Document NGKprODxXxL265ZEprmyNMrpQ

HEATING VENTILATING AIR CONDITIONING GUIDE 1943 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 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. CHAPTER 25. REFRIGERATION 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. This law applies strictly, only to what is known as an ideal solution, that is, one in which the inter-molecular forces between the substances present in the solution are equal. Actually, no such solutions exist, so that deviations from Raoult's Law are always found in practice. The deviation is called positive when the observed pressure is greater than that calculated from Raoult's Law, while the term negative deviation refers to the opposite case. Negative deviations are found wherever, chemical attraction exists between the solvent and the solute. Positive Fig. 5. Performance Characteristics of Compression Refrigeration Machines at Constant Speed '\ 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 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- 486 Fig. 6. Closed Absorption System deviation occurs when there is a. difference in the internal pressure of the components, chemical attraction between them being absent. In order to make an effective' absorption machine, large negative deviations from Raoult's Law must be shown by solutions of the refrig erant in the liquid absorbent, because the larger the negative deviation, the greater is the amount of refrigerant that can be cycled, using a given weight of absorbent. Cycling a large amount of refrigerant for a given weight of absorbent is important because of the heat required to raise the temperature of the mixture and disassociate the refrigerant and the absorbent. Only the latent heat of the refrigerant can be recovered for useful ;work. Many refrigerant-absorbent combinations have been proposed and quite a number have been tested. A diagrammatic representation of a typical closed absorption system is outlined in Fig. 6. In this system'a mixture of refrigerant and absorbent is evaporated in the generator, passes to an analyzer and rectifier where it is purified, and then to a con denser where the refrigerant and remaining absorbent is condensed. It . 487