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Description:
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Windback seals work similarly to labyrinth seals except for the effect of helical
groove . These seals are essentially a tooth on stator or tooth on rotor labyrinth seal
where the grooves are a continuous helical cut like a thread . Windback seals are used in
centrifugal gas compressor to keep oil out of the gas face seal area . These face seals
cannot be contaminated by oil . A purge gas is applied to the seal to help force the oil
back into the bearing area .
The windback seal should be designed to prevent any oil contamination into the
supply plenum and to reduce purge gas leakage . The CFD simulations have been
performed with the effect of clearance , tooth width , cavity shape , shaft rotation ,
eccentricity , and tooth location on the seal leakage performance and the flow field inside
the seal . The leakage flow rate increases with increasing the pressure differential , rotor
speed , radial clearance , cavity size , and shaft diameter and with decreasing the tooth
width . The eccentricity has a minimal effect for the windback seal . From oil simulations ,
the windback seal with 25 % rotor eccentricity has some of the journal bearing action and
drives back flow into the gas plenum . However the windback seal can be used to force the oil back into the bearing side before starting the compressor by applying a purge gas
flow since the positive axial velocity inside the cavity is larger than the negative axial
velocity . m A Rw cav & / ? is constant for varying shaft rotation since the leakage flow rate
for the windback seal increases linearly as the the rotor speed increases . The leakage
flow rate for the windback seal increases as the groove size increases due to the pumping
action of the windback seal . A windback seal design based upon the numerical
simulations that minimize gas leakage and help prevent gas face seal oil contamination
was optimized .
The windback seal has two leakage flow paths . Since the leakage flow rate under
teeth of windback seals is the same as for a similar geometry labyrinth seal , the flow
under the teeth can be predicted by two -dimensional labyrinth seal analysis . An
empirical model for the leakage rate through the cavity has been developed which fits
the data with a standard deviation of 0 .12 . |