COMPRESSOR WINDAGE MITIGATION
CROSS-REFERENCE TO RELATED APPLICATION
[0001] Benefit is claimed of US Patent Application Ser. No. 61/491,509, filed May 31, 201 1,
and entitled "Compressor Windage Mitigation", the disclosure of which is incorporated by
reference herein in its entirety as if set forth at length.
BACKGROUND
[0002] The disclosure relates to compressors. More particularly, the disclosure relates to
electric motor-driven hermetic or semi-hermetic compressors.
[0003] One particular use of electric motor-driven compressors is liquid chillers. An
exemplary liquid chiller uses a hermetic centrifugal compressor. The exemplary unit comprises a
standalone combination of the compressor, the cooler unit, the chiller unit, the expansion device,
and various additional components. The exemplary compressor includes a transmission
intervening between the motor rotor and the impeller to drive the impeller at a faster speed than
the motor.
[0004] The motor is exposed to a bypass of refrigerant flow to cool the motor. The exposure
of the motor rotor to refrigerant produces losses known as windage. Windage losses increase
with motor speed and with density of refrigerant exposed to the rotor.
[0005] In the United States, chillers are subject to American Refrigeration Institute (ARI)
Standard 550. This standard identifies four reference conditions characterized by a percentage of
the chiller's rated load (in tons of cooling) and an associated condenser water inlet/entering
temperature. Operation is to achieve a chilled water outlet/leaving temperature of 44F (6.67C).
The four conditions are: 100%, 85F (29.44C); 75%, 75F (23.89C), 50%, 65F (18.33C); and 25%,
65F (18.33C also). These conditions (or similar conditions along a curve of connecting them)
may provide relevant conditions for measuring efficiency. In API testing, the water flow rate
through the cooler is 2.4 gallons per minute per ton of cooling (gpm/ton) (0.043 liters per second
per kilowatt (1/s/kW)) and condenser water flow rate is 3gpm/ton (0.054 1/s/kW).
SUMMARY
[0006] One aspect of the disclosure involves a compressor having a housing assembly with a
suction port, a discharge port, and a motor compartment. An electric motor has a stator within
the motor compartment and a rotor within the stator. The rotor is mounted for rotation about a
rotor axis. One or more working elements are coupled to the rotor to be driven by the rotor in at
least a first condition so as to draw fluid in through the suction port and discharge the fluid from
the discharge port. A gap is between the rotor and stator. The compressor includes means for
isolating the gap from an outer portion of the motor compartment. The outer portion is exposed
to the stator. One or more passages are positioned to draw fluid from the gap in the first
condition.
[0007] The details of one or more embodiments are set forth in the accompanying drawings
and the description below. Other features, objects, and advantages will be apparent from the
description and drawings, and from the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a partially schematic view of a chiller system.
[0009] FIG. 2 is a longitudinal sectional view of a compressor of the chiller system.
[0010] FIGS. 2A and 2B are enlarged semi-schematic view of isolating sleeves in the
compressor of FIG. 2.
[0011] Like reference numbers and designations in the various drawings indicate like
elements.
DETAILED DESCRIPTION
[0012] FIG. 1 shows a vapor compression system 20. The exemplary vapor compression
system 20 is a chiller system. The system 20 includes a compressor 22 having a suction port
(inlet) 24 and a discharge port (outlet) 26. The system further includes a first heat exchanger 28
in a normal operating mode being a heat rejection heat exchanger (e.g., a gas cooler or
condenser). In an exemplary system based upon an existing chiller, the heat exchanger 28 is a
refrigerant-water heat exchanger in a condenser unit 29 where the refrigerant is cooled by an
external water flow.
[0013] The system further includes a second heat exchanger 30 (in the normal mode a heat
absorption heat exchanger or evaporator). In the exemplary system, the heat exchanger 30 is a
refrigerant-water heat exchanger for chilling a chilled water flow within a chiller unit 31.An
expansion device 32 is downstream of the heat rejection heat exchanger and upstream of the heat
absorption heat exchanger 30 along the normal mode refrigerant flowpath 34 (the flowpath being
partially surrounded by associated piping, etc.). The exemplary refrigerant-water heat exchangers
28 and 30 comprise tube bundles carrying water flow and in heat exchange relation with
refrigerant passing around the bundles within the shelves of the units 29 and 31. For ease of
illustration, the water inlets and outlets of the heat exchangers are not shown.
[0014] An exemplary compressor is a centrifugal compressor having a housing assembly
(housing) 40. The housing assembly contains an electric motor 42 and one or more working
elements 44 (an impeller for a centrifugal compressor; a scroll of a scroll compressor; or pistons
for a reciprocating compressor) drivable by the electric motor in the first mode to compress fluid
(refrigerant) to draw fluid (refrigerant) in through the suction port, compress the fluid, and
discharge the fluid from the discharge port. The exemplary centrifugal working element(s)
comprise a rotating impeller directly driven by the motor. As is discussed below, the exemplary
centrifugal compressor eliminates the transmission of the baseline prior art compressor. To
compensate, the motor is driven at a higher speed. With the higher speed driving, the windage
losses would otherwise be increased.
[0015] The housing defines a motor compartment 60 containing a stator 62 of the motor
within the compartment. A rotor 64 of the motor is partially within the stator and is mounted for
rotation about a rotor axis 500. The exemplary mounting is via one or more bearing systems 66,
68 mounting a shaft 70 of the rotor to the housing assembly. The exemplary impeller 44 is
mounted to the shaft (e.g., an end portion 72) to rotate therewith as a unit about the axis 500. The
exemplary bearing system 66 mounts an intermediate portion of the shaft to an intermediate wall
74 of the housing assembly. The exemplary bearing system 68 mounts an opposite end portion of
the shaft to an end wall portion 76 of the housing assembly. Between the walls 74 and 76, the
housing includes an outer wall 78 generally surrounding the motor compartment.
[0016] There is a gap 80 between the rotor and the stator (e.g., between a lamination 82 of
the rotor and a coil 84 of the stator). The exemplary motor directly drives the impeller in the
absence of a transmission. Therefore, the motor may be driven at a higher speed than the motor
of an equivalent compressor having a transmission. An exemplary operational speed range of the
rotor is 900-18,000rpm (15-300Hz). Especially at the higher portion of this range, the presence
of refrigerant in the gap may produce drag losses known as windage. Limiting pressure within
the gap limits these windage losses. With the exemplary semi-hermetic compressor, however, it
is desirable to introduce fluid into the motor compartment 60 to cool the motor. In the exemplary
system, this introduction is achieved by ports 90 and 92 in the housing. These ports receive
refrigerant via a branch line 94 (FIG. 1) extending from a port 96 along the condenser unit 29,
intermediate the condenser inlet 98 and outlet 100. This refrigerant flow is removed from the
motor compartment via drains 110 and 112 coupled to a line 120 (FIG. 1) which extends to a
port 122 of the cooler unit intermediate the cooler inlet 124 and cooler outlet 126. Thus, the
motor compartment would essentially be at the evaporator pressure which is essentially the
suction pressure Ps. The exemplary inlet 124 feeds a distributor 128 upstream of the intermediate
port 122. FIG. 1 further shows a compressor suction line 130 from the cooler unit outlet 126 to
the suction port 24 and a compressor discharge line 132 from the compressor discharge port 26
to the condenser unit inlet 98.
[0017] Accordingly, the exemplary compressor includes means for isolating the gap from an
outer/outboard portion of the motor compartment and for limiting pressure in the gap. Exemplary
means for isolating include one or more sleeves 140, 142 extending axially from ends of the
stator to isolate an outboard portion 144 of the motor compartment from an inboard portion 146
of the motor compartment which includes the gap 80. In addition to that means for isolating, the
exemplary means for limiting pressure further comprises one or more passageways 150
positioned to draw fluid (refrigerant) from the gap in at least a first condition of compressor
operation. Fluid may need to be drawn from the gap either due to leakage into the gap (discussed
below) or due to intended metering of fluid into the inboard portion of the motor compartment
for purposes of: (a) lubrication; and/or (b) cooling the rotor or an inboard (inner diameter (ID))
surface of the stator (albeit at a lower pressure than in the outboard portion 144 of the motor
compartment).
[0018] For example, an exemplary leakage path may pass through seals associated with the
bearing systems. An exemplary passageway 150 provides communication between the motor
compartment inboard portion to a suction housing plenum 160. In the exemplary compressor
configuration, the suction housing plenum surrounds the ring 162 of inlet guide vanes 166 and is
essentially at a pressure the same as a pressure Py immediately downstream of the vanes. The
exemplary compressor is modified from a baseline transmission-equipped compressor wherein a
vent 164 communicates with the suction housing plenum to vent the transmission. In the
exemplary compressor, this vent 164 is re-used as part of the passageway 150. The net effect of
the passageway 150 is to maintain the gap at a pressure PGwhich is lower than a pressure PH of
the housing outboard portion. With a suction pressure Ps and a discharge pressure PD, exemplary
PH may be at or close to the suction pressure Ps and PG at or close to the pressure Py. For
example, a difference between PH and PGmay vary over an operational range of the compressor.
A typical value of PH is 50 to 55 psia (345-379kPa). The lower pressure PH is achieved by
closing inlet guide vanes 166. At lower loads and speeds, the inlet guide vanes are relatively
closed, thereby increasing their flow restriction and increasing D R .
[0019] Table I shows exemplary D R for the four ARI load conditions in an exemplary system
using R134a refrigerant:
Table I
[0020] Centigrade temperatures and kPa pressures are conversions from the listed Fahrenheit
and psi values and thus do not necessarily add and present excess precision. Although DR is
small at 100% load, chillers are mostly operated at low load conditions. Accordingly, the overall
benefit may more reflect the low load benefit. At very low speeds, it is seen that the DR is in
excess of 25psi (173kPa) and that Pv (which provides a good proxy for PG) is less than 50%> of
Ps (which provides the proxy for PH) . Even at 50%> load, the DR is at least 20psi (138kPa) and P
is still less than 70%> of Ps. Windage loss is proportional to density and to the third power of the
speed and to the fourth power of gap radius. Reduction in windage loss at the higher speed is
achieved by reduction in density. The windage loss reduction is not directly due to lower DR but
due to lower density in the rotor cavity as a result of maintaining the gap at a lower pressure.
This becomes particularly relevant if elimination of a transmission requires high motor speed at
loads which, in a compressor having a transmission, would otherwise have much lower motor
speed. In an exemplary reengineering from a transmission-type compressor, although speed is
increased, gap radius may be decreased. The pressure reduction may combine with the gap radius
reduction to fully or partially compensate for speed-related windage loss increases relative to the
baseline.
[0021] Exemplary sleeves are relatively rigidly connected to an adjacent portion of the stator
or housing and more compliantly connected to or engaging the other. The sleeve can be either
welded or fastened by fasteners such as bolts. In case of fasteners, the sleeve will likely have a
mounting flange at the proximal end. Each exemplary sleeve 140, 142 is formed of tubular metal
stock (e.g., carbon or stainless steel) and extends from a proximal end 200 to a distal end 202.
The exemplary proximal ends 200 are mounted to respective ends of the stator core. As one
example of mounting, a flange (e.g., also stainless steel) 210 is secured (e.g., welded) adjacent
the end 200 and is fastened to the core such as via screws 212 extending into core laminations.
The sleeves each have an inboard surface 204 and an outboard surface 206. The opposite distal
end 202 of the sleeve may bear a seal or engage a seal for sealing with an adjacent portion of the
housing structure. FIGS. 2A and 2B show seals 220 as elastomeric O-rings captured in
associated grooves of adjacent portions of the housing (e.g., of respective adjacent bearing
housings). The exemplary impeller end sleeve 140 has its proximal end secured to its adjacent
end of the stator lamination stack and distal end engaging a seal carried by the bearing housing.
The opposite sleeve 142 has a flange mounted to its adjacent end of the stator lamination stack
and a distal end engaging a seal carried by the motor cover of a bearing housing carried by the
motor cover. FIGS. 2A and 2B also show seals 222 (e.g., elastomeric O-rings) in channels in the
flanges 210 to seal the flanges to the stator core.
[0022] Alternative connections might simply involve welding the end 200 to the stator core
laminations. Yet further alternative variations might involve mounting one or both sleeves to
adjacent portions of the housing and having a seal engagement with the stator.
[0023] Exemplary sleeve materials are carbon steel or stainless steel or refrigerantcompatible
plastic. Exemplary materials for the seals are refrigerant-compatible elastomers (e.g.,
O-rings)
[0024] Although an embodiment is described above in detail, such description is not
intended for limiting the scope of the present disclosure. It will be understood that various
modifications may be made without departing from the spirit and scope of the disclosure. For
example, when applied to the reengineering of an existing compressor or a compressor in an
existing application, details of the existing compressor or application may influence details of
any particular implementation. Accordingly, other embodiments are within the scope of the
following claims.

CLAIMS
What is claimed is:
1. A compressor (22) comprising:
a housing assembly (40) having a suction port (24) and a discharge port (26) and a motor
compartment (60);
an electric motor (42) having a stator (62) within the motor compartment and a rotor (64)
within the stator, the rotor being mounted for rotation about a rotor axis (500);
one or more working elements (44) coupled to the rotor to be driven by the rotor in at
least a first condition so as to draw fluid in through the suction port and discharge said fluid out
from the discharge port;
a gap (80) between the rotor and stator;
means for (140, 142) isolating said gap from an outer portion (144) of the motor
compartment, said outer portion exposed to the stator; and
one or more passages (150) positioned to draw fluid from the gap in the first condition.
2. The compressor of claim 1 wherein:
the one or more passages are further positioned to pass said fluid from the gap to a
suction housing plenum.
3. The compressor of claim 1 wherein:
one or more additional passages are positioned to pass said fluid from the outer portion of
the motor compartment to the gap.
4. The compressor of claim 3 wherein:
the one or more additional passages comprise a leakage path through one or more seals.
5. The compressor of claim 1 further comprising:
a first external port and a second external port, said ports positioned to communicate with
the outer portion of the motor compartment.
6. The compressor of claim 1 being a centrifugal compressor wherein:
the one or more working elements comprises (44) an impeller.
7. The compressor of claim 6 wherein:
the impeller is a single impeller mounted to the rotor for direct coaxial rotation therewith.
8. A vapor compression system comprising:
the compressor of claim 1;
a first heat exchanger (28) coupled to the discharge port to receive refrigerant driven in a
downstream direction in the first operational condition of the compressor;
an expansion device (32) downstream of the first heat exchanger;
a second heat exchanger (30) downstream of the expansion device and coupled to the
suction port to return refrigerant in the first operating condition; and
wherein a refrigerant flowpath branch between the first and second heat exchanger passes
through the motor compartment outboard portion bypassing the expansion device.
9. The system of claim 8 wherein the refrigerant flowpath branch passes:
from a subcooler portion of the first heat exchanger;
to the motor compartment outboard portion; and
to a portion of the second heat exchanger downstream of a distributor (128).
10. The system of claim 9 wherein:
the first heat exchanger is a heat rejection heat exchanger; and
the second heat exchanger is a heat absorption heat exchanger.
11. A method for operating the compressor of claim 1 comprising:
driving the motor to draw the fluid in through the suction port and discharge the fluid
from the discharge port;
drawing the fluid through the one or more first passageways from the gap so as to place
the gap at a gap pressure (PG); and
passing fluid into the housing outboard portion so that the fluid in the housing outboard
portion is at a pressure (PH) which is greater than PQ.
12. The method of claim 11 wherein:
PGis less than 70% of PH in a portion of an operational range; and
PH is between a suction pressure Ps and a discharge pressure PD.
13. The method of claim 12 wherein the operating is at a first speed and the method further
comprises:
increasing the rotational speed of the motor to a second speed so as to increase a ratio of
PH and PG-
14. The method of claim 11 wherein:
the compressor is used in a vapor compression system having a heat rejection heat
exchanger, an expansion device, and a heat absorption heat exchanger,
wherein:
fluid is drawn through the suction port from the heat absorption heat exchanger;
fluid is discharged from the discharge port to the heat rejection heat exchanger;
fluid from the heat rejection heat exchanger is expanded in the expansion device;
fluid expanded in the expansion device is delivered to the heat absorption heat exchanger;
and
a portion of the fluid delivered to the heat rejection heat exchanger bypasses the
expansion device and passes as said fluid to the motor compartment outer portion; and
the fluid passed from the motor compartment outer portion is delivered to the evaporator.
15. The method of claim 14 wherein:
a leakage flow of the fluid in the motor compartment outboard portion becomes the fluid
drawn from the gap in the first condition.