Editing Compressor (section)

==={{Anchor|Hermetically-sealed compressor}} Hermetically sealed, open, or semi-hermetic===
[[File:Lednička Zanussi ZRA 319 SW, kompresor s odpařovací miskou (002).JPG|thumb|A small hermetically sealed compressor in a common consumer [[refrigerator]] or [[freezer]] typically has a rounded steel outer shell permanently welded shut, which seals operating gases inside the system, in this case an [[Isobutane|R600a]] [[List of refrigerants|refrigerant]]. There is no route for gases to leak, such as around motor shaft seals. On this model, the plastic top section is part of an [[auto-defrost]] system that uses motor heat to evaporate the water.]]

Compressors used in [[Vapor-compression refrigeration|refrigeration]] systems must exhibit near-zero leakage to avoid the loss of the [[refrigerant]] if they are to function for years without service. This necessitates the use of very effective seals, or even the elimination of all seals and openings to form a [[Hermetic seal|hermetic]] system. These compressors are often described as being either '''hermetic''', '''open''', or '''semi-hermetic''', to describe how the compressor is enclosed and how the [[Electric motor|motor]] drive is situated in relation to the gas or vapor being compressed. Some compressors outside of refrigeration service may also be hermetically sealed to some extent, typically when handling toxic, polluting, or expensive gasses, with most non-refrigeration applications being in the petrochemical industry.

In hermetic and most semi-hermetic compressors, the compressor and motor driving the compressor are integrated, and operate within the pressurized gas envelope of the system. The motor is designed to operate in, and be cooled by, the refrigerant gas being compressed. Open compressors have an external motor driving a shaft that passes through the body of the compressor and rely on rotary seals around the shaft to retain the internal pressure.

The difference between the hermetic and semi-hermetic, is that the hermetic uses a one-piece welded steel casing that cannot be opened for repair; if the hermetic fails it is simply replaced with an entire new unit. A semi-hermetic uses a large cast metal shell with gasketed covers with screws that can be opened to replace motor and compressor components. The primary advantage of a hermetic and semi-hermetic is that there is no route for the gas to leak out of the system. The main advantages of open compressors is that they can be driven by any motive power source, allowing the most appropriate motor to be selected for the application, or even non-electric power sources such as an [[internal combustion engine]] or steam [[turbine]], and secondly the motor of an open compressor can be serviced without opening any part of the refrigerant system.

An open pressurized system such as an automobile air conditioner can be more susceptible to leak its operating gases. Open systems rely on lubricant in the system to splash on pump components and seals. If it is not operated frequently enough, the lubricant on the seals slowly evaporates, and then the seals begin to leak until the system is no longer functional and must be recharged. By comparison, a hermetic or semi-hermetic system can sit unused for years, and can usually be started up again at any time without requiring maintenance or experiencing any loss of system pressure. Even well lubricated seals will leak a small amount of gas over time, particularly if the refrigeration gasses are soluble in the lubricating oil, but if the seals are well manufactured and maintained this loss is very low.

The disadvantage of hermetic compressors is that the motor drive cannot be repaired or maintained, and the entire compressor must be replaced if a motor fails. A further disadvantage is that burnt-out windings can contaminate the whole systems, thereby requiring the system to be entirely pumped down and the gas replaced (This can also happen in semi hermetic compressors where the motor operates in the refrigerant). Typically, hermetic compressors are used in low-cost factory-assembled consumer goods where the cost of repair and labor is high compared to the value of the device, and it would be more economical to just purchase a new device or compressor. Semi-hermetic compressors are used in mid-sized to large refrigeration and air conditioning systems, where it is cheaper to repair and/or refurbish the compressor compared to the price of a new one. A hermetic compressor is simpler and cheaper to build than a semi-hermetic or open compressor.