Diagnosis and Troubleshooting of Refrigeration Compressor Failure

Figure 1: A refrigeration compressor.

The main content of the following is the failure diagnosis and troubleshooting related to the refrigeration compressor system. After reading the following content, you will know how to diagnose a refrigeration compressor failure.

Compressor Cannot Start Normally

When the refrigeration compressor cannot start normally, first check the power supply, followed by the wires, electrical appliances, the electric motor, and compressor protection devices. If there is no problem with these items, check whether the refrigeration compressor itself is faulty.

1. Power failure: such as low voltage, abnormal current. The voltage is usually required to be ±10% of the rated voltage. Too high or too low voltage both will cause the refrigeration compressor not to start normally.

2. Wiring failure: Check whether the wiring is loose, causing poor contact, or open circuit.

3. Failure of electrical components: such as contactors, relays, air switches, and transformers are sometimes not fully closed or damaged, causing the refrigeration compressor to fail to start normally. However, some electrical components have a start-up delay. It is normal for the compressor not to start when the delay is not over. Observe carefully during inspection to see if it is the case mentioned above.

4. The motor burned out: it can be judged by measuring the resistance with a multimeter.

Figure 2: A FLUKE multimeter.

5. The relevant protection is not fully reset or faulty.

Temperature protection: The thermostat is faulty due to inappropriate set value, improper position of the sensing bulb, leakage of the sensing bulb, etc., or the thermostat itself has not been reset.

Pressure controller failure: the high and low pressure setting values are inappropriate, or the controller is not fully reset. Sometimes the valve is not fully opened , which also causes the pressure controller not to be reset.

The thermal relay or its built-in thermal protection delay time has not expired or has not been fully reset.

6. Mechanical failure of the refrigeration compressor: After troubleshooting the above problems, if it cannot be started, it may be due to a mechanical failure of the refrigeration compressor itself, for example the compressor being damaged or stuck.

Compressor Starts and Stops Frequently

If the refrigeration compressor starts and stops frequently, it is first necessary to check whether the power supply is abnormal. Then consider the possible reasons from the systematic and mechanical aspects.

Figure 3: Schematic diagram of refrigeration system.

1. Low pressure control problem:
One is that the setting value of the low pressure controller is unreasonable, the interval is too small, and the start and stop values are too close. It reaches the start value in a short time after shutdown, resulting in frequent start and stop of the refrigeration compressor. If the interval set for the thermostat is too small, it will also cause the same problem.

Second, refrigerant leakage causes the pressure at the low-pressure end to be too low, so the system starts and stops frequently. Generally, if the pressures of the high and low pressure ends both are low, it is caused by refrigerant leakage.

The third is insufficient liquid supply, caused by the insufficient opening degree and blockage of the liquid supply pipeline valve or expansion valve, too small throttling element, filter blockage, etc. The pressure of the low pressure end drops rapidly after starting up, and the low pressure protection will be triggered soon. “A big horse pulling a small cart" (referring to problems such as the system load is too small, or the refrigeration compressor is too large) will also cause this problem.

2. High pressure is too high: There are many reasons, such as poor heat transfer caused by too dirty condenser, small condenser, non-condensable gas entering the system, too much refrigerant, high pressure valve (exhaust check valve, etc.) not fully opened, too thin high-pressure pipeline. These cause sudden pressure rise in a short period of time after starting up, and trigger high-pressure protection, making the refrigeration compressor start and stop frequently.

Figure 4: Refrigeration compressor pressure control action diagram.

3. Oil pressure failure: Generally, the oil pressure is too low, caused by the failure or blockage of the oil pump or the differential pressure oil supply system. At the same time, if there is too much refrigerant dissolved in the oil, the pressure drop will cause the refrigerant to boil and too many bubbles produced in the oil.

The unstable pumping of the oil pump will result in insufficient lubrication. Large oil pressure fluctuations cause frequent triggering of oil pressure differential protection and frequent start and stop of the unit. Sometimes this phenomenon will cause cavitation in the oil pump.

4. Motor overload: The system load is too large, and the motor is overloaded, resulting in motor overcurrent or overheating protection. The same situation will occur when the power supply voltage drops and the circuit breaker trips.

5. The oil temperature is too high: when the oil temperature is too high, the refrigeration compressor will not stop immediately, and usually there will be a delay of several minutes. The common reasons are insufficient cooling of the oil cooling system, oil deterioration, etc., and the excessive friction resistance of the refrigeration compressor itself causes the oil temperature to be too high.

6. Too much oil leakage: It is mainly caused by the refrigeration compressor itself. If the oil return pipe is closed or blocked, the oil separator is damaged or the design is poor, these will also cause the oil return problem of the system.

Compressor is Laborious to Start

Generally, it is caused by overload operation, or it may be caused by the circuit problem. The phase loss causes the refrigeration compressor to be overloaded, the refrigeration compressor is damaged, or the internal resistance of the compressor is too large.

Compressor Noise Problem

1. The refrigeration compressor is short of oil and poorly lubricated.

2. Pipeline related reasons: for example, no shock absorber is installed on the exhaust pipe of the piston compressor or it is installed improperly. Usually the shock absorber should be parallel to the crankshaft. Or the pipeline is too thin, the flow rate of the refrigerant is too fast, and the noise is loud.

Figure 5: Refrigeration compressor shock absorber tubes.

3. The refrigeration compressor is not well fixed or has a foundation problem. The weight of the foundation or base of a small compressor should be close to that of the compressor (to facilitate shock absorption). If its weight is too light, there will be a lot of noise.

4. Mechanical reasons, such as the cylinders, pumps, cylinder liners, crankcases of the piston compressors, or foreign objects in the crankcase, will also cause high noise.

5. The system has too much oil or liquid return. When liquid slugging happens, the piston compressors are in a more dangerous situation. Screw and scroll compressors have a certain ability to resist liquid slugging, but when liquid slugging occurs, the noise will be relatively loud and the compressor will be damaged, so it should be avoided as much as possible.

Compressor Efficiency is Low

1. The cooling capacity is too small, and the selection of the refrigeration compressor is improper. It may be also caused by the mechanical reasons of the compressor itself, such as poor sealing and relatively small pressure difference between high and low pressure, so there is no large working pressure difference established.

2. Insufficient refrigerant. When the refrigerant is insufficient, the refrigeration compressor cannot work at 100% load. The lack of refrigerant is the cause of the system, but it feels like the efficiency of the compressor is low.

3. The liquid supply is not smooth. Whether the expansion valve is too small or the filter is blocked, the suction volume will be insufficient, the gas delivery volume will not be effectively utilized, and the efficiency of the refrigeration compressor will be low.

4. Unsmooth air return will also make the refrigeration compressor low efficiency and it is mainly because the suction filter is clogged. The pressure difference between the low-pressure gauge on the compressor body and on the evaporator is relatively large, the gas delivery volume of the compressor is insufficient, and the efficiency is low.

Energy Regulation Mechanism Fails

Generally, it is caused by the failure of the oil system, such as low oil pressure, blockage of the oil system, mechanical failure of the oil pump, etc. The mechanical failure of the four-way valve and solenoid valve in the regulation mechanism or the electrical failure will also cause the failure of the energy regulation mechanism.

Figure 6: Energy regulation devices for the refrigeration system.

The Body Temperature of the Compressor is Too High

1. For example, it is common that the water pipe of a water-cooled piston compressor is blocked.

2. The refrigeration compressor itself has too much friction and too much heat produced.

3. The suction superheat is too high.

4. The gas suction capacity is too small. Mainly for semi-hermetic or fully hermetic compressors, their electric motor cannot be effectively cooled, resulting in excessive oil temperature or body temperature.

5. The compression ratio is too large, or the oil cooling effect is too poor when there is oil cooling. Excessive oil temperature will also cause the refrigeration compressor body to overheat. Therefore, many systems with relatively large system load changes are equipped with liquid spray cooling or hot gas bypass to ensure that the compressor can adapt to greatly changing working conditions.

Reverse Rotation When the Compressor is Stopped

The common reason is that the suction and discharge check valves of the compressor are stuck. Especially for screw compressors, the stuck suction and discharge check valves will make high-pressure gas leak through the compressor to the low-pressure chamber, causing the compressor to reverse.

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