What is axial compressor? How does an axial compressor work? Axial compressors and centrifugal compressors are both turbine compressors. The post introduces the axial compressor working principle, parts, and difference between axial compressor vs centrifugal compressor.

Types of compressor

Positive displacement compressor and turbine compressor

A compressor is a machine that converts the mechanical energy of the prime mover into gas energy. According to different air compression methods, it can be classified into positive displacement compressors (wiki)and turbine compressors.

Positive displacement compressors are of two types: reciprocating compressors and rotary compressors. The meaning of positive displacement compressors is that the increase in air pressure in these air compressors is achieved by reducing the air volume.

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Turbine compressors are divided into four categories: centrifugal compressors, axial compressors, diagonal flow compressors, and mixed flow compressors, depending on the flow direction of air in the compressor.

The working principles of turbine compressors rely on high-speed rotating blades to work on the air, and first greatly increases the flow speed of the air, and then converts the kinetic energy of the air into pressure energy.

(The figure below lists the four flow forms of turbine compressors)

Centrifugal compressor: The air flow direction in the compressor is roughly perpendicular to the rotating axis, also known as radial compressor.

Axial compressor: The air flow direction in the compressor is roughly parallel to the rotating axis.

Diagonal flow compressor: The air flow direction in the compressor is between the centrifugal and axial flow, and the flow direction forms a certain angle with the rotating axis.

Mixed flow compressor: Refers to the same air compressor with both axial flow and centrifugal (diagonal flow) working impellers. Generally, the axial flow compression structure is in front and the centrifugal compression structure is in the back.

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Axial compressor and centrifugal compressor

Axial flow and centrifugal compressors are the most commonly used turbine compressors. Both compressors are mainly composed of two parts: the rotor and the stator. The rotor and the stator also constitute the entire compression channel of these two compressors.

The gas dynamics design of axial compressors adopts the most advanced three-dimensional flow theory and optimized design method, and various reaction blade combination designs are carried out. Under the same parameter conditions, the efficiency is 10% higher than that of general centrifugal compressors.

Axial compressor parts

Rotor:

The rotor blades are evenly arranged on the rotor. There are seals at both ends of the rotor.

The entire rotor is supported on radial bearings at both ends. One end is equipped with a thrust bearing to withstand the axial thrust of the compressed gas on the rotor. The rotor is a uniform inner diameter structure.

The lateral vibration and torsional vibration analysis and calculation are considered during the design. After assembly, it undergoes low-speed dynamic balancing inspection, high-speed dynamic balancing and overspeed testing to ensure the safety and reliability of the unit during operation.

Stator:

The inlet pipe, convergent, inlet guide, outlet guide, diffuser and exhaust pipe of the axial compressor are collectively called the flow section. The guide is fixed in the casing and constitutes the stator.

One rotor and one stator make up a stage in an axial compressor. Axial compressor generally has 10~18 compression stages.

Three-layer cylinder structure:

Greatly reduces deformation and stress concentration caused by thermal expansion, avoids damage caused by dust and external conditions, greatly reduces the noise generated when airflow passes through the blade grid, and plays a role in sound insulation.

Blade bearing cylinder:

The intake side of the bearing cylinder matches the inlet ring, and the exhaust side matches the diffuser, which together with the casing and the rotor constitutes the channel of the axial compressor.

The two ends of the blade bearing cylinder are supported on the casing, the intake end is a fixed support, and the exhaust end is a sliding support, which is conducive to the thermal expansion of the cylinder body. Casting horizontal split structure, center split surface bolt connection. Equipped with stationary blades, blade bearings, cranks, and sliders.

Blades:

Rotor blades and stationary blades are all made of stainless steel forgings with high strength and high vibration attenuation rate. The raw materials are tested for chemical composition, mechanical properties, and cracks, and the formed blades are then wet sandblasted to increase the fatigue strength of the blade surface.

After many improvements, the blade structure has gradually achieved the reduction of blade surface loss, increased strength, and reduced blade vibration amplitude. Roots of rotor blades are toothed structures with strong load-bearing capacity.

Adjustment cylinder:

The adjustment cylinder is equipped with respective guide rings corresponding to each level of stationary blades, which are divided into upper and lower halves and are installed on the upper and lower cylinder bodies respectively.

When adjusting the working conditions, the electric actuator drives the adjustment cylinder to make axial reciprocating motion. The purpose of adjusting the angle of the stationary blades is achieved through the crank slider mechanism. The size of the angle adjustment of each level of stationary blades is achieved through the length of the crank at each level.

Axial compressor working principle

Axial compressors are usually composed of several stages consisting of rotors and stators. The stage is the most basic unit for analyzing the working principle of axial compressors. When airflow through the stages of axial compressor, air is uniformly led from the inlet pipe to the convergent and inlet guide vane, and enters the first stage at a certain speed.

The air is subjected to the dynamic action of the blades in the stage, and the pressure increases due to the energy gained. The air is compressed along each stage in sequence, and the pressure gradually increases, and it is discharged through the outlet guide vane, diffuser and exhaust pipe.

Elementary stage: Imagine using a cylindrical surface whose axis coincides with the compressor axis to cut the blade row of the stage to obtain a section of two rows of blades (rotor and stationary blades). Such a rotor blade annular cascade and a stationary blade annular cascade constitute a elementary stage.

In each elementary stage of the axial compressor, the channel formed by the rotor and stationary blade cascades is curved, expanded, and streamlined. When the subsonic airflow flows through the expansion channel, its speed decreases and the static pressure increases. The air obtains kinetic energy and pressure energy under the push of the rotor blades, and moves from the rotor blades inlet to the outlet.

The expanded channel formed by the stationary blades causes the airflow velocity to decrease. Therefore, in the axial compressor, the rotor blades are in front and the stationary blades are in the back, forming an elementary stage. The air first obtains kinetic energy in the rotor blades, increasing the speed and pressure of the airflow. Part of the kinetic energy is further converted into pressure energy in the stationary blades at the back.

Axial compressor diagram

Positive displacement compressors are suitable for small flow applications and can produce the largest pressure range. Centrifugal compressors are suitable for large and medium flow applications and high and medium pressure occasions. Axial flow compressors are suitable for large flow and medium and low pressure occasions.

Regardless of the type of air compressor, losses are objective during operation. Therefore, the energy transferred from the outside to the gas cannot be fully converted into effective energy of the gas pressure.

The energy transferred from the outside to the gas can be expressed by the Bernoulli equation

When gas is compressed, the pressure of the gas increases:

The kinetic energy of the gas increases:

Energy lost:

From the Bernoulli equation, we know that only a part of the mechanical work added to the gas is used to increase the pressure energy of the gas. We call it compression work to indicate that it is the part of energy used to increase the static pressure energy of the gas, which is also the part of energy we want to get the most. Compression work is related to the nature of the compression process.

Adiabatic compression: refers to an ideal process with no heat exchange with the outside world and no loss. It is more precisely called “adiabatic isentropic compression process”.

Isothermal compression: refers to always having heat exchange with the outside world, and the temperature at the end of compression and the temperature between processes are equal to the initial temperature.

Variable compression: refers to a process with loss, which can have or have no heat exchange with the outside world.

For this reason, we use efficiency to characterize the perfection of the energy conversion of the compressor during operation. Efficiency has many forms of expression, but no matter which form of expression is used, they are essentially the same, that is, their power consumption is the same.

Adiabatic efficiency = adiabatic compression work / total power consumption

Isothermal efficiency = isothermal compression work / total power consumption

Polyvariable efficiency = polyvariable compression work / total power consumption

Difference between axial and centrifugal compressor

The rotating impeller in the centrifugal compressor drives the air flow to rotate at high speed, that is, the air flows radially and obtains a large amount of centrifugal force. Then the high-speed air collides with the diffuser, and the kinetic energy of the air is converted into pressure energy, so the air is compressed.

Compared with the centrifugal compressor working principle, since the air in the axial compressor does not flow in the radial direction, but in the axial direction, the biggest feature of the axial compressor is that the gas flow efficiency per unit area is higher.

Under the premise of processing the same amount of gas, the axial flow compressor is smaller in size, that is, the axial compressor is particularly suitable for occasions requiring large flow.

In addition, the axial flow compressor also has the advantages of simple structure and convenient operation and maintenance.

However, the axial flow compressor has complex blade profiles, high manufacturing process requirements, narrow stable operating area, and small flow adjustment range at a constant speed, which is obviously inferior to the centrifugal compressor.

Axial compressor vs centrifugal compressor

From the comparison in the above table, we can see that the efficiency of axial flow compressors is higher than that of centrifugal compressors. The reasons are analyzed as follows: As can be seen from the figure below, compared with centrifugal compressors, the compression channel of axial flow compressors is straight and the flow channel is short, so the flow loss of gas during the flow process is small, and the separation loss during the flow is small, so the flow efficiency is high.

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