Document 7OLbRgZ29m9R8wRLkNw3RGaGa

American Society of Heating and Ventilating Engineers Guide, 1936 flows to the evaporator through an expansion valve which reduces its pressure arid regulates its flow. The evaporator absorbs heat from a medium which is to be cooled. When this rnedium is water or brine, the evaporator is known as a water or brine cooler and the refrigeration system, if used for air cooling, is known as an indirect system. When the medium cooled is air, the evaporator is known as a direct expansion cooler and the system is known as a direct expansion system. Fundamentally, the function of the system is to absorb heat at one temperature and pump it to a higher temperature, where it may be re-' moved by an available cooling medium. In order to conserve refrigerant, ( Chapter 2--Refrigeration theoretical cycle starts with saturated vapor, operation is common at a condition of superheated vapor (as at di). Moreover, expansion may tart either with a mixture of liquid and vapor or with a sub-cooled liquid, s at ci with expansion to eu It is obvious that this latter is desirable as ft increases the refrigerating effect. Area aibicdaai represents the work of such a superheated cycle, while the area eidigifid represents the refriger ating effect of the cycle with superheated vapor and sub-cooled refrigerant liquid. It will be noted on the pressure-volume diagram the volume of the liquid is indicated by a dotted line close to and parallel to the ordinate. Heat of Compression Added to Gas 1 virtually all refrigeration systems are completely closed arid the same refrigerant is recirculated. Theoretical Mechanical Refrigeration Cycle The complete mechanical refrigeration cycle may be illustrated on' the temperature-entropy diagram, and also on the pressure-volume diagram both of which are shown.in Fig. 2. ' ' Considering the theoretical cycle, saturated vapor is drawn into the compressor at a and compressed at-constant entropy (adiabatically) and then delivered to the condenser at b. Condensation occurs at constant' temperature Tt from b to c with a contraction from the vapor to th*e liquid volume. The line cd represents cooling from the temperature of . the condenser to that of the evaporator by an external cooling means. At the same time, the pressure is lowered to Pi. Evaporation then occurs', from dto a at temperature Tu completing the work cycle dbcda. Since no, . external means of cooling the refrigerant liquid is normally available, the cooling is generally accomplished by evaporation of a portion qj the refrigerant. Since- the work of expansion' is usually used up as friction in the expansion valve, this process is carried on at constant total heat,' as represented by the line ce, on the temperature-entropy diagram. Thus the refrigerating effect is represented by an area eagfe. While the normal * 38 Fig. 2. Theoretical Dichlorodifluoromethane (F,,) Cycles It is obvious that it is necessary to include the work of pumping the liquid as well as pumping the vapor as part of the refrigeration work. Theoretical Work per Pound The temperature-entropy and pressure-volume diagrams are based, on one pound of the refrigerant. Likewise, the theoretical work and the refrigerating effects are conveniently based on a pound of refrigerant'. The compression work pier pound may be found by several methods. The temperature-entropy method starts with state point a. Since the quality of a is known, the heat content of the vapor -ffa is known, and also the entropy Sa. Since Sa = Sb and with 7* given, Hi> can be determined. If W = work in foot-pounds per piound of refrigerant, then W = (Hb - JZa) X 778 ~ (1) The pressure-volume method starts with state pioint a, whose pressure and specific volume are known. The work of compression is the adiabatic work of compression from Pi to Pi, plus the work of expelling the vapor at constant pressure Pt minus the external work' of evaporation of the , vapor to volume Fi at pressure Pi. r >