In large amounts, compressors for refrigeration often semi-hermetic, i.e. although the engine and compressor in one case, this shell can be unbolted, and refrigerant from flowing over the top of the motor windings. Access for maintenance and simple, but requires no external cooling of the engine, which contributes to the effectiveness of the systems of cooling no advantage in the refrigeration operations, and the cost is considerably higher than for sealed units. How big engines are more efficient than small ones, the overall efficiency of up to 70% or more theoretically possible, and in multi-cylinder compressors the capacity can be managed by one or more cylinders not work (e.g. holding the inlet valve open). Cylinder unloading at startup is also a convenient way to reduce torque.
These compressors (single or double-acting) were developed to avoid the disadvantages of hermetic compressors.
Semi-hermetic compressors are identical sealed type, but the motor and compressor built in manufactured housing with screw sections or access panels for ease of maintenance. These compressors are manufactured in small and medium capacities and their engine power may be up to 300 kW. For this reason, they are cheap, and another advantage is that they are compact. In addition, they have no problems with leaking. On Fig. 3.5 shows a new type of semi-hermetic reciprocating compressors for medium-and low-temperature commercial refrigeration equipment. They are issued to alternative refrigerants (e.g.. R-134a, R-404A and R-507). Fig. 3.5a shows a cutaway view of a single-stage octagon series semi-hermetic reciprocating compressors with a nominal engines with a capacity of 60 and 70 HP With integrated ripple mufflers and performance management (100-75-50%), smooth, efficient and compact piston semihermetics now available for this category of potential. They can work with refrigerants R-134a, R-407C, R-404A, R-507A, R-22. Fig. 3.5b shows used a two-stage semi-hermetic reciprocating compressors for extremely low temperatures and its main feature is the two-stage compression in a single package. A two-stage compression, the compression ratio of the share, thus avoiding extreme temperatures and achieve very reliable operation. In particular, for commercial refrigeration systems with high load variations, energy-efficient operation at full and partial load (capacity up to four stages) to all common refrigerants can be at a reasonable price. In addition, it is recognized features of the octagon, compressors, which even pay with a double in tandem configuration.
