Document EmaYLnwQXop6zLqkKBdVKYGNN

706 CHAPTER 39 1946_ Guide force; therefore, this type of compressor is inherently suitable for large volumes of refrigerant at low pressure differentials. Two or more stages are usually required and high speeds are necessary to obtain good efficiency. The evaporator is usually constructed as an integral part of the centrif ugal type condensing unit, to chill water which is then circulated to the air conditioning system. This is done because it would not be economical to pipe these large volumes of refrigerant any distance. Centrifugal compressors like reciprocating compressors can be divided into two general types, open and enclosed. In general, the open type compressor is geared to the driving mechanism, and operates at higher speed than the driving motor or turbine. A modern completely enclosed direct-driven, centrifugal compressor is illustrated in Fig. 6. The compressor capacity can be varied by controlling the condensing pressure. This is accomplished by regulating the quantity and tem- Refrigeration 707 the different types of compressors is shown in Fig. 8. It may be noted that the power required by the reciprocating compressor increases rapidly with increase in condenser temperature, while the power curve for the centrifugal compressor is relatively flat. It is also evident that the capacity of the steam jet compressor is independent of condenser tem perature until a certain point is reached where it drops to zero. As previously stated, steam jet equipment requires more condensing water than other types of compression systems. Consequently, steam jet 1 st stage compressor Chiller Fig. 6. Enclosed Type Centrifugal Condensing Unit perature of the condenser cooling water. The capacity falls off with increasing condensing pressure. Centrifugal compressors are seldom built for less than 50 tons capacity, since it is not practical to make impellers which pump much less than the volume of refrigerant required for this tonnage. Characteristics of Compression Systems The various types of compression systems have quite different charac teristics of capacity and power with varying evaporator and condenser temperatures, as may be noted from curves in Figs. 7 and 8. From Fig. 7 it may be observed that power requirements for the centri fugal compressor increase much more rapidly than for the reciprocating compressor with increase in evaporator temperature. Similarly, the capacities of the steam ejector and centrifugal compressors increase more rapidly than those of the reciprocating compressor with increase in evapor ator temperature. Thus, both the steam jet and centrifugal machines tend to be more self-regulating than the reciprocating. It is also evident from Fig. 7 that the steam jet equipment is best suited for operation at high evaporator temperatures. The effect of condenser temperature upon the power and capacity of Fig. 7. Performance Characteristics of Compression Refrigeration Machines at Constant Speed systems are well suited to those applications where condensing water is cheap, or where condensing water is rather high in temperature. Condensers Condensers used for liquifying the refrigerant are of three general de signs: (1) air cooled, (2) water cooled, and (3) evaporative (combination air and water). 1. Air cooled condensers are seldom used for capacities above 3 tons of refrigeration, unless an adequate water supply is extremely difficult to obtain, as, for instance, in railway air conditioning. Evpn on fractional tonnage installations, air is used as the condensing medium only where water is expensive or where simplicity of installation warrants the higher condensing pressure, and consequent higher power costs than would be obtained using water as the condensing medium. The conventional air cooled condenser consists of an extended surface coil across which air is blown by a fan. The hot discharge gas enters the