In larger sizes, refrigeration compressors are often semihermetic, i.e. although motor and compressor are within one casing, this casing may be unbolted, and the refrigerant does not flow over the motor windings. Access for maintenance is straightforward, but the need for external motor cooling which aids efficiency in cooling applications is no advantage in refrigeration operations, and the cost is substantially higher than for hermetic units. As large motors are more efficient than small ones, overall efficiencies of up to 70% or more are theoretically possible, and in multi-cylinder compressors, capacity may be controlled by making one or more cylinders ineffective (e.g. by holding the inlet valve open). Cylinder unloading at startup is also a convenient way of reducing starting torque.
These compressors (single or double acting) were developed to avoid the disadvantages of the hermetic compressors. Semihermetic compressors are identical to the hermetic types, but the motor and compressor are constructed in a fabricated enclosure with bolted sections or access panels to facilitate servicing. These compressors are manufactured in small and medium capacities and their motor capacities can reach 300 kW. For this reason they are cheap, and another advantage is that they are compact. Also, they do not have a leakage problem. Figure 3.5 shows new type semihermetic reciprocating compressors for medium and low-temperature commercial refrigeration applications. These compressors are available for alternative refrigerants (e.g. R-134a, R-404A and R-507). Figure 3.5a shows the cutaway view of a single-stage octagon series semihermetic reciprocating compressor with nominal motor powers of 60 and 70 hp. With integrated pulsation mufflers and capacity control (100-75-50%), smooth running, efficient and compact reciprocating semihermetics are now also available for this category of capacity. They can be operated with the refrigerants R-134a, R-407C, R-404A, R-507A and R-22. Figure 3.5b shows a twostage semihermetic reciprocating compressor for extremely low temperature applications and its main feature is the two-stage compression in one housing. In two-stage compression, the compression ratio is divided, thus avoiding extreme operating temperatures and achieving very reliable operation. Particularly for commercial refrigeration applications with high load variations, an energy efficient operation at full and part load (up to four capacity stages) with all common refrigerants is possible at reasonable cost. In addition to that, there are the recognised features of the octagon compressors which even pay off double in tandem configuration.