What is centrifugal compressor and the working principle of centrifugal compressor is not that complicated. Centrifugal compressor is a type of speed air compressor that differs from positive displacement air compressor, it increase the speed of air flow and convert the kinetic energy of air into its pressure through a diffuser, thus the air is compressed. All the centrifugal compressors are oil-free compressors because it drives air flows at high speed with only impellers in multi-stage compression and there is no oil involved.

What is centrifugal compressor?

Centrifugal compressor evolution

In the early 20th century, a centrifugal compressor with compression ratio of 4.5 had appeared, it was developed on the basis of ventilators. The centrifugal compressor manufacturing industry has developed since the 1950s. In 1963, the first 14.7 MPa high-pressure centrifugal compressor was produced in a synthetic ammonia plant in United States.

It used a barrel casing instead of a horizontally split casing, also known as a barrel compressor, which can withstand pressures above 10 MPa. When it came to the 1970s, the United States, Italy and the Federal Republic of Germany successively produced 60 MPa high-pressure barrel compressors with a barrel wall thickness of 280 mm. In the early 1980s, the exhaust pressure of centrifugal gas compressor reached 80 MPa.

Centrifugal compressor definition

Centrifugal air compressor belongs to speed air compressors and is a type of turbine compressor. It drives the air to rotate at high speed by the impeller, so that the gas obtains centrifugal force, thereby increasing the flow rate and pressure of the air after passing through the impeller.

After that, the gas is thrown out of the impeller and enters the diffuser to reduce speed and increase pressure. Repeating this process several times can make the compressed air reach the designed pressure. They have the characteristics of large gas processing volume, small size, simple structure, stable operation, easy maintenance, and gas is not polluted by oil. There are many driving forms that can be used such as motors, steam turbines, gas turbines, etc.

Centrifugal air compressor operation is stable and reliable, it has a compact structure, large exhaust volume range, few wearing parts, long service life, no oil pollution to compressed air, high air supply quality, and its multi-stage compression is closer to isothermal compression, which is conducive to energy saving.

How does a centrifugal compressor work?

Centrifugal compressor components

The centrifugal air compressor is mainly composed of three parts: stators, bearings and rotors.

The main body of the stator is the casing (cylinder), also includes partition, seal, intake chamber, volute chamber, diffuser, bend, return flow device, intake pipe, exhaust pipe and some shaft seals.

The rotor is installed on the bearing, and the bearing is driven by the motor to drive the rotor to rotate at high speed.

The rotor consists of an impeller, a balance plate and a part of the shaft seal, and there are blades on the impeller. The impeller is the only element in the centrifugal compressor that works on the air. There are three types of impellers: closed, semi-open and open. The closed impeller is composed of blades, wheel cover and wheel disc. The semi-open impeller has no wheel cover but a wheel disc. The open impeller has no wheel cover and wheel disc, and the impeller is on the shaft.

A diffuser is a device that decelerates high-speed air and converts its kinetic energy into pressure. Its principle is to change the flow direction of the gas, thereby reducing the flow rate of the gas.

Working principle of centrifugal compressor

The working principle of the centrifugal compressor is using mechanical working elements (high-speed rotating impellers) to work on the gas, so that the kinetic energy of gas is increased under the action of centrifugal force, and its pressure is also greatly increased. Subsequently, this part of the kinetic energy is converted into static pressure energy in the diffuser flow channel, which further increases the gas pressure.

Generally speaking, the main goal of increasing gas pressure is to increase the number of gas molecules per unit volume, that is, to shorten the distance between gas molecules.

In order to achieve this goal, the method of gas dynamics is adopted, when the rotor (impeller) inside the centrifugal air compressor rotates at high speed, the gas is driven by the impeller to flow at high speed, under the action of centrifugal force, the air gains kinetic energy, and its pressure, temperature, and flow rate increase.

Then, the gas flows through channels of centrifugal compressor stator components such as diffuser, and the air speed is reduced. According to the law of conservation of energy, the reduced speed will be converted into pressure (that is, kinetic energy is converted into pressure energy).

Multi-stage compression of centrifugal compressor

Centrifugal air compressors usually adopt a multi-stage compression structure, which is similar to the principle of multi-stage screw compressors. After the air is compressed in first stage, it will be compressed in second or more stages. One stage means one compression process.

Unlike screw compressors, centrifugal compressors generally have very large amounts of air delivery, which can reach hundreds or even thousands of cubic meters per minute. Since the temperature of air increases after compression, it takes more energy to compress the air with higher temperature in the next stage. In order to reduce the power consumption of centrifugal compressors, the temperature of compressed air needs to be lowered.

Therefore, in multi-stage centrifugal air compressor systems, a segmented inter-cooling structure is often used. The more compression stages there are, the closer it is to isothermal compression, and the centrifugal compressor is more energy-efficient and has higher compression efficiency.

The working process of centrifugal compressor

The air system of a centrifugal air compressor is not complicated. The outside air enters the compressor’s air inlet after being filtered by the air inlet guide vanes and air filters. The impeller rotates, and under the action of centrifugal force, the air flows along the blade channel (the channel between the blades) to the outer edge of the impeller. The impeller blades drive the air to rotate, transfer kinetic energy to the air, and the air pressure and speed increase accordingly.

The air is then thrown out along the gradually opening tangent on the impeller and enters the vortex chamber and diffuser. The diffuser will reduce the flow rate of the air and the air pressure will be further increased.

The air flowing out of the diffuser flows through the inter-cooler for cooling under the guidance of the stator parts such as the bend and return flow device, and then enters the next stage impeller for further compression until its air pressure reaches the system design pressure and is discharged from the centrifugal compressor outlet.

During the entire compression process, the specific volume of the gas decreases and the temperature increases. When the temperature increases, it takes more energy to compress the same unit of gas. Therefore, in order to save energy and power, multi-stage centrifugal compressors often use inter-cooler when the pressure ratio is greater than 3. The gas enters the inter-cooler from the previous stage, and after being cooled it enters the next stage for further compression.

While the impeller rotates continuously, the gas is continuously sucked in and thrown into the diffuser, thus maintaining the continuous flow of gas. For each centrifugal gas compressor, in order to achieve the required design pressure, each compressor has a different number of stages and sections, and may even consist of several cylinders.

Centrifugal compressor diagram

Centrifugal compressor working conditions

Centrifugal compressors are designed for large flow, high pressure and continuous operation. Theoretically, the pressure-flow characteristic curve of a centrifugal compressor is a straight line, but due to the friction resistance and other losses inside the centrifugal compressor, the actual pressure and flow characteristic curve decreases gently with the increase of flow, and the corresponding compressor power-flow curve increases with the increase of flow.

When the centrifugal compressor runs at a constant speed, its operating point will move along the pressure-flow characteristic curve. The operating point of the centrifugal compressor during operation depends not only on its own performance, but also on the characteristics of the system.

When the pipe network resistance increases, the pipeline performance curve will become steeper.

The basic principle of centrifugal compressor regulation is to obtain the desired operating conditions by changing its own performance curve or the external pipe network characteristic curve.

Isothermal compression in centrifugal compressors

In principle, isothermal compression has high compression efficiency and can compress the gas to the target volume while keeping the compressed gas temperature unchanged, thus avoiding some energy losses in the compression process. Compared with isobaric compression and adiabatic compression, isothermal compression has less energy loss and can effectively improve compression efficiency.

The more compression stages there are, the closer it is to isothermal compression, and the higher its efficiency. Considering the pressure loss of the inter-cooler and the leakage of the shaft seal, the efficiency difference between 2-stage compression and 3-stage compression is about 3.5%, and the efficiency difference between 3-stage compression and 4-stage compression is about 1%.

The working principle of a single-stage high-speed centrifugal compressor is as follows: the prime mover drives the impeller to rotate at high speed through the shaft. The airflow enters the high-speed rotating impeller from the inlet axial direction and becomes radial flow and is accelerated.

Then it enters the expansion chamber, changes the flow direction and decelerates. This deceleration effect converts the kinetic energy in the high-speed rotating airflow into pressure energy (potential energy), so that the compressor outlet maintains a stable pressure.

What is the use of centrifugal compressor?

Application of Centrifugal Air Compressor

Centrifugal compressor features large amount of air delivery and high air pressure and is widely used in industries such as automobiles, chemicals, pharmaceuticals, mining and gas synthesis and separation, various industrial manufacturing fields that require high standard compressed air. Centrifugal air compressors are suitable for large and medium flow, medium and high pressure occasions.

Industrial field: In industrial production, centrifugal compressors are widely used in ventilation, exhaust, dust removal and high pressure fluids transportation. For example, in chemical, pharmaceutical, metallurgical, oil refining, natural gas processing and other industries, centrifugal compressors can be used to transport gases, flue gases and powders, compress natural gas as CNG compressors, and can also play a role in cooling, dust reduction and air purification.

Environmental protection field: In the field of environmental protection, centrifugal compressors are used for dust removal, smoke exhaust and sewage treatment. For example, in equipment such as chimneys, boilers and incinerators, centrifugal compressors can help exhaust flue gases and reduce air pollution.

Agricultural field: In agriculture, centrifugal compressors can be used for ventilation and cooling in greenhouses, poultry and livestock farms and other places. It can provide sufficient air circulation, help regulate temperature and humidity, and improve the growth environment of crops.

Food processing industry: The food industry uses centrifugal compressors in refrigeration systems for cooling and food preservation. The compressor is part of the refrigeration cycle and works by increasing the refrigerant pressure and raising its temperature. High reliability and energy efficiency are desirable features in this industry.

In addition to the above scenarios, centrifugal compressors are also used in many other fields, such as ventilation and smoke exhaust in public facilities like tunnels and subways. In short, as an efficient and stable ventilation and air compressing equipment, centrifugal air compressors have a very wide range of applications.

Centrifugal compressor manufacturers

The speed of centrifugal compressors is generally more than several thousand rpm, and some have reached more than 25,000 rpm. The required power can reach tens of thousands of kilowatts, and the flow rate has reached 10,000 m3/min. The main centrifugal compressor manufacturers in the world include Ingersoll Rand, Atlas Copco, IHI, Siemens Energy, CompAir, etc. In 2023, the top three manufacturers in the world accounted for about 32.0% of the market share.

According to the latest report of the QYResearch research team, “Global Centrifugal Compressor Market Report 2023-2029”, the global centrifugal compressor market is expected to reach US$2.22 billion in 2029, with a compound annual growth rate of 2.5% in the next few years.

Advantages of centrifugal compressor

Structural characteristics of centrifugal air compressors

The main components of the centrifugal air compressor unit are simple in structure and reliable in operation;

The manufacturing of the main components, aerodynamic part, inter-cooler, integral transmission, lubrication system and operating controls of the centrifugal air compressor can ensure reliable performance;

The centrifugal air compressor does not have any lubricated parts in the compressed air duct, ensuring pure and oil-free compressed air;

The centrifugal air compressor’s precisely balanced flexible disc coupling minimizes vibration and does not require coupling lubricant, and other connection methods can also be provided;

The compact complete base of the centrifugal air compressor integrates the frame, intercooler and lubricating oil tank, thus having excellent torsional stiffness;

Centrifugal compressors 100% oil-free compressed air

Advanced technology ensures the best performance of components. Pneumatic components such as impellers and diffusers are designed to ensure efficient operation within the working range, and various tests are carried out on site. Meet all aspects of user needs.

Backward-curved three-dimensional flow impeller optimizes blade angle. The backward-curved impeller design and unique shaft-wheel connection make the compressor efficient and stable throughout the entire operating range. The use of anti-corrosion and wear-resistant materials prolongs the service life of the impeller.

Bearings The inclined pad journal bearings installed on the pinion shaft prevent oil film vibration during high-speed operation and extend service life. The four-lobe tapered thrust shaft bearings installed on the large gear shaft reduce friction losses.

Horizontally split gearbox The horizontally split gearbox is easy to disassemble and assemble. There is no need to disassemble the piping system when repairing the gears. This rigidly designed gearbox absorbs vibration and reduces noise.

Centrifugal compressor and screw compressor comparison

The centrifugal compressor is a speed compressor. Compared with other compressors, it has large exhaust volume, uniform exhaust, and pulse-free airflow; it has a high speed, does not require lubricating oil in the machine, has good sealing effect, and has less leakage; it has a flat performance curve, a wide operating range, and is easy to automate and scale up; it has fewer wearing parts, less maintenance, and a long operating cycle.

Centrifugal air compressors compress and discharge air through high-speed rotating centrifugal wheels, and have the advantages of large exhaust volume, uniform exhaust, pulse-free airflow, high speed, no need for lubrication inside the machine, good sealing performance, few wearing parts, less maintenance, and long operating cycle.

This type of compressor is suitable for occasions with high compression ratios, such as chemical, petroleum, and electric power industries, and can provide higher output pressures, but the energy consumption is relatively high.

The screw air compressor compresses and discharges air through the rotation of two screws inside. It has the advantages of simple structure, high reliability and easy maintenance. It is suitable for occasions with low compression ratio, such as air conditioning, painting, food processing and other industries. It has low energy consumption, but provides low output pressure.

The characteristics of screw air compressors also include small size, light weight, low vibration and low noise, but they require continuous oil supply and lubrication. The oil circuit system is relatively complex, and there is a leakage gap in the screw rotor, after long time use, it may also need to remove oil sludge and carbon deposits. The volumetric efficiency is slightly lower than that of the reciprocating type.

In general, the choice of centrifugal air compressor or screw air compressor should be determined according to the specific application and needs. If a large amount of gas needs to be processed and the gas purity is high, a centrifugal air compressor is a better choice.

If the application scenario has stricter restrictions on equipment size, energy consumption and cost, a screw air compressor may be more suitable.

Maintenance and troubleshooting of centrifugal compressor

The maintenance and overhaul of centrifugal air compressors mainly include daily maintenance, regular maintenance, preventive maintenance and troubleshooting.

‌Daily maintenance‌

Check and replace the filter, regularly check the filter for blockage or reduced filtering effect, and replace it in time.

Clean the cooler, regularly clean the dust and dirt on the surface of the cooler to ensure good heat dissipation.

Check the electrical system, regularly check the electrical system to ensure that the terminal is not loose, there is no heating phenomenon, the fuse is not blown, and the electrical circuit is not damaged.

‌Regular maintenance‌

Replace wearing parts, according to the use and manufacturer’s recommendations, regularly replace wearing parts such as bearings, gears, seals, etc.

Check the screw rotor and cylinder, regularly check whether the screw rotor and the inner wall of the cylinder have wear, cracks or scratches.

Calibrate the pressure gauge, regularly calibrate the pressure gauge to ensure its accuracy.

‌Preventive maintenance‌

Develop a preventive maintenance plan, regularly perform performance tests on the compressor according to the manufacturer’s recommendations or based on the number of hours the equipment is running, to ensure that its efficiency and output meet the standards.

‌Troubleshooting‌

The high exhaust temperature of the unit may be caused by the coolant level being too low: The oil cooler being dirty, or the oil filter element being clogged. The corresponding parts need to be checked and cleaned or replaced.

The high oil consumption of the unit or the high oil content of the compressed air may be caused by too much coolant, the return oil pipe being clogged, or the return oil pipe being improperly installed. The corresponding parts need to be checked and adjusted.

The low pressure of the unit may be caused by the actual gas consumption being greater than the unit output gas volume, the vent valve failure, or the intake valve failure. The corresponding parts need to be checked and repaired.

The high exhaust pressure of the unit may be caused by the failure of the temperature control valve, the oil cut-off solenoid valve not being energized, or the coil being damaged. The corresponding parts need to be checked and repaired.

In short, the maintenance and overhaul of centrifugal air compressors are of great significance to ensure the normal operation of the equipment, extend the service life, and reduce maintenance costs.

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