CHAPTER V SIMPLE. COMPARISONS (Part 1)

CHAPTER V SIMPLE. COMPARISONS I. Elementary Compression and Absorption Refrigerating Systems The fundamental natural laws on which artificial refrigeration depends can be readily understood by comparing their action with better known processes. The flow of heat from a higher to a lower thermal level, the operation upon which mechanical refrigeration depends, is clearly illustrated by the flow of water from a higher to a lower level. Heat can no more be made to flow up hill than can water. By the use of appropriate mechanical devices, both heat and water can be raised from lower to higher planes. When water is raised to a given hight, its ability to do work, or its energy, is proportionately increased. When heat is raised to a higher thermal level its ability to do work, or its energy, is also proportionately increased. Pump for Raising Water A simple machine for the performance of work on a gas usually takes the form of a pump similar to that used for pumping liquids. Such a machine is represented diagrammatically in Fig. 25. This machine is so constructed that successive strokes of the piston will effect the raising of the liquid from tank 1 to tank 2. To pre- pare for the comparison which is to be drawn later, let the cylinder be supposed to be filled with sponges, which will not materially afifect the operation of the machine. In this case foot-pounds of work are delivered through the piston for the purpose of raising the water from a lower to a higher level and thereby increasing its po- tential energy. To speak more practically, it transfers the liquid from a place where it is not wanted to some other place where it is. Pump for Raising Temperature Only slight modifications are necessary in this mechanism to make it an appropriate machine for illustrating the principle of the compression system as used in the most modem processes of mechanical refrigeration. A refrigerating machine is then a device Digitized by V^OOQIC SIMPLE COMPARISONS 63 for removing heat from a place where it is not wanted to some other place where it can be conveniently disposed of. Fig. 26 shows a machine similar in construction to that shown in Fig. 25, but instead of having an inlet and an outlet for the passage of a liquid, its cylinder walls are constructed of a very thin sheet of metal which readily allows the passage of heat whenever a difference in temperature exists between the substance inside and that outside of its walls. This cylinder employs as a working medium ammonia gas instead of sponges. In this analogy the action of ammonia, or in Fig. 25. — Conventionalized Diagram of Pump for Raising Water fact any refrigerating medium, in absorbing heat, is compared to that of the sponges in absorbing water. After the first charge of heat has been squeezed out of the ammonia and that of water out of the sponges, both the ammonia and the sponges still possess their original capacity for absorbing more. Both the heat and the water are drawn in or absorbed only because of the outside in- fluence exerted by the ammonia and the sponges. Instead of two tanks at different levels representing difference in "head" or potential energy, in Fig. 26 there are two tanks, a and 6, on the same level, but containing liquids of different tem- peratures and representing difference in thermal energy. In study- ing the operation of the elementary refrigerating engine repre- sented in Fig. 26, it must be assumed that the cylinder is to be Digitized by V^OOQIC 64 ELEMENTARY MECHANICAL REFRIGERATION filled with ammonia gas at a temperature ti, and a pressure pi, the piston occupying that position in its path farthest from X. If the crankshaft be turned through one-half of a revolution the piston will arrive at X. The volume of the gas having become greatly diminished, its temperature and its pressure will have been proportionately increased to fe P2*. At the end of stroke the piston is allowed to stop and by means of the three-way cock C, a spray of water is allowed to Fig. 26. — Conventionalized Diagram of Compressor for Raising Temperature flow over the end of the cylinder in which the hot gas is confined, absorbing a portion of the heat of compression, and consequently reducing its temperature and pressure to tz ps. The machine is now made to complete its revolution and the gas, by expanding with the increasing cylinder volume, cools itself to the conditions of <4 P4. At the other end of the stroke the piston is again allowed to stop, and by reversing the three-way cock a spray of water is allowed to flow over that portion of the cylinder through which the ♦ The ratio of pressure to volume in the case of adiabatic compression for P Vi*'* ammonia is expressed by the equation p- = yiri* which means that the pressure will vary inversely in the 1.3 power of the volume. Digitized by V^OOQIC SIMPLE COMPARISONS 65 piston has just passed. The gas within having now become colder than the water, because of having expanded after reaching the temperature of the water, again absorbs heat from it as it passes in a thin sheet over the comparatively large surface of the cylin- der, and its temperature and pressure are raised to their original conditions of ti pi, after which the cycle is again traversed. Working Mediums In this example the ammonia gas within the cylinder may be compared to the sponges in 'the preceding case. During the ex- pansion the gas absorbs heat and the sponges water. During compression both the heat and the water are expelled and directed into "places where they can be used, or conveniently disposed of.'' In the operation of the mechanism represented in Fig. 26, heat is taken away from a part of the water passed over the cylinder and given to the other part. The former, or '* refrigerated" water, being conducted into tank fc, and the latter, which contains a double share of heat, into tank a. In order to more closely approach the mechanism employed in practical refrigeration systems, it will be possible, without in the least altering the principles involved, to slightly modify the sys- tem represented in Fig. 26. In practice the operation of the com- pressor must be continuous and the cylinder proper would have far too Uttle surface to allow of its being used either as a gas cooler or a gas heater, as was done in the foregoing example. Compressor and Expansion Engine Let it be assumed, as shown in Fig. 27, that a portion of the cylinder at the top and at the bottom be drawn out, forming a cooling or condensing coil B and a heating or expansion coil D and at some convenient point, Ej the passage between the two be contracted to a sufficiently small cross section to maintain a higher pressure in the condenser coil than in the expansion coily the com- pressor being in operation. If the two coils be sprinkled with water, or, to conform more closely to practice, if the lower coil be entirely submerged, we will have practically the same conditions as those in the foregoing example. Starting with the piston in the position shown in Fig. 26, the cylinder full of gas is under conditions of temperature and pressure <i pi. By compression these condi- tions are changed to fe V2, which the cooling of the spray of water Digitized by V^OOQIC 66 ELEMENTARY MECHANICAL REFRIGERATION over the condenser coil tends to reduce to Upz. In the compound machme, Fig. 28, ammonia gas, or whatever the working medium Pig. 27. — Conventionalized Diagram of Compression Refrigerating System may be, is compressed in the compression cylinder A, after which it passes to the condenser B, where it is relieved of a large part of its Fig. 28. — Conventionalized Diagram Showing Compression Cylinder for Eaising and Expansion Cylinder for Lowering Temperature Digitized by V^OOQIC SIMPLE COMPARISONS 67 heat. From the condenser the gas passes to the expansion cylmder C, where it loses more heat because of the work it does in expanding behind the moving piston. This work done by the gas expanding behind the moving piston in C, (which is an exact equivalent of the amount of thermal energy lost,) is recovered, since it assists in compressing the gas in cylinder A. Refrigeration by Change of Temperation of Gases The production of refrigeration by the method just described depends on the change in temperature of the working medium, and not on its liquefaction. Machines of this type are, accordingly, not limited in choice of working medium to those which are easily liquefiable, and air rather than ammonia is most commonly em- ployed. Such machines, known as cold air machines^ have been used extensively in the past on shipboard, their favor there being due to the dangers, real and imaginary, incident to the use of ammonia and other liquefiable refrigerants. Refrigeration is produced by cold air machines by the process just outlined. The air is first compressed, raising its temperature, and allowed to do work in expanding behind the piston of an air engine, just as steam is allowed to do work by expanding behind the piston of a steam engine. The air, as well as the steam, is cooled, because of the heat converted into mechanical work. The exhaust air escaping from the cylinder of the air engine into the atmosphere of the cold-storage room is often as low as from —50° to —85° Fahrenheit while exhaust steam, though similarly cooled by the performance of work in the steam cylinder, usually escapes to the atmosphere at a temperature of 212° Fahrenheit and upward. In practice the expanded air from a cold-air refrigerating ma- chine is returned to the compressor and used over and over again, as this reduces the losses in efficiency due to the presence of atmos- pheric moisture. The production of cold by the expansion of compressed and cooled air is often apparent in the operation of pneumatic machinery in cold weather, in which case the tempera- ture of the exhaust is often low enough to freeze the aqueous vapor, making it necessary to take precautionary measures to prevent tlie closing up of exhaust outlets by the accumulation of ice. In practical refrigerating systems, in which the working medium is carried through the liquid state in order to make use of the Digitized by V^OOQIC 68 ELEMENTARY MECHANICAL REFRIGERATION latent heat of liquefaction, the expansion cylinder is omitted as in Fig. 27. In the example cited in connection with Fig. 26 the available refrigeration is obtained by simply raising and lowering the tem- perature of the ammonia gas, of which, since one degree of sensible heat represents only one-half a British thermal imit, 288 pounds would have to be lowered one degree in order to produce an effect equivalent to the melting of one poimd of ice. This would necessi- tate handling tremendous volumes of gas to produce compara- tively small results of refrigeration. The specific heat of air being only about one-half that of ammonia vapor, approximately twice as many poimds of the former as of the latter refrigerant would have to be employed to produce a given cooling effect. At atmos- pheric pressure the latent heat of vaporization for ammonia is 573 and the specific heat of the liquid is imity, while that of the gas at constant pressure is only about .5080. Comparing these last values, we naturally look for means of producing refrigeration through the aid of the latent heat of vaporization because of the great heat absorbing capacity of this process. The gas, instead of expanding behind a piston, is allowed to escape through a restricted opening, Ej Fig. 27, into the expansion coil below, in which a considerably lower back pressure is maintained through the efforts of the compressor which is constantly drawing gas from the lower coil and discharging it into the upper coil. In passing the point E, the conditions of temperature and pressure of gas drop to t^ p^, which the heating effect of the liquid surrounding the expansion coil finally restores to ti pi, at which point it meets with a repetition of the cycle. The utilization of the latent heat of liquefaction requires no mechanical ^alterations in the system, and this elementary mechanism illustrated in Fig. 27, when put in operation with a proper charge of anhydrous ammonia and cooling water for the condensers, is capable of pro- ducing commercial results of artificial refrigeration. An elementary absorption machine, corresponding to the com- pression machine illustrated in Fig. 27, is shown in Fig. 29. As is pointed out in another chapter, the absorber of the absorptipn machine performs the function of the suction stroke of the com- pressor in the compression system, and the generator that of the compression stroke. The remaining portion of the cycle, including Digitized by VjOOQIC SIMPLE COMPARISONS 69 expansion at E, and evaporation in the expansion coil also remains as in the compression system. a^^^^^^^ «M lai|'.i|i!Vi,'lli'.l Expansion Coil Absorber Fig, 29. — Conventionalized Diagram of Absorption Refrigerating System II. SIMPLE COMPARISONS Water and Ammonia Systems It has already been shown that the amount of heat required to raise the temperature of either a solid, a liquid or a gas through one degree is very small compared to that required to effect a change either from the solid to the liquid or from the liquid to the gaseous state. In the case of anhydrous ammonia, for example, the specific . TABLE I. COMPARATIVE PROPERTIES OF WATER AND ANHYDROUS AMMONIA Hz O Steam and Ammonia Oas NHs 4500. Temperature of decomposition 900 . 698. Critical Temperature 266. .480 Specific heat. Constant pressure 5080 .346 Specific heat. Constant temperature 3911 966. 1 Latent heat of evaporation. Atmospheiic pressure 573 . .0376 Weight per cubic foot at atmospheric pressure 055 Water and Ammonia Liquid 212. Temperature of evaporation. Atmospheric pressure — ^28 . 5 1 . Specific heat of the liquid 1 to 1 .23 144. Intent heat of fusion ( ) 62.42 Weight per cubic foot at 32' F 42.02 Ice and Ammonia Solid 32. Temperature of fusion. Atmospheric pressure — 115 . .5 Specific heat of the solid ( ) 57.50 Weight per cubic foot at 32* ( ) Digitized by V^OOQIC 70 ELEMENTARY MECHANICAL REFRIGERATION heat of the liquid is about unity, that of the gas about .51 under constant pressure and about .39 at constant volume, while the latent heat of evaporation under atmospheric pressure is about 673 B.t.u. For comparison of these characteristics with similar ones for water see Table I. It has also been shown that refrigeration or cooling is always effected by drawing heat out of one substance into some other substance at a lower temperature. In this case the former sub- stance is refrigerated and the latter heated. In fact refrigeration and heating are two different views of the same operation. Refrigeration and heating may take place at any temperature. Metals, for example, are melted in a furnace. To effect the change from the sohd to the Uquid state considerable amounts or heat have to be supplied to satisfy the latent heats of fusion, and we may state just as accurately that the melting metals refrigerate the furnaces as we can that melting ice refrigerates a refrigerator. If the average temperature of our terrestrial atmosphere were a few hundred degrees higher on the thermometric scale, we might actually employ the latent heat of fusion of lead, tin or other metals as we now employ ice, i.e., as a convenient vehicle for absorbing and carrying away comparatively large quantities of heat. With the low temperatures that we can now produce we might at the present time actually employ congealed mercury in our refrigerators in the place of ice were there any advantage to be gained in producing cold by the fusion of a metal. Any substance might be used for absorbing heat for the purpose of cooling other substances. The limits of temperature between which practical use of refrigeration can be made are so narrow, however, that the latent heat of fusion of only a very few substances can be employed and, as a matter of fact, only that of* one substance, ice, is em- ployed. Similarly the latent heat of evaporation of only a com- paratively few substances falls within that range, and since, imfor- tunately, none of these can be employed because of other reasons, only the specific heats of available natural cooling media, which as has already been shown afford only limited heat absorbing capacities, can be employed. Natueal Cooling Mediums Even the so-called refrigerating mediums such as ammonia, carbon dioxide, etc., which substances are liquefiable at such tem- Digitized by VjOOQIC SIMPLE COMPARISONS 71 peratures as to make their respective latent heats of evaporation readily available for the production of artificial cold, must in turn be cooled by some natural cooling medium after they have ab- sorbed their fill of heat in the cold-storage compartment. As will now be shown the most commonly employed natural cooling me- dium is water. As there are no mediums having their boiling points so located as to make their latent heats of evaporation available for cooling the primary refrigerants such as ammonia, carbon dioxide, etc., we must of necessity employ the more limited heat absorbing capacities available in the specific heat of some convenient natural cooling medium. These heat absorbing capacities available in changes of temperature without change of state are so very lim- ited that only the most inexpen ive substances such as air and water can be considered for cooling on the large scale necessary in the case of steam and ammonia condensers. As a matter of fact, these two agents are practically the only mediums we have that can be employed as primary cooling agents Air is everywhere