Document 1raeYd9LokN4xnMmqEL4r5JK

American Society of Heating and Ventilating Engineers Guide, 1937 Fortunately, the trouble can be concentrated in the compressor and the problem relieved of many complications. It is comparatively easy t0 furnish condensers and evaporators to carry the maximum load so arranged that they will function properly at small demands. They affect the compressor performance to some extent but most of the compressor problems are in the machine itself. Variations in load are usually effected by lowering the suction tem perature and pumping a larger volume of gas per ton through a greater pressure range. This is possible because the latent heat of the refrigerant remains nearly constant throughout the small range used and the specific volume varies rapidly with change in pressure. As the compressor must remoye the refrigerant evaporated, the evaporator temperature fixes the displacement required. The objection to such method is that the total power consumed remains nearly constant and the power per unit of cooling increases rapidly as the total output is reduced. Such operation is satisfactory as long as the load is kept within 10 per cent of the rating of the compressor but this condition does not commonly occur in air conditioning applications. Operating Methods It is possible to divide the entire refrigeration system into a number of small units, which will allow cutting in and out of compressors and con densers as the load fluctuates. A second method of providing for economy of operation is to have storage capacity which can be utilized during the peak period. A further reference to the Weather Bureau records indicates that maximum con ditions prevail during the day for not more than three hours, and con sequently the refrigerating system can be run for a longer period at maximum efficiency with tanks to store cold water or brine for supple menting the actual output of the refrigerating equipment when the load is more than the machine will carry. This situation brings complications. Storage tanks require space and extra apparatus, which increase the cost of the entire system, and further, it is difficult to determine what the size of the compressor should be because of the other variables which enter the problem. Depending upon the availability of storage space, the com pressor could be designed for any reasonable percentage of the maximum load, so the smaller the compressor, the larger the storage space, and vice versa. A third method is to provide in the compressor itself some means of reducing the capacity. This can be done by varying the speed and con sequently the displacement of the compressor, or by varying the dis placement, either by a partial by-pass of the cylinder or by a clearance pocket in the head of the cylinder when reciprocating compressors are used. It might be assumed that the efficiency would remain practically constant. This is not correct, inasmuch as the machine friction remains constant with the by-pass or clearance pocket method and this raises the power required per ton of refrigeration developed. Also, the volumetric efficiency of the machine falls off rather rapidly when the clearance pocket or partial by-pass is used. By varying the speed of the compressor, the efficiency of the power unit falls off as the speed is reduced, while the compressor friction remains constant. 204 Ghapter 11--GooLING N4ethods A other method of operation is the automatic starting and stopping of l refrigerating machine, with the automatic control designed to function "'the joa(j varies. All of the methods described are used from time to time. ^The methods of varying the output of a refrigeration system which have outlined apply to the reciprocating type q(_ compressor, although riations in the speed of the compressor to change the refrigerating output are common to all types of mechanical refrigeration. There is a further method of controlling the compressor output which is Particularly adaptableto the centrifugal type of machine. This is accom plished by varying the amount of condensing water used with the fluctu ation in demand load. Because of the characteristics of the centrifugal type of apparatus, as the condensing water quantity is reduced and the condensing temperature consequently raised, the discharge pressure of the centrifugal machine rises correspondingly and the horsepower input to the machine falls off. While this reduces the total power input to the machine, it does not necessarily reduce the power input per ton of re frigeration developed, as the power input does not drop with a rising dis charge pressure as fast as the refrigerating effect produced drops. It is a method, however, which shows marked economies over the method generally used by the operating engineer, which is to lower the suction pressure in order to reduce the refrigerating output of the system. STEAM JET SYSTEM So far the discussion has been confined to reciprocating, centrifugal, and rotary compressors. The steam jet type of compressor, under certain circumstances, is desirable for use in air conditioning. A complete flow diagram of the system is shown in Fig. 4 of Chapter 2 on Refrigeration. The power used for compressing the refrigerant is steam, taken directly from the boiler, thus eliminating the mechanical losses of manufacturing electric current. As the compression ratio between the evaporator and condenser under normal circumstances is large, the mechanical efficiencies of the equipment are somewhat lower than those of the positive mechanical type of compressor; also the condensing water requirements are con siderably greater, as both the refrigerant and the impelling steam must be condensed. The steam jet system functions on the principle that "water under high vacuum will vaporize at low temperatures, and steam ejectors of the type - commonly used in power plants for various processes will produce the necessary low absolute pressure to cause evaporation of the water. In Chapter 2 on Refrigeration Fig. 4 shows a typical water cooling application. The water to be cooled enters the evaporator and is cooled to a temperature corresponding to the vacuum maintained. Because of the high vacuum, a small amount of the water introduced in the,evaporator is flashed into steam, and as this requires heat and the only source of. heat is the rest of the water in the evaporator tank, this other water is almost instantly cooled to a temperature corresponding to the boiling point, determined by the vacuum maintained. "The amount of water flashed into steam is a small percentage of the total water circulated through the evaporator, amounting to approximately 11 lb per hour per ton of refrigeration developed. The remainder of the water at the desired 205