Answer / keshav verma

First We need to understand "what is Surge" ...Surge refers
to the reverse flow of fluid which a pump or compressor is
pumping. This happens when the pressure at discharge point
is suddenly becomes too high...which may be due to closing
or partially closing of valves...by doing so static head
inceares so much that the pressure at discharge point is
more than the thrust given by the centrifugal impeller, so
due pressure difftial at discharge and suction fluid flows
in reverse direction....since centrifugal compressors are
non positive displacement(NPD) compressors..which means no
displacing member is there which can prevent the reverse
flow (as in reciprocating), in NPD comp suction and
discharge are not physically separated. So, reverse flow
happened which is called Surging. Once the surging occurs,
the excess pressure at dis point is released and system
tries to stable again flow start in forward direction but
due restriction of closed valves the same thing happen
again..surging...this repeatition is called hunting.

Answer / vairaprakash

CENTRIFUGAL COMPRESSOR IS A NON DISPLACEMENT TYPE BECAUSE
AIR IS NOT GOING TO BE TRAPPED BETWEEN ANY VANES OR
SURFACES. BUT LIKE FILLING AIR IN THE TYRES, IN A CLOSED
VESSEL CONTINUOUSLY AIR IS PUSHED INTO IT THE PRESSURE OF
AIR INSIDE THE STORAGE TANK IS INCREASED(AIR IS
COMPRESSIBLE)

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