CHARACTERISTIC CURVE OF CENTRIFUGAL COMPRESSOR

 

Characteristic curve or Performance curve

Character or performance of equipment represented in the form of curves or graph.

Characteristic curve of centrifugal compressor

Centrifugal Compressor Performance or characteristic curve is the curve between Polytrophic Head and Inlet Volume Flow.

Performance curve also known as compressor map.

Sample compressor map for centrifugal compressor is as shown below.

 

 Boundaries of a centrifugal compressor map are shown in below figure.

                       

Minimum speed and maximum speed are the limits maintained by the driver of the compressor.

Maximum flow limit of the curve is known as stone wall or choke. It occurs at high flow and low pressure.

Minimum flow limit of the curve is known as stone surge. It occurs at low flow and high pressure.

Both stonewall and surge effects are the unstable operating points of a compressor.

Compressor shall be operated between all the limits and that zone is called stable zone of operation.

 

 

CENTRIFUGAL COMPRESSOR

Centrifugal

Move away from the centre.

Centrifugal compressor

Compress fluid by using the principle of dynamic compression.

Fluid enters at the centre of the impeller i.e. impeller eye along its axis and exits along the circumference through the impeller vane tips.

Inlet guide vanes are installed before fluid entering the impeller which can be used for compressor control.

The part of the impeller which penetrates the inlet nozzle end is called the inducer. The inducer blades are angled towards the direction of rotation. In the centrifugal section of the impeller, fluid enters from the inducer and exit the impeller radially as mentioned above. The blades at the exit may be curved in a backward- or forward swept manner, away from or against the angle of rotation.         

Then the fluid flows into the diffuser, where the velocity is reduced in order to gain pressure. The diffuser channels are therefore usually diverging. Pressure increase also occurs due to redirection of flow.

Types of compressors

·         Single stage compressor

·         Multi stage compressor

Main components of centrifugal compressors

·         Impeller

·         Diffuser

·         Shaft

·         Gearbox (Gears and pinions)

·         Bearings

·         Seal housing.

 

 

 

COMPRESSORS

 

Introduction

Compressor – A Mechanical device used for following function.

Function

 Increase the pressure of the fluid (gases) and transport from one location to other.

Applications

Gas Transport, Power generation, Refrigeration & Air conditioning, Refineries / Oil & Natural gas processing plants / petrochemical & chemical plants for product  processing applications, Air craft engines, etc.

Principle

1.       Increase in pressure by reducing the volume of the gases. (Positive displacement compression)

2.       Increase in fluid velocity, followed by a reduction of velocity to increase the fluid pressure. (Dynamic compression)

Main types of compressors & selection criteria

1.       Reciprocating compressors:         High pressure Low flow and service.

2.       Centrifugal compressors:             Medium pressure and medium flow service.

3.       Axial compressors:                       Medium pressure and high flow service.

 A compressor selection criterion is not only based upon pressure and flow, but also suction temperature, gas composition & mode of operation shall be taken into consideration.

  

                Classification of compressors