Gas Turbine Handbook : Principles and Practices

Microturbines 179
While this is a reliable design guideline for air bearings, lower
air pressures, increased damping, increased stability and increased
stiffness are all improved with increased surface area.
A refinement of the air bearing design is the “foil” bearing.
In a “foil” bearing design, when the shaft is stationary there is a
small preload between the shaft and the bearing caused by the foils.
When the shaft starts to turn, hydrodynamic pressure builds up that
pushes the foil away from the shaft causing the shaft to become
airborne
Another particular bearing design can be described as follows:
Water or water vapor discharged through ultra-fine porous medium is
used as the working fluid. The bearing surface is virtually hydraulically
smooth. The bearing is enclosed by the water storage, and the
water pressure is controlled so as to be higher than the atmospheric
pressure. At rest the clearance is filled with liquid water. As the
speed of rotation increases a phase change occurs through viscous
dissipation and heat transfer from the high-temperature journal to
the bearing surface converting the “liquid water” to “water vapor.”
Water vapor evaporation from the bearing surface elevates local
pressure in the bearing clearance area; this stabilizing effect due
to “hydrostatic” pressure is than superimposed on the conventional
stabilization due to “hydrodynamic” pressure. 6
The Elliott Energy Systems TA 100 microturbine (Figure 11-6)
utilizes two primary bearings. The inboard bearing is a conventional
hydrodynamic design and the outboard bearing is a rolling element
bearing utilizing ceramic balls. Both bearings are lubricated and
cooled by synthetic oil.
References
1 “Microturbines: installation and Operation” presented by Jim Watts,
Ingersoll-Rand, Distributed Energy Road Show—Sturbridge, Mass, May
2003
2 “Industrial Applications for Micropower: A Market Assessment” Prepared
for Office of Industrial Technologies, DOE and Oak ridge National
Laboratory, Prepared by Resource Dynamics Corp., November 1999.
3 “Industrial Applications for Micropower: A Market Assessment” Prepared
for Office of Industrial Technologies, DOE and Oak ridge National
Laboratory, Prepared by Resource Dynamics Corp., November 1999
4 “Clean Distributed Generation Performance and Cost Analysis”
Subcontract # 400002615, Prepared for Oak Ridge National Laboratory