Document dn5RzKbGz8e0X6BEjRR6o1aj0

American Society of Heating and Ventilating Engineers Guide, 1936 The balance of the overall volumetric efficiency is known as the super heat volumetric efficiency even though it includes some other sources of capacity loss. The mechanical efficiency of a reciprocating and rotary compressor must be multiplied by the superheat volumetric efficiency to give the overall efficiency of the compressor. Eff. overall = TM Eff-overa" x Mech.Eff. = Super. Vol. Eff. X Mech. Eff. Vol. ttl.reexp- , (7) Normally, the volumetric efficiency of a compressor varies with the Chapter 2--Refrigeration ired a two-stage purge is usually used,sometimes, a combination steamand water and occasionally a two-stage steam purge. The vapor which is drawn from the evaporator is the result of spraying water which has been warmed by the cooling load into the flash evaporator. Evaporation of a art of this water occurs, which cools the remainder, which is then again returned to the cooling load to receive more heat. The performance of the steam ejector may be studied theoretically by the use of the temperature-entropy diagram, Fig. 5. Unlike its usual application, however, the amount of working fluid is different for one portion of the cycle than for the other. Dry saturated steam under high pressure, for example 100 lb per square inch gage, at a, is expanded ratio of compression, while the mechanical efficiency remains virtually fixed. Good standard practices for Fi2 compressors are: Low comp, ratio = 2.5 to 1 Vol eff.reexp. Vol. eff.super- Vol. eff-overall Mech. eff. 94 to 96 per cent 75 to 85 per cent 70 to 81 per cent 75 to 85 per cent High comp, ratio = 5 to 1 88 to 92 per cent 73 to 77 per cent 64 to 71 pier cent 75 to 85 per cent These values are for one ton or larger compressors. Part of the dif ference expresses the change with capacity.. With other refrigerants.and other types of compressors there will be some further variation. \ Steam Ejector Refrigeration System .- Fig. 4 is a diagrammatic representation of . a typical steam ejector refrigeration system. Live saturated steam is supplied to a nozzle which discharges at a high vacuum into the steam ejector. The expansion of the steam imparts a tremendous velocity to it, which entrains vapor from a flash evaporator and compresses it, with a conversion from velocity to static pressure. The mixture of vapor is then delivered to a condenser. The condensate is removed by suitable means, some of it being used in some cases to supply make-up water to the evaporator circuit. The con denser is maintained free of air by means of a suitable purge. This may. take the form of a water ejector if only a small amount of air is to be removed, as in a completely closed system. If a larger capacity is re- 46 Fig. 5. Steam Ejector Temperature-Entropy Diagram through the nozzle of the steam ejector. With 100 per cent efficiency, the expansion would occur along isentropic line ab. Actually, however; most nozzles are only about 90 per cent efficient, the real expansion being along the line abx. The resulting velocity is determined from the work area abxgea when multiplied by the mechanical efficiency. At the evaporator pressure or slightly below, the vapor from the nozzle mixes with virtually dry saturated vapor from the evaporator. An impact loss also occurs at this point due to the mixture of vapors at different velocities. This results in bringing the state point of the mixture to c. Compression then occurs along the line cd, the work of compression per pound being cdfgc. In computing the work area, however, the point c is not actually known. Therefore, the work area CidfgCi is used in expres sing the efficiency of the ejector, the line cfd being an isentropic. The losses are expressed by nozzle efficiency, impact loss and diffuser efficiency. The work of compression, however, is performed on the mass of the mixture;. Thus, the available work is reduced in proportion to: 3/ primary M mixture' 47 -