It is important for the designer and operator of a system to know how the suction and discharge pressures influence the capacity and power, because few plants operate with these pressures constant. An awareness of the influence on power requirements is needed both for analyses of energy requirements and for selection and operation of the motor without overloading it.
For the reciprocating compressor, Figure 4.10 showed the effect of evaporating and condensing temperatures on the refrigerating capacity, and Figure 4.13 the effect of these temperatures on the power requirement. Figure 5.9 shows the influence of the evaporating and condensing temperatures on the refrigerating capacity, with the evaporating temperature exerting the major influence. The power required by the screw compressor appears in Fig. 5.10, exhibiting the characteristic peak in the curve at a given condensing temperature. These trends for the screw compressor are similar to those of the reciprocating compressor, but there are also some differences worth noting. For both types of compressors the refrigerating capacity is influenced by the volumetric efficiency, the specific volume of the suction vapor, and the pressure ratio. The power is affected by the volumetric efficiency and work of compression.
Because the screw compressor completes its expulsion of gas with virtually no volume remaining, there is no clearance volume to reexpand, as is the case with the reciprocating compressor. It would be expected, then, that the volumetric
efficiency and refrigerating capacity drop off less as the pressure ratio increases. Table 5.2 shows the comparison of refrigerating capacity and power of a screw and reciprocating compressor as the evaporating temperature changes. Indeed at the higher condensing temperature of 35°C (95° F) there is a greater dropoff in capacity of the reciprocating compressor as the evaporating temperature decreases. But at the lower condensing temperature of 20°C (68°F), F), the percentage reduction in capacity is about the same for the two compressors.
The modest drop in refrigerating capacity of the screw compressor at the higher condensing temperature is accompanied by very little reduction in the power requirement as the evaporating temperature drops. In contrast to the reciprocating compressor, the screw compressor operates more favorably in a temperture pulldown situation. The capacity of the reciprocating compressor drops off, even though the motor would have the capability of providing additional power.