CENTRIFUGAL COMPRESSOR SHORT QUESTION ANSWERS

CENTRIFUGAL COMPRESSOR

SHORT QUESTION ANSWERS 

Question answers make the concept

clear and well understood. This helps

in handling difficulties in design and

operation.

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

Slip factor

There are pressure and velocity differences between the leading and trailing faces of the impeller blades. There is a high pressure and low velocity on the leading edge. Pressure is low & 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 reduces; the reduction in the angle and hence reduction in the tangential velocity is  slip. Slip factor is the ratio of actual whirl velocity to the theoretical whirl velocity at the outlet of the impeller. Represent it by σs.

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

 Calculate theoretical whirl velocity analytically and measure the actual whirl velocity experimentally.

The value of the slip factor nearly 0.9.

Due to slip, actual work input in the compressor increases.

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  causes less damage. It is found in axial & centrifugal pumps and blowers. It does not exists in centrifugal 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
 Interrelate it.
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. Surge, stall and choking cause instability in the flow of air through the compressor. These is highly undesirable and unwanted.