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780 CHAPTER 71 1962 Guide Arid Data. Book * Fig. 28. 2. For fv0& head pump dttatb fee Fig. 25. * 3. For coodeofer-recerver detail ee Fig. 11. 4. For eoaipmsor coafag coo fig. 7, 9, or 9. Fig. 31 .... Arrangement for Compound System with Rash Type Intercooler Fig. 31 illustrates a two-etage system with a low-pressure receiver and a low-head liquid ammonia pump, for circulating to the evaporators and with a horizontal flash type inter cooler. A high-head liquid ammonia pump can also take a suc tion from the low-pressure receiver and pump any excess ammonia back to the high-pressure receiver during unusual operation of the evaporators. An oil separator is used on both the low- and high-stage compressors. It returns oil to the crankcase. A check valve is used on the line from the top of the inter cooler to the top of tiie discharge piping from' the low-stage compressor to prevent liquid from flowing back through the discharge line when the low-stage compressor is-shut down for any reason. This check valve is required because the dis charge end suction sides of tiie compressor are equalized when it stops. When starting a system with only the high-stage compressor in operation, the liquid feed should bypass the intercooler. This is required winm the intercooler would be at a lower pressure th*n the low-pressure receiver. Under this condition liquid from the.intercooler would not be able to'flow to the higher-pressure vessel. CASCADE SYSTEMS Cascade systems employ two different refrigerant circuits to produce a desired temperature. fig. -32 ... i Cascade System Using a Centrifugal Refrigerant 12 Compressor for Condensing Ammonia A series cascade system could be illustrated by an ammonia system operating from -- -40 F evaporator temperature to a condensing temperature of plus 5 F with a Refrigerant 12 sys tem providing an evaporator temperature'of -- 5 F for con densing the ftmmnnk A parallpl cascade system would be one with an- ammonia system operating at an evaporator temperature of plus 5 F and with the evaporatedgas being recondensed'at a slightly lower pressure in a cascade condenser by a Refrigerant 12 sys tem at an evaporator temperature of -- 5 F. As an added parallel type cascade system, an illustration would be .with only part of the ammonia at the plus 5 F tem perature being re-condensed by the Refrigerant 12 system, and with the remaining.part being.taken into an ammonia compression system and discharged into the condensers. Fig. 32 is a cascade system using a centrifugal Refrigerant 12 compressor to condense ammonia at plus 5 F. Purging of non-condensables from the low-pressure receiver is usually accomplished.by a small purge- compressor that takes its suction from the top of the low-pressure receiver and discharges .into a water-cooled - ammonia condenser and re; ceiver from which the air can be-purged as shown in Fig. 11- CHAPTER 72 REFRIGERANT PIPING SYSTEMS FOR MULTI-STAGE APPLICATIONS Compound Systems; Cascade Systems; Reasons for Multi-Stage Systems; Refrigerants; Equipment Considerations, Compressors, Condenser, Evaporator,- Accessory Equipment, Subcoolers, Gas Intercooler, Heat Exchangers, Oil Return Equipment, Expansion Tanks, Refrigerant Liquid Pumps, Liquid Level Indicators and Sight Glasses, Knock-Out Drums; Pipe Sizing, Arrangement and Size of Piping for 03 Return; Liquid Feed Control, Controlling Load During Pull Down, Operation at Varying Loads and Temperatures; Moisture; Insulation and Vapor Barriers THIS chapter discusses multi-stage systems using positive by possibly reducing the displacement of the lower stage, displacement compressors. Piping systems for ammonia line sizes, etc. The cascade system makes possible the elimina (Refrigerant 717) are discussed in Chapter 71 and.therefore tion of problems of balancing the oil in the crankcases be only mentioned in this chapter. tween separate stage compressors. Although it does require A refrigeration system which consists of more than one an additional temperature difference between the condenser stage of compression is defined as a multi-stage system. In of the first cascade and the evaporator of the second cascade, general, there are two types of multi-stage systems, com no desuperheater or subcooler is required. pound and cascade. Each refrigerant circuit b the cascade system may in itself COMPOUND SYSTEMS be either a single-stage or a compound system. An example, might be the use of Refrigerant 13 in a single-stage compres A compound system is a multi-stage application where sor with its condenser rejecting heat to an evaporator which compressors are interconnected in series in the same refrig erant system. Fig. 1 shows a simple outline of a two-stage forms a part of a two-stage Refrigerant 22 system. In the discussion which follows the term multi-stage refers compound system. In addition to the major components to both compound and cascade systems. shown, compound systems are-likely to .contain additional components such as desuperheaters and liquid coolers, as REASONS FOR MULTI-STAGE SYSTEMS shown b Fig. 2. Knock-out drums, oil recovery devices and other accessories are used when necessary or desirable. In some cases two or more stages of compression may be con tained b a single compressor, (internally compounded). In these cases one or more of the cylinders will be isolated from the others so that they may act as an independent stage of compression. There are several factors which limit the use of single-stage systems. The evaporating and condensing temperatures necessary and available may be such as to require a compression ratio high enough to result b very poor volumetric efficiency of the compressor, with the result that the total displacement for two or more stages may be less than the displacement CASCADE SYSTEMS required for single-stage operation. Thus a multi-stage system can result b 'lower compressor costs and almost certainly A cascade system is a multi-stage application b which two Bparate refrigerant systems are interconnected b such a lower operating costs, than single-stage systems for the same conditions of service. Pinner that one provides the means of heat rejection (con denser) for the other. The lower system may, therefore, operate at a much lower temperature. Cascade systems have the additional feature, over compound systems, of permitting the use of different refrigerants b each cycle of the cascade, The difference between the high pressure and low pressure may be more than can be handled by the available com pressor. When a refrigerant is selected that has an acceptable evap- fig. 3 shows a simple outirne of a cascade system. As b the compound systems, additional equipment beyond the major components such as knock-out drums, oil recovery devices, etc., may be used. The cascade system is most often used where the range of pressures (using a single refrigerant) from evaporating to final condensing temperatures is greater than r-an be handled with commercial compressors or shells, or where the conditions of service are such as to make a cascade system more economical 781