What's the Difference between Semi-Hermetic Compressors and Hermetic Compressors?

An Overlook of Hermetic Compressors

In the hermetic compressor, the motor and the compressor share the main shaft (reciprocating type), and the compressor unit is suspended in the housing by spring. The casing is divided into upper and lower parts. After the internal components are assembled, the upper and lower interfaces will be sealed by welding. The suction pipe, exhaust pipe, and process pipe are drawn out of the outer shell.

Figure 2: A Bristol hermetic compressor H23B223DBEA.

The power line of the motor is fixed on the wall of the shell through the sintering process of the terminal. The shell bottom is equipped with an elastic gasket, so the vibration and noise of the compressor can be greatly reduced through the coordination of the internal anti-vibration spring and the external rubber pad. From the perspective of structure, a hermetic compressor is mainly composed of a shell, a piston connecting rod group, a crankshaft, a cylinder, an air valve, and a lubrication system.

Pros of Hermetic Compressors

The refrigeration compressor and the motor are both installed in a fuse-welded or a brazed shell, sharing a spindle, which does not need the shaft seal device, and greatly reduces the size and weight of the whole compressor. Only suction and exhaust pipes, process pipes, and other necessary pipes (such as spraying pipes), input power terminals, and compressor brackets are welded outside.

Cons of Hermetic Compressors

Since the electric unit is placed in a sealed casing that cannot be opened, internal maintenance is difficult to perform. Therefore, it requires high reliability and long service life of the compressor, and high installation requirements for the entire refrigeration system. The hermetic compressor is generally used in mass production volume of the refrigeration compressor with a small capacity of refrigeration.

An Introduction to Semi-Hermetic Compressors

Figure 3: A semi-hermetic compressor.

Semi-hermetic refrigeration compressor adopts a structure of crankcase-integrated cylinder block. The motor shell is an extension of the crankcase to reduce the connection surface and ensure the concentricity between the compressor and motor. The crankcase and the motor room are connected by holes to facilitate the backflow of lubricating oil.

The main shaft of the semi-hermetic refrigeration compressor is a crank or an eccentric shaft. The motor inside is cooled by the air or water, or by inhaling the low-temperature working medium vapor. For the semi-hermetic compressor with a small power range, the lubricating oil is often supplied centrifugally. This lubrication mode is simple, but when the compressor power increases and the oil supply is insufficient, you should adopt the pressure lubrication method.

Semi-hermetic compressors are widely used in cold storage, refrigerated transportation, frozen processing, display cabinets, and refrigerators for home use and other fields.

Semi-Hermetic Compressor Advantages

Figure 4: A water-cooled semi-hermetic compressor.

Adapt to a wide range of pressure and cooling capacity requirements.

High thermal efficiency, less power consumption per unit, especially when the air valve makes the compressor deviate from the design operating condition.

Low requirements for materials that are usually ordinary steel materials. Easier processing and relatively low cost.

Mature technology and rich experience in production and use.

Relatively simple system.

Characteristics of Semi-Hermetic Screw Compressors

The semi-hermetic screw compressor is defined compared to the open screw compressor and the hermetic screw compressor. A screw compressor can also maintain good thermal performance and reliable operation in small-capacity models and adapts to harsh working conditions. With the rapid growth of the demand for air-cooled heat pump units in the field of air conditioners, screw compressors are rapidly developing to semi-hermetic and hermetic structures.

Structural Features

Rolling bearings

Figure 5: Rolling bearings.

Radial bearings are cylindrical type, thrust bearings are ball thrust type to bear the axial thrust of the rotor. The clearance of these rolling bearings is smaller than that of sliding bearings, which can keep the axle of the rotor stable, so as to reduce the rotor clearance as well as leakage losses.

Differential pressure oil supply

Using the difference between the exhaust pressure and the bearing pressure to supply oil, the compressor is not set up with the oil pump to simplify the lubricating oil supply system.

Oil mist eliminator

The inhaled gas passes through the motor first and is compressed and discharged in the cylinder after cooling the motor. The oil mist eliminator is set in the exhaust cylinder to separate the oil droplets from the gas. Therefore, there is no need to set another oil separator in the system, and the unit device has a compact structure.

Stepless adjustment of the gas transmission volume

Moving slide valve is used to adjust the gas flow of the compressor, which is better than the step adjustment of the cooling capacity of the reciprocating compressor.

Spraying cooling

When the exhaust temperature of the semi-hermetic screw compressor is high, the temperature of lubricating oil also rises, leading to the compressor motor burning down. Therefore, you can adopt the method of spraying the refrigerant to cool down the motor, so as to ensure normal working conditions.

Strengths

Compared with reciprocating compressors, screw compressors have many outstanding advantages. On the one hand, a screw rotor rotates and the speed can be increased. Therefore, among compressors with the same displacement, the screw compressor would be smaller, lighter, especially with no reciprocating inertia force during its operation, and low requirements for the foundation.

On the other hand, it is a simple structure with fewer parts. There are few wearing parts, especially it has no suction and exhaust valves and no expansion process with a large single-stage pressure ratio. Besides, the screw compressor is not sensitive to the liquid strike. Last not but least, it can adapt to a wide range of working conditions and is suitable as the main engine of a heat pump unit.

Summary

The semi-hermetic compressor covers less space than the hermetic compressor. In addition, it consumes less power that is energy-saving.

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