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448 Chapter 24 1945 Guide - condenser is then necessary to condense the steam in the secondary jet. - While a single booster of smaller than 15 tons capacity is difficult to build, steam jet vacuum cooling units have been built for as small as 5 to 6 tons capacity. They can readily be built for steam pressures of from 5 to 200 lb per square inch and condenser water temperatures as high as . 90 F. The steam consumption in pounds per hour per ton of refrigeration increases rapidly as the booster steam pressure is lowered. Faf example, the lowering of .the booster steam pressure from 200 to 90.1b per square inch results in an increase in steam consumption of approximately 5 per cent, whereas a further decrease in booster steam pressure to 10 lb per square inch increases the steam consumption by approximately 72 per cent over that required at 200 lb per square inch. The capacity of a steam jet system is usually controlled by controlling Refrigeration 449 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. 5 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 the different types of compressors is shown in Fig. 6. 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 Fig. 3. Diagrammatic Arrangement of Steam Jet Vacuum Cooling Unit the number of boosters in use since the unit usually has several boosters operating on the same evaporator. Usually one booster is automatically controlled whereas the others are manually operated. The capacity is dependent, as for all compressors, upon the evaporator temperature, or in other words, the suction pressure. For example, the capacity is lowered approximately 17 per cent if the evaporator or chilled water temperature is lowered from 50 to 4;5 F. The capacity therefore can be controlled to some extent by regulating the evaporator temperature. 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. 4 and 5. From Fig. 5 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 Fig. 4. Performance Characteristics of Compression Refrigeration Machines at Constant Speed 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 systems are well suited to those applications where condensing water is cheap, or where condensing water is rather high in temperature. ABSORPTION SYSTEMS The fundamental rule governing the absorption (in a closed system) of a gas by a liquid is Raoult's Law, which states that at any given tem perature the ratio of the partial pressure of a volatile component in' a solution to the vapor pressure of the pure component at the same tem perature is equal to its mol fraction in the solution. The mol fraction in turn is equal to the number of mols of substance divided by the total number of mols present. The number of mols in a given weight of a compound is'equal to the weight divided by the molecular weight.