Fault Analysis of Working Principle of Screw Chiller Unit

Working Principle of Screw Chiller Unit
The screw chiller unit is a type of vapor compression refrigeration unit. Its refrigeration principle is that by applying energy to the refrigerant vapor through the compressor, its pressure and temperature are increased. Then, through the condensation and throttling processes, it becomes low-pressure and low-temperature refrigerant liquid. The refrigerant evaporates into vapor in the evaporator. At the same time, it obtains heat from the surrounding environment (secondary refrigerant, such as cold water) to reduce the temperature of the secondary refrigerant, thus achieving the purpose of artificial refrigeration. Thus, it can be seen that the vapor compression refrigeration cycle includes four essential processes: compression, condensation, throttling, and evaporation.
I. Compression process: After the refrigerant vapor in the evaporator is sucked in by the screw compressor, the prime mover (generally an electric motor) applies energy to it through the compressor impeller to increase the pressure of the refrigerant vapor and enter the condenser. At the same time, the temperature of the refrigerant vapor also increases correspondingly at the end of compression.
II. 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. At the same time, under the saturation pressure (the condensation pressure corresponding to the condensation temperature), it condenses into a liquid. At this time, the temperature of the cooling water will increase because it has absorbed heat from the refrigerant vapor. The temperature of the cooling water is directly related to the condensation temperature (condensation pressure).
III. Throttling process: The high-temperature and high-pressure refrigerant liquid from the bottom of the condenser undergoes decompression and expansion when flowing through the throttling device. The pressure and temperature both decrease and become low-pressure and low-temperature liquid and enter the evaporator.
IV. Evaporation process: The low-pressure and low-temperature refrigerant liquid absorbs heat from the secondary refrigerant (such as cold water) in the evaporator and then evaporates into gas. At the same time, the temperature of the secondary refrigerant is reduced, thus realizing artificial refrigeration. The refrigerant vapor in the evaporator is sucked in by the compressor again for compression. Repeat the above compression, condensation, throttling, and evaporation processes. In this way, it goes round and round to achieve the purpose of continuous refrigeration (heating). The cooling capacity is proportional to the suction flow of the compressor. The screw compressor is equipped with a slide valve mechanism inside it. It is used to control the suction flow of the compressor and also the evaporation amount of the refrigerant, so that the cooling capacity can be adjusted steplessly within a certain range.
After understanding the main working principle of the screw chiller, let's analyze the common fault problems of the screw chiller unit for your reference.
High pressure fault
The exhaust pressure of the compressor is too high, resulting in the action of the high-pressure protection relay. The exhaust pressure of the compressor reflects the condensation pressure. The normal value should be 1.40~1.60 MPa, and the protection value is set at 2.00 MPa.
If the pressure is too high for a long time, it will cause the compressor operating current to be too large, which is easy to burn the motor. It is also easy to damage the valve plate at the compressor exhaust port. What should be done naturally is to control the size of the compressor exhaust pressure within a safe range!
The reasons for high pressure faults are as follows:
The cooling water temperature is on the high side and the condensation effect is poor;
The cooling water flow is insufficient and cannot reach the rated water flow;
The condenser is fouled or blocked;
Too much refrigerant is charged;
There are non-condensable gases such as air and nitrogen mixed in the refrigerant;
False alarms caused by electrical faults.
Low pressure fault
The suction pressure of the compressor is too low, resulting in the action of the low-pressure protection relay. The suction pressure of the compressor reflects the evaporation pressure. The normal value should be 0.40~0.60 MPa, and the protection value is set at 0.20 MPa.
If the suction pressure is low, the return gas volume is small and the cooling capacity is insufficient, resulting in waste of electric energy. For the compressor motor cooled by return gas, poor heat dissipation is easy to damage the motor! The solution is the same as for high pressure faults. Try to keep the compressor within the normal pressure range.
The reasons for low pressure faults are as follows:
Insufficient refrigerant or leakage;
Insufficient flow of refrigerant water and less heat absorption;
False alarms caused by electrical faults;
The outside temperature is relatively low.

Low valve temperature fault
The outlet temperature of the expansion valve reflects the evaporation temperature and is a factor affecting heat exchange. Generally, there is a temperature difference of 5.0~6.0°C between it and the outlet temperature of the refrigerant water.
When a low valve temperature fault occurs, the compressor will stop. When the valve temperature rises back, it will automatically resume operation. The protection value is -2.0°C.
The reasons for low valve temperature faults are as follows:
A small amount of refrigerant leakage is 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 flow of refrigerant water or blockage of the evaporator;
False alarms caused by electrical faults, such as poor contact of the valve temperature wire.
Compressor overheating fault
There is a thermistor embedded in the motor winding of the compressor. The resistance value is generally 1 kΩ. When the winding is overheated, the resistance value will increase rapidly. When it exceeds 141 kΩ, the thermal protection module SSM will act to cut off the operation of the unit. At the same time, an overheating fault will be displayed and the fault indicator light will be on.
The reasons for compressor overheating faults are as follows:
The compressor load is too large and operates with overcurrent;
Overcurrent operation of the compressor caused by electrical faults;
The overheating protection module M is damp or damaged, the intermediate relay is damaged, and the contact is poor.
Communication fault
The computer controller controls each module through the communication line and the main interface board. The main reason for communication faults is poor contact or open circuit of the communication line.
In particular, poor contact caused by moisture oxidation of the interface. In addition, unit electronic board or main interface board failure, improper selection of address DIP switch, and power failure can all cause communication faults.