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.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/ETD-TAMU-2009-05-506 |
| Date | 16 January 2010 |
| Creators | Lim, Chae H. |
| Contributors | Morrison, Gerald L. |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Book, Thesis, Electronic Dissertation |
| Format | application/pdf |
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