Types of compressor can be summarized as positive displacement compressor and dynamic compressor according to the compression methods. This article will discuss their working principles, pros and cons, and applications.

Types of compressor

There are two basic methods for air compressor to compress air, dynamic compression and positive displacement compression. Air compressors can be divided into two categories according to the two different compression methods used: dynamic compressors and displacement compressors.

Dynamic compressors compress gas into high-pressure gas through dynamic compression. Positive displacement compressors compress gas into a space with reduced volume through mechanical movement.

Positive displacement compressors are the most common air compressors. Piston air compressors, screw air compressors, scroll compressor, and rotary lobe compressors are all displacement compressors.

Dynamic compressors include centrifugal compressors, axial compressors, mixed flow compressors, magnetic suspension compressors, and air suspension compressors.

Air compressors are widely used in various industrial fields. Common usage scenarios include the production of building raw materials, processing and manufacturing of automobiles and tools, paint and coating spraying, production and packaging of food and beverages, etc.

In addition, air compressors are also widely used in fields such as electricity, ships, and industrial machinery.

What is a dynamic compressor?

Dynamic compressors rely on high-speed rotating impellers to increase the gas velocity, and then convert part of the velocity energy into pressure energy in the diffuser to achieve gas compression.

This type of compressor is also called a speed compressor. It first increases the gas flow velocity, that is, increases the kinetic energy of the gas molecules. Then the gas flow velocity is orderly reduced, the kinetic energy is converted into pressure energy, and at the same time the gas volume is correspondingly reduced. Its typical feature is an impeller that drives the gas to obtain flow velocity.

Dynamic air compressor types

Dynamic air compressors can be divided into centrifugal compressors, axial flow compressors, and mixed flow compressors according to their working principles and structural characteristics. Magnetic suspension compressors and air suspension compressors are a type of centrifugal compressor.

‌Centrifugal compressor

It is mainly composed of a rotor and a stator. The rotor includes an impeller and a shaft. The impeller has blades, a balance disk and a part of the shaft seal. The main body of the stator is a cylinder, and there are diffusers, bends, return pipes, exhaust pipes and other devices. The main working parts of a centrifugal compressor are a high-speed rotating impeller and a diffuser with a gradually increasing flow area.

(‌Centrifugal compressor impeller)

(Impeller of centrifugal compressor)

In short, the centrifugal compressor working principle is to work on the gas through the impeller.

In the flow channel of the impeller and the diffuser, the mechanical energy is converted into gas pressure energy by using the centrifugal boosting effect and the speed reduction and pressure expansion effect.

When the impeller rotates at high speed, the gas is thrown into the diffuser behind for pressure increase under the action of centrifugal force as it rotates, and a negative pressure is formed at the air inlet.

At this time, the outside air is sucked into the impeller, thereby realizing continuous compression of the gas.

In more popular terms, when the gas flows through the impeller of a centrifugal compressor, the high-speed rotating impeller causes the gas to increase its pressure on the one hand and greatly increase its speed on the other hand under the action of centrifugal force. That is, the mechanical energy of the prime mover is first converted into the static pressure energy and kinetic energy of the gas through the impeller.

(Single stage centrifugal compressor air end)

After that, when the gas flows through the diffuser channel, the flow channel cross-section gradually increases, the flow velocity of the gas molecules in front decreases, and the gas molecules behind continue to flow forward, so that most of the kinetic energy of the gas is converted into static pressure energy, which further plays the role of pressurization.

Advantages: large and continuous gas transmission, stable operation; small unit size, light weight, small footprint; few vulnerable parts of the equipment, long service life, small maintenance workload; due to the high speed, it can be directly driven by the steam turbine, which is relatively safe and easy to realize automatic control.

Disadvantages: the efficiency is not as good as axial flow compressors and reciprocating compressors; the stable operating area is relatively narrow and surge occurs.

‌Axial flow compressor

Similar to centrifugal compressors, but the impeller blades of axial compressors are arranged axially, suitable for large flow and low pressure compression requirements. Axial compressors are large air compressors with a maximum power of 150,000 kW and an exhaust volume of 20,000 m3 per minute.

Its compressor energy efficiency ratio can reach about 90%, which is more energy-efficient than centrifugal compressors.

The moving blade row and the guide vane behind it form a stage, and usually several stages constitute the stage group of the compressor. The outlet guide vane, convergent vane, inlet guide vane, stage group, diffuser, exhaust pipe and intake pipe of the axial compressor are collectively referred to as the flow section.

The guide vane is fixed in the casing and forms a stator, and the moving blades are evenly placed on the rotating shaft or wheel to form a rotor.

The gas is evenly led from the intake pipe to the convergent vane and the inlet guide vane, and enters the first stage at a certain speed. The gas is subjected to the dynamic action of the blades in the stage, and the pressure is increased due to the energy gained. The gas is compressed in sequence along each stage, gradually increasing the pressure, and is sent out through the outlet guide, diffuser and exhaust pipe.

Advantages: high efficiency, single-unit efficiency can reach 86%~92%, 5%~10% higher than the centrifugal compressor, large flow capacity per unit area, small radial size, suitable for flow greater than 1500m³/min, single stage vs two stage air compressor pressure is relatively low, single-cylinder multi-stage pressure ratio can reach 11, relatively simple structure, easy maintenance.

Mixed flow compressor

Mixed flow compressor (From: wiki) is actually a combination or variant of centrifugal and axial flow compressors.

Combination – two types of compressors are connected in series, low-pressure axial flow at the front end and high-pressure centrifugal flow at the rear end. They can be connected in series with only the gas path or use the same shaft. The structure is more compact when installed in one casing.

Variant – the airflow direction is different from that of centrifugal compressors (radial) and axial compressors (axial). The airflow direction is about 45° to the radial and axial directions. This is the so-called “diagonal flow compressor or diagonal compressor”.

What is a turbine compressor?

Centrifugal and axial compressors are also called “turbine compressors or turbo compressors”. Turbine comes from the Latin word turbo, which means “rotating object” and “blade-type machine” in Chinese, so the meaning of “turbo compressor” is also blade-type compression machinery.

Not only compressors, but also steam turbines, turbines, flue gas turbines, and expanders can be called turbines as long as they have rotating blades.

Dynamic air compressor applications

Dynamic air compressors are widely used in the following scenarios due to their efficient and continuous compression characteristics:

Industrial field: such as the production of building raw materials, processing and manufacturing of automobiles and tools, paint and coating spraying, etc.

Power industry: Dynamic air compressors are used for compressed air supply to support the operation of various industrial equipment.

Shipping industry: Dynamic air compressors provide compressed air for the operation of various equipment and systems on ships.

In the camp of dynamic compressors, centrifugal compressors are the most widely used, and axial flow compressors with larger air volume are mainly used in large projects.

Advantages and disadvantages of dynamic compressors

• ‌Advantages

‌High efficiency‌: Dynamic compressors can compress gas efficiently and are suitable for compression needs with large flow and low pressure.

‌Continuous operation‌: They run smoothly and can provide continuous gas supply.

‌Low maintenance cost‌: Some models use magnetic suspension or air suspension technology to reduce mechanical friction and extend service life.

• ‌Disadvantages

‌High cost‌: Dynamic compressors using advanced technology (such as magnetic suspension and air suspension) are expensive and have relatively high maintenance costs.

‌Limited scope of application‌: They are suitable for scenarios with large flow and low pressure, but not for high pressure occasions.

Positive displacement compressor

Positive displacement compressors are the most common type and they increase the pressure of a gas by reducing its volume. This type of compressor can be further subdivided into piston compressors, screw compressors, and rotary lobe compressors.

Piston compressor working principle

Piston air compressor is a traditional type of air compressor that uses one or more reciprocating pistons to compress air. The crank-connecting rod mechanism is used to convert the rotary motion of the prime mover into the linear reciprocating motion of the piston.

When the piston moves downward, the volume in the cylinder increases, forming a negative pressure, and the external air is sucked in; when the piston moves upward, the volume in the cylinder decreases and the air is compressed.

(KT series PET piston air compressor)

Compression process: The piston moves upward from the bottom dead center, the suction and exhaust valves are in the closed state, and the gas is compressed in the closed cylinder. As the cylinder volume gradually decreases, the pressure and temperature gradually increase until the gas pressure in the cylinder is equal to the exhaust pressure.

Exhaust process: The piston continues to move upward, causing the gas pressure in the cylinder to be greater than the exhaust pressure, and the exhaust valve opens. The gas in the cylinder is pushed by the piston and discharged from the cylinder into the exhaust pipe at equal pressure until the piston moves to the top dead center. At this time, due to the exhaust valve spring force and the gravity of the valve plate itself, the exhaust valve closes and the exhaust ends.

Advantages and disadvantages: Piston air compressors are cheap, simple and cheap to maintain, but the compressed gas of piston compressors will inevitably contain lubricating oil, so they are not suitable for industries with high gas precision requirements. Secondly, there are also problems such as high noise, gas pulses, and easy damage to the main engine.

Screw compressor working principle

The screw compressor is a positive displacement compressor with spiral rotation, and is one of the most widely used positive displacement compressors in the world. There are two kinds of rotary screw compressors: single screw compressors and double screw compressors.

The basic structure of the screw compressor is shown in the figure below. In the compressor body, a pair of mutually meshing spiral rotors are arranged in parallel. The larger rotor is usually called the male rotor or male screw; the smaller rotor is called the female rotor or female screw.

Generally, the male rotor is connected to the prime mover, and the male rotor drives the female rotor to rotate. The ball bearing on the exhaust end of the rotor enables the rotor to achieve axial positioning and withstand the axial force in the compressor.

(Screw compressor rotors)

As shown in the figure below, the working cycle of the screw compressor can be divided into three processes:

(1) suction, (2) (3) compression, and (4) exhaust.

As the rotor rotates, each pair of meshing teeth completes the same working cycle one after another.

(Screw compressor compression process)

The volume occupied by the gas sealed in the volume between the teeth decreases as the teeth move, resulting in an increase in pressure, thereby realizing the compression process of the gas. When the volume between the teeth is connected to the exhaust port, the exhaust process begins.

Advantages and disadvantages of screw compressors: large gas output, low noise, high reliability, but relatively high cost and cannot be used in ultra-high pressure applications.

Rotary lobe compressor working principle

This type of air compressor uses a rotating cylinder and vanes(lobes) that slide radially inside the cylinder. The movement of the vanes divides the compression chambers, and as the cylinder rotates, the air is compressed and discharged.

Rotary lobe compressors use traditional technology and are driven directly at a lower speed. The rotor is a continuously running part with several slots cut along the length, in which the vanes that slide on the oil film are inserted.

The rotor rotates in the stator of the cylinder. During the rotation, the centrifugal force throws the vanes out of the slots to form separate compression chambers. The rotation causes the volume of the compression chamber to decrease continuously, and the air pressure to increase continuously.

The heat generated by the compression is controlled by injecting pressurized oil, and the air is compressed.