Popularization of cold chain technology analysis of the advantages and disadvantages of 5 types of compressors

There are mainly five types of compressors, piston compressors, rolling rotor compressors, turbo compressors, screw compressors, and centrifugal compressors. This article briefly introduces the advantages and disadvantages of various compressors.

   1. Piston compressor

   When the crankshaft of the piston compressor rotates, the piston reciprocates through the transmission of the connecting rod, and the working volume formed by the inner wall of the cylinder, the cylinder head and the top surface of the piston changes periodically. When the piston of a piston compressor starts to move from the cylinder head, the working volume in the cylinder gradually increases. At this time, the gas moves along the intake pipe and pushes the intake valve to enter the cylinder until the working volume reaches z*. At this point, the intake valve is closed; when the piston of the piston compressor moves in the reverse direction, the working volume in the cylinder decreases and the gas pressure increases. When the pressure in the cylinder reaches and is slightly higher than the exhaust pressure, the exhaust valve opens and the gas is discharged Cylinder, until the piston moves to the limit position, the exhaust valve is closed. When the piston of the piston compressor moves in the reverse direction again, the above process repeats.

   In short, the crankshaft of a piston compressor rotates once, the piston reciprocates once, and the process of air intake, compression, and exhaust is realized in the cylinder, that is, a working cycle is completed.

   advantages

  (1) Regardless of the flow rate, the required pressure can be obtained, and the exhaust pressure range is wide, and the high pressure can reach 320MPa (industrial applications) or even 700MPa (in the laboratory);

  (2) The single machine capacity is any flow below 500m3/min;

  (3) In the general pressure range, the requirements for materials are low, and ordinary steel materials are mostly used, which is easier to process and lower in cost;

  (4) The thermal efficiency is relatively high, and the adiabatic efficiency of general large and medium-sized units can reach about 0.7~0.85;

  (5) When the air volume is adjusted, the adaptability is strong, that is, the exhaust range is wide, and it is not affected by the pressure level, and can adapt to a wider pressure range and cooling capacity requirements;

   (6) The severity and characteristics of the gas have little effect on the performance of the compressor, and the same compressor can be used for different gases;

  (7) The driving machine is relatively simple, most of which use electric motors, and generally do not adjust the speed, and the maintainability is strong;

  (8) The piston compressor is relatively mature in technology, and has accumulated rich experience in production and use;

   disadvantages

  (1) The structure is complex and heavy, there are many vulnerable parts, the area is large, the investment is high, the maintenance workload is large, and the service cycle is short, but after hard work, it can reach more than 8,000 hours;

  (2) The speed is not high, the machine is large and heavy, and the air displacement of a single machine is generally less than 500m3/min;

   (3) There is vibration during machine operation;

  (4) The exhaust is discontinuous and the airflow is pulsating, which is easy to cause pipeline vibration. In severe cases, the pipe network or mechanical parts are often damaged due to airflow pulsation and resonance;

  (5) Flow adjustment adopts auxiliary volume or bypass valve, although simple, convenient and reliable, but the power loss is large, and the efficiency is reduced during partial load operation;

   (6) For compressors lubricated with oil, the oil in the gas needs to be removed;

  (7) When multiple compressor units are used in a large-scale factory, there are more operators or greater work intensity.

  2. Rolling rotor compressor

  Rolling rotor compressor is a positive displacement rotary compressor. The change of cylinder working volume is realized by the rolling of a cylindrical rotor of an eccentric device in the cylinder.

  (1) The rotor makes one full revolution to 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 steam inlet valve, there is a strict correspondence between the timing of the start of suction and the position of the suction port on the cylinder, which does not change with changes in working conditions.

  (3) Since the exhaust valve is installed, the timing of the end of compression will vary with the pressure in the exhaust pipe.

   advantages

   This type of compressor is now widely used in household refrigerators and air conditioners. Its advantages are:

  (1) Simple structure, small size and light weight. Compared with piston compressors, the volume can be reduced by 40%-50%, and the weight can be reduced by 40%-50%;

  (2) There are few parts, especially the wearing parts, and the friction loss between the relative moving parts is small, so the reliability is high;

  (3) Only the sliding plate has a small reciprocating inertia force, the rotating inertia force can be fully balanced, the speed can be higher, and the vibration is small, and the operation is stable;

  (4) There is no suction valve, the suction time is long, the clearance volume is small, and the direct suction reduces the harmful overheating of the suction, so its efficiency is high. But its processing and assembly precision requirements are high. Since there is no gas valve, it can be used to transport dirty, droplet, and dust-containing process gas;

   disadvantages

   The main disadvantage of the rotary compressor is that the leakage, friction and wear between the sliding vane and the cylinder wall are large, which limits its working life and the improvement of efficiency; and this type of compressor requires high machining accuracy. If double-layer sliding vanes are used, the ends of the two sliding vanes are kept 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 ends of the sliding vanes. Reduce friction and friction loss, so that the working life and efficiency of the machine are improved.

Once this kind of compressor wears on its bearings, main shaft, rollers or sliding vanes, and the gap increases, it will immediately have a significant adverse effect on its performance. Therefore, it is usually used in refrigerators and air conditioners that are assembled in factories. In the machine, the system also requires high cleanliness.

  3. Scroll compressor

The main components in the scroll compressor are two involute scrolls with the same shape but with a relative offset of 180° in angular position. One is a fixed scroll, and the other is driven by an eccentric shaft with its axis around An orbiting scroll in which the axis of the fixed scroll revolves. During work, the two scrolls are tangent to each other to form a sealing line, and the end faces of the two scrolls are properly sealed to form a tooth-shaped air cavity for several months. The sealing line at the common tangent point between the two scrolls continuously shifts along the vortex curve with the revolution of the orbiting scroll, so that the shape and size of these crescent-shaped air chambers are constantly changing. The suction port of the compressor is opened in the upper part of the fixed scroll shell. When the eccentric shaft rotates clockwise, the gas enters the suction cavity from the suction port, and is successively taken into the peripheral crescent-shaped air cavity communicating with the suction cavity. As these peripheral crescent-shaped air chambers are closed and no longer communicate with the suction chamber, their enclosed volume is gradually shifted to the center of the fixed scroll and shrinks continuously. The gas is continuously compressed and the pressure increases.

   advantages

  From the specific structure, the scroll compressor has no suction and discharge valves, which greatly improves the reliability of high-speed operation. Taken together, scroll compressors have the following main features:

(1) It belongs to the third generation compressor, multiple compression chambers work at the same time, the gas pressure difference between adjacent compression chambers is small, the gas leakage is small, and the volumetric efficiency is high, up to 98%, which is higher than the second generation compressor rotor The compressor efficiency is about 5% higher;

  (2) The eccentric shaft that drives the moving scroll can rotate at a high speed, so the scroll compressor is small in size and light in weight;

  (3) The force changes of moving parts such as the movable scroll and the main shaft are small, and the vibration of the whole machine is small;

  (4) There are no suction and exhaust valves, the scroll compressor runs reliably, and it is especially suitable for variable speed movement and frequency conversion speed regulation technology;

(5) The compression chamber of the scroll compressor is composed of a scroll profile, which is a multi-chamber compression mechanism. When the center of the movable scroll moves in a circle around the center of the stationary scroll, the volume of each compression chamber changes with the rotation angle of the main shaft. It will be reduced or enlarged accordingly, thereby realizing the gas suction, compression and exhaust process. Since the suction and exhaust process is almost continuous, the noise of the whole machine is very low;

(6) The axial and radial flexible structure improves the production efficiency of the scroll compressor, and ensures the sealing effect of the axial gap and the radial gap, which is not reduced due to friction and wear, that is, the scroll compressor is reliable and effective The airtightness, so its refrigeration coefficient does not decrease with the increase of operating time, but slightly increases;

(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 range of discharge pressure and is suitable for various indoor and outdoor environments to ensure that the compressor is always running at a high energy efficiency ratio. , So as to ensure the energy efficiency ratio of the air conditioning unit. In the heat pump air conditioning system, it is especially manifested in high heating performance, good stability and high safety;

(8) The scroll compressor has no clearance volume and belongs to multi-chamber compression in structure. The pressure difference between adjacent chambers is not very large (approximately continuous change). At the same time, the sealing strips at the contact parts of the moving and static scrolls The axial back pressure is compressed to make the moving and static scrolls come into close contact, and with the help of refrigerating oil to achieve a good sealing effect, so that there is almost no internal leakage; when the end plane of the sealing strip is worn, it can be along the axis Automatic compensation in the direction to ensure that the end surface of the movable scroll and the bottom surface of the fixed scroll are always close, and the longer the compressor works, the better the cooperation of these close relative moving surfaces and the better the sealing effect. These advantages make the scroll The rotary compressor does not have the problem of secondary compression of refrigerant gas, which is 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, and the operation is stable, so the operation is simple and easy to realize automation;

  (10) Because of its own few moving parts and no reciprocating mechanism, it has simple structure, small size, light weight, few parts (especially few wearing parts), high reliability, and a life span of more than 20 years.

   disadvantage

  (1) The surface of the moving parts is mostly curved. The processing and inspection of these curved surfaces are complicated. The manufacturing requires high-precision processing equipment and precise centering assembly technology, so the manufacturing cost is high.

(2) Between the moving parts or between the moving parts and the fixed parts, sealing is often achieved by maintaining a certain movement gap. Gas passing through the gap is bound to cause leakage, which limits the difficulty of the rotary compressor to achieve larger Compression ratio, therefore, most rotary compressors are mostly used in air conditioning conditions.

  4. Screw compressor

In the screw compressor, a tooth-shaped space is formed through the meshing of a male rotor composed of 5 convex teeth (hereinafter referred to as M rotor) and a female rotor composed of 6 concave teeth (hereinafter referred to as F rotor), and the refrigerant is sucked in by reducing The tooth-shaped space compresses the refrigerant to a predetermined pressure. The working process of the screw compressor is as follows:

  〔Inhalation stroke〕

   The refrigerant is sucked in from the suction port that opens in the axial direction to the radial direction. As the rotor rotates, the lower side of the rotor meshes and separates, the length of the tooth groove increases, and the refrigerant is sucked into the tooth space.

  〔Compression stroke〕

  The tooth profile meshes from the suction side of the tooth groove, and the seal line gradually moves to the discharge side, reducing the tooth profile space and compressing.

  〔Compression stroke〕

   Through the lubricating oil sucked in along with the refrigerant, an oil film seal is formed in the rotor gap, and the rotor is lubricated at the same time. The male and female rotors gradually approach the discharge port for compression, and the pressure increases.

  〔Discharge stroke〕

  The tooth-shaped space is connected with the discharge port, and the discharge stroke starts. This stroke continues to the discharge end until the refrigerant in the toothed space is completely sent out.

   advantages

  (1) Few parts, less wearing parts, high reliability;

  (2) Convenient operation and maintenance;

  (3) No unbalanced inertial force. Smooth and safe operation, small vibration;

  (4) It has the characteristics of forced air delivery, the exhaust volume is almost not affected by the exhaust pressure, and the working conditions are adaptable;

   (5) The rotor tooth surface of the screw compressor is actually a gap. Therefore, it is not sensitive to wet stroke and can withstand liquid shock;

  (6) The exhaust temperature is low, and it can be operated under higher pressure ratio conditions:

  (7) It can realize stepless adjustment of the refrigeration condition, adopting a sliding valve mechanism, so that the refrigeration capacity can be adjusted steplessly from 15% to 100%, saving operating costs;

  (8) It is easy to realize automation and can realize remote communication.

   disadvantage

  (1) The rotor tooth surface is a spatial curved surface, which needs to be processed on expensive equipment with special tools. The machining accuracy of body parts also has high requirements, and high-precision equipment must be used:

   (2) Because the inter-tooth volume is periodically connected to the suction and exhaust ports, the compressor noise is high;

  (3) Due to the limitations of rotor stiffness and bearing life, the compressor can only rely on gap sealing inside, so screw compressors can only be used in the medium and low pressure range, and cannot be used in high pressure applications;

   (4) Due to the large amount of fuel injected and the complicated oil processing system, there are many auxiliary equipment for the unit;

  (5) The screw compressor relies on the gap sealing gas and does not have superior performance in a small volume range.

   5. Centrifugal compressor

   The structure and working principle of centrifugal refrigeration compressors are very similar to those of centrifugal blowers. However, its working principle is fundamentally different from that of a piston compressor. Instead of increasing the gas pressure by reducing the cylinder volume, it 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 obtain kinetic energy, and then convert part of the kinetic energy into pressure energy to increase the pressure of the gas. This type of compressor is called a centrifugal compressor because it constantly sucks in refrigerant vapor when it works, and is constantly thrown out along the radius.

   advantages

  (1) The centrifugal compressor has large air volume, simple and compact structure, light weight, small unit size, and small footprint. Compared with the piston compressor, the weight is 5 to 8 times lighter than the piston compressor when the cooling capacity is the same.

  (2) Because it has no vulnerable parts such as steam valve piston rings, and no crank and connecting rod mechanism, it has balanced operation, reliable operation, high operation rate, and few friction parts. Because of the small amount of spare parts required, maintenance costs and fewer personnel.

  (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 the chemical medium.

  (4) Centrifugal compressor is a rotary motion machine, which is suitable for direct drive of industrial steam turbine or gas turbine. For general large-scale chemical plants, by-product steam is often used to drive industrial steam turbines, which provides the possibility for comprehensive utilization of thermal energy.

   disadvantage

(1) Centrifugal compressors are currently not suitable for occasions where the gas volume is too small and the pressure ratio is too high, and because it is suitable to use refrigerants with a relatively large molecular weight, it is only suitable for large refrigeration capacity, generally between 250,000 and 300,000. Cards/hour or more.

  (2) The stable working condition area of centrifugal compressors is narrow, and its air volume adjustment is convenient, but the economy is poor.

  (3) At present, the efficiency of centrifugal compressors is generally lower than that of piston compressors.

  (4) Generally, speed-increasing gears are used for transmission. The speed is relatively high and the requirements for shaft end sealing are high. These all increase the difficulty of manufacturing and the complexity of structure.