Common Types of Refrigeration Compressors: A Comprehensive Guide

When mentioning refrigeration, everyone in this modern world may be familiar with it. People's daily use of air conditioners, refrigerators, whether small household or large commercial ones, are refrigeration appliances.

In the hot summer, without air conditioning, city people who are used to the coolness of air conditioners may don't know how to survive, and restaurant owners may be extremely anxious because they don't know where to put their daily food.

However, refrigeration compressors will be unfamiliar to many people. In fact, refrigeration compressor is the heart of refrigeration system. In a sense, the design and matching of refrigeration system is to bring out the best capacity of compressor.

Figure 1:Refrigeration compressors manufactured by Bristol, Mitsubishi, Hitachi and Tecumseh of different types and sizes

One compressor accounts for 30% - 40% of the cost of an air conditioner. So it is necessary to know more about the compressor used when buying an air conditioner. A good compressor will be quieter, and have better performance in service life and energy efficiency ratio.

The refrigeration industry all over the world has invested a lot in the research of refrigeration compressor, and new research directions and results are constantly emerging. The technology and performance level of compressor change with each passing day.

Here we will introduce the common types of refrigeration compressors in the marketplace.

1. Classified by Technical Parameters

1.1 Classified by Exhaust Pressure

In the refrigeration system, only by converting the electric energy into mechanical work through the compressor, and compressing the low temperature and low pressure gaseous refrigerant into high temperature and high pressure gas, can the refrigeration cycle be realized.

If the air pressure lifted by a compressor is less than 0.2MPa, we call it a blower, if less than 0.02MPa, it’s called a fan.

The discharge pressure standard of the compressor is referred as following table.

Table 1: The discharge pressure standard of the Refrigeration compressors

1.2 Classified by Compression Stages

Table 2: The Stages of the Refrigeration Compression

There are more obvious advantages of multi-stage compressors compared with single-stage ones.

* Reduce the Exhaust Temperature

If a single-stage compressor is used when the exhaust pressure of the compressor is high, the pressure ratio will be large, which will make the exhaust temperature far higher than the allowable value, causing improper operation of the machine.

Adopting multi-stage compression, the pressure ratio of each stage can be reduced. Intermediate cooling measures can be applied between stages to make the suction temperature of each stage very low. That is usually similar to the first stage suction temperature. And the exhaust temperature at the end of compression will be greatly lowered.

* Save Power Consumption

By using multi-stage compression, the interior cooler can be set for different stages. Compressed gas is cooled under equal pressure after the first stage compression to reduce the temperature. Thereafter, it enters the next cylinder. The temperature is lowered and the density is increased, so it is easy for further compression, and the energy consumption can be reduced greatly.

Figure 2: high efficiency and energy saving two stage screw compressor.

* Improve the utilization of cylinder volume

Due to the influence of manufacturing, installation and operation, the residual clearance volume in the cylinder is inevitable. That directly reduces the effective volume of the cylinder. What’s worse, the residual high-pressure gas must be expanded to the suction pressure, so the cylinder can start to inhale fresh gas, which further reduces the effective volume of the cylinder.

If the pressure ratio is larger, the more residual gas expansion is in the residual volume, the smaller the effective volume of the cylinder. Under the limit conditions, even after the gas in the residual clearance volume expands completely in the cylinder, the pressure is still higher than the suction pressure. At this time, the suction and exhaust can not be continued, and the effective volume of the cylinder becomes zero.

If multi-stage compression is adopted, the compression ratio of each stage is very small, and the residual gas in the residual volume can be expanded slightly to achieve suction pressure. Thus, the effective volume of the cylinder can be increased naturally, thus improving the utilization of the cylinder volume.

* Reduce the maximum gas force on the piston

When the compression ratio is high, if the single-stage compression is adopted, the final pressure acts on the larger piston area will be higher, so the force transmitted to the moving mechanism is also larger.

If it is multi-stage compression, the gas pressure increases step by step, the diameter of the cylinder will decrease step by step. In this way, while the higher gas pressure acts on the smaller piston area in the high-pressure stage, the gas pressure acting on the larger piston area in the low-pressure stage will be smaller. Therefore, the forces acting on the motion mechanism at all levels are relatively small.

If all stages are arranged reasonably, the force acting on the moving parts can be reduced.

Generally speaking, if a compressor is to be designed in multi-stage compression, the number of stages should be determined by the following principles.

a. The compression temperature of each stage is within the allowable range.

b. The total power consumption of the compressor is at the lowest.

c. The compressor structure is as simple as possible, making it easy to manufacture and reliable in operation.

1.3 Classified by Volume Flow

Table 3: The Volume Flow of the Refrigeration Compressors

2. Classified by Working Principle

There are many different names of compressors according to different classification. This is confusing enough even to the professionals.

It is found in various compressor articles that compressors are classified into positive displacement compressors and dynamic compressors, piston (reciprocating) compressors, screw compressors, and scroll compressors, etc. In fact, all of them are classified according to the working principle.

Hopefully the following table can help you understand common compressor types easily.

Table 4: Refrigeration compressors classified by working principles, features, and motor structures.

2.1 Positive Displacement Compressor

As shown in the above table, most compressor types in the market are positive displacement compressors.

Because there are too many kinds of subdivision, we will briefly introduce the most common types of positive displacement compressors in the market.

* Piston Compressor

Piston compressor is a type of compressor which relies on reciprocating motion of piston to pressurize and transport gas. It is mainly composed of working cavity, transmission part, fuselage and auxiliary parts.

After the motor starts, the crankshaft is driven to rotate. Through the transmission of connecting rod, the piston moves reciprocally. The working volume composed of the inner wall of cylinder, cylinder head and top surface of piston will be changed periodically.

Figure 3: han engineering drawing of the interior parts of a piston compressor,
including crankshaft, connecting rod, crosshead, coupling, cylinder, piston, valve, fuselage.

When the piston starts to move from the cylinder head, the working volume in the cylinder increases gradually. At this time, the steam body pushes the intake valve along the intake pipe and enters the cylinder until the working volume reaches the maximum, and the intake valve closes. When the piston moves in the opposite direction, the working volume in the cylinder decreases and the gas pressure increases. When the pressure in the cylinder reaches and slightly exceeds the exhaust pressure, the exhaust valve opens The gas is discharged from the cylinder until the piston moves to the limit position, and the exhaust valve is closed.

When the piston reverses again, the above process repeats. In a word, the crankshaft rotates a circle, the piston reciprocates once, and the cylinder successively achieve the process of air intake, compression and exhaust, that is, completing a working cycle.

* Screw Compressor

Screw compressor belongs to rotary compressor, including single screw compressor and twin screw compressor. It was first proposed by German H. Krigar in 1878, and it was not until 1934 that the technology was established by A. Lysholm of the Royal Swedish Institute of Technology. Then it began to be applied in industry, and achieved rapid development.

Screw compressor is composed of a pair of parallel and mutually meshed female and positive screw, which is the most widely used type of rotary compressor. Generally speaking, screw compressors usually refers to twin screw compressors.

Figure 4: 55kW screw compressor

Screw compressors can also be divided into dry and wet types.

For dry screw compressors, there is no liquid spray in the working chamber, therefore the compressed gas will not be polluted.

On the contrary, when a wet screw compressor works, there will be lubricating oil or other liquid injected into the working chamber to cool the compressed gas, improve the seal and lubricate the anion and cation rotors and bearings. After the compressor finishes its self transmission, the oil or other liquid impurities in the compressed air will be removed through a high-precision filter to get high-quality compressed gas.

Dry screw compressors are generally used in environments with high requirements for gas quality, and the requirement for gas volume is not large. This type is characterised as with complex structure, difficult maintenance, loud noise and high cost. Wet type is more widely used for its simple structure, easy maintenance, stable and reliable performance, and is also commonly used in aerodynamic engineering.

* Scroll Compressor

Scroll compressor also belongs to rotary compressor, and its compression parts are composed of moving scroll and static scroll. It has a number of structural features, including that the air passing through the compressor housing is shunted to reduce the entrained oil.

The unique design makes it energy-saving. It has a longer service life because of no wear, and is known as compressor of maintenance free.

Figure 5: silent and oil-free scroll compressor

Scroll compressors are known as "super static compressors" for their smooth operation and quiet working environment. This type has many advantages, such as novel and precise structure, small size, light weight, low noise, small vibration, low energy consumption, long service life, continuous and stable gas transmission, and reliable operation and clean gas source.

Known as the "new revolution compressors" and "maintenance free compressors", scroll compressors are the ideal power source for wind driven machinery, and is widely used in industry, agriculture, transportation, medical equipment, food manufacturing, decoration and textile and other industries and occasions requiring compressed air.

2.2 Dynamic Compressor

* Centrifugal Compressor

Centrifugal compressor is a kind of vane rotary compressor (turbine compressor). The increase of air pressure in centrifugal compressor is achieved by impeller rotation and diffuser expansion.

In centrifugal compressor, the centrifugal force of the high-speed rotating impeller and the expansion of the gas in the expanding channel can increase the gas pressure.

In early days, since this type of compressors was only suitable for low, medium pressure and high flow conditions, it was not noticed by people. With the development of chemical industry, and the establishment of various large chemical plants and refineries, centrifugal compressor has become the key machine for compressing and transporting various gases in chemical production, and has been playing an important role.

Figure 6: complete unit of samsung centrifugal compressor requiring control power of three-phase 380V, 50 kV

* Axial Compressor

Axial compressor is a kind of large air compressor with a maximum power of 150,000kw, and an exhaust volume of 20,000 m3/min. Its compressor energy efficiency ratio can reach about 90%, which is more energy-saving than centrifugal compressors.

Axial compressors are usually composed of three parts: one is the rotatable part with the rotating shaft as the main body (called "rotor" for short); the second is the stator with the casing and the static parts installed on the casing; the third is the casing, sealing body, bearing box, adjusting mechanism, coupling, base and control protection.

Figure 7: a axial compressor with outermost shell removed showing substantial steel turbines

Axial compressors and centrifugal compressors are both speed compressors, which are called turbine compressors. It means that their working principle is to depend on the turbines to work on the gas, and the flow speed of the gas is greatly improved, and then the kinetic energy is converted into pressure energy.

All turbine compressors have vanes with high speed rotation.

3.How to Choose the Right Type of Compressor

To select a reasonable type, users of compressors need to know the working principles of compressors.

Compressor is a kind of machine with pressure, so safety is the primary consideration. When selecting a compressor, follow the principle of safety, economical efficiency, reliability to ensure the quality of the equipment.

Here is a quick look of how to choose compressors that suit different uses.

1. The compressor is selected according to the cooling capacity, and under common circumstances, there will be no standby compressor.
2. If you need to select more than one refrigeration compressor, try to use the same series of compressors.
3. The compressors with different cooling capacity should be used together to ensure the economy and rationality during peak load or low load.
4. For different refrigeration systems, it is necessary to consider the possibility of compressor substitution among different systems.
5. For general freon refrigerant system, when PK / P0 > 10, choose two-stage compressors, otherwise use single-stage compressors.

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