Open reciprocating compressors with a shaft seal and an external drive motor suitable for a range of prime movers are also available up to about 2 MW duty (e.g. compressor in the condensing unit given in Figure 3.6b). In these compressors, the crankshafts, which are externally coupled with electric motors, extend through the compressor housings. Appropriate seals must be used where the shafts come through the compressor housings to prevent refrigerant gas from leaking out or air from leaking in (when the crankcase pressure is lower than atmospheric pressure). In order to prevent leakage at the seal, the motor and compressor are rarely enclosed in the same housing.
Figure 3.6a shows an open type reciprocating compressor which is suitable for all kinds of refrigerants, including NH3, and Figure 3.6b shows a compact air-cooled condensing unit with an open reciprocating compressor.