Comparison of Ammonia Compression and Absorption Systems, Tests have shown that the economy oa compression system when driven by an engine requiring three pounds of coal per hour per indicated horsepower is about equal to that of the best absorption system when used in connection with a boiler of good efficiency provided the suction pressure of the i ammonia gas is about 20 pounds per square inch above the atmosphere — that is when the tem perature of the substance to be refrigerated is about 20° F. For higher temperatures and higher suction pressures the compression ma chine is more economical in fuel, and for lower temperatures the absorption machine is more economical, unless the compression system em ploys a multiple expansion engine so as to reduce the coal consumption below the three pounds on which this comparison is based.
Ammonia Compression Machines with Brine Circulation.— Fig. 4 shows a compres sion system for a cold storage warehouse. A is the compressing cylinder which is driven by the steam engine B. The gas on being com pressed in A passes up through the pipe as to the condenser C, where it is liquefied. The liquid anhydrous ammonia passes from the con pressed and returned to the condenser. The brine which is reduced in temperature in the cooler F is pumped by means of the pump G through the pipes dtt, which are connected with coils HH in the cooling rooms II. After the brine has passed through the cooling coils it is returned to the cooler F. In passing through the expansion cock E about 10 per cent of the weight of the anhydrous ammonia will be va porized and it is the heat required to produce this vaporization that causes the liquid to be re duced in temperature from say 80° F. before throttling to 0° F. after throttling. Ninety per cent of the ammonia is, therefore, in the liquid state when it has attained the tempera ture of ebullition corresponding to the pressure existing in the cooler. The brine in the cooler is at a slightly higher temperature than the liquid ammonia contained in the coils and trans nuts heat to it so as to boil it and convert it into gas. The brine in parting with its heat is thus cooled to the extent of the heat required to evaporate 90 per cent of the liquid ammonia which, passes through the throttling cock. In some types of compression machines all of the ammonia is not allowed to vaporize in the cooler, whereas in other systems care is taken to secure a complete evaporation. This gives rise to two classes of machines, one of which is known as the wet or cold compression and the other as the dry compression system. The brine returns to the cooling tank after passing through the storage rooms at about higher temperature than that at which it leaves the cooler and the mean temperature of the brine is about 10° higher than the boiling point of the ammonia corresponding to the suction-pressure.
The mean temperature of the brine is 6° less than that of the air in the storage-rooms. For the storage of beer a temperature of about 36° F. is required, and this is secured with a pressure of about 28 pounds per square inch above the atmosphere in the coils of the cooler. Slaughter-houses require about 25° F. in their storage-rooms, which must be obtained by a pressure of about 22 pounds per square inch in the cooler. The storage of fish requires a tem perature of about 0° F., for which a pressure in the brine coils of the cooler of about five pounds per square inch above the atmosphere must be used. Instead of a brine made with ordinary salt one formed from chloride of cal cium is often used as a circulating medium be cause it produces less corrosion in the pipes than salt brine and may be brought to a lower temperature before it freezes or becomes partly congealed. In another system air is blown around the coils of the cooler and is circulated through the storage-rooms, and a chamber freshly filled with material can be more quickly cooled than by the use of cooling coils. The air system appears to be more expensive to operate than the brine system.
Direct Expansion Systems.— In some ma chines instead of expanding the ammonia in coils placed in the cooler and forcing cold brine through the coils in the refrigerating-rooms the anhydrous ammonia is expanded directly in coils placed in the refrigerating-rooms. In this sys tem we eliminate the loss due to the difference in temperature between the brine and the coils in the cooler. The use of brine is, however, re garded by many as a safeguard against damage to the material in the storeroom by the acci dental escape of ammonia from the circulating pipes; the brine circulation is for this reason much used, notwithstanding the fact that the cost of the piping is less for the direct eitisair sion system and that there is a saving in the cost of operation. This saving is due to dis pensing with the brine circulating pump and permitting from five to 10 pounds higher pres ence to be used in the coils of the cooler. In the wet system of compression, known also as the Linde system, the presence of liquid am amnia in the compression cylinder limits the highest temperature attained by the ammonia during to the boiling point of the ammonia liquid corresponding to the highest pressure, or say to F. With the dry system of compression the maximum temperature in the compression cylinder is about F. higher than this, and if the cylinders were non-con dnctors of heat the wet process would be more economical than the dry. It appears by tests, however, that the influence of the conductivity of the cylinder walls in abstracting heat in one part of the cycle and giving it back in part in another, makes the two systems about equal in economy.