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Compressors Unit 1 – Dynamic Compressors

DYNAMIC COMPRESSORS

 

In a dynamic compressor, the pressure rise is obtained by imparting kinetic energy to a continuous flowing gas stream and converting this energy to pressure energy by decreasing the velocity of the gas in a widening passage.

 

There are two general types of dynamic compressors namely radial-flow and axial compressors.

Radial-flow / centrifugal compressors

The radial-flow compressor, also known as a centrifugal compressor, is widely used and is probably second only to the reciprocating compressor in process industry application.

Centrifugal compressors raise the pressure of air by imparting velocity energy, using a rotating impeller, and converting it to pressure energy.

Figure 1: Cut off view of a single stage centrifugal with a closed type impeller

Centrifugal compressors, originally built to handle only large volumes of low pressure gas and air (maximum inlet pressure of 300 kPa), have been improved over the years to move large volumes of gas with discharge pressures up to 24,000 kPa.  These high pressures are easily achieved using a multistage compressor having several impellers on a common shaft.

Figure 2: Multi-stage radial compressor with flow from bottom to top

 

Types of centrifugal compressors

There are two main types of centrifugal compressors namely:

The single-stage centrifugal compressor:

The single stage compressor consists of a single rotating disk or impeller.

The multi-stage centrifugal compressor:

This compressor consists of many impellers that turn on one rotor as shown in the figure below.

Figure 3: Multi-stage radial compressor with flow from bottom to top

Components of centrifugal compressors

The centrifugal compressor consists of an impeller which turns and forces the gas up to the rim.  The impeller is either driven by an electric motor or it is driven by the exhaust fumes of a combustion chamber, or the gas from a turbine.  The compressor is situated inside a casing with the inlet port towards the centre of the impeller and the out ward port towards the sides of the impeller.

Figure 4: Picture showing the single-stage centrifugal compressor and the impeller

Figure 5: Impellers for single-stage (left) and multi-stage (right) compressors

Operating principles of a centrifugal compressor

The centrifugal force utilised by a centrifugal compressor is the same force utilised by a centrifugal pump.  Therefore a simple diagram of a centrifugal pump is used to explain the functioning of a centrifugal compressor.  Even though there are two different types of centrifugal compressors they both work the same way.

Figure 6:  Simplified diagram of a centrifugal pump

The air or gas enters the eye of the impeller, indicated as D in the figure.  As the impeller rotates, air is thrown against the casing of the compressor.

The air becomes compressed as more and more air is thrown out to the casing by the impeller blades.

The air is pushed along the path designated A, B, and C.  The pressure of the air is increased as it is pushed along this path.

Centrifugal compressors are oil-free by design and have few moving parts, hence decreasing maintenance requirements and costs.  They are designed to handle a base or continuous load in compressed air systems because they have limited turn-down or reduced output capability.

Advantages and disadvantages of a centrifugal compressor

Advantages

  • Centrifugal compressors are relatively simple to manufacture,
  • They cost less than most compressors,
  • The starting power requirements are low,
  • They are relatively light in weight,
  • They have a good operating efficiency over a wide range of speeds

Disadvantages

  • One of the disadvantages of a centrifugal compressor is that it can be surged if it is throttled down below 75-80% maximum capacity.  Surge is a phenomenon associated with the reversal of airflow back into the compressor when it cannot maintain a steady flow of air and can cause excessive vibration and mechanical damage in a short period of time.  Other disadvantages include:
  • High rotational speeds require special bearings
  • Complicated monitoring and control systems
  • High Initial cost

Applications of centrifugal compressors

Centrifugal compressors are used for heavy duty applications in the industry; it is used for high pressure applications (where the pressure exceeds 69 MPa).  It is used as turbo chargers and super chargers in internal combustion engines and it is also used in gas turbine engines.

Axial-flow compressors

Axial-flow compressors are characterised by the fact that the air is compressed while continuing its original direction of flow parallel to the axis of the compressor rotor.  The purpose of the axial compressor, like any other compressor, is to take in ambient air, increase the speed and pressure thereof and then discharge it.

Components of axial compressors

The rotor and stator are the two main elements of an axial-flow compressor.  The rotor is a rotating part of a compressor that consists of multiple rows of blades / impellers.  The stator is the fixed element of the compressor.

Figure 7: Picture showing the rotors of an axial flow compressor

Operating principles of axial compressors

The energy from the rotation of the rotor is transferred to the gas by blading of the impellers as the gas flows past.  The rotor and stator are enclosed in the compressor case.  The stator has fixed blades that force the air rearwards towards the rotor stage.

The inlet guide vanes (IGVs) are in front of the first rotor stage.  These vanes direct the intake air toward the first set of rotor blades.  Directly behind each rotor stage is a stator which directs the air to the next rotor stage for compression until the desired pressure is reached.  A simplified diagram of an axial-flow compressor is shown in the figure below.  The inlet guide vanes are not shown in the figure.

Figure 8:  Axial flow compressor

Axial compressors can accommodate more airflow than a centrifugal design of the same size.  A single rotor / stator stage generally does not provide enough compression for most applications.  For this reason, multistage devices for axial flow compressors are generally employed.

As the temperature rises, the compressor loses power.  This is avoided by compressing the air or gas partly of its final pressure, this removes most of the heat generated and then the air is compressed to its final pressure.  This is known as multi-stage compression.  The compression is approached by increasing the number of stages used.  For each pressure level there is a certain number of stages, depending on the use for which the compressor is intended.

Advantages and disadvantages of axial compressors

Advantages

  • The axial compressor multi-stages are very compact,
  • The compressor does continuous compression,
  • The compressor is easy to drive.

Disadvantages

The axial compressor experiences a low increase in pressure at each stage.

Applications of axial compressors

Axial compressors are used in applications that require high flow, applications such as large gas turbine engines need axial compressors.

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