Air Refrigeration machines which employ air as the working substance filled an important place in the development of mechanical refrigeration, and are still doing useful work; though the more efficient and more compact machines of the vapor-compres sion-type, using ammonia or carbonic acid, have now to a great extent driven them from the field. Dr. Gorrie of New Orleans invented an air machine in 1845. It had a compression cylinder in which the air was compressed, and from which it passed into a chamber which was kept cool by surrounding water. This chamber was maintained at a pressure of about 15 pounds per square inch above the pressure of the atmosphere. The air was partly cooled during compression by the injection of water, so that what passed into the receiver was really air and water together. The air was further cooled in the chamber by the application of cold water outside, and then it passed on to another cylinder in which it was allowed to expand down to something like atmospheric pressure. While it was expanding it was mixed with a quantity of brine injected into the expansion cylinder. By expansion the air became much cooled and its low temperature was communi cated by direct contact to the brine in the ex pansion cylinder. The air leaving the expan sion cylinder was allowed to escape to the at mosphere, while the brine cooled to about F. was conveyed into a tank and was usefully applied to ice-making or other refrigerating purposes. Many improvements were made in Gorrie's machine, which was followed in 1862 by Kirk's machine, an air machine of the closed cycle type. These types of machine and their successors are substantially the same. Their essential parts are a compression cylinder and an expansion cylinder. The first takes air from the room which is to be kept at a low temperature, compresses it; the air is then cooled by circulating water while compressed and is then returned to the cooled room by the expansion cylinder. Air machines have played a large part in developing the commer dal applications of cold, although now the first place is taken by other types. Air can be employed as a medium for cooling itself more efficiently than it can be employed as a medium for cooling anything else. Also, air machines use a working substance that is simple and harmless and that costs nothing. For use on shipboard the air machine still retains the favor of some engineers although the more efficient vapor-compression machines dispute its sway even there.
Absorption Machines.— In machines of this type cold is produced as a direct result of the utilization of high-temperature heat, without the intermediate step of converting the heat into mechanical work. In any machine of this type there are two substances used which have an affinity for one another so that one tends strongly to unite with or dissolve in the other when they are in the cold state, but they can be separated by applying heat. In the action of the machine they are alternately allowed to unite and made to separate. By the direct ap plication of heat one of them is driven off in the form of a vapor. It is then condensed, is allowed to re-evaporate, and is absorbed by the other substance as soon as it does so. While it takes up heat from bodies round it, thus becoming effective for refrigera tion. Of practical machines of the absorption type there are two kinds. In one kind the vapor used is water, and it is absorbed by sul phuric acid. In the other kind the vapor is am monia, and it is absorbed by water. When water vapor is used, the pressure at which the vapor is formed must be very low, and hence machines using water and sulphuric acid are often described as vacuum machines. The method was known from about 1810 when Leslie described the process. In 1878 Windhausen made an improved machine of this type for the making of ice.
Ammonia absorption machines employ am monia dissolved in water and are in much wider use than the sulphuric acid or vacuum machine. The action is in general respects the same, but as ammonia has a much higher vapor pressure than water, there is no vacuum, but pressure higher than the atmosphere throughout the cycle. The ammonia absorption machine consisted merely of two vessels, in one of which was placed a strong solution of am monia in water ; the other in the first instance was empty, and was surrounded by cold water. The vessel containing the solution of ammonia was heated and the gas passed over under pres sure into the other vessel, where it was con densed, giving up its latent heat to the water outside. Liquid anhydrous ammonia collected in the cold vessel while the solution in the first vessel became more and more dilute. After some time the source of heat was removed, and the vessel containing dilute solution was sur rounded by cold water. The condensed am monia in the other vessel then evaporated and ber or cooler F, where it is evaaorated through heat supplied by the coils contained therein through which a calcium chloride brine is circu lated. The cold brine is circulated through the pipes dd to and from coils in the cold storage rooms. The ammonia gas leaves the cooler
through the pipe cc and enters near the top of the absorber J, where it meets a spray of weak ammonia liquor, which is conducted to it through the pipe cc. The ammonia gas is taken up by the weak ammonia liquor in the absorber and forms the strong liquor which leaves the absorber through the pipe ff and is forced by the pump K through the pipe //, the exchanger I. and the pipe mm into the top of the generator, thus completing the cycle. Heat is given off by was reabsorbed by the dilute solution. In re evaporating, the ammonia exerted a refrigerat ing effect. This machine was intermittent in action but a continuous machine was soon de veloped from it by Reece, Stanley, Mart and others adding features which are found in the ammonia absorption machine of the present day. As has already been explained, the essential difference in an absorption machine and a com pression machine consists in the method of in creasing the pressure of the ammonia gas leav ing the refrigerating cogs. For the compress ing pump and the steam-engine in the compres sion system there is substituted a vessel called an absorber, and a pump for pumping the liquid into a steam still. The condenser, liquid am monia reservoir, expansion cock and cooler are the same in the two systems. A section of an absorption refrigerating machine is shown in Fig. 3. A is the still or generator in which the aqueous ammonia is heated in order to drive off the gas. The ammonia gas passes off through the pipe oa to the rectifier B, where the gas is cooled to a point where any water vapor in it will be condensed. The ammonia gas free from water vapor passes to the condenser C, where it is condensed and passes through the pipe bb to the ammonia reservoir D. From the reservoir D the liquid ammonia passes through the valve, or expansion-cock E, into a refrigerating chain .the ammonia on absorption, which is taken up by water circulated through coils contained in the absorber. The exchanger L saves heat which would otherwise be lost, and thus increases the efficiency of the machine. The weak liquor on leaving the generator is at the highest tem perature at which the aqueous ammonia is brought in the system, whereas the strong liquor leaving the absorber is at the lowest tempera ture. It is necessary to cool the weak liquor before it enters the absorber and to heat the strong liquor to the temperature of the gen erator, and this is accomplished, in part, by the exchanger by making the weak liquor heat the strong liquor and the strong liquor cool the weak liquor. The weak liquor at about 270° F. is taken off near the bottom of the generator and passes through the pipe nn to the top of the exchanger L and thence through a coil contained therein. From the bottom of the exchanger it passes through the pipe a to the absorber I. The strong liquor at about 130° F. passes through the exchanger L in the reverse direc tion to that of the weak liquor, and the inter changing of heat is such that the weak liquor leaves the exchanger say 100° F. cooler than it entered and the strong liquor is heated a corre sponding amount. The strong liquor is made to enter the top of the generator and pass through what is called an analyzer M. The analyzer adds to the efficiency of the generator by causing the strong liquor to meet the outgo ing gas generated at the bottom of the still. An exchange of heat takes place, which causes much of the ammonia contained in the strong liquor to be driven off at a lower temperature than that which exists at the lower part of the still and produces ammonia gas more free from water vapor than could be secured if the strong liquor were introduced into the lower part of the still. The cooling water passes first through the condenser C. entering through the pipe oo and from thence to the absorber J through the pipe pp. It finally leaves the absorber through the pipe qq. Heat is supplied to the generator by means of steam which enters a coil contained denser through the pipe bb to the liquid monia reservoir D. From the reservoir D the liquid ammonia passes through the valve B, termed °the expansion cock,) where it is duced in pressure from say 140 pounds per square inch above the atmosphere to a pressure of say 15 pounds per square inch above the at mosphere, and thereby is reduced in tempera ture from about 80' F. to about 0° F. The anhydrous ammonia at a temperature of about 0° F. flows in coils of pipe in the brine cooling tank F, and abstracts heat from the brine by being evaported. The gas produced by the evaporation of the ammonia passes from the condenser coils through the pipe cc to the com pressing cylinder A, where it is again com therein through the pipe yr. The water vapor condensed in the rectifier together with the am monia which it carries with it is returned to the analyzer by means of the pipe ss. The drip water from the generator coil is discharged through the steam trap N.