U.S. patent application number 15/067318 was filed with the patent office on 2017-09-14 for centrifugal compressor with casing treatment bypass.
The applicant listed for this patent is Daikin Applied Americas Inc.. Invention is credited to Fumiaki Onodera.
Application Number | 20170260987 15/067318 |
Document ID | / |
Family ID | 58387919 |
Filed Date | 2017-09-14 |
United States Patent
Application |
20170260987 |
Kind Code |
A1 |
Onodera; Fumiaki |
September 14, 2017 |
CENTRIFUGAL COMPRESSOR WITH CASING TREATMENT BYPASS
Abstract
A centrifugal compressor for a chiller system includes a casing,
an inlet guide vane, an impeller disposed downstream of the inlet
guide vane, a motor and a diffuser. The casing has inlet and outlet
portions with the inlet guide vane disposed in the inlet portion.
The impeller is attached to a shaft rotatable about a rotation
axis, and the motor rotates the shaft in order to rotate the
impeller. The centrifugal compressor further includes a casing
treatment bypass having an entrance port and an exit port. The
casing treatment bypass injects refrigerant from a gap between the
impeller and the inlet portion of the casing toward an area between
the impeller and the inlet guide vane. The exit port of the casing
treatment bypass is positioned upstream in a direction of a
refrigerant flow with respect to the entrance port of the casing
treatment bypass.
Inventors: |
Onodera; Fumiaki;
(Minnetonka, MN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Daikin Applied Americas Inc. |
Minneapolis |
MN |
US |
|
|
Family ID: |
58387919 |
Appl. No.: |
15/067318 |
Filed: |
March 11, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F04D 29/462 20130101;
F25B 49/022 20130101; F04D 27/0246 20130101; F04D 29/284 20130101;
F04D 27/0215 20130101; F04D 29/685 20130101; F04D 29/4213 20130101;
F04D 29/444 20130101; F25B 1/10 20130101; F25B 31/026 20130101;
F04D 29/4206 20130101 |
International
Class: |
F04D 27/02 20060101
F04D027/02; F04D 29/44 20060101 F04D029/44; F25B 49/02 20060101
F25B049/02; F04D 29/28 20060101 F04D029/28; F25B 1/10 20060101
F25B001/10; F25B 31/02 20060101 F25B031/02; F04D 29/42 20060101
F04D029/42; F04D 29/46 20060101 F04D029/46 |
Claims
1. A centrifugal compressor adapted to be used in a chiller system,
the centrifugal compressor comprising: a casing having an inlet
portion and an outlet portion; an inlet guide vane disposed in the
inlet portion; an impeller disposed downstream of the inlet guide
vane, the impeller being attached to a shaft rotatable about a
rotation axis; a motor arranged and configured to rotate the shaft
in order to rotate the impeller; a diffuser disposed in the outlet
portion downstream of the impeller; and a casing treatment bypass
having an entrance port and an exit port, the casing treatment
bypass being arranged and configured to inject refrigerant from a
gap between the impeller and the inlet portion of the casing toward
an area between the impeller and the inlet guide vane, and the exit
port of the casing treatment bypass being positioned upstream in a
direction of a refrigerant flow with respect to the entrance port
of the casing treatment bypass.
2. The centrifugal compressor according to claim 1, wherein the
impeller includes an impeller hub and an impeller blade surrounding
the impeller hub, the entrance port of the casing treatment bypass
faces the impeller blade, and the exit port of the casing treatment
bypass opens to the area between the inlet guide vane and the
impeller.
3. The centrifugal compressor according to claim 2, wherein a
diameter of the entrance port of the casing treatment bypass is
determined based on a diameter of the impeller blade.
4. The centrifugal compressor according to claim 1, wherein a
cross-sectional area of the exit port of the casing treatment
bypass is equal to or greater than a cross-sectional area of the
entrance port of the casing treatment bypass.
5. The centrifugal compressor according to claim 1, wherein the
casing treatment bypass includes a hole formed in the inlet portion
of the casing.
6. The centrifugal compressor according to claim I, wherein the
casing treatment bypass includes a plurality of holes formed in the
inlet portion of the casing.
7. The centrifugal compressor according to claim 1, wherein the
casing treatment bypass has a ring shape.
8. The centrifugal compressor according to claim I, wherein a flow
path area of the casing treatment bypass is fixed.
9. The centrifugal compressor according to claim 1, wherein a flow
path area of the casing treatment bypass is adjustable.
10. The centrifugal compressor according to claim 9, wherein a
movable adjusting member is disposed in the inlet portion of the
casing so as to at least partly block the flow path area of the
casing treatment bypass, and the flow path area of the casing
treatment bypass is adjusted by moving the movable adjusting member
in a radial direction perpendicular to an axial direction
corresponding to the rotation axis of the impeller.
11. The centrifugal compressor according to claim 7, wherein an
flow path area of the casing treatment bypass is adjustable.
12. The centrifugal compressor according to claim 11, wherein the
inlet portion of the casing includes a movable sub-portion which is
separated from the inlet portion of the casing by the casing
treatment bypass, and the flow path area of the casing treatment
bypass is adjusted by moving the sub-portion in an axial direction
corresponding to the rotation axis of the impeller.
13. The centrifugal compressor according to claim 1, wherein the
refrigerant is low global warming potential refrigerant.
14. The centrifugal compressor according to claim 13, wherein the
low global warming potential refrigerant is low pressure
refrigerant.
15. The centrifugal compressor according to claim 14, wherein the
low pressure refrigerant includes R1233zd.
16. The centrifugal compressor according to claim 1, wherein the
impeller is a mixed flow type impeller.
17. The centrifugal compressor according to claim 1, wherein the
impeller is a radial flow type impeller.
Description
BACKGROUND
[0001] Field of the Invention
[0002] The present invention generally relates to a centrifugal
compressor used in a chiller system. More specifically, the present
invention relates to a centrifugal compressor with a casing
treatment bypass.
[0003] Background Information
[0004] A chiller system is a refrigerating machine or apparatus
that removes heat from a medium. Commonly a liquid such as water is
used as the medium and the chiller system operates in a
vapor-compression refrigeration cycle. This liquid can then be
circulated through a heat exchanger to cool air or equipment as
required. As a necessary byproduct, refrigeration creates waste
heat that must be exhausted to ambient or, for greater efficiency,
recovered for heating purposes. A conventional chiller system often
utilizes a centrifugal compressor, which is often referred to as a
turbo compressor. Thus, such chiller systems can be referred to as
turbo chillers. Alternatively, other types of compressors, e.g. a
screw compressor, can be utilized.
[0005] In a conventional (turbo) chiller, refrigerant is compressed
in the centrifugal compressor and sent to a heat exchanger in which
heat exchange occurs between the refrigerant and a heat exchange
medium (liquid). This heat exchanger is referred to as a condenser
because the refrigerant condenses in this heat exchanger. As a
result, heat is transferred to the medium (liquid) so that the
medium is heated. Refrigerant exiting the condenser is expanded by
an expansion valve and sent to another heat exchanger in which heat
exchange occurs between the refrigerant and a heat exchange medium
(liquid). This heat exchanger is referred to as an evaporator
because refrigerant is heated (evaporated) in this heat exchanger.
As a result, heat is transferred from the medium (liquid) to the
refrigerant, and the liquid is chilled. The refrigerant from the
evaporator is then returned to the centrifugal compressor and the
cycle is repeated. The liquid utilized is often water.
[0006] A conventional centrifugal compressor basically includes a
casing, an inlet guide vane, an impeller, a diffuser, a motor,
various sensors and a controller. Refrigerant flows in order
through the inlet guide vane, the impeller and the diffuser. Thus,
the inlet guide vane is coupled to a gas intake port of the
centrifugal compressor while the diffuser is coupled to a gas
outlet port of the impeller. The inlet guide vane controls the flow
rate of refrigerant gas into the impeller. The impeller increases
the velocity of refrigerant gas. The diffuser works to transform
the velocity of refrigerant gas (dynamic pressure), given by the
impeller, into (static) pressure. The motor rotates the impeller.
The controller controls the motor, the inlet guide vane and the
expansion valve. In this manner, the refrigerant is compressed in a
conventional centrifugal compressor.
[0007] When the pressure next to the compressor discharge is higher
than the compressor discharge pressure, the fluid tends to reverse
or even flow back in the compressor. This happens when the lift
pressure (condenser pressure-evaporator pressure) exceeds the
compressor lift capability. This phenomenon, called surge, repeats
and occurs in cycles. The compressor loses the ability to maintain
its lift when surge occurs and the entire system becomes unstable.
A collection of surge points during varying compressor speed or
varying inlet gas angle is called a surge surface. In normal
conditions, the compressor operates in the right side of the surge
surface. However, during startup/operation in part load, the
operating point will move towards the surge line because flow is
reduced. If conditions are such that the operating point approaches
the surge line, flow recirculation occurs in the impeller and
diffuser. The flow separation will eventually cause a decrease in
the discharge pressure, and flow from suction to discharge will
resume. Surging can cause damage to the mechanical impeller/shaft
system and/or to the thrust bearing due to the rotor shifting back
and forth from the active to the inactive side. This is defined as
the surge cycle of the compressor.
[0008] Therefore, techniques have been developed to control surge.
See for example U.S. Patent Application Publication No.
2014/0260385 and U.S. Patent Application Publication No.
2014/0260388.
SUMMARY
[0009] In a conventional centrifugal compressor as shown in FIG.
10, a hot gas bypass may be provided to connect the discharge side
of the compressor and the suction side of the compressor to expand
the operation range of the compressor. While this technique works
relatively well, this system requires a pipe of a large diameter
for the hot gas bypass, which results in increased costs.
Especially in a case where a centrifugal compressor uses low
pressure refrigerant such like R1233zd, the specific volume of the
refrigerant is large compared to a case of conventional refrigerant
such like R134a. This requires a large-diameter pipe for the hot
gas bypass, which results in increased costs.
[0010] Also, a conventional centrifugal compressor with a hot gas
bypass requires sensitive control using a variable frequency drive
(VFD) to prevent surge.
[0011] Therefore, one object of the present invention is to provide
a centrifugal compressor that expands the operation range of the
compressor to prevent surge without increased costs.
[0012] Another object of the present invention is to provide a
centrifugal compressor that prevents surge from occurring without
performing sensitive control.
[0013] One or more of the above objects can basically be attained
by providing a centrifugal compressor adapted to be used in a
chiller system, the centrifugal compressor including a casing
having an inlet portion and an outlet portion, an inlet guide vane
disposed in the inlet portion, an impeller disposed downstream of
the inlet guide vane, the impeller being attached to a shaft
rotatable about a rotation axis, a motor arranged and configured to
rotate the shaft in order to rotate the impeller, a diffuser
disposed in the outlet portion downstream of the impeller, and a
casing treatment bypass having an entrance port and an exit port,
the casing treatment bypass being arranged and configured to inject
refrigerant from a gap between the impeller and the inlet portion
of the casing toward an area between the impeller and the inlet
guide vane, and the exit port of the casing treatment bypass being
positioned upstream in a direction of a refrigerant flow with
respect to the entrance port of the casing treatment bypass.
[0014] These and other objects, features, aspects and advantages of
the present invention will become apparent to those skilled in the
art from the following detailed description, which, taken in
conjunction with the annexed drawings, discloses preferred
embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] Referring now to the attached drawings which form a part of
this original disclosure:
[0016] FIG. 1 is a schematic diagram illustrating a chiller system
in accordance with an embodiment of the present invention which
includes a casing treatment bypass;
[0017] FIG. 2 is a perspective view of the centrifugal compressor
of the chiller system illustrated in FIG. 1, with portions broken
away and shown in cross-section for the purpose of
illustration;
[0018] FIG. 3 is a schematic longitudinal cross-sectional view of
the impeller, motor and magnetic bearing of the centrifugal
compressor illustrated in FIG. 2;
[0019] FIG. 4A is a graph illustrating expanding the operation
range of a compressor by shilling a surge line, FIG. 4B is a
schematic diagram illustrating surge and a stall at the outlet, and
FIG. 4C is a schematic diagram illustrating a stall at the
inlet;
[0020] FIG. 5A illustrates the static pressure, the flow velocity,
and the flow rate of refrigerant without a casing treatment bypass,
and FIGS. 5B-5D illustrate the static pressure, the flow velocity,
and the flow rate of refrigerant with a casing treatment bypass of
various load;
[0021] FIG. 6 is an enlarged schematic diagram inside circle 6 in
FIG. 2, illustrating the inlet guide vane, the impeller and the
casing of the centrifugal compressor of FIGS. 1-3 with a casing
treatment bypass;
[0022] FIGS. 7A and 7B are schematic diagrams illustrating other
embodiments of the casing treatment bypass;
[0023] FIG. 8A is a schematic diagram illustrating a mixed flow
impeller and FIG. 8B is a schematic diagram illustrating a radial
flow impeller;
[0024] FIG. 9 is a schematic diagram of the chiller controller of
the chiller system of FIG. 1; and
[0025] FIG. 10 illustrates a conventional chiller system which
includes a hot gas bypass.
DETAILED DESCRIPTION OF EMBODIMENT(S)
[0026] Selected embodiments will now be explained with reference to
the drawings. It will be apparent to those skilled in the art from
this disclosure that the following descriptions of the embodiments
are provided for illustration only and not for the purpose of
limiting the invention as defined by the appended claims and their
equivalents.
[0027] Referring initially to FIG. 1, a chiller system 10, which
includes a casing treatment bypass 60 (60a, 60b), is illustrated in
accordance with an embodiment of the present invention. The chiller
system 10 is preferably a water chiller that utilizes cooling water
and chiller water in a conventional manner. The chiller system 10
illustrated herein is a two-stage chiller system. However, it will
be apparent to those skilled in the art from this disclosure that
the chiller system 10 could be a single stage chiller system or a
multiple stage chiller system including three or more stages.
[0028] The chiller system 10 basically includes a chiller
controller 20, a compressor 22, a condenser 24, an expansion valve
26, and an evaporator 28 connected together in series to form a
loop refrigeration cycle. In addition, various sensors (not shown)
are disposed throughout the circuit of the chiller system 10. The
chiller system 10 is conventional except that the chiller system
has the casing treatment bypass 60 (60a, 60b) in accordance with
the present invention.
[0029] Referring to FIGS. 1-3, the compressor 22 is a two-stage
centrifugal compressor in the illustrated embodiment. The
compressor 22 illustrated herein is a two-stage centrifugal
compressor which includes two impellers. However, the compressor 22
can be a multiple stage centrifugal compressor including three or
more impellers. The two-stage centrifugal compressor 22 of the
illustrated embodiment includes a first stage impeller 34a and a
second stage impeller 34b. The centrifugal compressor 22 further
includes a first stage inlet guide vane 32a, a first
diffuser/volute 36a, a second stage inlet guide vane 32b, a second
diffuser/volute 36b, a compressor motor 38, and a magnetic bearing
assembly 40 as well as various conventional sensors (only some
shown). A casing 30 covers the centrifugal compressor 22. The
casing 30 includes an inlet portion 31a and an outlet portion 33a
for the first stage of the compressor 22. The casing 30 also
includes an inlet portion 31b and an outlet portion 33b for the
second stage of the compressor 22.
[0030] The chiller controller 20 receives signals from the various
sensors and controls the inlet guide vanes 32a and 32b, the
compressor motor 38, and the magnetic bearing assembly 40 in a
conventional manner, as explained in more detail below. Refrigerant
flows in order through the first stage inlet guide vane 32a, the
first stage impeller 34a, the second stage inlet guide vane 32b,
and the second stage impeller 34b. The inlet guide vanes 32a and
32b control the flow rate of refrigerant gas into the impellers 34a
and 34b, respectively, in a conventional manner. The impellers 34a
and 34b increase the velocity of refrigerant gas, generally without
changing pressure. The motor speed determines the amount of
increase of the velocity of refrigerant gas. The diffusers/volutes
36a and 36b increase the refrigerant pressure. The
diffusers/volutes 36a and 36b are non-movably fixed relative to the
casing 30. The compressor motor 38 rotates the impellers 34a and
34b via a shaft 42. The magnetic bearing assembly 40 magnetically
supports the shaft 42. Alternatively, the bearing system may
include a roller element, a hydrodynamic bearing, a hydrostatic
bearing, and/or a magnetic bearing, or any combination of these. In
this manner, the refrigerant is compressed in the centrifugal
compressor 22.
[0031] In operation of the chiller system 10, the first stage
impeller 34a and the second stage impeller 34b of the compressor 22
are rotated, and the refrigerant of low pressure in the chiller
system 10 is sucked by the first stage impeller 34a. The flow rate
of the refrigerant is adjusted by the inlet guide vane 32a. The
refrigerant sucked by the first stage impeller 34a is compressed to
intermediate pressure, the refrigerant pressure is increased by the
first diffuser/volute 36a, and the refrigerant is then introduced
to the second stage impeller 34b. The flow rate of the refrigerant
is adjusted by the inlet guide vane 32b. The second stage impeller
34b compresses the refrigerant of intermediate pressure to high
pressure, and the refrigerant pressure is increased by the second
diffuser/volute 36b. The high pressure gas refrigerant is then
discharged to the chiller system 10.
[0032] Referring to FIGS. 2 and 3, the magnetic bearing assembly 40
is conventional, and thus, will not be discussed and/or illustrated
in detail herein, except as related to the present invention.
Rather, it will be apparent to those skilled in the art that any
suitable magnetic bearing can be used without departing from the
present invention. The magnetic bearing assembly 40 preferably
includes a first radial magnetic bearing 44, a second radial
magnetic bearing 46 and an axial (thrust) magnetic bearing 48. In
any case, at least one radial magnetic bearing 44 or 46 rotatably
supports the shaft 42. The thrust magnetic bearing 48 supports the
shaft 42 along a rotational axis X by acting on a thrust disk 45.
The thrust magnetic bearing 48 includes the thrust disk 45 which is
attached to the shaft 42.
[0033] The thrust disk 45 extends radially from the shaft 42 in a
direction perpendicular to the rotational axis X, and is fixed
relative to the shaft 42. A position of the shaft 42 along
rotational axis X (an axial position) is controlled by an axial
position of the thrust disk 45. The first and second radial
magnetic bearings 44 and 46 are disposed on opposite axial ends of
the compressor motor 38. Various sensors detect radial and axial
positions of the shaft 42 relative to the magnetic bearings 44, 46
and 48, and send signals to the chiller controller 20 in a
conventional manner. The chiller controller 20 then controls the
electrical current sent to the magnetic bearings 44, 46 and 48 in a
conventional manner to maintain the shaft 42 in the correct
position.
[0034] The magnetic bearing assembly 40 is preferably a combination
of active magnetic bearings 44, 46, and 48, which utilizes gap
sensors 54, 56 and 58 to monitor shaft position and send signals
indicative of shaft position to the chiller controller 20. Thus,
each of the magnetic bearings 44, 46 and 48 are preferably active
magnetic bearings. A magnetic bearing control section 71 uses this
information to adjust the required current to a magnetic actuator
to maintain proper rotor position both radially and axially.
[0035] Referring to FIGS. 1 and 9, the chiller controller 20
includes a magnetic bearing control section 71, a compressor
variable frequency drive 72, a compressor motor control section 73,
an inlet guide vane control section 74, and an expansion valve
control section 75. The compressor variable frequency drive 72 and
the compressor motor control section 73 can be a single
section.
[0036] In the illustrated embodiment, the control sections are
sections of the chiller controller 20 programmed to execute the
control of the parts described herein. The magnetic bearing control
section 71, the compressor variable frequency drive 72, the
compressor motor control section 73, and the inlet guide vane
control section 74, and the expansion valve control section 75 are
coupled to each other, and form parts of a centrifugal compressor
control portion that is electrically coupled to an I/O interface of
the compressor 22. However, it will be apparent to those skilled in
the art from this disclosure that the precise number, location
and/or structure of the control sections, portions and/or chiller
controller 20 can be changed without departing from the present
invention so long as the one or more controllers are programed to
execute control of the parts of the chiller system 10 as explained
herein.
[0037] The chiller controller 20 is conventional, and thus,
includes at least one microprocessor or CPU, an Input/output (I/O)
interface, Random Access Memory (RAM), Read Only Memory (ROM), a
storage device (either temporary or permanent) forming a computer
readable medium programmed to execute one or more control programs
to control the chiller system 10. The chiller controller 20 may
optionally include an input interface such as a keypad to receive
inputs from a user and a display device used to display various
parameters to a user. The parts and programming are conventional,
and thus, will not be discussed in detail herein, except as needed
to understand the embodiment(s).
[0038] As mentioned above, the chiller system 10 has the casing
treatment bypass 60 (60a, 60b) in accordance with the present
invention. In the illustrated embodiment, the compressor 22 is a
two-stage centrifugal compressor. A first stage casing treatment
bypass 60a and a second stage casing treatment bypass 60b are
provided in the first stage and the second stage of the compressor
22, respectively, as shown in FIG. 1. It will be apparent to those
skilled in the art from this disclosure that the structures of the
first stage casing treatment bypass 60a and the second stage casing
treatment bypass 60b are identical, except that they are mirror
images of each other. Therefore, the first stage casing treatment
bypass 60a and the second stage casing treatment bypass 60b are
collectively referred to as the casing treatment bypass 60
hereinafter.
[0039] In the same manner, the elements of the first stage and the
second stage of the compressor 22 are collectively referred to
hereinafter without being distinguished. For example, the inlet
portion 31a of the casing 30 for the first stage and the inlet
portion 31b of the casing 30 for the second stage are collectively
referred to as the inlet portion 31 of the casing 30. The first
stage inlet guide vane 32a and the second stage inlet guide vane
32a are collectively referred to as the inlet guide vane 32. The
first stage impeller 34a and the second stage impeller 34b are
collectively referred to as the impeller 34.
[0040] In accordance with the present invention, the casing
treatment bypass 60 is provided in the chiller system 10 to inject
refrigerant from a gap between the impeller 34 and the inlet
portion 31 of the casing 30 toward an area between the impeller 34
and the inlet guide vane 32, as explained in more detail below.
[0041] Referring to FIG. 4A, the operation range of a compressor is
expanded at low load (i.e., at a small flow rate) by shifting a
surge line to the high pressure side. The surge line is a line
connecting pressure limit values in which the compressor cannot be
operated at a predetermined flow rate. As shown in FIG. 4B, when
surge occurs, the flow moves forward or backward repeatedly at the
outlet, which results in severe vibration of the pressure and the
flow rate. The severe vibration causes a stall of the flow at the
outlet. As shown in FIG. 4C, a stall of the flow occurs also at the
inlet in a case where the flow rate is reduced. More specifically,
when the flow rate is reduced, the flow goes into the blade of the
impeller with a larger angle of incidence compared to the ideal
flow inlet angle, which causes the flow separation from the front
edge of the blade. The flow separation causes the stall of the flow
at the inlet. The casing treatment bypass 60 in accordance with the
present invention is provided to prevent the flow separation and
the stall of the flow at the inlet so as to expand the operation
range of the compressor 22, as explained in more detail below.
[0042] FIG. 5A illustrates the static pressure, the flow velocity,
and the flow rate of refrigerant without a casing treatment bypass,
and FIGS. 5B-5D illustrate the static pressure, the flow velocity,
and the flow rate of refrigerant with a casing treatment bypass of
various load. In each of FIGS. 5A-5D, the upper graph shows the
static pressure of the refrigerant at positions "a" to "g"
illustrated in the diagram at the top. Here, the static pressure at
the inlet is 0.0, and the static pressure at the outlet is 1.0. The
negative value means that the static pressure is lower than that at
the inlet. In each of FIGS. 5A-5D, the lower graph shows the flow
velocity of the refrigerant at positions "a" to "g" illustrated in
the diagram at the top. Here, the flow velocity at the inlet is
0.0. The negative value means that the flow velocity is lower than
that at the inlet. In each of FIGS. 5A-5D, the table at the bottom
shows the flow rate of the refrigerant at positions (A) to (E)
illustrated in the diagram at the top.
[0043] Referring to FIG. 5A, when the compressor is operated at
100% load without a casing treatment bypass, the static pressure of
the refrigerant decreases, and then increases as the flow
approaches the outlet. Referring to FIG. 5B, when the compressor is
operated at 100% load with a casing treatment bypass, the static
pressure of the refrigerant decreases, and then increases as the
flow approaches the outlet, similarly to the case shown in FIG. 5A.
In the case shown in FIG. 5B, the exit port of the casing treatment
bypass is arranged at the edge of the impeller blade, and thus, the
ability of increasing the pressure does not significantly change
even with the casing treatment bypass. In this manner, when the
compressor is operated at 100% load, the static pressure of the
refrigerant changes similarly regardless of whether a casing
treatment bypass is provided or not.
[0044] As mentioned above, the stall of the flow at the inlet will
occur when the flow rate of the refrigerant is reduced. In other
words, the stall of the flow at the inlet will not occur when the
flow rate of the refrigerant is sufficiently large as in the cases
shown in FIGS. 5A and 5B. The casing treatment bypass 60 in
accordance with the present invention is provided to prevent the
stall of the flow at the inlet when the flow rate of the
refrigerant is reduced as in the cases shown in FIGS. 5C and
5D.
[0045] Referring to FIG. 5C, when the compressor is operated at 50%
load with a casing treatment bypass, the static pressure of the
refrigerant at position "c" is larger than the static pressure of
the refrigerant at the inlet. The pressure difference between the
static pressure at position "c" and the static pressure at the
inlet at 50% load refers to .DELTA.P.sub.50. Here, 50% is an
estimated value. In this instance, it is assumed that surge
partially occurs at the inlet because the flow rate of the
refrigerant is small. As a result of the pressure difference
.DELTA.P.sub.50, the flow toward the inlet (toward position "f" in
the diagram) is created in the casing treatment bypass. Referring
to FIG. 5D, when the compressor is operated at 20% load with a
casing treatment bypass, the static pressure of the refrigerant at
position "c" is much larger than the static pressure of the
refrigerant at the inlet. The pressure difference between the
static pressure at position "c" and the static pressure at the
inlet at 20% load refers to .DELTA.P.sub.20. Here, 20% is an
estimated value. As seen from FIGS. 5C and 5D, the pressure
difference .DELTA.P.sub.20 is larger than the pressure difference
.DELTA.P.sub.50.
[0046] When the compressor is operated at low load such as 50% or
20% with the casing treatment bypass, the flow rate of the
refrigerant increases at the inlet due to the flow introduced by
the casing treatment bypass, which prevents the stall at the inlet
from being caused by the flow separation at the front edge of the
impeller blade. In this manner, the operation range of the
compressor 22 is expanded when the flow rate of the refrigerant is
small.
[0047] Referring to FIGS. 6, FIGS. 7A and 7B, the casing treatment
bypass 60 in accordance with the present invention will be
explained in more detail.
[0048] FIG. 6 is an enlarged schematic diagram inside circle 6 in
FIG. 2, illustrating the inlet guide vane 32, the impeller 34 and
the casing 30 of the centrifugal compressor 22 of FIGS. 1-3 with
the casing treatment bypass 60.
[0049] In the illustrated embodiment, the casing treatment bypass
60 is a hole formed in the inlet portion 31 of the casing 30 of the
compressor 22. The casing treatment bypass 60 may include a
plurality of holes, with each hole as illustrated in FIG. 6 being
circumferentially disposed of the inlet portion 31 of the casing
30. For example, the number of the holes may be eight, and the
diameter "a" of each hole may be 46.1 mm.
[0050] The casing treatment bypass 60 includes an entrance port 61
and an exit port 63. The entrance port 61 of the casing treatment
bypass 60 is connected to a gap between the impeller 34 and the
inlet portion 31 of the casing 30. The exit port 63 of the casing
treatment bypass 60 is connected to an area between the impeller 34
and the inlet guide vane 32. As shown in FIG. 6, the exit port 63
of the casing treatment bypass 60 is positioned upstream in a
direction of the refrigerant flow with respect to the entrance port
61 of the casing treatment bypass 60. In this manner, the casing
treatment bypass 60 injects the refrigerant from the gap between
the impeller 34 and the inlet portion 31 of the casing 30 back to
the area between the impeller 34 and the inlet guide vane 32. In
the illustrated embodiment, the exit port 63 of the casing
treatment bypass 60 is located in the area between the impeller 34
and the inlet guide vane 32. In other words, the exit port 63 of
the casing treatment bypass 60 is located downstream of the inlet
guide vane 32 in the refrigerant flow direction. Alternatively, the
exit port 63 of the casing treatment bypass 60 may be located
upstream of the inlet guide vane 32 in the refrigerant flow
direction.
[0051] As shown in FIG. 6, the impeller 34 includes an impeller hub
35 and impeller blades 37. The impeller blades 37 are disposed to
surround the impeller hub 35. The entrance port 61 of the casing
treatment bypass 60 faces the impeller blade 37. The exit port 63
of the casing treatment bypass 60 opens to the area between the
inlet guide vane 32 and the impeller 34. The diameter "a" of the
entrance port 61 of the casing treatment bypass 60 is determined
based on the diameter "d" of the impeller blade 37. It is
preferable, however, that the diameter "a" of the entrance port 61
of the casing treatment bypass 60 does not exceed 25% of the inlet
area of the impeller 34.
[0052] It is also preferable that the cross-sectional area of the
exit port 63 of the casing treatment bypass 60 is equal to or
greater than the cross-sectional area of the entrance port 61 of
the casing treatment bypass 60. For example, the diameter "b" of
the exit port 63 of the casing treatment bypass 60 can be arranged
to be greater than the diameter "a" of the entrance port 61 of the
casing treatment bypass 60 as illustrated in FIG. 6. With this
arrangement, the refrigerant can stably flow in the casing
treatment bypass 60 from the entrance port 61 toward the exit port
63.
[0053] Referring to FIGS. 7A and 7B, the casing treatment bypass 60
in accordance with another embodiment will be explained.
[0054] The casing treatment bypass 60 illustrated in FIG. 7A has an
annular ring shape extending the whole circumference of the inlet
portion 31 of the casing 30. In this embodiment, the inlet portion
31 of the casing 30 includes a sub-portion 31s which is separated
from the inlet portion 31 of the casing 30 by the ring-shaped
casing treatment bypass 60. For example, the sub-portion 31s is
connected to the inlet portion 31 with a linkage mechanism 66 which
is attached to the outside of the inlet portion 31. The linkage
mechanism 66 may include a connecting ring, and a connecting rod
which is rotatably attached to the connecting ring. The linkage
mechanism 66 is driven by a driving mechanism 67 such as a stepper
motor or a hydraulic cylinder. In operation, the connecting ring of
the linkage mechanism 66 is driven by the driving mechanism 67 to
move in the axial direction corresponding to the rotation axis of
the impeller 34, and rotate the connecting rod of the linkage
mechanism 66. The connecting rod of the linkage mechanism 66 then
moves the sub-portion 31s in the axial direction corresponding to
the rotation axis of the impeller 34 as shown with the dotted line
in FIG. 7A. In this manner, the sub-portion 31s is arranged to be
movable relative to the non-movable inlet portion 31 in the axial
direction corresponding to the rotation axis of the impeller 34. In
this embodiment, the flow path area of the casing treatment bypass
60 can be adjusted by moving the sub-portion 31s in the axial
direction corresponding to the rotation axis of the impeller 34.
Alternatively, the flow path area of the casing treatment bypass 60
may be fixed. For example, the width in the radial direction of the
ring-shaped casing treatment bypass 60 may be 15.75 mm in a case
where the diameter at the inlet of the impeller 34 is 270 mm.
[0055] The casing treatment bypass 60 illustrated in FIG. 7B
includes an adjusting member 64. The adjusting member 64 is a
movable ring disposed in the inlet portion 31 of the casing 30 so
as to at least partly block the flow path area of the casing
treatment bypass 60. For example, the adjusting member 64 is
connected to the inlet portion 31 with a linkage mechanism 68 which
is attached to the outside of the inlet portion 31. The linkage
mechanism 68 may include a connecting ring which is rotatably
attached to the adjusting member 64. The linkage mechanism 68 is
driven by a driving mechanism 69 such as a stepper motor or a
hydraulic cylinder. In operation, the connecting ring of the
linkage mechanism 68 is driven by the driving mechanism 69 to move
the adjusting member 64 in the radial direction perpendicular to
the axial direction corresponding to the rotation axis of the
impeller 34 as shown with the dotted line in FIG. 7B. In this
manner, the adjusting member 64 is arranged to be movable relative
to the non-movable inlet portion 31 in the radial direction
perpendicular to the axial direction, and the flow path area of the
casing treatment bypass 60 can be adjusted by moving the adjusting
member 64 in the radial direction perpendicular to the axial
direction. The adjusting member 64 can be applied to both of the
case in which the casing treatment bypass 60 has a single hole and
the case in which the casing treatment bypass 60 has a plurality of
holes.
[0056] As mentioned above, the flow path area of the casing
treatment bypass 60 can be optimized by moving the sub-portion 31s
in the axial direction corresponding to the rotation axis of the
impeller 34 (see FIG. 7A) or by moving the adjusting member 64 in
the radial direction perpendicular to the axial direction (see FIG.
7B). The casing treatment bypass 60 in accordance with the present
invention can use either one of the movable sub-portion 31s and the
movable adjusting member 64 to optimize the flow path area of the
casing treatment bypass 60.
[0057] Referring to FIGS. 8A and 8B, various types of impellers can
be used as the impeller 34 of the compressor 22 in accordance with
the present invention. FIG. 8A is a schematic diagram illustrating
a mixed flow impeller and FIG. 8B is a schematic diagram
illustrating a radial flow impeller. However, the present invention
is not limited to the type of the impeller 34. The case treatment
bypass 60 in accordance with the present invention can be applied
to various types of impellers including the mixed flow impeller as
shown in FIG. 8A or the radial flow impeller as shown in FIG.
8B.
[0058] In terms of global environment protection, use of new low
GWP (Global Warming Potential) refrigerants such like R1233zd,
R1234ze are considered for chiller systems. One example of the low
global warming potential refrigerant is low pressure refrigerant in
which the evaporation pressure is equal to or less than the
atmospheric pressure. For example, low pressure refrigerant R1233zd
is a candidate for centrifugal chiller applications because it is
non-flammable, non-toxic, low cost, and has a high COP compared to
other candidates such like R1234ze, which are current major
refrigerant R134a alternatives. Especially in a case of using low
pressure refrigerant, the compressor 22 including the casing
treatment bypass 60 in accordance with the present invention has
advantages because the operation range of the compressor 22 can be
expanded to prevent surge without requiring a large-diameter pipe
for a conventional hot gas bypass.
General Interpretation of Terms
[0059] In understanding the scope of the present invention, the
term "comprising" and its derivatives, as used herein, are intended
to be open ended terms that specify the presence of the stated
features, elements, components, groups, integers, and/or steps, but
do not exclude the presence of other unstated features, elements,
components, groups, integers and/or steps. The foregoing also
applies to words having similar meanings such as the terms,
"including", "having" and their derivatives. Also, the terms
"part," "section," "portion," "member" or "element" when used in
the singular can have the dual meaning of a single part or a
plurality of parts.
[0060] The term "detect" as used herein to describe an operation or
function carried out by a component, a section, a device or the
like includes a component, a section, a device or the like that
does not require physical detection, but rather includes
determining, measuring, modeling, predicting or computing or the
like to carry out the operation or function.
[0061] The term "configured" as used herein to describe a
component, section or part of a device includes hardware and/or
software that is constructed and/or programmed to carry out the
desired function.
[0062] The terms of degree such as "substantially", "about" and
"approximately" as used herein mean a reasonable amount of
deviation of the modified term such that the end result is not
significantly changed.
[0063] While only selected embodiments have been chosen to
illustrate the present invention, it will be apparent to those
skilled in the art from this disclosure that various changes and
modifications can be made herein without departing from the scope
of the invention as defined in the appended claims. For example,
the size, shape, location or orientation of the various components
can be changed as needed and/or desired. Components that are shown
directly connected or contacting each other can have intermediate
structures disposed between them. The functions of one element can
be performed by two, and vice versa. The structures and functions
of one embodiment can be adopted in another embodiment. It is not
necessary for all advantages to be present in a particular
embodiment at the same time. Every feature which is unique from the
prior art, alone or in combination with other features, also should
be considered a separate description of further inventions by the
applicant, including the structural and/or functional concepts
embodied by such feature(s). Thus, the foregoing descriptions of
the embodiments according to the present invention are provided for
illustration only, and not for the purpose of limiting the
invention as defined by the appended claims and their
equivalents.
* * * * *