The Chemistry Student COOLING AND REFRIGERATION OTTORBINMUTH, ASSOCIATEEDITOR Refrigeration is, perhaps, more a matter of physics than of chemistry and, in its practical aspects, is certainly more the province of the engineer than of either the physicist or the chemist. Nevertheless, the principles involved are among those often employed by the physical chemist and even, to some extent, by the general chemist. At any rate, teachers of chemistry are frequently besieged by such questions as: "How can a refrigeration machine operate without any moving parts?" "How is it possible to cool a household refrigerator by heating i t with a gas flame?" "What cooling medium is used in the Blank household refrigerator?" etc. We have, therefore, been led to believe that a brief review of the principles involved and of the means by which they may be adapted to practical problems of refrigeration might prove welcome to inquiring students and bedeviled instructors alike. Methods of Cooling The simplest method of cooling consists in the interchange of heat between the object to he cooled and a practically unlimited supply of C cooling medium which is available a t a temperature somewhat lower than the final temperature desired. This is the method employed in cooling the ordinary laboratory condenser, where tap water acts as the cooling medium. It is also the method utilized by airplane and automobile engines, both air- and "water-cooled." Both types of engines are actually air-cooled; in the latter type water acts as an intermediate, absorbing heat from the motor and being in turn cooled by the air which isfanned through the radiator. Numerous other methods of cooling are theoretically possible. C. W. Kanolt' lists the following (although not in the order here given): Peltier cooling is produced when an 1. The Peltier Cooling.-"The electric current is passed in the proper direction through a junction between two different metals. Under ordinary conditions difficulties in using this as a source of cold would be presented by the generation of heat as the result of resistance, and by the conduction of heat by the metals. The method has never been employed in practice,"' hut has been suggested for use a t extremely low temperature^.^ "The Production of Cold," J . O$diticd Soc. Am., 9, 411-53 (Oct., 1924). Martin, Chem. News, 84, 73 (1901); Nature, 64, 376 (1910); 107, 43 (1921); Swinton, Ibid.,106, 828 (1921).
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example of cooling by chemical 2. Cooling by Chemical Action.-"An action without the use of latent heats is afforded by a mixture of ammonium chloride and potassium nitrate in the form of powders, in the presence of a trace of m o i ~ t u r e . " ~ 3. The Cooling of a Fluid by the Doing of Work in Exfiansion against an Electrical Force.-"Meissner3 has proposed to charge electrically the gas issuing from an expansion nozzle and cause it to carry the charge in a direction opposed to a restraining force, thereby diminishing the velocity and preventing part of the kinetic energy from being converted into heat."' a 4. The Conversion of IIeat Energy into Kinetic Energy of Flow.-If gas could be caused to leave a chamber a t a very high velocity, a corresponding amount of heat energy would have been removed from the gas and converted into mechanical energy and retained in this condition until the cold so produced could be utilized, in part a t least. Nothing practical has been done along this line as yet. 5 . The Cooling of a Fluid by the Doing of Mechanical Work in Expansion against a Moving Mechanism.Students of thermodynamics will recognize this process as a reversed Joule cycle. (The Joule cycle hears somewhat the same relation to the Carnot cycle as van der Waals' equation hears to the pelfect gas law.) Students who have never had the misfortune of encountering the'Joule or Carnot cycles may content themselves by recalling that steam, introduced under pressure into the cylinder of a steam engine, loses heat not only by radiation through the cylinder walls but by converting some of its heat energy into work by expanding against the moving piston. A suitable refrigerant gagwould, of course, act in the same manner.4 This method is feasible in large refrigeration installations and has, in fact, been employed. 6. The Joule-Thompson Effect.-When a compressed gas is allowed to expand into a vacuum or into a lower pressure without doing any work, it produces a cooling effect merely by expansion. This principle is employed in the Hampson gas liquefier and also operates to some extent in the ordinary refrigeration machine-particularly the large installation of the compression type. 7. The Absorption of Ifeat as Latent Ifeat.-This includes heats of fusion, of solution, and of vaporization. We employ the former two when we freeze ice cream with the aid of rock salt and cracked ice, or when we use any of the ordinary laboratory freezing mixture^.^ Deutsch. phys. Gesell., 21, 369 (1919). Such a gas would not lose any heat by radiation, since it would already be cooler than its swroundings. On the contrary, it would gain some heat through thecylinder walls. 6 In the article already cited' Kanalt discusses a number of freezing mixtures and gives a table (p. 415) showing the temperatures which may be attained and the available cooling in gram calories per gram of mixture at various temperatures. a 4
The melting of ice in an ordinary old-fashioned ice-box is an example of cooling by the absorption of heat of fusion alone. Cooling with "dry ice" or by means of a spray of volatile liquid is accomplished through absorption of heat of vaporization. The latter is also the principle to which all of the ordinary refrigeration installations owe a great part of their cooling effect.
Types of Refrigeration Installations Although the absorption of .heat of vaporization is a process common to all industrial and household refrigeration devices, there is great variety in the means employed to put this process to practical use. Obviously, the problem to be solved is how to maintain a constant supply of fluid available for vaporization. Since all suitable fluids are expensive, it is impossible to allow them to escape into the atmosphere and to continually replace them with fresh supplieseven if we disregard the irritant, toxic, or inflammable characteristics which many of them possess. The problem therefore resolves itself into the necessity of devising practical means for reducing the vapor of the cooling medium to liquid form. The means which have been developed so far may he roughly classified under two heads-mechanical compression and absorption. Refrigerating Fluids Refrigeration employing mechanical covpression is so well known and so thoroughly understood by every one that there could be little point in undertaking a detailed discussion here. It may not be amiss, however, to include a note on refrigeration fluids. "The choice of a fluid for refrigeration by the compression system will depend upon the temperature desired, the size of the installation, the thermodynamic properties of the fluids, the operating pressures they require, and upon their corrosiveness, their harmfulness when inhaled, their inflammability, and their cost. "Industrially, ammonia is most used in large installations, and gives, in general, greater actual coefficient of performance than other refrigerants; sulfur dioxide is adapted to very small units, as in household refrigerators, because of the low pressures required, though ammonia is also used in small units, while carbon dioxide is especially suitable for use on shipboard or in other confined spaces where the accidental release of large quantities of ammonia or especially sulfur dioxide would be very- dangerous to life."' Table I6 lists some of the properties of the more important refrigerants. 'This is a portion ol a tahle included in the paper by Kanolt already cited.'
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Critical Centigrade
Boiling point
Ammonia
132 4'
-33.4"
Sulhtr dioxide
157.2'
-10.0'
temperature,
Material
,
Methyl chloride
143.1'
-78.5' (sublimation) -24.1'
Ethyl chloride
183 0 '
14.0'
Carbon dioxide
31.1"
Heat of vaporization at boiling Prrrzing point. calorler point per gram
-77.0"
327
-73.0'
91(0")
-57.0"
86(-57') (liquid) - 103 . G o 97(0")
-142.0"
83
1771
Ofher pm~~rtie.
Harmful except in small quantities; corrodes copper and brass if not dry; not readily combustible. Harmful; corrodes copper and brass if not dry. Nearly harmless, .not corrosive. Anesthetic, inflammable, not corrosive. Anesthetic, inflammable, not corrosive.
A recent note by Science Service touching upon the hazards associated with various refrigerants may be of interest here. Deaths as a result of gas leaks from automatic refrigerators in Chicago recently are unlikely to he repeated very extensively elsewhere, in the opinion of government experts. Of sixty-three makes of electric and automatic refrigerators compiled in a recent list, only twenty-three make use of methyl chlogde, the gas responsible for the Chicago fatalities, as a refrigerant. Of these, only one is a nationally advertised make. Furthermore, the only danger comes from central refrigerating plants, used in apartment houses, where the refrigerant is piped t o a numher of refrigerators throughout the building. Even when methyl chloride is used in a small independent home unit, a leak would not liberate enough of the gas t o cause danger. Sulfur dioxide, the choking . gas . t h a t results from the burning of sulfur, is the rnwt populnr cooling comgound, and is employed in the two must widely used elrctric it is not actually poisonous. refrigerators. \\'bile this is irritating to the nasal -passages, in the strictest sense of the term. I t s pungent smell is a safeguard because its presence is recognized before i t reaches dangerous concentrations. Ammonia, used in many refrigerators, is safe for the same reason. Methyl chloride is odorless, and, while it is not poisonous in itself, it is anesthetic and a large concentration would exclude the necessary oxygen, and death would result if a person were kept in a dosed room with it. Many manufacturers use it in combination with either ammonia or sulfur dioxide, which have characteristic odors that reveal leaks. For a similar reason, manufacturers of illuminating gas, which consists largely of odorless and poisonous carbon monoxide, mix other gases with it t h a t give th;characteristic odor. Carbon dioxide, which makes up a large percentage of our every breath, is used in one make of refrigerator. This is also the refrigerant used in cooling systems of battleships. For such purposes the other gases would be highly dangerous, because damage t o the refrigerating ystem in a storm, or a hit by a shdl in an engngprnpnt, would produce an effect similar to a well-placed gas homh from an airplane. ~
~
Absorption Systems The earliest refrigerating machines were of the type where operation depends upon the absorption of the refrigerant gas.? For various reasons systems of the compression type overcame the early lead of the absorption cycle and are now by far the most numerous in practical use. The absorption system, however, has been the subject of a great deal of intensive research and of some highly ingenious inventive thought, and it is again coming into its own. I t now bids fair to find application in certain special situations to which the better-known compression system has never been satisfactorily adapted, and even to supplant the latter in many instances. Each system has advantages peculiar to it, and future usage will no doubt be guided by the advantages which weigh most heavily in any given situation. So far we have spoken of the absorption cycle as though it were as distinct a process as the compression cycle. This, perhaps, is misleading. As a matter of fact there are a t least three different processes which are generally loosely classified as employing absorption cycles. One involves the solution of the refrigerant gas, generally in a liquid of relatively low vapor density. A second process depends upon the adsorption of the refrigerating vapor by means of a suitable adsorbent, such as charcoal or silica gel. Still another system employs as absorbents salts which form ammine- or addition-compounds with ammonia. All of these processes are actually in practical use todiy, though only the first has invaded the household field to any extent. Let @s consider some examples in a little more detail. The Water-Ammonia Absorption Cycle In principle the water-ammonia absorption system is exceedingly simple, "requiring but two pressure vessels with means for applying cooling water, a certain amount of piping, and no moving part . . . . ."' One vessel acts as an ammonia reservoir and evaporator and would ordinarily be placed within a refrigerator or other space which it is desired to cool. The other vessel contains water (or an ammonia-water solution) and serves alternately as an absorber and a generator. To the best knowledge and belief of the present writer, the simplest and most compact apparatus of this type intended for actual practical operation is the " I ~ y b a l l . " ~I t consists, as shown in Figure 1, of a "hot ball" and a "cold ball," connected by piping. The "cold ball" is the evaporator; the "hot ball" acts as generator and absorber. The device is fitted 7 Frederick G. Keyes, "Renaissance of the Absorption Refrigeration Cycle," I d . En&VICE DIAGRAMMED INFIGURE2 JO-Generator:
76-Absorber; 70-Evaporator; 85 and 85-Condenrerr; Rectifier. For flirther explanllion see text.
61-Heating
unit; 55-
a t 127. The purpose of this vessel is to force into the evaporator any vapor of ammonia which mizht . pass into the propane condenser and which would not condense in the propane condenser. The gas assembles in vessel 125 and due to the restricted opening 127 alternating gas and liquid slugs are formed which serve to carry the gas entering between portions of liquid through conduit 92 into the evaporator. The evaporator 70 comprises a n outer shell 74 and a series of plates or discs over which the mixed liquid runs downward, evaporating as it goes. I n the absorber 75 the mixed vapors enter above the liquid level and diffuse upward against a current of down-trickling water which enters through conduit 86 into a pre-cooling chamber ~
~
~
"The reader will note that 93 occurs twice in these diagrams. from the context, however, which character is meant in each case.
I t is apparent
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89 and flows downward over a series of discs 106. The outer shell of the absorber is designated as 88 and surrounding this is a cooling water jacket 93," corresponding to N, with inlet a t 111 and outlet a t 98. Insoluble propane passes through conduit 72, corresponding to H, to the condenser. The lower part of the absorber containing concentrated absorption liquid is connected with a conduit 112 (corresponding to D) through a passageway 113 in a filling member 114. Surrounding the major portion of conduit 112 is a conduit 117 which is welded to conduit 112 a t the points 118. A conduit 119 connects the lower part of generator 50 with one end of conduit 117 and the other end of conduit 117 is connected to conduit 86 which enters into pre-cooling chamber 89. (This arrangement replaces conduit B ,in Figure 2.)
I t is obvious that, although this particular design employs electricity for heating, the device could be modified to permit the use of gas. As a matter of fact, gas-operated machines are available, even in household sizes. The Salt Absorption Cycle In discussing the water-ammonia absorption system we should not overlook the fact that other refrigerants and other solvents are possible. Methylamine might be substituted for ammonia and used with water. On the other hand, we might dispense with water entirely and employ as a solvent a solid salt in which ammonia is extremely soluble. (Ammonium nitrate is a good example.) It would seem that a system of this type might have to operate intermittently rather than continuously, hut it would have the following advantages: (1) an absolutely non-volatile solvent, dispensing with the necessity for a rectifier; (2) the possibility of employing absolutely anhydrous ammonia whicff would permit the use of copper tubing in constructing the unit; (3) the use of a solvent which cools rather than warms during the process of solution-a distinct aid to efficiency. So far as the writer knows, there are as yet no units of this type on the market. Dr. F. G. Keyes of the Massachusetts Institute of Technology has, however, taken out patents14covering the idea. Ammine-Compound Absorption Cycle The ammine-compound absorption cycle operates in somewhat the same fashion as the one just described. Two salts which are suitable for use as absorbing agents are calcium chloride and strontium chloride, the former being preferable from the point of view of cost. This system also has the advantage of being able to employ anhydrous ammonia. It presents several problems, however. Among the more important factors involved in using a n ammine-forming salt may be mentioned the large volume change occurring when the ammine passes to the salt during the decomposition or heating part of the cycle, the maintenance of a desirable porosity t o permit easy diffusion of the refrigerant throughout the material,
" U. S. Patents 1,258,017 and 1,267,772.
the avoidance of segregation in the material, and the elimination of the tendency of fine partides of the salt to be carried out of the generator by the rapidly moving gas stream. The material is characterized by poor heat conductivity and the design of the generator must be such as t o permit a uniform heating of the ammine through the generating part of the cycle, as well as uniform cooling during absorption. If a good design is not realized, local overheating will (at 750'F.) gradually decompose the ammonia, resulting in a serious lowering of the efficiency of the refrigerating cycle . The automatic control of the cycle of operations is the remaining portion of the task in realizing noiseless, and cheaply refrizeratine .a lona-lived. . . . omrated . . .unit. The cycle of operations to be automatically performed consists of the turning on of the pas (or electricity) a t the moment that the refriaerant has been com~letely . absorbed by the generator or, if the temperature of the refrigerator is sufficiently low, the turning on of the gas must be responsive to a temperature-controlled signal. I n any event, a t the instant the gas is lighted the cooling means must be transferred from the generator to the condenser. At the conclusion of the heating peiiod, which must be timed t o just empty the generator of its refrigerant content, the gas must extinguish and the c w l i means transfer from the condenser to the generator, which now hecomes an absorber.'
..
All these problems and others have been successfully and ingeniously solved and there are simple, reliable machines of this type on the market. Figure 4 is a diagram of one of the "Ice-0-Lator" units, which operates on this principle. The Adsorption Cycle
In the adsorption cycle the refrigerant is adsorbed in some suitable adsorbent material, such as charcbal or silica gel. The latter has been found to be especially well adapted to reffigeration needs. The adsorption machine operates by virtue of the fact that an adsorbed gas is held very tenaciously and at very low pressures. The application of heat increases the pressure and the gas may he driven out and condensed to be re-evaporated and re-adsorbed in the adsorbent. An entirely successful system of this kind has been devised for the artificial refrigeration of freight cars and is described in detail by Hulse.ls Silica gel is the adsorbent, and water vapor the refrigerating, medium, with liquid propane as a fuel supply. Obviously, this system must operate a t a pressure considerably lower than atmospheric, in contrast to the systems already described which operate under pressures considerably greater than atmospheric. The action, is, of course, intermittent rather than continuous. Figure 5 is a diagram illustrating the very interesting apparatus described by Hulse. Household units are in process of development. Conclusion
It has been claimed for the compression-type machine that it is more efficient than any of the absorption types.' However, the absorption systems have been tremendously improved of late, and the reader must
" Hulse, Refrigerating Eng., 17, 41 (1929).
also remember that the highest degree of thermodynamic efficiency is not always synonymous with lowest operating cost. Recent data7seem to indicate that a well-designed, gas-heated absorption system may be operated a t alower cost than an electrical compression system. Costs will naturally depend somewhat upon the respective costs of gas and current in a n y given community. Water costs and temperatures also enter into the picture with water-cooled machines. Aside from the matter of operating costs, the absorption systems are free from moving parts and, hence, presumably from wear and service costs, as well as from noise and vibration. In the latter respect, however, they have hut little advantage over the latest models of the best electrical compression machines. ' Acknowledgment In addition to the quotations and statements specifically accredited in the foregoing review, the writer acknowledges indebtedness for additional information to the sources already cited and to a lecture on "Adsorption and Its Relation to Refrigeration" delivered before the American Gas Association by Dr. F. G. Keyes.
