INTERVIEW SHORT QUESTION ANSWERS–CENTRIFUGAL COMPRESSOR

INTERVIEW SHORT QUESTION ANSWERS–CENTRIFUGAL COMPRESSOR

Explain slip factor and work input factor in a centrifugal compressor.

Slip factor

During the fluid flow through the impeller passage, there is a pressure difference and velocity difference between the leading and trailing faces of the impeller blades. There is relatively a high pressure and low velocity on the leading edge while there is low pressure and high velocity on the trailing face. Due to this, There is a non-uniform velocity distribution at any radius of the impeller. Therefore mean direction of flow at the impeller outlet changes from the blade angle at outlet. Due to this, the tangential velocity component at outlet is reduced; the reduction in the angle and hence reduction in the tangential velocity is called slip. Slip factor is the ratio of actual whirl velocity at the outlet to the theoretical whirl velocity at the outlet of the impeller. It is represented by σs.

σs = (Actual whirl velocity/theoretical whirl velocity) impeller outlet

The theoretical whirl velocity is calculated analytically and the actual whirl velocity is measured experimentally.

The value of the slip factor nearly 0.9.

Due to slip, actual work input in the compressor is increased.

Work input factor= actual work input/theoretical work input

Work input factor is greater than 1.

2. What are surging, stalling and choking in an axial as well as in centrifugal compressors?

A.

 

Process Surging Stalling/stagnation Choking
Condition Minimum flow rate under maximum pressure  through the impeller complete breakdown in flow Maximum flow and minimum pressure
Feature Reversal of flow from diffuser to impeller.

Surge is a dynamic as well as mechanical instability that occurs in dynamic compressors. It is very dangerous. Surge occurs at low flow rates. Surge line is the line joining points of least flow rates at various speeds.

However low level of surge which causes less damage is also found in axial pumps, centrifugal pumps and blowers but not in compressors

 

Separation of flow over the impeller blades is stalling.

It is an instability.

Any further increase in flow is not possible i.e. it is under maximum flow.

It is an instability.

Cause Reversal of flow (i)           Improper  angle position of fixed vanes

(ii)         When laminar flow changes to turbulent flow over the moving blade

 

Mach number is unity
repetitive Partial reason for stall is surge or back flow Under maximum flow condition
Total disruption In between disruption Local disruption
Effects of (i)                High vibrations

(ii)             Abnormal noise

(iii)           Rapid changes in thrust cause Potential damage the bearings, rotor seals, and compressor driver (motor or turbine)

(iv)           Increase in temperature

(v)              Flow reversal to impeller outlet

 

Engine power drops, almost no compression taking place and needs reduction in fuel flow to the combustion chamber Surge Versus stall

 

This is very high speed and may cause severe damage to the compressor.

 

Steps to reduce (i)           By reducing the speed

(ii)         By reducing difference of [pressures

(iii)       By using a surge valve to send flow from diffuser end to suction side

 

It is interrelated. This can be prevented by using anti-chock valves at the compressor outlet line. These valves maintaining minimum flow resistance to the fluid flow. These valves close to restrict the flow to keep compressor from stonewalling and hence prevent Choking.

 

 

 

FINAL COMMENTS

  1. Compressor stall is a LOCAL disruption of the airflow in a gas turbine or turbocharger.
  2.  Surge which is a COMPLETE disruption of the flow through the unit.
  3. All the three (surge, stall and choking) cause disruption (instability) in the flow of air through the compressor and hence in the whole cycle. These are highly undesirable and unwanted.