How Compressor Works in Refrigeration

 

1. Function of Compressor

In the vapor compression refrigeration system, compressor is used to raise the pressure of refrigerant and to keep it continuously recycling so that interior heat will discharge out of system to environment where temperature is higher than system. Compressor is virtually the heart of refrigeration. System inputs power through compressor to drive refrigerant flowing in the system, then low-pressure steam is compressed into high-pressure with heat going from low temperature to high temperature. So a cycle finishes. EER (energy efficiency ratio) of refrigeration depends on the EER of compressor. Function of compressor comes down to: provide impetus for refrigerant flow, compress refrigerant steam and raise pressure and temperature.

 

Below is the cycle reflection if pressure enthalpy diagram, number 1 and 2 stands for compressing stage of refrigerant.

 

 

Variation of refrigerant in compressor:

Refrigerant steam enters compressor air suction port from evaporator end, during which the higher pressure, the higher temperature and vice versa. Steam in the compressor then is compressed into overheating steam, with pressure rising up to condensation pressure Pk from evaporation pressure P0. This is called adiabatic compression. External energy acts on refrigerant to further raise steam temperature. Thus, steam temperature exhausted from compressor is higher than condensation temperature.

 

2. Classification of Compressor

Based on working principle, compressor falls into two categories: volume type and velocity type, each of which has many models. Shini Water Chillers adopts scroll compressor, which we are going to introduce with advantages as below:

• Free of reciprocating mechanism, simple structure, compact size, light weight and few components.

• Small change of torque, high equilibrium, few vibrations make a stable running, easy to operate and to realize automation.

• High efficiency in a suitable refrigeration capacity.

• Low noise level.

 

3. Connection of Compressor

 

 

During the pipe welding, avoid overheating compressor housing in case inner components would be badly damaged. It is therefore recommended to adopt heat shield or heat absorption compound. As for the big diameter of pipes and joints, it is recommended to adopt dual-nozzle acetylene welding gun to conduct welds:

• Uniformly heat A spot until welding temperature is achieved. Then move welding gun to B spot to heat uniformly until welding temperature is achieved, next begin to use welding flux.

• Move welding gun to the joint evenly. And do not use too much flux, just uniformly cover interface circle.

• Move welding gun to C spot, gun should not stay here too long in case flux comes into compressor.

• After finishing interface welding, use wire brush or wet cloth to clean residual flux in case of pipe erosion. Make sure no flux entering pipe or compressor. Welding flux is acidic, which may cause major damage to system or interior compressor.

 

4. Cautions for Compressor Operation

Compressor Safe Operation Area

Every compressor has one safe operation range as shown in the picture below. Blue range inside the black box is safety area. If temperature exceeds this area, compressor is highly likely to damage (safety area varies based on different compressor models).

 

 

Example 1: R410A, degree of superheat 11k, low pressure LP1=3.3bar (-15.5°C) and high pressure HP1=38bar (62°C). It is clearly that cross-point between evaporation and condensation temperature exceeds the safety range, which means compressor will be in under damage.

 

Example 2: R410A, degree of superheat 11k, low pressure LP2=4.6 bar (-10.5°C) and high pressure HP2=31bar (52°C). Pressure range does not exceed the safety range so the system is under secure service.

 

In addition, as shown in picture above, evaporating temperature can not exceed 15°C otherwise it will go beyond safety range.

 

Poor Lubrication of Compressor

Damages of compressor derive from poor system design and poor environment. Poor lubrication in compressor will cause bearing abrasion or seizure of scroll due to lack of oil, as well as abnormal operation inside device, finally to the detriment of compressor performance.

 

 

Below are major causes of poor compressor lubrication:

Return of Liquid

Abrupt or continuous return of liquid is caused by bad reaction of expansion valve or system load sudden changes. We provide an oil dilution table for you judge the abnormality. Horizontal axis is evaporating temperature and vertical axis oil groove temperature. If the cross-point between evaporating and oil groove temperature exceeds safety range, compressor damage would taka place.

 

Poor Pipeline Design

A poorly designed pipeline would fail lubrication oil to flow with refrigerant after draining out of compressor or make lubrication oil abound in condenser or evaporator, hindering oil return to device therefore lubrication oil in groove become less.

 

Lack of Lubrication Oil

Leakage of refrigeration system would exhaust a part of lubrication oil, causing a shortage of oil supply.

 

Lubricating Oil Goes Bad or Polluted

This phenomenon is mainly due to poor cleanness inside system or long term compressor running in a severe environment.

 

5. Maintenance of Compressor

It is a good way to fill liquid refrigerant to high pressure side and fill gas to low pressure side. For any compressor, it is not recommended to fill liquid refrigerant from steel bottle to compressor crankcase. Do not start compressor when system is in high vacuum state due to the possible interior stretching. Do not operate compressor until system reaches the air suction pressure at least 0.05Mpa (gage pressure) (7psig). Also do not operate compressor when low pressure controller is short circuited. Keeping lower air suction pressure to 0.05Mpa (gage pressure) (7psig) for only a few seconds would cause scroll superheat and damage driving bearing. If compressor motor burnouts and need to be replaced, most of polluted oil will be extracted along with broken compressor. The rest of oil would be cleaned through using drying filter of air suction and liquid pipeline. It is recommended to adopt 100% activated aluminum air suction drying filter, which should be dismantled after 72 hours. Replace gas-liquid separator if there is one in system because during the short time of compressor damage, return oil port or screen in the separator may be blocked by scraps. And this will cause compressor oil shortage then make compressor damaged again.