Note: Descriptions are shown in the official language in which they were submitted.
BEAkING COOLING ARRANGEMENT FOR AIR CYCLE MACHINE
Technical Field
This invsntion relates to air cycle machines having
hydrodynamic bearings.
Background Art
Environmental control systems for aircraft typically
employ air cycle machines and heat exchangers to cool and
condition high pressure air supplied by either the engines
or the auxiliary power unit. A compressor and fan in these
machines are powered by a shaft connected to a turbine. The
pressurized supply air passes first into the compressor.
Outlet flow from the compressor, heated and further
pressurized by the compression step, is chilled as it passes
through the warm path of a heat exchanger. To sufficiently
reduce the temperature of the air passing through the warm
path, the fan draws cooler ambient air through the cooling
path of the heat exchanger. Chilled air exiting the warm
path of the heat exchanger is then expanded in the turbine
to further cool it before it enters the aircraft cabin.
Since the cabin air is maintained at a lower pressure than
the supply air, properly designed systems pro~ide
conditioned air at temperatures low enough to cool both the
cabin and the aircraft avionics.
To support the shaft connecting the turbine to the
compressor and the fan, air cycle machines typically use
three bearings. Two of these three bearings are journal
bearings, and are configured to prevent the shaft from
shifting radially~ The third, a thrust bearing, fixes the
axial orientation of the shaft. For optimum machine
performance, very small clearances ~etween the stators fixed
to the machine housing and the tips of the fan and
compressor blades must be maintained. Since the compressor
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and turbine rotors, to which the blades attach, are
connected to the shaft, should the baarings allow more than
slight amounts of free play, the shaft would shift when
loaded and the blade tips would contact the stator surfaces
encircling them.
As they offer minimal free play and reliable operation
at high speed, hydrodynamic fluid film journal and thrust
bearings are used to locate the shaft radially and axially,
respectively. The inner race of each of these bearings
connects to, or is a part of, the shaft, and the outer race
of each attaches to the housing. When the shaft rotates,
hydrodynamic forces are generated in fluid contained in the
space between the inner and outer races of each bearing.
These forces combine to yield a high pressure region in each
bearing sufficient to oppose loads applied to the shaft.
To ensure that the magnitude of these hydrodynamic
bearing forces remains constant during operation, the
clearance between the inner and outer races must be
maintained within a fairly narrow range. However, the
hydrodynamic effect responsible for producing the high
pressure region between the races of a rotating hydrodynamic
bearing also generates heat. To minimize nonuniform thermal
expansion and regulate inner race-outer race clearance,
coolant is used to carry this heat away from the bearings.
US Patent 4,500,143 describes a roller bearing and
journal assembly that employs oil and air both to regulate
the clearance between inner and outer races and to lubricate
the system. Cool pressurized oil circulates through
passages adjacent to both the inner race of the roller
bearing and the journal encircling the bearing. The flow
rate of the cooling oil is selected to limit, during hot
operation, the thermal expansion of the inner and outer
surfaces to within a specified range. Holes drilled
radially into the cooling passages at periodic intervals
bleed a portion of this oil flow into the bearing chamber,
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directly lubricating and cooling the rollers comprising the
bearing. To prevent the inner race from being overcooled to
the point where the roller bearing and journal clearance
increases beyond the specified range, warm air is introduced
in~o a second passageway adjacent to the inner race. By
applying air in this fashion, only the inner race expands,
and the bearing-journal clearance r~mains su~ficiently
small.
In US Patents 4,503,683 and 4,507,939 a shaft
lG supporting a turbine, compressor, and fan in an air cycle
machine is axially and radially constrained by one air
thrust, and and two air journal, bearings. A portion of the
turbine inlet air is extracted, serving as a coolant that
lubricates, cools, and supports these three bearings. A
first portion o~ this coolant flows first into the thrust
bearing cooling flowpath. A labyrinth seal at one end of
the thrust bearing forces the coolant to exhaust from the
other end. The slightly warmed coolant then flows directly
into the inlet of the first journal bearing cooling
flowpath. A labyrinth seal at the outlet of this cooling
flowpath meters the mass flow rate of air passing through
both the thrust and the journal bearing cooling flowpaths.
This seal is critical, as flow exiting the first journal
bearing cooling flowpath exhausts directly into the fan
circuit. Without this facility for metering the cooling
circuit flow, an excessive mass of air is extracted from the
turbine inlet and wasted. Additionally, with no seal, the
pressure of the coolant in both bearing flowpaths drops to
the air pressure in the fan circuit, which is approximately
equivalent to ambient pressure. As the density of the
coolant at ambient pressure is too low to adequately support
the bearings, the inner race contacts the outer rac~,
causing excessive friction and potentially damaging wear.
A second portion of the coolant extracted from the
inlet of the turbine is delivered to the second journal
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bearing cooling flowpath. The second journal bearing has
labyrinth seals at both ends. The first of these seals
allows no flow, and the second seal meters the amount of
coolant allowed to flow throuyh this second journal bearing,
similar to the way the seal on the first journal bearing
meters flow through the thrust and first journal bearings.
The inlet to the second journal bearing flowpath is located
adjacent to the first seal. Coolant therefore flows along
the length of the bearing, exhausting through the second
seal into the fan circuit.
Other, less relevant patents that generally relate to
hydrodynamic bearing applications are US Patents 4,306,755
and 4,580,406.
Disclosure of Invention
Objects of the invention include improvements in
hydrodynamic bearing cooling circuits.
Further objects of the invention include extracting
entrained particulates from bearing coolant air prior to
injection into the bearing cooling ~lowpath.
According to the present invention, the mass flow rate
and pressure of coolant flowing through a hydrodynamic
bearing cooling circuit is metered upstream of the cooling
circuit.
According further to the invention, coolant circulates
through an annular plenum upstream of the bearing cooling
circuit, wherein the coolant velocity is reduced to cause
any entrained particulate matter to drop out of the coolant
flow.
In prior art systems, ~he flow of coolant in
hydrodynamic bearing cooling circuits is throttled by a
labyrinth seal, or metering holes encircling a labyrinth
seal, located at the outlet of the cooling circuit.
Regardless of the throttliny means chosen in these systems,
a labyrinth or other rotating seal must always be included
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at the circuit outlet to seal the space between the inner
and outer bearing races and prevent excessive flow through
the circuit. The pressure of the coolant flowing through
the circui~ is t~erefore maintained by properly adjusting
the flow through these labyrinth seals or metering holPs.
In the present invention, the mass flow rate of the
coolant is metered before it enters the final bearing
cooling flowpath in the circuit. The metering or throttling
orifi~es are sized to ensure adequate, but not excessive,
coolant flow through the entire circuit. Since metering is
done before the final bearing cooling flowpath in the
circuit, the labyrinth seal and/or metering hole arrangement
at the circuit outlet, as taught in the prior art, is no
longer necessary. In the present invention, therefore,
pressure in this final bearing flowpath is nearly equal to
the air pressure in the region receiving the exhausted
coolant. The operating pressure of these bearings is
critical, since a minimum coolant density is required to
generate hydrodynamic forces sufficient to oppose the
anticipated shaft loads. To ensure proper bearing
operation, the cooling circuit outlet therefore exits into a
region upstream of the compressor having pressure no lower
than the minimum desired coolant pressure.
The benefits of reducing the total number of seals
required to ensure proper flow through a hydrodynamic
bearing cooling circuit are numerous. Since fewer parts are
required to produce air cycle machines comprising these
bearings, both the complexity and cost of manufacturing
these machines decrease. Unlike static seals, rotating
seals attempt to seal a region defined by two bodies in
motion with respect to each other. Reducing the number of
these seals therefore results in fewer contact and wear
points, improving the overall reliability of the machine.
Reducing the number of fragile labyrinth seals also reduces
the risk of damaging them when the shaft is installed or
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remove~, resulting in easier machine assembly and
disassembly. Elimination o~ seals also allows the overall
shaft length to be shortened. This both lowers the weight
of the machine and increases the fundamental natural
frequency of the shaft, raising the maximum speed at which
the machine can be run without exciting shaft oscillations.
The foregoing and other objects, features, and
advantages of the present invention will bacome more
apparent in the light of the following detailed description
of exemplary embodiments thereof, as illustrated in the
accompanying drawings.
Brief Description of Drawings
FIG. 1 is schematic diagram of an air cycle machine
incorporating the present invention;
FIG. 2 is a broken away side view showing a cooling
circuit for hydrodynamic bearings in the air cycle machine;
FIG. 3 is an enlarged view taken along the line 3-3 in
FIG. 2, showing a second gas foil journal bearing in greater
detail;
FIG. 4 is a sectional view, taken on the line 4-4 in
FIG. 3, showing, not to scale, a schematic end view of the
second gas foil journal bearing; and
FIG. 5 is a sectional view, taken on the line 5-5 in
FIG. 2, showing the configuration of a flow tube that
extracts coolant to supply the cooling circuit.
Best Mode for Carrying Out the Invention
Referring now to FIG. 1, in an aircraft air cycle
machine, a hollow shaft 10 connects a turbine 12, a
compressor 14, and a fan 16. The compressor 14 further
compresses supply air 18 delivered to the compressor inlet
from either the aircraft engine compressor bleed system (not
shown~ or an auxiliary power unit (not shown). Outlet air
20 exiting the compressor 14, heated in the compression
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step, passes subsequently to the warm path of a heat
exchanger 22. To lower ~he temperature of the outlet air
20, cooler ambient air 24 is drawn through the cooling path
of the heat exchanger 22 by the fan 16. Cooled air 26
exiting the warm path of the heat exchanger 22 then passes
to the turbine 12. Expanding ~his air 26 in the turbine 12
not only produces the power necessary to drive the
compressor 14 and ~an 16, but chills it as well, allowing it
to be used to cool and condition the aircraft cabin (not
shown~.
Referring now to FIG. 2, a first gas foil journal
bearing 28, located between the compressor 14 and the
turbine 12, and a second gas foil journal bearing 30,
located at the forward end of the machine between the
compressor 14 and the ~an 16, radially locate and support
the shaft 10. A gas foil thrust bearing 32, located at the
aft end of the machine near the inlet of the turbine 34,
ensures that proper orientation is maintained when the shaft
10 is axially loaded. Each bearing is composed of an inner
and an outer race. FIGS. 3 and 4 show expanded views of the
second journal bearing 30. The inner race 31 is formed
integrally with the shaft 10. The outer race 33 has groves
cut into its outside surface. Compression of O-rings 35
installed in these grooves 33 against an inner surface of
the housing 39 fixes the second journal bearing 30 with
respect to the housing. A foil pack 41 separates the inner
31 from the outer 33 raceO
The first journal bearing 28 has an inner race/outer
race and foil pack configuration identical to the second
journal bearing 30. Thrust beaxing 32 construction is based
upon similar principleq, but the inner race, outer race, and
foil pack are all planar disks, instead of cylindrical
sleeves. When the shaft 10 rotates, hydrodynamic forces
inside these gas foil bearings 28, 30, 32 combine to produce
a pressuri2ed region at the inner race-outer race interface,
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both driving the foil pack away from contact with the inner
race and opposing any axial or radial loads tending to force
the shaft ~0 from its desired orientation.
Both to produce the necessary hydrodynamic pressures
and to minimize axial and radial free play, during shaft
rotation the inner race-outer race clearance in each bearing
is small. Should this clearance increase, the hydrodynamic
pressures developed may be insufficient to support the shaft
10. Should the clearance decrsase, the inner race could
contact the outer race, causing wear and friction.
Clearance fluctuation is a concern, as developing and
maintaining the pressurized region within each of these
bearings generates heat which could cause the inner and
outer races to expand. Coolant 36 therefore passes over the
races in each bearing, controlling the thermal expansion of
each component to maintain inner race-outer race clearance
within some predetermined critical range.
Referring to FIG. 2, a portion of the air in the
turbine inlet 34 serves as this coolant 36. Turbine inlet
air is both the coolest, having passed through the heat
exchanger 22l as shown in FIG. 1, and the highest pressure
air available in the system. Typically, when the machine is
operated at sea level conditions, the pressure of the air in
the turbine inlet 34 ranges between 40 and 50 psig (280 to
350 kPa). The pressure and density of the coolant 36 is
important, since it not only cools the bearing, but
lubricates and supports it as well. The greater the coolant
pressure, the greater the hydrodynamic forces generated
within each bearing, and the larger the loads that can be
handled by the bearing. Based upon anticipated peak bearing
loading, therefore, each bearing 28, 30, 32 has a critical
coolant pressure which must be met or exceeded to ensure
proper operation of the machine.
Referring to FIGS. 2 and 5, coolant 36 flows from the
turbine inlet through a flow tube 38. The design and
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orientation o~ this flow tuhe 38 prevent dirt, water, and
other particulates entrained in the turbine inlet air from
entering and clogging the cooling circuit 40 that delivers
coolant 36 to each ~earing 28, 30, 32 in series. The inlet
end 42 of the flow tube 38 is scarf cut and extends
perpendicularly into the ~low of air 44 circulating through
the turbine inlet 34. The flow tube 38 is rotated such that
the opening 42 formed by the scarf cut faces away from the
airflow 44. With this configuration, only entrained
particles able to rapidly change direction can remain in the
coolant flow 36 passing through the tube 38. Since only
lighter and smaller particles are able to change direction
quickly enough, orienting the inlet 42 of the flow tube 38
in this fashion removes a significant percentage of the
particles large enough to clog the bearing cooling circuit
~0 .
To further minimize the concentration of air~orne
contaminants, the outlet 46 of the flow tube is directed
into an annular plenum 48 that separates the turbine 12 from
the compressor section 14. Before entering the inlet holes
50 in the gas foil thrust bearing 32 located adjacent to the
flow tube outlet 46, the coolant 36 circulates
circumferentially through nearly the entire plenum 48. As
the flow area of the plenum 48 is considerably greater than
the flow area of the bearing cooling circuit 40, the
velocity of the coolant 36 passing through the plenum 48 is
very low. The coolant 36 no longer moves quickly enough to
support any but the lightest and smallest particles, so most
of the remaining airborne contaminants fall into a
collection reservoir 52 at the lowest point in the plenum
48.
Coolant 36 enters the thrust bearing inlet hole 50 and
splits at the outside edge of the inner race to flow over
both the forward 56 and the aft 54 inner race surfaces. Air
flowing over the aft 54 surface exhausts into a cavity at
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the backface of the turbine rotor 58 through a labyrinth
seal 60 on the a~ side of the bearing 32. To ensure
adequate cooling flow through the other bearings 28, 30, the
gap between the labyrinth seal 60 and the shaft 10 7 S sized
to allow only roughly one-third of the coolant to pass over
the aft surface 54 of thP thrust bearing 32. The remaining
two-thirds of the coolant passes over the the forward inner
race surface 56 before exiting through the thrust bearing
outlet holes 62.
Coolant 36 leaving the thrust bearing outlet holes 62
flows directly in~o inlet holes 64 on the aft end of the
compressor journal bearing 2~, traversing forward over the
length of the bearing. A labyrinth seal 66 at the forward
end of the bearing 2~ prevents coolant 36 from passing into
a cavity at the backface of the compressor rotor 67. All
coolant 36 therefore exhausts through outlet holes 68 in the
inner race, passing directly into the hollow interior of the
shaft 10.
Referring to FIGS. 2, 3, and 4, through the hollow
interior of the shaft 10, coolant 36 flows forward from the
compressor journal bearing 28 to the fan journal bearing 30.
Throttle orifices 70 in the shaft wall serve as fan journal
bearing inlet holes, allowing the coolant 36 to enter the
fore end of that bearing. A labyrinth seal 74 at the
forward end of the fan journal bearing 30 prevents coolant
36 from entering the fan circuit 76. As there is no seal at
the aft end of the fan journal bearing 30, coolant 36 flows
from the forward to the aft end of the bearing, exhausting
directly into the compressor inlet 72.
Sizing the throttle orifices 70 so that a desired mass
flow rate through then is maintained when operating at
baseline conditions requires that the pressure at both the
inlet and outlet of the orifices be known at these operating
conditions. The inlet pressure is the pressure of coolant
contained within the hollow shaft lO~ As the coolant has
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passed through the thrust 32 and first journal bearing 28
cooling flowpaths be~ore reaching the hollow shaft cavity,
the coolant 36 pressure at the orifice inlets is typically
two to three psi (15 to 20 kPa) below the turbine inlet
pressure. To determine the pressure at the outlets of the
orifices 70, the pressure drop across the second journal
bearing cooling flowpath needed to maintain the desired mass
flow rate of coolant 36 through that flowpath is calculated.
The pressure at the outlet of the orifi~es 70 is therefore
the compressor inlet pressure typically around 35 psig (240
kPa~, plus the calculated pressure drop. Based on these
inlet and outlet pressures, throttling orifices 70 are
selected that allow the desixed mass flow rate of coolant 36
to circulate through the bearing cooling circuit ~0.
Although the invention has been shown and described
with respect to exemplary embodiments thereof, it should be
understood by those skilled in the art that various changes,
omissions, and additions may be made therein and thereto,
without departing from the spirit and scope of the
invention.
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