Working principle of water-cooled screw chiller

In the air conditioning systems of modern industrial production and large commercial buildings, water-cooled screw chillers play a vital role, providing efficient and stable refrigeration services for various environments. To deeply understand its working principle, it is necessary to analyze it from two major aspects: its system composition and operation process.

System Configuration

The water-cooled screw chiller is mainly composed of several key parts such as screw compressor, condenser, evaporator, expansion valve and control system.

Screw compressor: As the heart of the entire refrigeration system, the screw compressor takes on the important task of sucking in low-temperature and low-pressure refrigerant gas and compressing it into high-temperature and high-pressure gas. It works through a pair of intermeshing spiral rotors, and the tooth grooves of the rotors and the inner wall of the casing form multiple closed working chambers. As the rotor rotates, the volume of the working chamber changes continuously, thereby realizing the process of sucking, compressing and discharging the refrigerant gas. The screw compressor has the advantages of compact structure, stable operation, large gas transmission capacity and adaptability to various working conditions.

Condenser: After the compressor compresses the refrigerant into high-temperature and high-pressure gas, the gas enters the condenser. The condenser is a heat exchange device that cools and condenses the high-temperature and high-pressure refrigerant gas into a high-pressure liquid. In water-cooled screw chillers, water is usually used as the cooling medium. The high-temperature and high-pressure refrigerant gas flows in the tubes of the condenser, while the cooling water flows outside the tubes, exchanging heat through the tube walls. The refrigerant gas transfers heat to the cooling water, its own temperature decreases and gradually condenses into a liquid, while the cooling water absorbs the heat and its temperature rises, and is then discharged to the cooling tower for heat dissipation and cooling.

Evaporator: After the high-pressure refrigerant liquid is condensed by the condenser, it enters the evaporator after being throttled and reduced in pressure by the expansion valve. The evaporator is also a heat exchange device, where the refrigerant liquid absorbs the heat of the cooled medium (such as chilled water in the air conditioning system) to achieve the cooling effect. The refrigerant evaporates into a low-temperature and low-pressure gas in the evaporator, and the temperature of the cooled medium is reduced to achieve the purpose of cooling. For example, in the air conditioning system, the chilled water flows in the tube of the evaporator, and the refrigerant evaporates outside the tube, and heat is exchanged through the tube wall. After the chilled water transfers the heat to the refrigerant, the temperature is reduced, and then it is transported to various areas that need cooling through the circulation pump.

Expansion valve: The expansion valve is installed between the condenser and the evaporator. Its main function is to throttle and reduce the pressure of the high-pressure refrigerant liquid coming out of the condenser. After the high-pressure refrigerant liquid passes through the expansion valve, the pressure drops sharply, part of the liquid vaporizes, and the temperature also drops, turning into a low-temperature and low-pressure gas-liquid mixed state, and then enters the evaporator for evaporative cooling. The expansion valve can also automatically adjust the flow of the refrigerant according to the load changes of the evaporator to ensure the normal operation of the evaporator and the stability of the cooling effect.

Control system: The control system is like the brain of the chiller. It monitors and controls the operating parameters of the entire unit in real time, including temperature, pressure, flow, etc. Through the precise control of these parameters, it ensures that the unit can operate efficiently, stably and safely under various working conditions. For example, when the system detects that the temperature of the chilled water at the outlet of the evaporator rises, the control system will automatically adjust the opening of the expansion valve and increase the flow of the refrigerant to increase the cooling capacity and restore the chilled water temperature to the set value. At the same time, the control system also has fault diagnosis and alarm functions. When the unit has an abnormal situation, it can issue an alarm in time and take corresponding protective measures to avoid equipment damage.

Operation process

Compression process: After the screw compressor is started, it sucks in low-temperature and low-pressure refrigerant gas from the evaporator. As the compressor rotor rotates, the volume of the working chamber gradually increases, forming a negative pressure, and the refrigerant gas is sucked into the working chamber. When the working chamber is filled with refrigerant gas, the rotor continues to rotate, and the volume of the working chamber gradually decreases, compressing the refrigerant gas, causing its pressure and temperature to continue to rise, becoming a high-temperature and high-pressure refrigerant gas and being discharged from the compressor.

Condensation process: After the high-temperature and high-pressure refrigerant gas is discharged from the compressor, it enters the condenser. In the condenser, the refrigerant gas exchanges heat with the cooling water flowing outside the tube, transferring heat to the cooling water. Since the temperature of the refrigerant gas is higher than the temperature of the cooling water, the heat is transferred from the refrigerant gas side to the cooling water side, and the refrigerant gas gradually cools and condenses into a high-pressure liquid. After absorbing the heat, the cooling water rises in temperature and is transported to the cooling tower for heat dissipation and cooling. The cooled water returns to the condenser for recycling.

Throttling process: After the high-pressure refrigerant liquid is condensed by the condenser, it is throttled and depressurized by the expansion valve. The throttling effect of the expansion valve causes the pressure of the refrigerant liquid to drop instantly. Part of the refrigerant liquid absorbs its own heat and vaporizes during the depressurization process, forming a low-temperature and low-pressure gas-liquid mixed state. This process is an isenthalpic process. The enthalpy value of the refrigerant remains basically unchanged, but the pressure and temperature are significantly reduced.

Evaporation process: After the low-temperature and low-pressure gas-liquid mixed refrigerant enters the evaporator, it absorbs the heat of the cooled medium (such as chilled water) in the evaporator. Due to the low pressure in the evaporator, the refrigerant liquid evaporates rapidly in this low-pressure environment, changing from liquid to gas. During the evaporation process, the refrigerant absorbs a large amount of heat, which reduces the temperature of the cooled medium, thereby achieving a refrigeration effect. The evaporated low-temperature and low-pressure refrigerant gas is sucked into the screw compressor again, starting a new round of refrigeration cycle.

The water-cooled screw chiller realizes the state change of the refrigerant in the refrigeration cycle through the coordinated work of the main components such as the compressor, condenser, expansion valve and evaporator, as well as the precise regulation of the control system, thereby transferring heat from the cooled medium to achieve the purpose of refrigeration and provide reliable low-temperature environment protection for many fields.