Hermetic and semi-hermetic compressor
Also known as: hermetic compressor, semi-hermetic compressor, accessible hermetic compressor
Short definitionA hermetic compressor has a welded shell and cannot be opened or repaired. A semi-hermetic compressor has a bolted housing; it can be opened and its parts replaced.
In both types the electric motor and the compression mechanism sit inside the same housing. The difference is how that housing is closed. A hermetic unit has a welded steel shell: it is small, cheap and unlikely to leak, but when it fails the whole unit is replaced. A semi-hermetic unit has a cast-iron body closed with bolts; the valve plate, pistons, bearings and motor windings can be removed and renewed on site.
For cold rooms below roughly 10 kW, a condensing unit with a hermetic compressor is both sufficient and economical. Semi-hermetic compressors are chosen for large stores that run continuously, frozen storage, blast freezers and central systems. They are heavier and dearer, and in return offer a 20–30 year service life, capacity control by cylinder unloading and the option of repair. In a quote, the price gap between the two is therefore weighed against the operating life.