A hermetic compressor has its motor and mechanism sealed inside a welded steel shell. It is small and cheap to produce and has little risk of refrigerant leakage, but when it fails it cannot be repaired and is replaced as a whole. Most commercial condensing units from the domestic refrigerator up to 10–15 kW have a hermetic reciprocating or scroll compressor.
In a semi-hermetic compressor the body is cast iron and closed with bolts: the valve plate, pistons, bearings and motor windings can be removed and replaced in place. It is heavier and more expensive, but offers a 20–30 year service life, capacity control (cylinder unloading), suitability for low-temperature applications and repairability. The Bitzer, Frascold and Copeland Stream series are well-known examples of this class.
Which to choose? For cold room and kitchen applications below 10 kW a unit with a hermetic compressor is adequate and economical. In continuously running, large or low-temperature stores (blast freezing, frozen storage, central systems) semi-hermetic is preferred; the first cost looks higher, but breakdown and energy costs are lower in the long run. In our catalog the compressor type is stated for every condensing unit.