Pressure Drop Between Evaporator And Compressor

The life of a manufacturer of any type of compressor is complicated when a designer or contractor fails to account for the pressure drop of refrigerant in the suction line between the evaporator and compressor. The person ordering 0°C for example, that the evaporating temperature is to be (-22 F) and orders a compressor to operate at this same saturated suction temperature of -30°C (-22°F). After the compressor is installed and in operation it proves to be short of capacity. The reason is that the pressure drop the suction line increases the specific volume of the suction gas, so the mass rate of flow drops. This phenomenon is particularly penalizing at low evaporating temperatures. Table 4.2 shows the percentage reduction in refrigerating capacity for ammonia and R-22 at two different evaporating temperatures and at drops saturation temperature of 0.5 and 1.0°C (0.9 and 1.8°F). While it is true that at the lower saturated suction temperature caused by pressure drop in the suction line, the power requirement also decreases, as was illustrated in Figure 4.13. However, if the compressor is already attempting to deliver full capacity it cannot take advantage of this reduction in power to increase the refrigerating capacity.

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