The Difference between Hermetic and Semi-Hermetic Compressors

Figure 1: Semi-hermetic compressor.

The refrigeration compressor is the primary equipment in the refrigeration system. It converts electrical energy into mechanical energy, compresses low-temperature and low-pressure refrigerant gas into high-temperature and high-pressure gas, and ensures the cycle of refrigeration.

In a vapor compression refrigeration system, the pressure of the refrigerant is raised from a low pressure to a high pressure and it circulates continuously in the refrigeration pipe, allowing the system to continuously dissipate internal heat to the environment.

1. Types of Compressor

Compressors can be divided into open, semi-hermetic and hermetic compressors according to the sealing form.

Open Compressor: The main shaft of the open type compressor extends out of the crankcase and is driven by the electric motor through a pulley or a coupling. Therefore, a shaft seal must be installed to prevent the refrigerant from leaking outward. However, if the sealing surface of the shaft seal is worn, it will cause refrigerant leakage and increase the difficulty of maintenance.

In order to overcome these shortcomings, a semi-hermetic compressor is manufactured, which is most suitable for use in small and medium-sized refrigeration devices.

Figure 2: Open compressor.

Semi-Hermetic Compressor: The compressor and the electric motor share the same shaft, so there is no need for a shaft seal device, and refrigerant leakage at the shaft seal is avoided.

Semi-hermetic refrigeration compressors can be detachable during maintenance as open type compressors, and their sealing surfaces are connected by flanges and sealed with gaskets or washers. Although these sealing surfaces are static sealing surfaces, leakage will inevitably occur, so they are called semi-hermetic compressors.

Hermetic Compressor: The characteristic of the hermetic compressor is that the compressor and the motor are assembled together in a closed casing. From the surface, there are only compressor inlet and outlet pipes and motor leads.

Simply put,
·Hermetic compressor: The motor and compressor are installed in a sealed chamber.
·Semi-hermetic compressor: The motor and compressor are installed in the same chamber, and their sealing surfaces are fixed by flanges.
·Open compressor: The compressor and motor are not placed in one chamber.

2. Differences between Hermetic and Semi-Hermetic Compressors

The difference between the hermetic compressor and the semi-hermetic compressor mainly includes the following aspects.

Figure 3: Semi-hermetic refrigeration compressor.

2.1 Maintenance

Since the compressor and electric motor unit of the hermetic compressor are installed in a sealed casing that cannot be disassembled, it is not easy to open for internal repairs. When the hermetic compressor motor burns out or other problems occur, the only solution is to replace the entire compressor. However, the semi-hermetic compressor is detachable and can be repaired.

2.2 Leakage

The shell contact surface of the hermetic compressor is welded. Only the suction and discharge pipes, process pipes and other necessary pipes, input power terminals and compressor brackets are exposed on the outside of the shell. The probability of refrigerant leakage is extremely low.
While the refrigerant leakage problem is more common in semi-hermetic compressors.

2.3 Application

Hermetic compressors are mainly used in areas with small cooling capacity requirements, such as household refrigeration systems (air conditioners, refrigerators), and semi-hermetic compressors are mainly used in areas with larger cooling capacity requirements, such as commercial and industrial applications (large cold storage, central air conditioning) .

2.4 Cost

Hermetic compressors are cheaper than semi-hermetic compressors. But this is not the decisive factor in deciding which compressor to choose.
When the hermetic compressor needs repairing, usually replace the compressor. In this way, the maintenance cost is relatively high. If a semi-hermetic compressor fails internally, it can be repaired, and the repair cost is lower than that of a fully hermetic compressor.

2.5 Other Differences

The compression parts of the hermetic compressor and the motor are installed in the same shell, share a main shaft, and the two are assembled in a welded airtight shell. This not only cancels the shaft sealing device, but also greatly reduces the size and weight of the entire compressor. This type of compressor has a more compact structure and more excellent sealing.

The hermetic compressor has a lower cooling capacity than the semi-hermetic compressor, and is generally used for working equipment with a small cooling capacity. It works with less noise.
Compared with the hermetic compressor, the semi-hermetic compressor has a larger cooling capacity and louder working sound, and is generally used for equipment with a relatively large cooling capacity requirement.

3. Which is Better, Hermetic Compressor vs. Semi-Hermetic Compressor?

Pros and cons of hermetic refrigeration compressors

Figure 4: Hermetic refrigeration compressor.

Advantage:
The hermetic refrigeration compressor has good sealing performance, small volume and compact structure.

Disadvantage:
It is not easy to open for internal repairs, so this type of refrigeration compressor is required to have high reliability, long life, and has high installation requirements for the entire refrigeration system.

Pros and cons of semi-hermetic refrigeration compressors
Advantage:
When a problem occurs, it can be disassembled for maintenance, and the maintenance cost is low; it can adapt to a wider pressure range and cooling capacity requirements.

Disadvantage:
There is a problem of refrigerant leakage.

Figure 5: Semi-hermetic refrigeration compressor.

Conclusion: Once the fully hermetic compressor is broken, it can only be replaced, and the maintenance cost is high. The semi-hermetic compressor is easy to repair and has low maintenance cost.
It is recommended to choose according to the needs. The fully enclosed structure is used in small cooling capacity refrigeration compressors. At present, semi-hermetic compressors are mainly occupying the small and medium cooling capacity refrigeration market, and are the most widely used and the largest production volume models used for refrigeration machines.

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