How a Refrigeration Compressor Works

Figure 1: Refrigeration compressor.

The refrigeration compressor is the core component of the refrigeration unit, and plays a decisive role in the running performance, noise, vibration and service life of the system. The following is an introduction to the working principles and characteristics of 6 common refrigeration compressors.

1. Centrifugal Compressor

1.1 Working Principle of Centrifugal Compressor

Impellers with blades rotate on the shaft of the compressor. The gas entering the impellers rotates led by the blades, with increased kinetic energy (speed) and static head (pressure), and then exits the impellers and enters the diffuser.

The velocity of the gas in the diffuser is converted to pressure, further increasing the gas pressure. The compressed gas enters the next-stage impeller through the bend and return channel for further compression to the required pressure.

There are two reasons why the pressure of the gas in the impeller can increase: one is that the gas rotates at a high speed with the impeller under the action of the impeller blades, and the gas pressure increases due to the centrifugal force generated by the rotation.
Secondly, the impeller shape gradually expands from the inside to the outside, and the gas diffuses and flows in the impeller, so that the pressure of the gas after passing through the impeller is increased.

Figure 2: The inner structure diagram of a centrifugal compressor.

1.2 Centrifugal Compressor Features

Advantages of Centrifugal Compressor:
1. Large exhaust volume, uniform exhaust, and airflow without pulses.
2. High speed.
3. No need for lubrication inside the machine.
4. Good sealing effect and less leakage.
5. Flat performance curve and wide operating range.
6. Easy to realize automation.
6. Less wearing parts, less maintenance, and long operating cycle.

Disadvantages of Centrifugal Compressor:
1. The adaptability of the operation is poor, and the nature of the gas has a great influence on the operation performance. During the start-up, stop and operation of the unit, the load changes greatly.
2. High air speed, causing a large friction loss of parts in the flow channel.
3. There is a surge phenomenon, which is extremely harmful to the machine.

Scope of application: occasions with large and medium flow rates and medium and low pressures.

2. Piston Compressor

2.1 Working Principle of Piston Compressor

When the crankshaft of the piston compressor (reciprocating compressor) rotates, the piston reciprocates through the transmission of the connecting rod. The working volume formed by the inner wall of the cylinder, the cylinder head and the top surface of the piston will change periodically.
When the piston 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, pushes the intake valve open and enters the cylinder until the working volume reaches the maximum, and then the intake valve will be closed.

When the piston moves in reverse, 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 from the cylinder until the piston moves to the limiting position, and then the exhaust valve closes.

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

Figure 3: The inner structure diagram of a piston refrigeration compressor.

2.2 Piston Compressor Features

Advantages of Piston Compressor:
1. Wide applicable pressure range. Regardless of the flow rate, it can reach the required pressure.
2. High thermal efficiency, small unit power consumption.
3. Strong adaptability, that is, the exhaust range is wide, not affected by the pressure level, and can adapt to a wider pressure range and cooling capacity requirements.

4. Low requirements for manufacturing materials, and ordinary steel materials are mostly used, which is easier to process and lower in cost.
5. Relatively mature technology and having accumulated rich experience in production and use.
6. Relatively simple installation system.

Disadvantages of Piston Compressor:
1. The speed is not high, and the machine is large and heavy.
2. Complex structure, many wearing parts, and large amounts of maintenance.
3. Discontinuous exhaust, resulting in air flow pulsation.
4. Large vibration during operation.

Piston compressors are used in various occasions, especially in the small and medium-sized refrigeration range, and have become the most widely used and the largest production volume model.

3. Screw Refrigeration Compressor

3.1 Working Principle of Screw Compressor

The screw compressor cylinder is equipped with a pair of intermeshing helical male and female rotors. Both rotors have several concave teeth, and the two rotate in opposite directions. The gap between the rotors and between the casing and the rotor is only 5 to 10 cmm (1 cmm = 0.01 mm).

The main rotor (also known as male rotor) is driven by an engine or an electric motor (mostly driven by an electric motor). The other rotor (also known as the female rotor) is driven by the main rotor through the oil film formed by oil injection, or by the synchronous gear at the main rotor end and the female rotor end. So there is no metal contact in the drive theoretically.

The length and diameter of the rotor determine the compressor displacement (flow rate) and discharge pressure. The longer the rotor, the higher the pressure; the larger the rotor diameter, the greater the flow.

Figure 4: The inner structure diagram of an oil-injected screw refrigeration compressor.

3.2 Screw Compressor Features

Advantages of Screw Compressor:
1. High reliability. The screw compressor has few parts and no wearing parts, so it operates reliably and has a long service life, and the interval between overhauls can reach 40,000 to 80,000 hours.

2. Easy operation and maintenance. The screw compressor has a high degree of automation, and the operator does not need to undergo a long period of professional training, and can realize unattended operation.

3. Good balance. The screw compressor has no unbalanced inertial force, can work stably at high speed, and can realize non-foundational operation, especially suitable as mobile compressors, with small size, light weight and less floor space.

5. Strong adaptability. The screw compressor has the characteristics of forced air delivery, the volumetric flow rate is hardly affected by the exhaust pressure, and can maintain high efficiency in a wide range. It is suitable for various working conditions without any change in the compressor structure.

Disadvantages of Screw Compressor:
1. The noise is loud, and a set of auxiliary equipment for lubricating oil separation, cooling, filtration and pressurization is needed, resulting in an excessively large unit volume.

2. High cost for processing, and high requirements for the machining accuracy of the screw compressor cylinder.
3. Not used in high-voltage occasions due to the limitation of rotor stiffness and bearing life.

4. Cannot be used in miniature occasions. The screw compressor relies on the clearance to seal the gas, and generally it has superior performance only when the volumetric flow rate is greater than 0.2m³/min.

4. Sliding Vane Compressor

4.1 Working Principle of Sliding Vane Compressor

The main parts of the rotary vane compressor are: the body 1 (also known as the cylinder), the rotor 2 and the vanes 3. The rotor of the sliding vane compressor is eccentrically arranged in the cylinder, and there are several longitudinal grooves on the rotor, and vanes installed in the grooves can slide freely in the radial direction.

Since the rotor is arranged eccentrically in the cylinder, a crescent-shaped space is formed between the inner wall of the cylinder and the outer surface of the rotor. When the rotor rotates, the sliding vanes slide out of the grooves by the centrifugal force, and their ends are close to the inner circular wall of the machine body, and the crescent-shaped space is divided into several fan-shaped small chambers by the sliding vanes.

Within one revolution of the rotor, every small chamber will gradually increase from the minimum value to the maximum, and then gradually decrease from the maximum value to the minimum. With the continuous rotation of the rotor, these small chambers change repeatedly according to the above law.

Figure 5: Working principle diagram of sliding vane compressor.

1. Sliding vane, 2. Rotor, 3. Cylinder

If the number of sliding vanes is z, then during each revolution of the rotor, there are z small chambers in sequence to perform the suction, compression, exhaust, expansion process respectively.

4.2 Sliding Vane Compressor Features

Advantages of Sliding Vane Compressor:
1. The vane compressor has a simple structure, is easy to manufacture, operate and maintain.
2. It is almost perfectly balanced and has no vibration, so the required foundation is small.
3. In addition, there are multiple chambers connected to the suction and exhaust pipes during one rotation, so the pressure pulsation of suction and discharge is small, and there is no need to install a large gas reservoir.

Disadvantages of Sliding Vane Compressor:
1. There is a lot of mechanical friction between the sliding vanes and the cylinder, resulting in a large energy loss, so the efficiency is low.
2. The service life of the vane can exceed 8000 hours now, but depending on the material, processing accuracy and operating conditions. So it is still an important factor affecting the operation cycle of the vane compressor.

Vane compressors are widely used in various medium and small compressed air devices and small air-conditioning and refrigeration devices. The sliding vane compressor can also be used as a vacuum pump.

5. Rolling Piston Compressor

5.1 Working Principle of Rolling Piston Compressor

Rolling piston compressor, also known as rolling rotor compressor or fixed vane compressor, is a type of rotary compressor. It uses the rotation of an eccentric cylindrical piston in the cylinder to change the working volume of the cylinder, so as to realize the suction, compression and exhaust of gas, so it also belongs to the positive displacement compressor.

When the rotor main shaft rotates under the drag of the prime mover, the eccentric piston rotates close to the inner wall of the cylinder, causing periodic changes in the volume of the crescent-shaped space, completing the process of suction, compression and exhaust.

Figure 6: The inner structure diagram of a rolling piston refrigeration compressor.

Rolling piston compressors are now widely used in home appliance refrigerators and air conditioners, and large and medium-sized rolling piston compressors are also used in cold storage.

5.2 Rolling Piston Compressor Features

Advantages of Rolling Piston Compressor:
1. Few parts and simple structure.
2. Few vulnerable parts, reliable operation.
3. There is no suction valve plate, the clearance volume is small, and the gas transmission coefficient is high.
4. Under the same cooling capacity, this compressor is small in size, light in weight and balanced in operation.

Disadvantages of Rolling Piston Compressor:
1. The machining accuracy is high.
2. Long sealing line, poor sealing performance, and large leakage loss.
3. The leakage, friction and wear between the sliding vane and the cylinder wall are relatively large, which limits its working life and efficiency.

6. Scroll Compressor

6.1 Scroll Compressor Working Principle

The scroll compressor is a new type, energy-saving, material-saving and low-noise positive displacement compressor. Its working principle is to use the relative revolution of the orbiting and fixed scrolls to form a continuous change of the closed volume to achieve the purpose of compressing gas.

It is mainly used in air conditioning, refrigeration, general gas compression, as well as in automotive engine superchargers and vacuum pumps, and can replace traditional medium and small reciprocating compressors in a wide range.

Figure 7: The inner structure diagram of the hermetic scroll compressor.

6.2 Scroll Compressor Features

Advantages of Scroll Compressor:
1. The pressure difference between two adjacent chambers is small, and the gas leakage is small.
2. Since the suction, compression, and exhaust processes are carried out continuously at the same time, and the pressure rises slowly, the range of torque changes and the vibration is small.
3. There is no clearance volume, so there is no expansion process that causes the gas transmission coefficient to drop.
4. No suction and exhaust valves, high efficiency, strong reliability and low noise.

Disadvantages of Scroll Compressor:
1. The scroll profile has high requirements in machining accuracy, and should be processed by special precision machining equipment, with high sealing requirements and complicated sealing structure.
2. Because there is no gas valve, over-compression and under-compression will be formed inside the compression chamber.

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