For a given temperature of the low-temperature evaporator and a given condensing temperature, the intermediate pressure is controlled by the relative pumping rates of the low- and high-stage compressors. The basic rules for controlling pressures, and thus the saturation temperatures, in a two-stage system are:
– Regulate the pressure of the low-temperature evaporator by the pumping capacity of the low-stage compressor
– Regulate the intermediate pressure by the pumping capacity of the highstage compressor
Following these rules for control of the pressures is not an unreasonable assignment, because suction pressure control of both reciprocating and screw industrial compressors is quite standard.
The two stages of compression must work in conjunction, because the highstage compressor must pump the same mass flow rate as the low-stage compressor plus the extra flow rate attributable to flash-gas removal and desuperheating, as well as that of an intermediate-temperature evaporator. The responses of mass flow rates and saturation temperatures to an increase in the refrigeration load in the low-temperature evaporator are shown in Figure 3.23. The system is assumed to have been in a steady-state operating condition, when at time T the refrigeration load on the low-temperature evaporator suddenly increases. This increase in load may appear as an abrupt increase in air temperature on the evaporator coil, The first response is for the evaporating temperature and pressure to increase. With a higher evaporating temperature the low-stage compressor is able to pump a higher flow rate which raises the intermediate pressure. Quickly the suction pressure controllers on both compressors sense values that are too high, so the controllers increase the flow rates of both compressors. The new steadystate conditions will settle at higher flow rates than original. A portion of the increase in flow rate is due to the initial increase in evaporating and intermediate pressures, and the other part is due to the controllers increasing the compressor capacities to restore the pressures to their settings.