Axial flow compressor
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,
- 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.
- It has more complexity and cost.
- High starting power requirements.
Applications of Axial flow Compressor :
Applications of axial flow compressor are as follows,
- Constant load applications such as in aircraft gas turbine engines.
- Fossil fuel power stations; where gas turbines are used for topping up the station output when normal peak loads are exceeded.
- Large marine gas turbine plant.
- Blast Furnace
- Air separation plants
- Nitric acid plants
Difference between Axial Flow compressor and Centrifugal Compressor :
Comparison between Centrifugal and Axial Flow Compressors as follows,
Sr. no Parameters Centrifugal Compressor Axial Flow Compressor 1. Direction of flow Radial flow Flow is Parallel to the direction of the axis of the machine 2. Pressure rise per stage Low-pressure ratio per stage i.e 4.5:1; thus compact unit (Supersonic compressor 10:1
but cost of efficiency)
Low-pressure ratio per stage i.e 1.2:1 due to the absence of centrifugal action.
10-20 stage required to achieve pressure ratio equal to centrifugal compressor
3. Isentropic Efficiency 80-82% 86-88% 4. Operation Wide range of operation between surging and choking limit.
The head capacity curve is flat. The part-load performance is better.
Narrow range of operation between surging and choking limit.
The Part load performance is poor.
5. Frontal area Larger frontal area. Smaller frontal area for the same mass flow rate than that of centrifugal compressor. 6. Deposits The accumulation of deposits on the surface of flow passage does not adversely affect the
The accumulation of deposits on the surface of flow passages adversely affects the performance. 7. Starting power Need low starting torque Need high starting torque 8. Construction & Cost Simple, rigid, and relatively cheap. At high altitude less sensitive to icing troubles. Complex and costly. It is sensitive to icing troubles at high altitudes. 9. Suitability for multistage Slightly difficult Most suitable for multistage 10. Applications Low-pressure Refrigeration, Large central air conditioning plant, Supercharging in I.C Engines Due to higher efficiency and smaller frontal area mostly used
in Jet engines & large marine gas turbine plant.
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