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 breakdownin 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 overthe impeller blades isstalling.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 angleposition of fixed vanes(ii) When laminarflow changes to turbulentflow over the movingblade |
Mach number is unity |
repetitive |
Partial reason for stall issurge 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 compressiontaking place and needsreduction in fuel flow tothe combustion chamberSurge 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
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Compressor stall is a LOCAL disruption of the airflow in a gas turbine or turbocharger.
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Surge which is a COMPLETE disruption of the flow through the unit.
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Surge, stall and choking cause instability in the flow of air through the compressor. These is highly undesirable and unwanted.