In catalogs showing refrigerating capacity and power requirements of reciprocating compressors, manufacturers do not show performance for all combinations of evaporating and condensing temperatures. Also, some manufacturers show data for conditions that are flagged and the user is asked to consult the manufacturer before selecting a compressor at those conditions. An important reason for avoiding some of these combinations is because of pressures—pressure ratio and/or the pressure difference. Ratios of discharge to-suction pressures higher than 8 or 9 normally are not recommended because the discharge temperature, particularly with ammonia, may rise too high. Another benefit of alerting a potential user to the prospect of high compression ratios is that two-stage compression should be considered for this application.
The reason for limiting the pressure differential is to restrict the load on the bearings and crankshaft. The maximum pressure difference varies between 1000 and 2000 kPa (150 to 300 psi) depending upon the bore, stroke, and other construction features of the compressor. The pressure difference restriction should be kept in mind, particularly in the range of high evaporating and condensing temperatures.