Troubleshooting of screw chillers

April 4, 2022
Latest company case about Troubleshooting of screw chillers

Working principle of screw chiller

 

The screw chiller is a type of vapor compression refrigeration unit. The refrigeration principle is that the compressor applies energy to the refrigerant vapor to increase its pressure and temperature, and then through the process of condensation and throttling, it becomes a low pressure, and the low-temperature refrigerant liquid evaporates into vapor in the evaporator, and at the same time from the evaporator. The surrounding environment (the refrigerant, such as cold water) obtains heat to reduce the temperature of the refrigerant, so as to achieve the purpose of artificial refrigeration. It can be seen that the vapor compression refrigeration cycle includes four essential processes including compression, condensation, throttling, and evaporation.

 

1. Compression process: After the refrigerant vapor in the evaporator is inhaled by the screw compressor, the prime mover (usually a motor) applies energy to it through the compressor impeller, so that the pressure of the refrigerant vapor increases and enters the condenser; at the same time , the temperature of the refrigerant vapor also increases accordingly at the end of compression..

 

2. Condensation process: The high-pressure and high-temperature refrigerant vapor from the compressor releases heat through the cooling water in the tube in the condenser, and the temperature drops. become liquid. At this time, the temperature of the cooling water will increase due to the heat absorbed from the refrigerant vapor. The temperature of the cooling water is directly related to the condensation temperature (condensation pressure).

 

3. Throttling process: When the high-temperature and high-pressure refrigerant liquid from the bottom of the condenser flows through the throttling device, it undergoes decompression and expansion, the pressure and temperature are reduced, and it becomes a low-pressure, low-temperature liquid into the evaporator.

 

4. Evaporation process: The low-pressure, low-temperature refrigerant liquid absorbs heat from the refrigerant (such as cold water) in the evaporator and evaporates into gas, and at the same time reduces the temperature of the refrigerant, thereby realizing artificial refrigeration and refrigeration in the evaporator. The agent vapor is inhaled by the compressor for compression, and the above-mentioned compression, condensation, throttling, and evaporation processes are repeated. This cycle is repeated to achieve the purpose of continuous cooling (heating). The cooling capacity is proportional to the suction flow of the compressor. The screw compressor is equipped with a slide valve mechanism inside, which is used to control the suction flow of the compressor and the evaporation of the refrigerant, so that the cooling capacity can be adjusted steplessly within a certain range.

 

High voltage fault

The compressor discharge pressure is too high, causing the high pressure protection relay to act. The compressor discharge pressure reflects the condensing pressure, the normal value should be 1.40~1.60MPa, and the protection value is set to 2.00MPa.

 

If the long-term pressure is too high, it will cause the compressor running current to be too large, which will easily burn the motor and cause damage to the valve plate of the compressor discharge port. What should be done is naturally to control the compressor discharge pressure within a safe range. Inside!

 

The reasons for the high voltage fault are as follows:

The cooling water temperature is too high and the condensation effect is poor;

The cooling water flow is insufficient and cannot reach the rated water flow;

Condenser scaling or clogging;

Excessive refrigerant charge;

The refrigerant is mixed with non-condensable gases such as air and nitrogen;

False alarms caused by electrical faults.

 

low voltage fault

The suction pressure of the compressor is too low, causing the low pressure protection relay to act. Compressor suction pressure reflects the evaporation pressure, the normal value should be 0.40 ~ 0.60MPa, the protection value is set to 0.20MPa.

 

The reasons for the low voltage failure are as follows:

Insufficient or leaking refrigerant;

Insufficient refrigerant water flow, less heat absorbed;

Electrical faults cause false alarms;

The outside temperature is low.

 

Low valve temperature fault

The outlet temperature of the expansion valve reflects the evaporation temperature, which is a factor affecting the heat exchange. Generally, the difference between it and the outlet temperature of the refrigerant water is 5.0~6.0°C.

When a low valve temperature fault occurs, the compressor will shut down. When the valve temperature rises, it will automatically resume operation with a protection value of -2.0°C.

 

The reasons for the low valve temperature failure are as follows

A small amount of refrigerant leaks, generally manifested as a low valve temperature fault rather than a low pressure fault;

The expansion valve is blocked or the opening degree is too small, and the system is not clean;

Insufficient refrigerant water flow or blocked evaporator;

False alarms caused by electrical faults, such as poor valve temperature wire contact.

 

compressor overheating failure

The thermistor is embedded in the compressor motor winding, and the resistance is generally 1kΩ. When the winding is overheated, the resistance value will increase rapidly. When it exceeds 141kΩ, the thermal protection module SSM will act to cut off the operation of the unit. At the same time, the overheating fault will be displayed, and the fault indicator will be on.

 

The reasons for compressor overheating failure are as follows:

Compressor load is too large, overcurrent operation;

Compressor overcurrent operation caused by electrical fault;

The overheat protection module M is damp or damaged, the intermediate relay is damaged, and the contacts are bad.

 

communication failure

The control of each module by the computer controller is realized through the communication line and the main interface board. The main reason for the communication failure is the poor contact or open circuit of the communication line.

In particular, the interface is wet and oxidized, resulting in poor contact. In addition, the unit electronic board or the main interface board is faulty, the address dial switch is improperly selected, and the power failure can cause communication failure.