Analyzing the Advantages and Disadvantages of 5 Types of Compressors in Cold Chain Technology

There are mainly five types of compressors: reciprocating compressor, rolling rotor compressor, turbo compressor, screw compressor, and centrifugal compressor. This article briefly introduces the advantages and disadvantages of various compressors.
1. Reciprocating Compressor
(1) Regardless of the flow rate, the required pressure can be obtained. The exhaust pressure range is wide, and the maximum pressure can reach 320MPa (in industrial applications), and even 700MPa (in laboratories).
(2) The single-unit capacity can be any flow rate below 500m³/min.
(3) In the general pressure range, the requirements for materials are low. Common steel materials are mostly used, which are relatively easy to process and have a lower cost.
(4) The thermal efficiency is relatively high. Generally, the adiabatic efficiency of large and medium-sized units can reach about 0.7 to 0.85.
(5) When adjusting the gas volume, it has strong adaptability, that is, the exhaust range is relatively wide, and it is not affected by the pressure level, and can adapt to a wide pressure range and cooling capacity requirements.
(6) The specific gravity and characteristics of the gas have little impact on the working performance of the compressor. The same compressor can be used for different gases.
(7) The drive is relatively simple, mostly using an electric motor. Generally, it does not need speed regulation, and it has strong maintainability.
(8) The technology of reciprocating compressors is relatively mature, and rich experience has been accumulated in production and use.
Disadvantages
(1) The structure is complex and bulky, there are many vulnerable parts, it occupies a large area, the investment is high, the maintenance workload is large, and the service life is relatively short. However, it can reach more than 8000 hours through efforts.
(2) The rotational speed is not high, the machine is large and heavy, and the single-unit exhaust volume is generally less than 500m³/min.
(3) There is vibration during the operation of the machine.
(4) The exhaust is discontinuous, and the gas flow has pulsation, which is likely to cause pipeline vibration. In severe cases, the pipeline network or machine parts are often damaged due to gas flow pulsation and resonance.
(5) The flow regulation adopts an auxiliary volume or a bypass valve. Although it is simple, convenient and reliable, the power loss is large, and the efficiency decreases during partial load operation.

(6) For compressors lubricated with oil, the oil carried in the gas needs to be removed.
(7) When multiple compressor units are used in a large factory, there are many operators or the work intensity is relatively high.
2. Rolling Rotor Compressor
The rolling rotor compressor is a positive displacement rotary compressor. The change of the working volume of the cylinder is realized by the rolling of a cylindrical rotor with an eccentric device in the cylinder.
(1) When the rotor rotates one revolution, it will complete the compression and exhaust process of the previous working cycle and the suction process of the next working cycle.
(2) Since there is no intake valve, the timing of the start of suction has a strict corresponding relationship with the position of the intake port on the cylinder, and it does not change with the change of working conditions.
(3) Since an exhaust valve is set, the timing of the end of compression will change with the change of the pressure in the exhaust pipe.
Advantages
This type of compressor is now also widely used in household refrigerators and air conditioners. Its advantages are:
(1) The structure is simple, the volume is small, and the weight is light. Compared with the reciprocating compressor, the volume can be reduced by 40% to 50%, and the weight can also be reduced by 40% to 50%.
(2) There are few components, especially few vulnerable parts. At the same time, the friction loss between the relative moving parts is small, so the reliability is relatively high.
(3) Only the sliding vane has a small reciprocating inertia force, and the rotational inertia force can be completely balanced. The rotational speed can be relatively high, and the vibration is small, and the operation is stable.
(4) There is no intake valve, the suction time is long, the clearance volume is small, and the direct suction reduces the harmful superheat of suction, so its efficiency is high. However, the requirements for processing and assembly accuracy are high. Since there is no gas valve, it can be used to transport dirty process gases with liquid droplets and dust.
Disadvantages
The main disadvantages of the rotary compressor are the large leakage, friction and wear between the sliding vane and the cylinder wall, which limit its service life and the improvement of efficiency; and the processing accuracy requirements of this compressor are relatively high. If a double-layer sliding vane is used, the ends of the two sliding vanes are in contact with the inner wall of the cylinder during operation, forming two sealing lines, and an oil seal is formed between the two sealing lines, which greatly reduces the leakage loss at the end of the sliding vane, reduces the friction force and friction loss, and improves the service life and efficiency of the machine. Once wear occurs at its bearings, main shaft, rollers or sliding vanes, and the clearance increases, it will immediately have a relatively obvious adverse impact on its performance. Therefore, it is usually used in factory-assembled refrigerators and air conditioners, and the system also requires a high degree of cleanliness.
3. Scroll Compressor
The main components of the scroll compressor are two involute scroll discs with the same shape but with an angular phase position offset by 180°. One is a fixed scroll disc, and the other is driven by an eccentric shaft, and its axis revolves around the axis of the fixed scroll disc. During operation, the two scroll discs are tangent at multiple points to form a sealing line. Coupled with the appropriate sealing at the end faces of the two scroll discs, several crescent-shaped air chambers are formed. The sealing line at the common tangent point between the two scroll discs continuously moves along the scroll curve with the revolution of the orbiting scroll disc, so that the shape and size of these crescent-shaped air chambers are constantly changing. The suction port of the compressor is opened at the upper part of the fixed scroll disc shell. When the eccentric shaft rotates clockwise, the gas enters the suction chamber from the suction port and is successively drawn into the crescent-shaped air chambers on the periphery that communicate with the suction chamber. As these peripheral crescent-shaped air chambers close and no longer communicate with the suction chamber, their closed volume is gradually transferred towards the center of the fixed scroll disc and continuously decreases, and the gas is continuously compressed and the pressure rises.
Advantages
From the perspective of the specific structure, the scroll compressor has no intake and exhaust valves, which greatly improves the reliability of high-speed operation.
Overall, the scroll compressor has the following main characteristics:
(1) It belongs to the third-generation compressor. Multiple compression chambers work simultaneously. The gas pressure difference between adjacent compression chambers is small, the gas leakage is low, and the volumetric efficiency is high, which can reach 98%, about 5% higher than that of the second-generation compressor, the rotor compressor.
(2) The eccentric shaft that drives the movement of the orbiting scroll can rotate at a high speed. Therefore, the scroll compressor is small in size and light in weight.
(3) The force change of the moving parts such as the orbiting scroll and the main shaft is small, and the vibration of the whole machine is small.
(4) Without intake and exhaust valves, the scroll compressor operates reliably and is especially suitable for variable speed motion and variable frequency speed regulation technology.
(5) The compression chamber of the scroll compressor is composed of scroll profiles and is a multi-chamber compression mechanism. When the center of the orbiting scroll moves in a circular motion around the center of the fixed scroll, the volume of each compression chamber changes with the rotation angle of the main shaft, correspondingly decreasing or expanding, thus realizing the processes of gas suction, compression, and exhaust. Since the suction and exhaust processes are almost continuous, the noise of the whole machine is very low.
(6) The axial and radial flexible structures improve the production efficiency of the scroll compressor and ensure the sealing effect of the axial clearance and radial clearance, which will not decrease due to friction and wear. That is, the scroll compressor has reliable and effective sealing performance. Therefore, its refrigeration coefficient does not decrease with the increase of running time but increases slightly.
(7) The scroll compressor has good working characteristics. Its performance is mainly affected by its own compression ratio and suction pressure. It has a wide exhaust pressure range and is suitable for various indoor and outdoor environments, ensuring that the compressor always operates at a high energy efficiency ratio, thus guaranteeing the energy efficiency ratio of the air conditioning unit. In a heat pump air conditioning system, it is particularly manifested in high heating performance, good stability, and high safety.
(8) The scroll compressor has no clearance volume. Structurally, it belongs to multi-chamber compression. The pressure difference between adjacent chambers is not very large (approximately continuous change). At the same time, the sealing strip at the contact part of the end faces of the orbiting and fixed scrolls is pressed tightly by the axial back pressure, making the orbiting and fixed scrolls in close contact, and a good sealing effect is achieved with the help of the refrigeration oil, so that the internal leakage is almost non-existent. When the end plane of the sealing strip is worn, it can be automatically compensated in the axial direction to ensure that the end face of the orbiting scroll and the bottom surface of the fixed scroll are always in close contact. Moreover, the longer the compressor works, the better the fit of these relatively moving surfaces in close contact, and the better the sealing effect. These advantages make the scroll compressor free from the problem of secondary compression of the refrigerant gas and are one of the important guarantee factors for maintaining high volumetric efficiency operation.
(9) The torque change is small, the balance is high, the vibration is small, the operation is stable, so the operation is simple and easy to achieve automation.
(10) Because it has few moving parts and no reciprocating motion mechanism, it has a simple structure, small size, light weight, and few parts (especially few vulnerable parts), high reliability, and a service life of more than 20 years.
Disadvantages
(1) The surfaces of its moving parts are mostly curved. The processing and inspection of these curved surfaces are relatively complex. High-precision processing equipment and accurate alignment assembly technology are required for manufacturing. Therefore, the manufacturing cost is relatively high.
(2) Between the moving parts or between the moving parts and the fixed parts, a certain movement clearance is often maintained to achieve sealing. The gas passing through the clearance will inevitably cause leakage, which limits the rotary compressor from achieving a large compression ratio. Therefore, most rotary compressors are used under air conditioning working conditions.
4. Screw Compressor
The screw compressor sucks in the refrigerant through the meshing of the male rotor (hereinafter referred to as the M rotor) composed of 5 convex teeth and the female rotor (hereinafter referred to as the F rotor) composed of 6 concave teeth, and compresses the refrigerant to a specified pressure by reducing the tooth space. The working process of the screw compressor is as follows:
[Suction Stroke] The refrigerant is sucked in from the suction port that opens axially to the radial direction. As the rotor rotates, on the lower side of the rotor, the meshing separates, the length of the tooth groove increases, and the refrigerant is sucked into the tooth space.
[Compression Stroke] The meshing of the tooth profile starts from the suction side of the tooth groove, the sealing line gradually moves towards the discharge side, the tooth space decreases, and compression occurs.
[Compression Stroke] Through the lubricating oil sucked in together with the refrigerant, an oil film seal is formed in the rotor clearance, and the rotor is lubricated at the same time. The male rotor and the female rotor gradually approach the discharge port for compression, and the pressure rises.
[Discharge Stroke] The tooth space is connected to the discharge port, and the discharge stroke starts. This stroke continues until the discharge end until the refrigerant in the tooth space is completely discharged.
Advantages
(1) There are few components, few vulnerable parts, and high reliability.
(2) It is convenient for operation and maintenance.
(3) There is no unbalanced inertia force. The operation is stable and safe, and the vibration is small.
(4) It has the characteristic of forced gas delivery, and the exhaust volume is hardly affected by the exhaust pressure, with strong adaptability to working conditions.
(5) There is actually a clearance on the tooth surface of the rotor of the screw compressor. Therefore, it is not sensitive to the wet stroke and can withstand liquid hammer.
(6) The exhaust temperature is low, and it can operate under working conditions with a relatively high pressure ratio.
(7) It can achieve stepless adjustment of the refrigeration capacity. By using a slide valve mechanism, the refrigeration capacity can be adjusted steplessly from 15% to 100%, saving operating costs.
(8) It is easy to achieve automation and can realize remote communication.
Disadvantages
(1) The tooth surface of the rotor is a spatial curved surface.

Special cutting tools need to be used, and it needs to be processed on expensive equipment. The processing accuracy of the machine body components also has high requirements, and high-precision equipment must be used.
(2) Since the inter-tooth volume is periodically connected to the suction and exhaust ports, the noise of the compressor is high.
(3) Due to the limitations of the rotor stiffness and bearing life, etc., the compressor can only rely on clearance sealing inside. Therefore, the screw compressor is only suitable for the medium and low-pressure range and cannot be used in high-pressure situations.
(4) Due to the large amount of oil injection and the complex oil treatment system, there are many auxiliary equipment for the unit.
(5) The screw compressor seals the gas relying on the clearance and does not have superior performance in the small volume range.
5. Centrifugal Compressor
The structure and working principle of the centrifugal refrigeration compressor are very similar to those of the centrifugal blower. However, its working principle is fundamentally different from that of the reciprocating compressor. It does not increase the gas pressure by reducing the volume of the cylinder but relies on the change of kinetic energy to increase the gas pressure. The centrifugal compressor has a working wheel with blades. When the working wheel rotates, the blades drive the gas to move or give the gas kinetic energy, and then part of the kinetic energy is converted into pressure energy to increase the gas pressure. Since this compressor continuously sucks in the refrigerant vapor during operation and continuously throws it out in the radial direction, this type of compressor is called a centrifugal compressor.

Advantages
(1) The centrifugal compressor has a large gas volume, a simple and compact structure, light weight, small unit size, and small floor space. Compared with the reciprocating compressor, when the cooling capacity is the same, it is 5 to 8 times lighter than the reciprocating compressor.
(2) Since it has no vulnerable parts such as steam valves and piston rings, and no crank connecting rod mechanism, it operates stably, is reliable in operation, has a high operation rate, few friction parts, so the demand for spare parts is small, and the maintenance cost and the number of maintenance personnel are small.
(3) There is no friction between the working wheel and the casing, and no lubrication is required. In the chemical process, the centrifugal compressor can achieve an absolutely oil-free compression process for chemical media.
(4) The centrifugal compressor is a rotating machine, and it is suitable for direct driving by an industrial steam turbine or a gas turbine. For general large chemical plants, the by-product steam is often used to drive the industrial steam turbine as the power, providing the possibility for the comprehensive utilization of heat energy.
Disadvantages
(1) The centrifugal compressor is currently not suitable for situations with too small a gas volume and too high a pressure ratio. And because it is suitable to use refrigerants with a relatively large molecular weight, it is only suitable for large cooling capacities, generally above 250,000 to 300,000 kcal/hour.
(2) The stable working condition area of the centrifugal compressor is relatively narrow. Although its gas volume adjustment is convenient, its economy is poor.
(3) At present, the efficiency of the centrifugal compressor is generally lower than that of the reciprocating compressor.

(4) Generally, a speed-increasing gear drive is required, the rotational speed is high, and the requirements for the shaft end seal are high, all of which increase the manufacturing difficulties and structural complexity.