Axial flow compressor

Introduction

Axial flow compressors are positive displacement type of compressor.
Axial flow compressors are turbomachines that increase the pressure of air or gas flowing continuously in the axial direction.

Construction of an Axial Flow Compressor

Axial Flow Compressor parts : 

• An axial-flow compressor consists of fixed and moving sets of blades in alternating sequence as shown in Fig.
• The sets of moving blades are attached to the periphery of a rotor hub and the sets of fixed blades are attached to the walls of the outer stationary casing called the stator.

Axial Flow Compressor Diagram :

• At the inlet of the compressor, an extra row of fixed vanes called inlet guide vanes are fitted; which guide the air at the correct angle onto the first row of moving blades.
• The rotor and stator blade banks must be as close as possible for smooth and efficient flow.
• The radius of the rotor hub and the length of the blades are designed so that there is only a very small tip clearance at the end of the rotor and stator blades.
• One set of the rotor blades and one set of the stator blades constitute a stage.
• The height of successive sets of blades is reduced to compensate for the reduction in volume resulting from increased pressure from stage to stage; thus the axial flow velocity constant through the compressor.
• The rotor and stator blades are so arranged that the spaces between them form diverging passages; hence the velocity of the air is decreased as it passes through them, and a rise in pressure takes place.

Working of axial flow compressor : 

• As the air passes through the moving blades, the kinetic energy added to the air and pressure rises at the expense of a reduction in the relative velocity of the air; by providing diffuser passages between blades.
• The absolute velocity of air increases due to work input to the moving blades through the rotor shaft. 

• Now, the air is then discharged at the proper angle to the first row of fixed blades; where the pressure is further increased by diffusion.
• Then, the air is directed to the second row of moving blades and the same process is repeated through the remaining compressor stages.
• It is usually arranged to have an equal temperature rise in the moving and the fixed blades; to keep the axial velocity of the air constant throughout the compressor.
• Thus each stage of the compression is exactly similar with regard to air velocity and blade inlet and outlet angles.

Advantages of Axial flow Compressor : 

Advantages of axial flow compressor are as follows,

1) It has higher efficiency than a centrifugal compressor.
2) It is more suitable for multi staging & increase in pressure with negligible losses.
3) The pressure ratio of 8:1 or even higher can be achieved using a multi-stage axial flow compressor.
4) It can handle a large amount of air inspired of small frontal area.
5) It has a high thrust per unit frontal area.

Disadvantages of Axial flow Compressor :

Disadvantages of axial flow compressor are as follows,

1. The performance is very sensitive to its mass flow rate at the design point and any deviation from the design condition causes the efficiency to drop off drastically.
2. It has more complexity and cost.
3. High starting power requirements.

Applications of Axial flow Compressor : 

Applications of axial flow compressor are as follows,

1. Constant load applications such as in aircraft gas turbine engines.
2. Fossil fuel power stations; where gas turbines are used for topping up the station output when normal peak loads are exceeded.
3. Large marine gas turbine plant.
4. Blast Furnace 
5. Air separation plants
6. Nitric acid plants 

Difference between Axial Flow compressor and Centrifugal Compressor :

Comparison between Centrifugal and Axial Flow Compressors as follows,

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