Quick comparison: Application scenarios and Core Differences of Screw and Centrifugal Refrigeration Compressors
Among the core equipment of refrigeration units, screw refrigeration compressors and centrifugal refrigeration compressors are the two most widely used types of models. Among them, centrifugal refrigeration compressors are mainly suitable for large-scale central air conditioning systems and various industrial refrigeration scenarios. They feature high operational efficiency and are particularly capable of meeting the actual demands of large flow rates and continuous operation without interruption. In contrast, screw refrigeration compressors have been widely popularized in medium to large-scale refrigeration systems due to their core advantages such as stable operation, outstanding efficiency under partial load conditions, and convenient maintenance. Their application fields cover multiple industries including refrigeration and freezing, central air conditioning, and chemical engineering. The specific details are as follows.
I. Working Principle of Screw Refrigeration Compressor
Screw refrigeration compressors belong to the category of positive displacement compressors. It relies on a pair of intermeshing helical rotors (male rotor and female rotor) to achieve gas compression. One of the rotors is directly driven by the motor, while the other rotor is driven to rotate through the meshing between them. As the rotors keep rotating, the space between them (i.e., the volume between the teeth) gradually shrinks, thereby compressing the inhaled refrigerant gas.
Screw-type refrigeration compressors are further classified into single-screw and twin-screw types, among which twin-screw compressors are more common. In a twin-screw compressor, two screws with helical grooves rotate inside the compressor casing. By changing the volume between the screws and between the screws and the casing, the processes of refrigerant intake, compression and discharge are completed. During the compression process, lubricating oil is sprayed into the compression chamber, playing multiple roles such as sealing, cooling and lubrication.
Suction stage: When the screw rotates, a large cavity is formed at one end between the screws. This cavity serves as the suction port, allowing the refrigerant gas to be drawn into the compressor from the low-pressure side of the system (evaporator). As the screw continues to rotate, the gap between the screw at the suction end is gradually occupied by the screw at the other end, isolating the refrigerant gas from the suction port. At this point, the refrigerant gas is enclosed in a continuously shrinking space and moves towards the other end of the compressor as the screw rotates.
Compression stage: During the conveying process, as the space between the screws gradually decreases, the enclosed refrigerant gas is compressed, and the pressure and temperature increase accordingly. This compression process is carried out continuously. The special design of the screw enables the gas to be compressed without obvious pulsation.
Exhaust stage: When the screw rotates to a specific position, the compressed refrigerant gas reaches the high-pressure end of the compressor (exhaust port) and is discharged to the high-pressure side of the system (condenser). At this point, the gas pressure is sufficient to overcome the back pressure of the system and is smoothly discharged into the condenser.
Throughout the entire working process, lubricating oil will be sprayed into the compression chamber to form a dynamic seal, reducing the friction between the screws, removing the heat generated by compression, and simultaneously cooling the compressor to extend its service life.
Advantages: Smooth operation, low vibration, which can reduce the impact on the surrounding environment and lower equipment maintenance costs. It features high partial load efficiency and can maintain a high cooling efficiency even under significant load variations, effectively reducing operational energy consumption. Maintenance is simple and convenient. The replacement and repair of components are relatively easy, which can shorten the downtime of equipment and improve the utilization rate of equipment. The requirements for refrigerants are relatively low, and it can adapt to a variety of different types of refrigerants, increasing the flexibility of its application.
Ii. Working Principle of Centrifugal Refrigeration Compressor
Centrifugal refrigeration compressors mainly rely on the conversion of kinetic energy into pressure energy to compress the refrigerant. The refrigerant vapor is first drawn into the center area of the impeller through the compressor's air inlet, at which point the refrigerant is in a low-pressure state.
The impeller is a disc with curved blades. When the impeller rotates at high speed, the refrigerant vapor is carried in by the blades and thrown outward along the radial direction of the impeller. During this process, the refrigerant vapor gains high speed, its kinetic energy increases, and the pressure is relatively low. Subsequently, the refrigerant vapor leaves the outer edge of the impeller and enters the diffuser or guide vane area.
A diffuser is a channel with a gradually increasing cross-sectional area. According to the principles of fluid mechanics, it reduces the velocity of the refrigerant vapor and increases its pressure. Finally, the refrigerant vapor, whose pressure has significantly increased after passing through the diffuser, enters the volute.
The volute is a gradually widening spiral channel that further homogenizes the refrigerant pressure and guides the refrigerant to the compressor outlet, which is then transported to the condenser for the next refrigeration cycle. If a higher pressure ratio is required, a multi-stage compression method can be adopted. That is, the refrigerant vapor is compressed in one stage and then returns to the next impeller through the return vane to continue the compression in the next stage until the required total pressure ratio is reached.
Suction stage: The refrigerant vapor is drawn into the center area of the impeller through the compressor's air inlet and is in a low-pressure state. The impeller rotates at high speed, and its curved blades carry the refrigerant vapor in and throw it radially outward. The refrigerant vapor gains high speed, its kinetic energy increases, and the pressure is relatively low. The refrigerant vapor leaves the outer edge of the impeller and enters the diffuser or guide vane area. The cross-sectional area of the diffuser gradually increases, and the velocity of the refrigerant vapor is reduced and the pressure is increased by the principle of fluid mechanics.
Discharge stage: After passing through the diffuser, the refrigerant vapor with increased pressure enters the volute. The volute further homogenizes the refrigerant pressure and guides it to the compressor outlet, where it is transported to the condenser for subsequent refrigeration cycles. During multi-stage compression, the refrigerant vapor is compressed in one stage and then returns to the next impeller through the return vane for further compression.
Advantages: Large cooling capacity, capable of meeting the demands of large-scale refrigeration systems, suitable for applications with extremely high cooling requirements. High efficiency, achieving a high energy efficiency ratio and reducing energy consumption during full-load operation. It operates smoothly with low vibration, which can ensure the long-term stable operation of the equipment and reduce the occurrence rate of equipment failures. However, when operating at low loads, centrifugal refrigeration compressors may experience surge, which can cause damage to the equipment. Corresponding anti-surge measures need to be taken.
