INDUSTRIAL AND ENGINEERING CHEMISTRY
Mav. 1929
477
Renaissance of the Absorption Refrigeration Cycle’ Frederick G. Keyes DEPARTMENT O F C H E M I S T R Y , MASSACHUSETTS INSTITUTE
H E earliest machines for producing refrigeration mechanically were of the type where operation depends upon the absorption of the refrigerant gas in a liquid. The possibility of an absorption refrigerating cycle will therefore be perceived to begin with the discovery of gases which dissolve abundantly in readily obtainable liquids of low vapor pressure a t ordinary temperatures. Thus ammonia, discovered by Joseph Priestley in 1774, and methylamine are possibilities to be considered in designing an absorption cycle wherein the dissolved gas in water may be driven out of one vessel (absorber-generator) and collected in a second vessel (evaporator), hermetically sealed to the first. Cooling of the generator causes the vessel first mentioned to function as an absorber and the liquid refrigerant in the evaporator is caused to evaporate, thereby cooling the evaporator. When the refrigerant has completely returned to the cooled absorber, heat is again applied to the absorber, whereupon it functions as a generator and the cycle of operations is recommenced,
T
Principle of Absorption Refrigeration
The general principle upon which the functioning of a typical absorption refrigerating machine rests is clearly the difference in solubility exhibited by gases in cold and hot absorbing liquids of relatively low vapor pressure. A low vapor pressure of the solvent liquid is desirable, since operation is simplified when during the heating cycle as little of the solvent as possible is carried over to the evaporator. Evidently the absorption machine is in principle exceedingly simple, requiring but, two pressure vessels with means for applying cooling water, a certain amount of piping, and no moving parts are involved. It will be made clear below that the effort to produce an entirely automatic absorption machine capable of “sensing” its cooling and fuel needs, a t such times and in such amounts as are required to maintain a specified temperature within a refrigerator, reasonably independent of the temperature of the surroundings, has resulted in a few accessories being added to the simple equipment described above. The automatic self-regulating absorption machines now on the market have already arrived a t the point, however, where the inherent simplicity of operation and structure is retained with a minimum of simple control apparatus of a highly ingenious design. The forms of energy which may be used to operate the absorption machine are as numerous as the sources of energy available. Heat is the form of energy directly applied to the generator, and this may of course be obtained electrically or from the combustion of gas, oils, etc. An absorption type of machine can therefore be adapted to many diversified requirements, although the machines on the market a t present operate with heat from electricity and gas only. The development of machines to operate with liquid fuel will eventually be completed and already one development (refrigerator car) is employing liquefied propane in tanks as a source of fuel. The cooling medium thus far used has been water. The companies interested in absorption refrigeration development have been designing air-cooled units for a considerable time and quietly testing the performance under all possible conditions of use. It is a safe prediction that within a few years a t least air-cooled machines will be available, which 1
Received February 21.1929.
O F TECHNOLOGY, C A M B R I D G E ,
MASSACHUSETTS
from the point of view of efficiency of operation and cost will be everything that can be desired. The water-cooled machine is preferred by the writer whether the refrigerating unit is of the absorption or motor-compressor type. A few of the reasons for this preference are that the unit is more compact, somewhat more efficient, more quickly responsive to sudden changes in outside temperatures, and less liable for service when once installed. The absorption machine thus far is the more common type wherein a liquid is used to absorb the refrigerant gas. There are, however, other and later types equally important and interesting. There has been an attempt to use a solid salt, such as ammonium nitrate2 or ammonium thiocyanate, either of which is exceedingly soluble in ammonia. Now it is well known that the vapor pressure of the solvent of a salt solution is lower than the vapor pressure of the pure solvent. With the salts mentioned, however, the vapor pressure of ammonia is tremendously lowered. There is presented, therefore, the prospect of obtaining not only an absolutely nonvolatile solvent for the absorption cycle but a solvent (ammonium nitrate) which absorbs heat or cools when the refrigerant dissolves, thereby leading to increased efficiencies. There are, of course, difficulties still to be overcome before the advantages of this type of absorption machine may be realized. The difficulties would probably yield, as usual, to sustained effort on the part of those especially apt in perceiving the practical applications of physico-chemical principles. The recent developments in the knowledge of the phenomena of adsorption (a special term for absorption on surfaces) and adsorptive materials have led to the introduction of still another type of absorption machine, which might very appropriately be termed the ‘Ladsorption” machine. This machine operates by virtue of the fact that a gas such as ammonia, absorbed, or better ‘Ladsorbed,’’by charcoal is retained very tenaciously and a t very low pressures. The application of heat increases the pressure and the ammonia may be driven out and condensed to be reevaporated and readsorbed in the charcoal (adsorbent). There are many adsorbents, but one whose application has been found especially well suited is silica gel. The preparation of this material was developed by Walter A. Patrick, of Johns Hopkins University, and is manufactured by the Silica Gel Corporation, which has assigned rights for the use of the material in small units to the Copeland Products Company, of Detroit. The latter company, it is understood, is developing practical units for household application. Meanwhile, the substance has been used in an interesting application-that of the artificially refrigerated freight car.* Silica gel is a hard, glasslike material composed of pure silicon dioxide. It is produced by precipitation fram sodium silicate and the process of its manufacture has been continuously improved to the point where a considerable augmentation in the adsorptive characteristics has been realized. There are other gels also which may in time be brought into a preferred physical state especially useful for certain important specific purposes. There remains to be mentioned the possibility of employing in a refrigeration cycle the chemical substances which ‘See, for example, Keyes, IJ. S. Patents 1,258,017 and 1,267,772 (1918). 8 Hulse, Refrigerating En&, 17, 41 (1929).
Vol. 21, No. 5
I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
478
form “associated compounds,” COOLING WATER known water-ammonia absorpamong which h y d r a t e s a n d tion machine into a constantYAPoRArOR pressure system by adding air ammines (BaC12.SNH3) are examples. Thus the a m m o n i a or a permanent gas. Geppert’s embodiment of the idea shows a pressure over the latter compump for circulating the air over pound has a relatively low presthe liquid ammonia in the evaposure at ordinary temperatures rator, bringing about a “forced” and is readily driven out by heat or accelerated evaporation. The to be reabsorbed when the salt inert gas also functions as a kind is cooled. It happens that the rate of absorption of ammonia of expansion valve, since the ammonia may be condensed by and other factors make barium the pressure of the inert gas and chloride not the most desirable passed directly to the receiver, of salts to choose for the purpose, or even the evaporator, dependbut there are other salts which ing on the size and design of the f o r m ammonia addition commachine. pounds, and one practical houseComparatively recently Geph ol d r e f r i g e r a t i n g machine4 pert’s idea has received modifiemploys a combination of abat the hands Of ‘laten sorbing elements based on the Figure 1-Diagram of Electrolux Serve1 Refrigerator a d d i t i o n - c o m p o u n d-forming and Munters, who made use property of certain salts.5 The special difficulties of poor of the small density of hydrogen relative to ammonia vapor heat conduction, slow absorption rate, permanency of preferred (1 t o 8.5) t o bring about a natural circulation of the inert physical state, and capacity have been entirely overcome gas over the surface of the liquid ammonia in the evaporator. without sacrificing in any way the important advantages pre- The circulating pump is thus eliminated and the water absented in the use of such addition compounds. Among the sorption machine converted into a continuously operating advantages to be noted is the possibility of employing am- machine as contrasted with the older intermittent distillation, monia, in many respects an ideal refrigerant, in the anhydrous water-absorption cycle unit. condition, thereby eliminating entirely any form of rectifier The constant-pressure hydrogen-water absorption machine as with a water absorption unit. The use of anhydrous am- as designed by the Electrolux-Serve1 Company is represented monia incidentally permits the use of copper tubing in the in diagram in Figure 1. The function of each part of the construction of the unit, and the compact and fireproof na- machine is labeled in the figure and circulation of the COOL ture of the absorbent is a further advantage. ing water, the ammoniacal liquor, liquid ammonia, ammonia vapor, and hydrogen may be readily followed. Through Improvements in Small Water Absorption Units the central tube of the ” generator a Bunsen flame is allowed to burn, causing the evaporation of the ammonia from the In a patent was issued to Geppert6 in which a strong liquor siphoning into the lower part of the generator idea was incorporated in the design of the water absorption from the absorber. The weak liquor from the generator machine* It was proposed, in essence, to convert the passes bv_ aavitv from the generator t o the upper part of 4 Ice-0-Lator unit, manufactured by the National Refrigerating &e absorber. s&ading itself over the travs. whilh arecooled Company of New Haven, Conn. by water flowing con2nuously in the coils surrounding the 1 Keyes, U. S. Patents 1,622,519 to 1,622,523, inclusive (March, 1927). evaporator. The cooled strong liquor collecting in the bot4 Geppert, U. S.Patents 622,690 (November, 1900) and 780,096 (Janutom of the absorber passes back to the generator through ary, 1905). ,
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Fiaure 2-Diagrammatic
Drawing of Ice-0-Lator
May, 1929
INDUSTRIAL A,YD ENGINEERING CHEMISTRY
479
a heat exchanger to be deprived of its ammonia in the generator. The ammonia evolved in the generator condenses at the temperature of the condenser and under the pressure exerted by the inert gas. After rectification, the liquid ammonia is led to the evaporator flowing over the trays, where the vapor of the ammonia mixes with the hydrogen forming an ammonia-rich mixture. This mixture is heavier than the corresponding mixture in the absorber, since the hydrogen in the latter has been deprived of its ammonia by absorp tion in the weak liquor flowing over the trays. A natural circulation due to difference in density accordingly sets in, the ammonia-rich mixture sinking in the evaporator to be replaced by relatively ammonia-free hydrogen entering a t the top of the evaporator from the absorber. It is easily seen that there results from this design a machine capable of continuous operation, completely or hermetically sealed, and possessing no moving or wearing parts. Of course, to control the amount of refrigeration delivered from such a machine to suit climatic or such conditions as the user desires, certain automatic control devices are required. Provision must also be made for automatic safety features, all of which have been worked out satisfactorily. Before entering into the operation costs of the absorption machines, one other type already referred to will be briefly described. It should be stated also that a considerable number of different types of absorption machines have been reported especially designed for the household field. Many of these are adaptations of the old intermittent water-absorption type. The present article would be over-extended, however, if these numerous units were described in detail. It may be remarked that no household machine of the purely adsorptive type is yet available for public purchase. A refrigerator car, using an adsorptive cycle, has been already mentioned and will again be referred to in concluding the paper.
of fine particles of the salt being 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 to permit a uniform heating of the ammine throughout 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. Assuming that the problem of putting the ammine in a preferred physical state by the addition of suitable substances and subsequent treatment is solved, together with the working out of an efficient generator, there exists the basis for a simple refrigerating unit. A rectifier is not required, for anhydrous ammonia of very high purity is circulated. Further, no moving parts are needed with their inevitable wear and ultimately noisy functioning.
Ammine-Compound Absorption Machine
The automatic control of the cycle of operations is the re maining portion of the task in realizing a long-lived, noiseless, and cheaply operated refrigerating unit. The cycle of operations to be automatically performed consists of the turning on of the gas (or electricity) a t the moment that the refrigerant has been completely 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 temperaturecontrolled 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 period, which must be timed to just empty the generator of its refrigerant content, the gas must extinguish and the cooling means transfer from the condenser to the generator, which now becomes an absorber. The refrigerator box interior once set for a desired temperature (45' F.)should not deviate more than a few degrees (+2' F.) night or day independently of the temperature prevailing outside the refrigerator. Of course, if the temperature of the room falls to zero, special provision would have to be made to heat the interior of the refrigerator box. In several instances coming under the writer's attention this has been done by suitably connecting a carbon filament lamp into the regulating circuit of the refrigerating unit. Suffice it to state that the marketed machines of the adsorption type are entirely automatic, self-regulating, and protected from the consequences of accidental failures in either gas or other energy applied t o heat the generator, or the cooling means. This has been rendered possible by the development of extraordinarily simple and ingenious devices, with the result that the absorption machines are reliable, efficient, durable, silent, and easily serviced. A diagram of one of the Ice-0-Lator units is given in Figure 2, where the
The proposal to employ an addition compound in a refrigerating cycle seems to have been first made by Puplett and Rigg.7 No practical utilization in the form of a marketed machine appears to have been made until the appearance of the Narco machine8 in Boston in 1923. Successful utilization of the ammine-forming salts in a refrigerating cycle depends on the existence of a salt ammine which possesses the proper pressure-temperature relation. Thus an ammine whose ammonia presqure was too high at ordinary temperature (ammonia boils a t 32' F. under a pressure of 65 pounds) would be impractical for producing temperatures low enough for the freezing of ice cubes, the preservation of ice cream, etc. On the other hand, one whose temperature of decomposition was too high would endanger the stability of the ammonia or make the refrigeration cycle inefficient owing to the excessive amount of heat required for operation. Two salts which are suitable are calcium chloride and strontium chloride, the former being preferable from the point of view of cost. The proper choice of salt settled, there remains the development of a process of putting the salt in the preferred physical state. Among the more important factors involved in using an ammine-forming salt may be mentioned the large volume change occurring when the ammine passes t o the salt during the decomposition or heating part of the cycle, the maintenance of a desirable porosity to permit easy diffusion of the refrigerant throughout the material, the avoidance of segregation in the material, and the elimination of the tendency
' Puplett and Rigg, British Prtent 507 (November, 1889). 'The Narco machine was taken over as a subsidiary (National Refrigerating Co.) of the Winchester Repeating Arms Company in 1926 and since marketed under the name "Ice-0-Lator."
Figure 3
INDUSTRIAL A N D ENGINEERING CHEMISTRY
480
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parts of the machine are labeled to facilitate following the cycle of operations. Costs of Operation and Efficiency Characteristics of Absorption Machines
The data presented are from tests on two types of refrigerating units-the Electrolux,s hydrogen-water-ammonia absorption, continuously operating unit, and the Ice-0-Lator intermittent automatic refrigerating unit. The heat inputefficiency'o capacity chart for cooling water a t 70" F. for the Electrolux unit is shown in Figure 3. It will be noted that both curves, efficiency-heat input and capacity-heat input, show a maximum. The maximum in the former is, however, very flat, showing but a relatively small variation in efficiency between an energy input of 1800 B. t. u. per hour e Data furnished by Robert S. Taylor, of the Electrolux Company. 1oThe eficiency percentage as used in this paper is the ratio of the energy corresponding to the refrigerating effect realized (energy of melting of one pound of ice times pounds of ice produced) divided by the total energy applied to the unit multiplied by 100.
and 2600 B. t. u. per hour. The capacity, on the other hand, shows a more abrupt maximum a t 2900 B. t. u. per hour. 0ver.a considerable range of the variables, however, the capacity increases nearly linearly with increasing energy input. ' Precise figures relative to the effect of varying the temperature of the inlet cooling water, for units of specific design, are not available for the Electrolux unit. As with every refrigerating device, however, whether of the absorption or compression type, the refrigerating effect diminishes as the temperature of the cooling means increases. The cost of operation for the Ice-0-Lator unit may be obtained from Figure 4.11 Here the cost of refrigeration based on 100 pounds of ice equivalent refrigerating effect is given for absorption machines of five different efficiencies; three gas units (Ice-0-Lator) and one electric absorption unit (Ice-0-Lator) are compared with data based on the results of tests on a number of compression units. The "20 per cent" gas unit may be taken as an average, and it is evident that the cost of operation is more favorable than either the compression unit or the electrically operated absorption unit. It is obvious that the average householder is not necessarily greatly concerned with costs of operation. Service costs might easily wipe out a considerable difference in running charges. It is, however, precisely in the element of durability and freedom from service that most is to be expected from the absorption machines, because of the absence of an electric motor and wearing parts. It may be added that, while the absolute cost of operation of the household machine as compared with silence of operation and freedom from service, may not primarily interest the householder, the grocer, butcher, florist, drink dispenser, etc., are concerned with costs, which again makes for interest in the gasoperated absorption unit. Adsorption Freight Car Refrigerating Unit
In conclusion, the attention of the reader is again called to the very interesting article by Hulse3 on the artificially refrigerated freight car. There is provided here an example of a system of refrigeration based on the property known as adsorption or surface condensation in which the adsorbent is silica gel. The description, illustrations, and details given by Mr. Hulse are very complete and the results obtained in trial trips are convincing evidence of the splendid results to be obtained by the installation of mechanical refrigeration in freight cars conveying perishable goods. Figure 53 is a diagram of the actual mode of installing the unit, and the original article giving complete details should be consulted by the reader interested in this important development. 11
Hainsworth, Refrigerating Eng., 13,245 (1927).
+ONDENSER
FUEL PRESS&?&
Figure 5-Diagram
Vol. 21, No. 5
of Refrigeration Equipment in Car
