of liquid and under these condi- tions particles of liquid are liable to be carried over with the vapour to the compressor. To prevent this an accumulator or liquid separator is often fitted to trap the liquid particles, which are allowed to drain back by gravity to the cooler shell. The shell and tube cooler may be of the “ multipass” type, where the biine is pumped backwards and forwards through the tubes, or it may be the “ single-pass ” type which is usually submerged in a tank and the brine is pushed through all the tubes together in one pass by a propeller.

The shell and tube evaporator whilst probably the cheapest and most efficient type is not generally used for water cooling a t temperatures close to the freezing point because of the risk of freezing the water in the tubes, which may result in burst tubes and loss of refrigerant.

With shell and tube or flooded coil type evapo- rators any oil which is carried over from the com- pressor into the condenser is trapped in the evapor- ator. Of course, an oil separator is normally fitted to catch as much oil as possible in the compressor dis- charge pipe, but no oil separator is completely effective. In an ammonia plant oil collects at the bottom of the evaporator and can be drained off at intervals. In the case of an Arcton or methyl chloride plant, however, the oil mixes with the liquid refrigerant in the evaporator and special devices are necessary to recover the oil and get it back to the compressor. This is one of the reasons why flooded type evaporators are less popular on Arcton plants, although on large Arcton plants they are frequently used.

CONDENSERS

All the heat which has been taken up in the evaporator plus the heat equivalent of the power used by the compressor has to be got rid of in the condenser. Small plants of up to two or even three h.p. rating may have direct air cooled con- densers. In the case of larger plants, however, the size of an air cooled condenser and the volume of air which must be circulated over it to carry away the heat becomes excessive. I t is usual therefore to use water as the cooling medium for

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condensers on plant of any size. The simplest and cheapest form of condenser is the multipass shell and tube type. It consists of a large dia- meter shell tube with tube plates welded into each end. A number of small bore tubes run longi- tudinally through the outer tube and are expanded or welded into the tube plates. Cast iron covers fitted with baffles are bolted to each end and arranged so that the water passes backwards and forwards through the tubes while the Iefrigerant vapour condenses on the outside of these.

While the end covers of this type of condenser can be removed without difficulty and the tubes cleaned through with a brush it is found in places where the water is dirty that the condenser be- comes fouled very quickly and that to keep the condenser reasonably efficient it must be cleaned at inconveniently frequent intervals. In such cases the vertical shell and tube condenser may be used. This condenser is set up vertically, usually in the open air. Water is delivered to a tray at the top and runs down through the tubes by gravity, a nipple being fitted into the top of each tube to ensure reasonably equal distribution. The water falls from the bottom into a collecting tank or trough whence it runs to waste, or in some cases to a recirculation pump. This type of con- denser is easily cleaned whilst in use by passing a brush up and down through the tubes from above.

When the water is foul, also, the ordinary atmospheiic condenser is often used. This con- sists of coils built up into a stack or stacks of norizontal pipes mounted one above the other in the form of a flat grid. The refrigerant vapour is inside the pipes and the water is caused to trickle down over the outside. This condenser occupies much more space and is more expensive, for a given capacity, than the shell and tube type, but is sometimes preferred since the tubes are exposed foi inspection and for maintenance.

In most places where refrigerating plant is installed, however, i t is necessary to make some piovision for recirculating and recooling the water owing to the absence of an adequate supply of water at low cost. When water is taken from the town mains, even if economic considerations do not make it necessary, the water supply authority probably insists that provision be made to conserve water. Fortunately the latent heat of evaporation of water is large and a large cooling effect can be obtained by the evaporation of only a small quantity. Water is therefore cooled by spreading it out over a large surface, or breaking it up into a fine spray, and bringing it into contact with atmospheric air. A small amount of water is evaporated to cool the remainder. The lowest temperature to which water can be cooled in this way is the " wet bulb" tempeiature of the atmosphere and is of course higher in warm

humid weather than in cold dry weather. In actual practice water should be recooled to within about 10" to 20°F. of the wet bulb temperature.

When it is necessary to recirculate the cooling water the shell and tube condenser may be used in conjunction with a separate cooling tower or spray pond. The type of cooling tower generally used is the open natural draught type which depends largely upon the wind for its air supply. The chimney type of tower, such as is used at power stations, is not favoured for refrigeration plants as the water is not heated to a high enough temperature to get a large enough chimney effect. Ordinal y atmospheric condensers installed in the open air are also freqaently used. The water trickling over the outside of the coils is cooled by evaporation at the same time that heat is taken up from the condensing refrigerant. When a large battery of atmospheric condensers is used in this way, with the same water being recircu- lated, it is essential that a generous cooling surface be provided and that the condenser stacks be well spaced out so as to catch as much air as possible. They must be installed in an exposed position and not tucked away in a corner protected from the wind.

Atmospheric condensers used in this way, or a natural draught cooling tower occupy a large amount of space. When such space is not con- veniently available a forced draught cooling tower may be used or a forced draught or induced draught evaporative condenser. In this type of condenser the condenser coils are nested closely together and enclosed in a casing of galvanised sheet steel or timber. Water from a tank below the coils is showered over them by a pump while air is drawn or blown over them by a fan. While it is more economic to install this form of con- denser in the open air it can when necessary be made very compact and installed inside a building with air intake and discharge ducts for the air

COLD ROOMS Most dairies have insulated cold rooms in which

the milk after cooling and bottling can be held at a low temperature until delivered. It is not perhaps generally realised how heavy is the cooling load on such cold rooms. The weight of an ordinary glass bottle of one pint capacity is about 1.15 lbs. or very nearly equal to the weight of milk which it holds. The specific heat of glass is 0.2 so that if milk at 40" is filled into bottles at say 80" the milk will be heated to about 45". There is also the heat in the crates. The internal surface of the room also, particularly walls and floor has probably been warmed up to 50" or an even higher temperature before the room is loaded and closed up. For the best results the cooling equipment must be capable of bringing

supply.

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the temperature down to 40” within a compara- tively few hours. For this ample capacity and a vigorous air circulation is necessary. For cooling

however, probably because owing to the compara- tively small volumes of liquid to be handled and the higher pressure drop, it is less reliable.

such rooms air cooling units a;e now almost invariably used. As the temperature is well above freezing point finned or gilled cooling coils may be used without trouble due to frosting up. Small cooling units are generally equipped with propeller type fans. Large cooling units, how- ever, usually have centrifugal fans, frequently with two or three fan runners mounted on a common shaft. These discharge an air stream a t high velocity which will carry right across a large room. The cooling coils may be arranged either for brine circulation or for direct evapora- tion of the refrigerant. Brine is usually preferred if the main milk cooling plant uses brine, and except in the case of small plants is preferable from the point of view of simplicity of control.

KEGULATING VALVES From the point of view of efficiency and satis-

factory service, one of the most important parts of a refrigeration plant is the regulating valve, or valves, which controls the feed of liquid refrigerant from the condenser or liquid receiver to the evaporator. Such valves are often referred to by the misleading name of “ expansion valves.” Until some twenty years ago these were usually hand adjusted throttle valves. As pointed out earlier in the paper, however, it is practically impossible to set such a valve by hand so as to give the conditions necessary for full efficiency.

Nowadays practically all refrigerating plants are fitted with some form of automatic regulating valve. As this is a most important part of the plant it is proposed to describe the various forms which this takes.

Small plants having simple coil type evapora- tors, as opposed to the flooded type, are often fitted with “ thermostatic expansion valves.” This type of regulator consists of a simple needle valve which is actuated by the temperature of the suction line carrying the vapour away from the coil. A phial containing a volatile liquid is clamped to the suction line and connected by a capillary tube to a metallic bellows or diaphragm at the valve. As the temperature of the vapour in the suction line rises the bellows expands and opens the valve. A s the temperature falls the bellows contracts, and the valve closes. The valve is usually adjustable by a screw and spring which acts on the bellows and is set to maintain a superheat of some 6” to 10” in the vapour leaving the coil. This type of valve is used for nearly all small Arcton or methyl chloride plants. I t is frequently used also on ammonia plants for the control of air cooling coils, and other small evapo- rator circuits. I t is not so popular for ammonia,

For the ordinary ammonia plant having a flooded type evaporator or shell and tube brine cooler and only one evaporator circuit the “ high pressure float valve ” is generally used. This consists of a chamber into which the liquid drains from the condenser or receiver. In the chamber is a float, which, as the liquid collects, rises and opens a valve which allows the liquid to drain out into the evaporator. No vapour, however, is allowed to pass. The valve thus feeds back into the evaporator as liquid all the refrigerant which is withdrawn from it as vapour and so maintains a constant liquid level in the evaporator. For this system of control to be satisfactory the evaporator must bave capacity for holding some extra refrigerant when the plant is stopped and its refrigerant charge should not be very critical. Most shell and tube or flooded type evaporators satisfy these conditions, A plant fitted with a high pressure float valve should be charged with refrigerant until the liquid level in the evaporator has reached the most efficient working level. No further adjustment is neces- sary or possible. This condition is usually judged by the operating pressures and temperatures, but sometimes a level gauge is fitted. - -

Where there are two or more evaporators, particularly at different levels, the high pressure float valve cannot be used as it has no means of distributing the liquid refrigerant between the evaporators, and if the evaporators are of the shell and tube or flooded type “low pressure float valves ” must be used. The low pressure float valve controls the liquid level in the evapo- rator just like the ball cock in the ordinary household water cistern. Except in the smallest sizes, however, the float itself is usually arranged in a separate vessel or chamber connected to the evaporator by balance pipes at top and bottom. These balance pipes being fitted with valves the float chamber can be isolated for overhaul or adjustment of the valve without pumping out the evaporator. The liquid after passing through the float controlled regulator is not admitted to the float chamber but is taken into the evaporator or accumulator by a separate pipe. As the liquid mixed with vapour enters at a high velocity it is liable to cause disturbance and the outlet should be arranged a t a point where such disturbance will not cause trouble. Although the low pressure float valve might be thought to be simpler than the high pressure float valve it is in fact a more troublesome and complicated piece of equipment, and greater skill and knowledge is necessary, both in its application and mainten- ance.

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Mr. L9gie : ‘‘ I have listened with much interest to Mr. Brown’s paper and to open the discussion would ask him t o amplify one or two points he has made. With regard to the use of calcium chloride brine would Mr. Brown give his views on how corrosion may best be avoided ? The use of chilled water as a secondary refrigerating medium continues to gain ground in the dairy industry and I would be interested to hear what method is used t o control the evaporator in such systems. Mr. Brown referred to the difficulty of cooling milk to 40°F. with chilled water a t 36°F. Could this not be met by increasing the rate of flow ? ”

Mr. Brown : From my practical experience I would say that by far the most important precaution to take to avoid corrosion is to prevent aeration of the brine. Where possible the brine circuit should be arranged on the closed system with a small balance tank to take care of expansion and contraction. In dairies, however, an open top brine storage tank must usually be fitted, The pipes leading brine back into this tank must be carried well below the brine surface. The proper brine level in the tank should be maintained. It is not unusual to find that the brine level has been allowed to drop so that an open stream of brine falls from the return pipe into the tank causing serious aeration. It is often stated that a strong brine solution is less corrosive than a weak one. This is not strictly true, but i t is a fact that a strong solution has less capacity for absorbing oxygen from the atmosphere than a weak solution. Whilst the strength of the brine should always be maintained a t a point which will avoid risk of freezing I do not advocate an unnecessarily strong solution as this reduces the capacity of the plant owing to the greater resistance to heat transfer. Chemically the brine should be kept in a slightly alkaline con- dition and any tendency t o acidity should be neutralised. Corrosion is always liable to occur on any pipes or tank plates at the brine surface. It is worth while taking special precau- tions to protect these places with a good coating of bituminous compound.

It is not usually considered necessary to take any special precautions to control the evaporator of a chilled water plant to prevent freezing. Normally a coil type evaporator is used and no harm is done if some ice is frozen on the surface of the coils. If in a particular case special precautions are considered necessary a “ back pressure valve ” can be fitted which will prevent the evaporating temperature of the refrigerant in the evaporator falling below a predetermined point, or a low pressure cut-out switch which can be set to stop the compressor driving motor if the evaporator pressure, and therefore the refrigerant temperature, falls below the level which is con- sidered safe.

If milk is to be cooled to 40” with water a t 35” an efficient heat exchanger is needed &B 6” is not a large temperature difference for heat transfer. Other things being constant the efficiency of the heat exchanger is increased by increasing the water circulation, but this means greater pumping power and as the power increases as the cube of the quantity circulated this cannot be carried very far. It is usual to circulate five or six times the volume of milk cooled as against three or four times for brine.”

Mr. Launders : ‘‘ Would the speaker explain the function of a liquid receiver in a refrigerating system operating with a high pressure float valve? Can he suggest a method of freeing brine from ammonia ? ”

Mr. Brown : “ The liquid receiver on a system fitted with a high pressure float valve serves no useful purpose while the plant is in operation. The sole reason for its inclusion is to provide space for the charge of ammonia when pumping out the evaporator, if and when this has to be done. The liquid receiver is sometimes arranged to by-pass the float valve so

tha t it is not in circuit when the plant is in normal operation, but this is not often done on small plants.

None of the authorities or experts that I have consulted has been able to suggest any satisfactory method of ridding brine of ammonia. To neutralise the animonia with hydro- chloric acid results in the formation of sal ammoniac whirh is most undesirable. However, where copper equipment is involved, as it may well be in a dairy, it may be preferable to doing nothing. If only steel tubing and tanks are involved i t is probably better to do nothing. The only real cure that I know is to run off the brine and mix a fresh solution, having first made certain that the ammonia leak has been stopped.”

MI. P. Clerkin : “With reference to small scale farm refrigeration I would ask Mr. Brown’s opinion on the surface cooler and the churn immersion type. I understand that the manufacturers are tenduig to concentrate on the four-can size of immersion cooler which is, of course, more expensive than the two-can type which would appeal to the smaller farmer.”

Mr. Brown : “ I am glad that this question has been raised &B I neglected t o say anything about the farm milk cooling plant in my paper. The advantage of the churn immersion type of plant is, of course, its simplicity and the absence of the surface cooler and associated equipment requiring cleaning and sterilisation. The disadvantage is the comparatively slow rate of cooling of the large body of milk in the churn when immersed in water at even 35°F. This can be speeded up by fitting some form of stirrer in the churn, but any device of this sort to a great extent takes away from tlic essential simplicity.

There appears to be no great demand for the churn type cooler in Scotland as opposed to plants with surface roolers of which a large number are installed.

If there is a demand for a small two-churn cooler i t could unquestionably be made. I fear, however, that the rewon that it is not made is that the cost is beyond tho means of the average producer a t tha t level of production.”

Mr. Hyde : “ I was interested in the speaker’s coninients on direct expansion. With regard to the cooling of milk does it offer advantages in respect of cost of plant, saving of spare and operating efficiency, over a system using a secondary refrigerant ? ”

Mr. Brown : “ It is difficult to give a diroct answer to this question. From the point of view of refrigcration efficiency direct expansion is desirable and in so far as the brine cooler or water cooler and pumps are eliminated cost and space occupied should be reduced. The difficulty, however, is to design and construct an acceptable and eficicnt type of milk cooler which is also an efficient evaporator. It appears that the most popular type of milk cooler at the present time is the plate type heat exchanger, which is not very adaptable to use as a refrigerant evaporator. The problem is complicated by the fact that refrigeration is only one step in tho complete heat treatment cycle and I know of only one make of heat treatment equipment which employs direct expansion for tho h a 1 refrigeration cooling stage and that one does not appear to be very popular.

It may well be, of course, that an inventor may de\ ise a form of direct expansion milk cooler which meets the requirements better than anything which has so far been thought of. but I know of no such prospect at present.

On farm plants, when cooling only is required, diroct expansion coolers are frequently used. The cooler is of the ordinary tubular surface type, the refrigerant being Arcton or methyl chloride. The direct expansion type of plant ifl usually the cheapest and simplest in cases where no cold room is required for holding tile milk after cooling.”