The small hermetic compressors in ordinary household refrigerators or freezers usually have a permanently welded circular steel shell, which can seal the working gas inside the system. There is no way to leak gas, such as around the motor shaft seal. In this model, the plastic top is part of an automatic defrost system that uses the heat of the electric motor to evaporate water.
Compressors used in refrigeration systems are usually described as fully enclosed, open or semi-enclosed to describe the position of the compressor and motor drive relative to the compressed gas or vapor. The industrial name of hermetic is hermetic compressor, and semi-hermetic is usually called semi-hermetic compressor.
In hermetic and most semi-hermetic compressors, the compressor and the motor driving the compressor are integrated and run in the pressurized gas envelope of the system. The motor is designed to operate in and be cooled by refrigerant gas.
The difference between airtightness and semi-airtightness is that the airtightness uses an integrated welded steel shell, which cannot be opened for maintenance. If the seal fails, just replace it with a brand new unit. The semi-hermetic uses a large cast metal housing with a screwed sealing cover that can be opened to replace the motor and compressor components.
The main advantage of airtight and semi-airtight is that there is no way for gas to leak out of the system. Open compressors rely on shaft seals to maintain internal pressure. These seals require lubricants (such as oil) to maintain their sealing performance.
Open pressurized systems such as car air conditioners are more likely to leak their working gas. An open system relies on the lubricant in the system being splashed onto the pump components and seals. If it is not operated frequently enough, the lubricant on the gasket will slowly evaporate, and then the gasket will begin to leak until the system no longer functions and must be recharged. In contrast, closed or semi-closed systems can be idle for many years and can usually be restarted at any time without maintenance or any system pressure loss.
The disadvantage of hermetic compressors is that the motor drive cannot be repaired or maintained. If the motor fails, the entire compressor must be replaced. Another disadvantage is that the burned-out winding will contaminate the entire system, which requires the system to be completely evacuated and replaced with gas (this may also occur in semi-hermetic compressors where the motor runs in refrigerant). Generally, hermetic compressors are used in consumer goods assembled in low-cost factories. Compared with the value of the equipment, the repair and labor costs are higher, and it is more economical to only purchase new equipment or compressors. Semi-hermetic compressors are used in medium to large-scale refrigeration and air-conditioning systems, where it is cheaper to repair the compressor than to buy and install a new compressor. Hermetic compressors are simpler and cheaper than semi-hermetic compressors.
The advantage of open compressors is that they can be driven by non-power sources, such as internal combustion engines or turbines. However, the open compressor that drives the refrigeration system is generally not completely maintenance-free during the entire life cycle of the system, because some gas leakage occurs over time.