c. Inefficiency of economizer, by reason of which heat obtained by the expenditure of steam in the generator is passed on to the absorber and there uselessly imparted to the cooling water.
d. The entrance of water into the refrigerator, due to the liquid being not perfectly anhydrous.
e. The useless evaporation of water in the generator. With regard to the amount of heat used, it will have been seen that the whole of that required to vaporize the ammonia, and whatever water vapor passes off from the generator, has to be supplied from without. Owing to the fact that the heating takes place by means of coils, the steam passed through may be condensed, and thus each pound can be made to give up some 950 units of heat. With the absorption process worked by an efficient boiler, it may be taken that 200,000 thermal units per hour may be eliminated by the consumption of about 100 lb. of coal per hour, with a brine temperature in the refrigerator of about 20° Fahr.
Compression Process.—In this process ammonia is used in its anhydrous form. So far as the action of the refrigerator is concerned, it is precisely the same as it is in the case of the absorption apparatus, but instead of the vapor being liquefied by absorption by water, it is drawn from the refrigerator by a pump, by means of which it is compressed and delivered into the condenser at such pressure as to cause its liquefaction at the temperature of the cooling water. It must be borne in mind, however, that allowance must be made for the rise of temperature of the water passing through the condenser, and also for the difference in temperature necessary in order to permit the transfer of heat from one side of the cooling surface to the other. In a compression machine the work applied to the pump may be accounted for as follows:
a. Friction.
b. Heat rejected during compression and discharge.
c. Heat acquired by the ammonia in passing through the pump.
d. Work expended in discharging the compressed vapor from the pump.
But against this must be set the useful mechanical work performed by the vapor entering the pump. The heat rejected in the condenser is the heat of vaporization taken up in the refrigerator, less the amount due to the higher pressure at which the change in physical state occurs, plus the heat acquired in the pump, and less the amount due to the difference between the temperature at which the vapor is liquefied in the condenser and that at which it entered the pump. An ammonia compression machine, as applied to ice making, contains ice-making tanks, in which is circulated a brine mixture, uncongealable at any temperature likely to be reached during the process. This brine also circulates around coils of wrought iron pipes, in which the liquid ammonia passing from the condenser is vaporized, the heat required for this vaporization being obtained from the brine. A pump draws off the ammonia vapor from the refrigerator coils, and compresses it into the condenser, where, by means of the combined action of pressure and cooling by water, it assumes a liquid form, and is ready to be again passed on to the refrigerator for evaporation. The ammonia compression process is more economical than the absorption process, and with a good boiler and engine about 240,000 thermal units per hour can be eliminated by the expenditure of 100 lb. of coal per hour, with a brine temperature in the refrigerator of about 20° Fahr.