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Screw compressor A screw compressor is also known as a rotary screw compressor. As early as the 1950s, oil-injected screw compressors were used in refrigeration systems. Thanks to their simple structure, few vulnerable components, the ability to operate under high pressure differences or pressure ratios with low exhaust temperatures, insensitivity to refrigerants containing large amounts of lubricating oil (a condition often referred to as wet stroke), and good capacity control capabilities, they quickly replaced large-capacity reciprocating compressors. Their use expanded further into the medium-capacity range, and they are now widely employed in various refrigeration applications such as freezing, refrigeration, air conditioning, and chemical processes. Screw heat pumps using it as a host have been used for heating and air conditioning since the early 1970s, and there are types based on air as a heat source, water-based heat pumps, heat recovery systems, and ice thermal storage systems. In the industrial sector, screw heat pumps are also used for heat recovery in order to save energy. 2 Centrifugal compressors A centrifugal compressor is a type of blade-driven compressor (i.e., a turbine compressor). In a centrifugal compressor, the high-speed rotating impeller exerts a centrifugal force on the gas, and the diffuser section applies a diffusing effect to the gas, thereby increasing the gas pressure. In the early days, this type of compressor went unnoticed because it was suitable only for applications involving low to medium pressures and high flow rates. With the development of the chemical industry and the construction of various large-scale chemical plants and refineries, centrifugal compressors have become key machines for compressing and transporting various gases used in chemical production, thus holding an extremely important position. As advancements in gas dynamics have led to increased efficiency in centrifugal compressors, and thanks to the development of key technologies such as high-pressure sealing, the manufacturing of impellers with small flow rates and narrow dimensions, and multi-oil- wedge bearings, a range of issues associated with the operation of centrifugal compressors at higher pressures and wider flow ranges have been resolved. This has greatly expanded the application scope of centrifugal compressors, allowing them to replace reciprocating compressors in many situations and thus **significantly broadening their range of uses. The 3-reciprocating piston compressor is the earliest type of compressor to have been developed. The wooden bellows invented in China in 1500 BC served as the prototype for the reciprocating piston compressor. At the end of the 18th century, Britain built the first industrial reciprocating piston air compressor. Labyrinth compressors appeared in the 1930s, followed by various oil-free lubrication compressors and diaphragm compressors. The dynamic structure that emerged in the 1950s significantly reduced the size of large reciprocating piston compressors and enabled multiple functions per unit. Piston compressors have a long history of use and are currently the most widely used type of compressor in China. Due to its wide pressure range, it can operate across a broad energy spectrum, and it boasts advantages such as high speed, multiple cylinders, adjustable energy output, high thermal efficiency, and suitability for various operating conditions. Its disadvantages include a complex structure, numerous wear-prone components, short maintenance intervals, sensitivity to wet strokes, pulse vibrations, and poor operational stability. The screw compressor is a new type of compression device. Compared with reciprocating compressors, its advantages are: ① It has a compact structure, small size, low floor space requirement, and light weight. ②It has high thermal efficiency, requires fewer components, and the total number of parts in the compressor is only 1/10 that of a piston-type compressor. It has few vulnerable parts, operates safely and reliably, and is simple to operate and maintain. ③The gas does not pulsate, and the operation is smooth; the unit does not require a special foundation as it does not sit high above the base. ④ Oil is injected into the rotor chamber during operation, which results in a low exhaust temperature. ⑤It is not sensitive to wet conditions; the entry of wet steam or a small amount of liquid into the machine poses no risk of liquid slugging. ⑥It can operate at a higher pressure ratio. ⑦The effective compression stroke can be adjusted using a slide valve, enabling stepless cooling capacity control from 10% to 100%. Disadvantages: It requires complex oil treatment equipment, including oil separators and oil coolers with high separation efficiency; it generates significant noise, usually above 85 decibels, thus sound insulation measures are necessary. Compared to piston-type systems, centrifugal types feature higher rotational speeds, larger air handling capacity, less mechanical wear, fewer vulnerable components, simpler maintenance, longer continuous operation time, lower vibration, smoother operation, and lower requirements for the foundation. At high air volumes, units with a given power output are lighter in weight, smaller in size, and require less space. The air handling capacity can be adjusted continuously within a range of 30% to 100%, making it easy to implement multi-stage compression and throttling. These systems can meet the requirements of certain chemical processing processes, and they are easy to automate. For large-scale applications, industrial steam turbines with high cost efficiency can be used for driving them, which offers economic advantages for enterprises that have waste heat steam available. The disadvantages are: high noise frequency, high consumption of cooling water, and surge can occur if operated improperly. A comparison of the performance characteristics of three common compression refrigeration machines (reciprocating, screw, and centrifugal) is presented. In refrigeration systems, due to the differences in their working principles, reciprocating refrigeration compressors and screw refrigeration compressors have different factors that affect their pressure losses and leakage losses. For reciprocating refrigeration compressors, the main factors affecting their pressure loss and leakage loss are the quality of the gas valves and their sealing performance when closed. This is because when the suction valve opens, it must overcome the resistance of the spring (the compressed spring); moreover, as the gas flows through the valve, the relatively small cross-sectional area results in a high flow velocity, which in turn creates certain flow resistance. As a result, during the suction process in a reciprocating refrigeration compressor, the pressure of the gas inside the cylinder remains always lower than the pressure of the gas in the suction pipe ； Similarly, in a reciprocating refrigeration compressor, the pressure of the gas inside the cylinder remains higher than the pressure of the gas in the suction line during the exhaust process. The smaller the cross-sectional area of the valve passage, the greater the resistance loss. If the weight of the valve disc is large and the spring force of the air valve is also high, the resistance loss increases, which in turn reduces the pressure coefficient value. For screw refrigeration compressors, the main factor affecting their pressure loss and leakage loss is the gas flow velocity. In screw refrigeration compressors, the performance of the screw is crucial. If the tooth profile of the screw is a symmetrical arc shape, then it is simple to manufacture. If the tooth profile of the screw is asymmetric, it has a large air delivery capacity and high efficiency. By reducing the length-to-diameter ratio of the screw, it is possible to achieve good strength in the screw, increase the reliability of operation of the screw-type refrigeration compressor, and facilitate the development of such compressors for higher pressure ratios. In screw refrigeration compressors, among the two pairs of screws with identical diameter and length, the pair with a higher rotor area utilization factor has a larger exhaust volume. On the surface, the larger the rotor area utilization factor, the better the performance of the screw refrigeration compressor. However, if the rotor area utilization factor is too high, it will reduce the strength and stiffness of the screw. In screw refrigeration compressors, reducing the number of teeth on the screws increases the area between the teeth, thereby enhancing the discharge volume of the screw refrigeration compressor. On the surface, the fewer teeth on the screw, the better the performance of the screw-type refrigeration compressor. However, if the number of teeth on the screw is too low, it will reduce the screw’s bending strength and stiffness. In screw refrigeration compressors, increasing the circumferential speed of the screws allows for a reduction in the external dimensions and weight of these compressors. This reduces the relative leakage of gas through the gaps in the compressor, thereby improving its volumetric efficiency and thermal efficiency. On the surface, the higher the circumferential speed of the screw, the better the performance of the screw-type refrigeration compressor. However, if the circumferential speed of the screw is too high, it will correspondingly increase the flow losses of the gas at the suction and exhaust ports as well as between the teeth. Among the three common types of refrigeration compressors (reciprocating, screw, and centrifugal), the inertia generated by reciprocating motion is the main drawback of reciprocating refrigeration compressors. Due to the inertia generated by the back-and-forth motion, the gas valves and crank-slider mechanism in reciprocating refrigeration compressors are most prone to damage. Among the three common types of refrigeration compressors (reciprocating, screw, and centrifugal), the high noise generated during operation is the main drawback of screw refrigeration compressors. Due to the effects caused by the periodic high-speed flow of refrigerant gas through the suction and discharge ports, as well as leakage through gaps, it is necessary to select an appropriate screw rotation speed in screw-type refrigeration compressors. Among the three common types of refrigeration compressors (reciprocating, screw, and centrifugal), surge is the main drawback of centrifugal refrigeration compressors. The cause of surge in centrifugal refrigeration compressors is that when the flow rate of cooling water into the condenser decreases to a certain level, the flow rate of the centrifugal refrigeration compressor drops significantly, leading to severe gas backflow within its passages and a sudden drop in its outlet pressure. Although the centrifugal refrigeration compressor and the condenser operate together, the pressure of the gas in the condenser does not decrease simultaneously; as a result, the pressure of the gas in the condenser becomes higher than the outlet pressure of the centrifugal refrigeration compressor. This causes the gas in the condenser to flow back into the centrifugal refrigeration compressor until the pressure there drops to equal the outlet pressure of the compressor. Thus, the centrifugal refrigeration compressor begins to send air to the condenser again; the flow rate increases, and the centrifugal refrigeration compressor returns to normal operation. However, when the gas pressure in the condenser also returns to its original level, the flow rate of the centrifugal refrigeration compressor decreases again, the outlet pressure of the centrifugal refrigeration compressor starts to drop, and the gas flows back in reverse. This repeats over and over, resulting in oscillatory phenomena of periodic air currents. Therefore, in centrifugal refrigeration compressors, the amount of cooling water used in the condenser should not be too low; otherwise, it can cause severe vibrations during operation of the compressor, and in serious cases, it may even lead to damage to the compressor.