U.S. patent application number 15/266403 was filed with the patent office on 2018-03-15 for centrifugal compressor.
The applicant listed for this patent is Daikin Applied Americas Inc.. Invention is credited to Phillip A. Johnson, Fumiaki Onodera, Tsuyoshi Ueda.
Application Number | 20180073779 15/266403 |
Document ID | / |
Family ID | 59969245 |
Filed Date | 2018-03-15 |
United States Patent
Application |
20180073779 |
Kind Code |
A1 |
Johnson; Phillip A. ; et
al. |
March 15, 2018 |
CENTRIFUGAL COMPRESSOR
Abstract
A centrifugal compressor includes a casing, a first compression
mechanism and a second compression mechanism. The casing has a
first inlet portion, a first outlet portion, a second inlet portion
and a second outlet portion. The first compression mechanism
includes a first inlet guide vane disposed in the first inlet
portion, a first impeller disposed downstream of the first inlet
guide vane, a first diffuser disposed in the first outlet portion
downstream from the first impeller, and a first motor. The second
compression mechanism includes a second inlet guide vane disposed
in the second inlet portion, a second impeller disposed downstream
of the second inlet guide vane, a second diffuser disposed in the
second outlet portion downstream from the second impeller, and a
second motor. The first and second motors are arranged to rotate
first and second shafts in order to rotate the first and second
impellers.
Inventors: |
Johnson; Phillip A.;
(Staunton, VA) ; Onodera; Fumiaki; (Minnetonka,
MN) ; Ueda; Tsuyoshi; (Plymouth, MN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Daikin Applied Americas Inc. |
Minneapolis |
MN |
US |
|
|
Family ID: |
59969245 |
Appl. No.: |
15/266403 |
Filed: |
September 15, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F25B 1/053 20130101;
F04D 29/286 20130101; F04D 29/444 20130101; F04D 25/16 20130101;
F04D 29/058 20130101; F04D 29/051 20130101; F04D 25/06 20130101;
F04D 23/003 20130101; F04D 17/12 20130101; F04D 27/004
20130101 |
International
Class: |
F25B 1/053 20060101
F25B001/053; F04D 17/12 20060101 F04D017/12; F04D 23/00 20060101
F04D023/00; F04D 25/06 20060101 F04D025/06; F04D 27/00 20060101
F04D027/00; F04D 29/051 20060101 F04D029/051; F04D 29/058 20060101
F04D029/058; F04D 29/28 20060101 F04D029/28; F04D 29/44 20060101
F04D029/44 |
Claims
1. A centrifugal compressor comprising: a casing having a first
inlet portion, a first outlet portion, a second inlet portion and a
second outlet portion; a first compression mechanism including a
first inlet guide vane disposed in the first inlet portion, a first
impeller disposed downstream of the first inlet guide vane, the
first impeller being attached to a first shaft rotatable about a
first rotation axis, a first diffuser disposed in the first outlet
portion downstream from the first impeller, and a first motor
arranged to rotate the first shaft in order to rotate the first
impeller; and a second compression mechanism including a second
inlet guide vane disposed in the second inlet portion, a second
impeller disposed downstream of the second inlet guide vane, the
second impeller being attached to a second shaft rotatable about a
second rotation axis, a second diffuser disposed in the second
outlet portion downstream from the second impeller, and a second
motor arranged to rotate the second shaft in order to rotate the
second impeller.
2. The centrifugal compressor according to claim 1, wherein
rotation speeds of the first and second motors are independently
controllable.
3. The centrifugal compressor according to claim 2, further
comprising a controller programmed to independently control the
rotation speeds of the first and second motors.
4. The centrifugal compressor according to claim 3, further
comprising a first Variable Frequency Drive (VFD) connected to the
first motor and the controller to variably control the rotation
speed of the first motor; and a second Variable Frequency Drive
(VFD) connected to the second motor and the controller to variably
control the rotation speed of the second motor.
5. The centrifugal compressor according to claim 1, further
comprising a first magnetic bearing rotatably supporting the first
shaft; and a second magnetic bearing rotatably supporting the
second shaft.
6. The centrifugal compressor according to claim 5, wherein the
first and second rotation axes are coincident with each other.
7. The centrifugal compressor according to claim 6, wherein the
first shaft has a first inlet end with the first impeller mounted
thereon and a first remote end with the first motor mounted on the
first shaft between the first impeller and the first remote end,
the second shaft has a second inlet end with the second impeller
mounted thereon and a second remote end with the second motor
mounted on the second shaft between the second impeller and the
second remote end, the first magnetic bearing includes a first
impeller side radial magnetic bearing axially disposed between the
first impeller and the first motor, and the second magnetic bearing
includes a second impeller side radial magnetic bearing axially
disposed between the second impeller and the second motor.
8. The centrifugal compressor according to claim 7, wherein the
first magnetic bearing includes a first remote side radial magnetic
bearing axially disposed on a side of the first motor that is
remote from a side where the first impeller is mounted, and the
second magnetic bearing includes a second remote side radial
magnetic bearing axially disposed on a side of the second motor
that is remote from a side where the second impeller is
mounted.
9. The centrifugal compressor according to claim 8, wherein the
first magnetic bearing includes a first axial thrust magnetic
bearing, and the second magnetic bearing includes a second axial
thrust magnetic bearing.
10. The centrifugal compressor according to claim 8, wherein the
first axial thrust magnetic bearing is axially disposed adjacent to
the first remote side radial bearing, and the second axial thrust
magnetic bearing is axially disposed adjacent to the second remote
side radial bearing.
11. The centrifugal compressor according to claim 6, wherein the
first shaft has a first inlet end with the first impeller mounted
thereon and a first remote end with the first motor mounted on the
first shaft between the first impeller and the first remote end,
the second shaft has a second inlet end with the second impeller
mounted thereon and a second remote end with the second motor
mounted on the second shaft between the second impeller and the
second remote end, the first magnetic bearing includes a first
remote side radial magnetic bearing axially disposed on a side of
the first motor that is remote from a side where the first impeller
is mounted, and the second magnetic bearing includes a second
remote side radial magnetic bearing axially disposed on a side of
the second motor that is remote from a side where the second
impeller is mounted.
12. The centrifugal compressor according to claim 6, wherein the
first magnetic bearing includes at least one first radial magnetic
bearing and at least one first axial thrust magnetic bearing, and
the second magnetic bearing includes at least one second radial
magnetic bearing and at least one second axial thrust magnetic
bearing.
13. The centrifugal compressor according to claim 12, wherein the
first axial thrust magnetic bearing is axially disposed at the
first remote end, the second axial thrust magnetic bearing is
axially disposed at the second remote end, and the first and second
remote ends and the first and second axial thrust magnetic bearings
are spaced from each other to form a gap therebetween.
14. The centrifugal compressor according to claim 1, wherein the
first diffuser is connected to the second impeller such that
refrigerant compressed in the first compression mechanism is
further compressed in the second compression mechanism.
15. A chiller system including the centrifugal compressor according
to claim 1, the chiller system further comprising: an evaporator; a
condenser; and an expansion device, the compressor, the evaporator,
the condenser and the expansion mechanism being connected together
to form a refrigerant circuit.
16. A chiller system according to claim 15, further comprising: an
economizer connected between the first compression mechanism and
the second compression mechanism in the refrigerant circuit.
Description
BACKGROUND
Field of the Invention
[0001] The present invention generally relates to a centrifugal
compressor. More specifically, the present invention relates to a
centrifugal compressor with a pair of compressors and a pair of
motors.
Background Information
[0002] 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.
[0003] 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.
[0004] 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. A conventional centrifugal
compressor may have one or two stages. A motor drives the one or
more impellers.
[0005] See U.S. Pat. No. 7,942,628 and U.S. Patent Application
publication No. 2010/0251750 as examples of conventional
technology.
SUMMARY
[0006] The chiller industry began to offer variable speed
compressors during the 1990's for improved efficiency.
[0007] The chiller industry has also developed 2-stage type
centrifugal compressors for higher chiller efficiency.
[0008] In the case of a two stage centrifugal compressor structure,
one motor is used to drive both impellers. It has been discovered
that this structure has issues with (1) operating range and (2)
efficiency.
[0009] Regarding (1) Operating range, it has been discovered that
there is relationship between operating range and rotational speed
in each impeller. Since the current technology only allows each
impeller to rotate at the same speed as the other, it has been
discovered that the compressor will be difficult and/or impossible
to operate when it is attempted to operate either impeller outside
the range.
[0010] Regarding (2) Efficiency, it has been discovered that once
either impeller does not operate at the point designed, the
efficiency of compressor will decline.
[0011] Regarding (1) Operating range, it has been discovered that
by rotating each impeller in different rotational speed, the
impellers may not be operated outside of their operating ranges
(i.e., may be maintained within their operating ranges).
[0012] Regarding (2) Efficiency, it has been discovered that
1.sup.st and 2.sup.nd stage impeller's rotational speed may be
adjusted to increase their efficiency, and it may therefore be
possible to improve overall compressor efficiency.
[0013] For current technology's operation, each compressor's
operating range will be dominated by impeller's operating range. A
two stage compressor can have more limited operating range
capability due to the fact that each stage can have its own
boundary limits (choke flow limits, stall and surge limit, minimum
unloading limit). Therefore, the compressor cannot or should not be
operated when either impeller operates outside of the range.
[0014] For current technology's operation, compressor efficiency
will decline if either impeller does not operate at its designed
point. The reason for this is due to the change of head coefficient
and flow coefficient. Once these values are changed, the compressor
cannot operate at its designed (Highest efficiency) point.
[0015] In addition, it has been discovered that two stage cycle
efficiency advantage over one stage is reduced or mitigated when
operating away from the design point if the two stages are not well
matched, causing more severe efficiency reduction when moving away
from the design point and the peak efficiency point. It has been
discovered that his can result in chillers spending a significant
proportion of operating time away from the "full load design
point".
[0016] Therefore one object of the present invention is to provide
a centrifugal compressor that can maintain operation within the
operating range.
[0017] Another object of the present invention is to provide a
centrifugal compressor that can improve efficiency.
[0018] Yet another object of the present invention is to provide a
centrifugal compressor that addresses one or more of the other
discovered problems mentioned above or already known to those
skilled in the art.
[0019] One or more of the foregoing objects can basically be
achieved by providing a centrifugal compressor including a casing,
a first compression mechanism and a second compression mechanism.
The casing has a first inlet portion, a first outlet portion, a
second inlet portion and a second outlet portion. The first
compression mechanism includes a first inlet guide vane disposed in
the first inlet portion, a first impeller disposed downstream of
the first inlet guide vane, a first diffuser disposed in the first
outlet portion downstream from the first impeller, and a first
motor. The first impeller is attached to a first shaft rotatable
about a first rotation axis. The first motor is arranged to rotate
the first shaft in order to rotate the first impeller. The second
compression mechanism includes a second inlet guide vane disposed
in the second inlet portion, a second impeller disposed downstream
of the second inlet guide vane, a second diffuser disposed in the
second outlet portion downstream from the second impeller, and a
second motor. The second impeller is attached to a second shaft
rotatable about a second rotation axis. The second motor is
arranged to rotate the second shaft in order to rotate the second
impeller.
[0020] 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
[0021] Referring now to the attached drawings which form a part of
this original disclosure:
[0022] FIG. 1 is a schematic diagram illustrating a two stage
chiller system (with an economizer) having a centrifugal compressor
in accordance with an embodiment of the present invention;
[0023] 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;
[0024] FIG. 3 is a perspective view of internal parts (e.g.,
shafts, impellers, magnetic bearings and motors) of the centrifugal
compressor illustrated in FIGS. 1-2;
[0025] FIG. 4 is an elevational view of the internal parts (e.g.,
shafts, impellers, magnetic bearings and motors) of the centrifugal
compressor illustrated in FIG. 3;
[0026] FIG. 5 is a schematic longitudinal partial cross-sectional
view of the internal parts (e.g., shafts, impellers, magnetic
bearings and motors) of the centrifugal compressor illustrated in
FIGS. 3-4, with additional parts such as sensors, back-up bearings,
coils, etc. schematically illustrated in more detail;
[0027] FIG. 6 is a flow chart illustrating a normal control of the
centrifugal compressor illustrated in FIGS. 1-5;
[0028] FIG. 7A is a graph illustrating operating range of a two
stage compressor (overall compressor operation), with A
representing an overall operating point outside the overall
operating range;
[0029] FIG. 7B is a graph illustrating operating range of a first
stage impeller of the two stage compressor illustrated in FIG. 7A,
with A1 representing a first stage operating point outside the
first stage operating range;
[0030] FIG. 7C is a graph illustrating operating range of a second
stage impeller of the two stage compressor illustrated in FIG. 7A,
with A2 representing a second stage operating point inside the
second stage operating range;
[0031] FIG. 8A is a graph illustrating operating range of a two
stage compressor (overall compressor operation), with A
representing an overall operating point outside the overall
operating range (like FIG. 7A) and with B representing a shifted
operating point within the overall operating range in accordance
with the present invention;
[0032] FIG. 8B is a graph illustrating operating range of a first
stage impeller of the two stage compressor illustrated in FIG. 8A,
with A1 representing a first stage operating point outside the
first stage operating range (like FIG. 7B) and with B1 representing
a shifted first operating point by decreasing the rotation speed of
the first stage impeller in accordance with the present
invention;
[0033] FIG. 8C is a graph illustrating operating range of a second
stage impeller of the two stage compressor illustrated in FIG. 7A,
with A2 representing a second stage operating point inside the
second stage operating range (like FIG. 7C);
[0034] FIG. 9A is a graph illustrating efficiency of a two stage
compressor (overall compressor efficiency), with E representing a
designed highest efficiency point and with D and E representing
shifted lower efficiency operating points;
[0035] FIG. 9B is a graph illustrating efficiency of a first stage
impeller of the two stage compressor illustrated in FIG. 9A, with
E1 representing a designed highest efficiency point of the first
stage and with D1 and E1 representing shifted lower efficiency
operating points of the first stage;
[0036] FIG. 9C is a graph illustrating efficiency of a second stage
impeller of the two stage compressor illustrated in FIG. 9A, with
E2 representing a designed highest efficiency point of the second
stage and with D2 and E2 representing shifted lower efficiency
operating points of the second stage;
[0037] FIG. 10A is a graph illustrating efficiency of a two stage
compressor (overall compressor efficiency) like FIG. 9A, with E
representing a designed highest efficiency point and with D and E
representing shifted lower efficiency operating points;
[0038] FIG. 10B is a graph illustrating efficiency of a first stage
impeller of the two stage compressor illustrated in FIG. 9A, with
E1 representing a designed highest efficiency point of the first
stage and with D1 and F1 representing shifted lower efficiency
operating points of the first stage, and with arrows illustrating
how the efficiency can be increased from points D1 or F1 by
reducing or increasing the first impeller speed, respectively;
and
[0039] FIG. 10C is a graph illustrating efficiency of a second
stage impeller of the two stage compressor illustrated in FIG. 9A,
with E2 representing a designed highest efficiency point of the
second stage and with D2 and F2 representing shifted lower
efficiency operating points of the second stage, and with arrows
illustrating how the efficiency can be increased from points D2 or
F2 by reducing or increasing the second impeller speed,
respectively.
DETAILED DESCRIPTION OF EMBODIMENT(S)
[0040] 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.
[0041] Referring initially to FIG. 1, a chiller system 10 having a
centrifugal compressor 22 in accordance with an embodiment of the
present invention is illustrated. The centrifugal compressor 22 of
FIG. 1 is a two stage compressor, and thus, the chiller system 10
of FIG. 1 is a two stage chiller system. The two stage chiller
system of FIG. 1 also includes an economizer. FIG. 1 merely
illustrates one example of a chiller system in which the
centrifugal compressor 22 in accordance with the present invention
can be used. For example, it will be apparent to those skilled in
the art from this disclosure that the economizer of the chiller
system 10 can be eliminated. However, in the illustrated embodiment
the economizer is present for the reasons discussed below.
[0042] The chiller system 10 is conventional, except for the
centrifugal compressor 22 and the manner in which the centrifugal
compressor 22 is controlled. Specifically, in the illustrated
embodiment, the centrifugal compressor 22 includes a structure and
is controlled to maintain operation within its operating range and
to improve efficiency, as explained in more detail below. Therefore
the chiller system 10 will not be discussed and/or illustrated in
detail herein except as related to the centrifugal compressor 22
the manner in which the centrifugal compressor 22 is controlled.
However, it will be apparent to those skilled in the art that the
conventional parts of the chiller system 10 can be constructed in
variety of ways without departing the scope of the present
invention. In the illustrated embodiments, the chiller system 10 is
preferably a water chiller that utilizes cooling water and chiller
water in a conventional manner.
[0043] Referring still to FIG. 1, the components of the chiller
system 10 will now briefly be explained. The chiller system 10
basically includes a chiller controller 20, the centrifugal
compressor 22, a condenser 24, an expansion valve or orifice 25, an
economizer 26, an expansion valve or orifice 27, and an evaporator
28 connected together in series to form a loop refrigeration cycle.
The economizer 26 is connected between the first compression
mechanism and the second compression mechanism in the refrigerant
circuit (e.g., to the intermediate stage). Various sensors (not
shown) are disposed throughout the circuit of the chiller system
10. Such sensors and use of information from such sensors to
control the chiller system 10 is conventional, and thus, will not
be explained and/or illustrated in detail herein except as related
to controlling the centrifugal compressor 22 in accordance with the
present invention. Therefore, it will be apparent to those skilled
in the art from this disclosure that normal operation of the
chiller system 10 will be omitted for the sake of brevity, except
as related the structure and operation of the centrifugal
compressor 22.
[0044] Referring now to FIGS. 1-5, the compressor 22 will now be
explained in more detail. The compressor 22 is a two-stage
centrifugal compressor in the illustrated embodiment. Thus, the
compressor 22 illustrated herein includes two impellers. However,
the compressor 22 may include three or more impellers (not shown).
The two-stage centrifugal compressor 22 of the illustrated
embodiment is conventional except that the compressor 22 includes
separate motors used to drive the two impellers. In addition, the
motors are controlled in accordance with the present invention.
[0045] Thus, the centrifugal compressor 22 includes first and
second stages fluidly connected in series so that refrigerant
compressed in the first stage is subsequently compressed in the
second stage. The first stage includes a first stage inlet guide
vane 32a, a first impeller 34a, a first diffuser/volute 36a, a
first stage compressor motor 38a and a first stage magnetic bearing
40a. Similarly, the second stage includes a second stage inlet
guide vane 32b, a second impeller 34b, a second diffuser/volute
36b, a second stage compressor motor 38b and a second stage
magnetic bearing 40b. In addition, the centrifugal compressor 22
includes various conventional sensors (only some shown).
[0046] While magnetic bearings are described herein, it will be
apparent to those skilled in the art from this disclosure that
other types and forms of compressor bearings maybe used with this
invention. A casing 30 covers the other parts of 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. Thus, the casing 30
has a first inlet portion 31a, a first outlet portion 33a, a second
inlet portion 31b and a second outlet portion 33b.
[0047] Thus, in the illustrated embodiment, the centrifugal
compressor 22 includes the casing 30, a first compression mechanism
(first stage) 23a and a second compression mechanism (second stage)
23b. The first compression mechanism 23a includes the first inlet
guide vane 32a disposed in the first inlet portion 31a, the first
impeller 34a disposed downstream of the first inlet guide vane 32a,
a first diffuser 36a disposed in the first outlet portion 33a
downstream from the first impeller 34a, and the first motor 38a
arranged to rotate the first shaft 42a in order to rotate the first
impeller 34a. The first impeller 34a is attached to a first shaft
42a rotatable about a first rotation axis X.sub.1. The second
compression mechanism 23b includes the second inlet guide vane 23b
disposed in the second inlet portion 31b, the second impeller 34b
disposed downstream of the second inlet guide vane 32b, the second
diffuser 36b disposed in the second outlet portion 33b downstream
from the second impeller 34b, and the second motor 38b arranged to
rotate the second shaft 42b in order to rotate the second impeller
34b. The second impeller 34b is attached to a second shaft 42b
rotatable about a second rotation axis X.sub.2.
[0048] The chiller controller 20 receives signals from the various
sensors and controls the inlet guide vanes 32a and 32b, the
compressor motors 38a and 38b, and the magnetic bearings 40a and
40b, 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 speeds determine the amount of
increase of the velocity of refrigerant gas. The diffusers/volutes
36a and 36b increase the refrigerant pressure.
[0049] The diffusers/volutes 36a and 36b are non-movably fixed
relative to the casing 30. The compressor motors 38a and 38b rotate
the impellers 34a and 34b via first and second shafts 42a and 42b,
respectively. The first diffuser 36a is connected to the second
impeller 34b such that refrigerant compressed in the first
compression mechanism 23a is further compressed in the second
compression mechanism 23b, as best understood from FIG. 1. The
first and second magnetic bearings 40a and 40b magnetically
rotatable support the shafts 42a and 42b, respectively.
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.
[0050] 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. In the illustrated embodiment,
because the impellers 34a and 34b are driven by separate motors 38a
or 38b, the rotation speeds of the first and second impellers 34a
and 34b are independently variable.
[0051] Referring to FIGS. 2-5, the first and second magnetic
bearings 40a and 40b will now be explained in more detail. The
first and second magnetic bearings 40a and 40b are conventional,
except as explained herein. Thus, the first and second magnetic
bearings 40a and 40b 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 first magnetic bearing 40a preferably includes a
first impeller side radial magnetic bearing 44a, a first remote
side radial magnetic bearing 46a and a first axial (thrust)
magnetic bearing 48a. Similarly, the second magnetic bearing 40b
preferably includes a second impeller side radial magnetic bearing
44b, a second remote side radial magnetic bearing 46b and a second
axial (thrust) magnetic bearing 48b.
[0052] At least one radial magnetic bearing 44a or 46a rotatably
supports the first shaft 42a, and at least one radial magnetic
bearing 44b or 46b rotatably supports the second shaft 42b. The
thrust magnetic bearing 48a axially supports the first shaft 42a
along a first rotational axis X.sub.1 by acting on a first thrust
disk 45a. The thrust magnetic bearing 48a includes the thrust disk
45a which is attached to the first shaft 42a. Similarly, the thrust
magnetic bearing 48b axially supports the second shaft 42b along a
second rotational axis X.sub.2 by acting on a second thrust disk
45b. The thrust magnetic bearing 48b includes the thrust disk 45b
which is attached to the second shaft 42b.
[0053] The first thrust disk 45a extends radially from the first
shaft 42a in a direction perpendicular to the first rotational axis
X.sub.1, and is fixed relative to the first shaft 42a. The second
thrust disk 45b extends radially from the second shaft 42b in a
direction perpendicular to the second rotational axis X.sub.2, and
is fixed relative to the second shaft 42b. A position of the first
shaft 42a along the first rotational axis X.sub.1 (an axial
position) is controlled by an axial position of the first thrust
disk 45a. Likewise, a position of the second shaft 42b along the
first rotational axis X.sub.2 (an axial position) is controlled by
an axial position of the second thrust disk 45b. The first radial
magnetic bearings 44a and 46a are disposed on opposite axial ends
of the first compressor motor 38a, while the second radial magnetic
bearings 44b and 46b are disposed on opposite axial ends of the
second compressor motor 38b. In the illustrated embodiment, the
first and second rotation axes X.sub.1 and X.sub.2 are coincident
with each other. Moreover, in the illustrated embodiment, the first
and second rotation axes X.sub.1 and X.sub.2 are parallel.
[0054] Referring still to FIGS. 2-5, various sensors detect radial
and axial positions of the shafts 42a and 42b relative to the
magnetic bearings 44a, 44b, 46a, 46b, 48a and 48b, 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 the magnetic bearings 44a, 44b, 46a, 46b, 48a and
48b in a conventional manner to maintain the shafts 42a and 42 in
the correct positions. Thus, the magnetic bearing 40a is preferably
a combination of active magnetic bearings 44a, 46a, and 48a, which
utilizes gap sensors 54a, 56a and 58a (FIG. 5) to monitor shaft
position and send signals indicative of shaft position to the
chiller controller 20. Thus, each of the magnetic bearings 44a, 46a
and 48a are preferably active magnetic bearings. Likewise, the
magnetic bearing 40b is preferably a combination of active magnetic
bearings 44b, 46b, and 48b, which utilizes gap sensors 54b, 56b and
58b (FIG. 5) to monitor shaft position and send signals indicative
of shaft position to the chiller controller 20. Thus, each of the
magnetic bearings 44b, 46b and 48b are preferably active magnetic
bearings.
[0055] Therefore, the centrifugal compressor 22 includes a first
magnetic bearing 40a rotatably supporting the first shaft 42a, and
a second magnetic bearing 40b rotatably supporting the second shaft
42b. The first shaft 42a has a first inlet end with the first
impeller 34a mounted thereon and a first remote end with the first
motor 38a mounted on the first shaft 42a between the first impeller
34a and the first remote end, and the second shaft 42b has a second
inlet end with the second impeller 34b mounted thereon and a second
remote end with the second motor 38b mounted on the second shaft
42b between the second impeller 34b and the second remote end.
[0056] As mentioned above, the first and second magnetic bearings
40a and 40b include a combination of radial and axial magnetic
bearings. Specifically, the magnetic bearing 44a is the first
impeller side radial magnetic bearing axially disposed between the
first impeller 34a and the first motor 38, and the magnetic bearing
44b is the second impeller side radial magnetic bearing axially
disposed between the second impeller 34b and the second motor 38b.
The magnetic bearing 46a is the first remote side radial magnetic
bearing axially disposed on a side of the first motor 38a that is
remote from a side where the first impeller 34a is mounted, and the
magnetic bearing 46b is the second remote side radial magnetic
bearing axially disposed on a side of the second motor 38b that is
remote from a side where the second impeller 34b is mounted. In any
case, the first magnetic bearing 40a includes at least one first
radial magnetic bearing 44a or 44b and at least one first axial
thrust magnetic bearing 48a, and the second magnetic bearing 40b
includes at least one second radial magnetic bearing 44b or 46b and
at least one second axial thrust magnetic bearing 48b.
[0057] In the illustrated embodiment, the first axial thrust
magnetic bearing 48a is axially disposed adjacent to the first
remote side radial bearing 46a, and the second axial thrust
magnetic bearing 48b is axially disposed adjacent to the second
remote side radial bearing 46b. Thus, the first axial thrust
magnetic bearing 48a is axially disposed at the first remote end of
the first shaft 42a, and the second axial thrust magnetic bearing
48b is axially disposed at the second remote end of the second
shaft 42b. In addition, the first and second remote ends (of the
shafts 42a and 42b) and the first and second axial thrust magnetic
bearings 48a and 48b are axially spaced from each other to form a
gap therebetween.
[0058] The gap sensors 54a, 54b, 56a, 56b, 58a and 58b are only
shown schematically in FIG. 5. Likewise, back-up bearings
(unnumbered), which are located at each end of each shaft 42a and
42b are provided in the illustrated embodiment as only shown in
FIG. 5. It will be apparent to those skilled in the art from this
disclosure that the back-up bearings (unnumbered) could be
eliminated. Likewise, it will be apparent to those skilled in the
art from this disclosure that one or more of the gap sensors could
be eliminated in order to simplify the magnetic bearings 40a and
40b. Moreover, it will be apparent to those skilled in the art from
this disclosure that if the gap sensors are illuminated, the
magnetic bearings could be controlled passively by the chiller
controller 20.
[0059] Referring to FIGS. 1-5, the motors 38a and 38b in accordance
with the present invention will now be explained in more detail.
The first motor 38a includes a first stator 60a and a first rotor
62a. Likewise, the second motor 38b includes a second stator 60b
and a second rotor 62b. The stator 60a is fixed to an interior
surface of the casing 30, while the rotor 62a is fixed to the shaft
42a. Likewise, the stator 60b is fixed to an interior surface of
the casing 30, while the rotor 62b is fixed to the shaft 42b. The
stators 60a and 60b and the rotors 62a and 62b are conventional.
Thus, when electricity is sent to the stator 60a, the rotor 62a is
caused to rotate at a speed according to the supplied electricity.
Moreover, when electricity is sent to the stator 60b, the rotor 62b
is caused to rotate at a speed according to the supplied
electricity. Since the rotors are fixed to the shafts, the shafts
are also caused to rotate, and thus, the impellers 34a and 34b are
also cause to rotate.
[0060] As mentioned above, because two separate motors 38a and 38b
are provided to rotate the first and second impellers 34a and 34b,
the first and second impellers 34a and 34b can be rotated
independently at different speeds. More specifically, the motors
38a and 38b preferably receive electricity from separate Variable
Frequency Drives (VFDs) 64a and 64b, respectively. The Variable
Frequency Drives (VFDs) 64a and 64b receive control signals from
the chiller controller 20 to independently control the speeds of
rotation of the first and second impellers 34a and 34b,
respectively. The manner in which the Variable Frequency Drives
(VFDs) 64a and 64b are controlled will be explained below with
reference to the control flow chart illustrated in FIG. 6 and the
graphs illustrated in FIGS. 7A-10C.
[0061] Referring now to FIG. 6, independent control of the motors
38a and 38b to independently control the rotations speeds of the
first and second impellers 34a and 34b will now be explained in
more detail. As mentioned above, rotation speeds of the first and
second motors 38a and 38b are independently controllable.
Specifically, the controller 20 is programmed to independently
control the rotation speeds of the first and second motors 38a and
38b in accordance with the flow chart of FIG. 6. This loop
illustrated in FIG. 6 starts and repeats with following triggers:
(1.) Compressor discharge pressure changes more than 10%/min;
and/or (2.) Compressor suction pressure changes more than 10%/min.
However, if the customer changes setting of chiller (i.e., the
leaving/exiting water temperature setting), this could be trigger
of FIG. 6 too. The first Variable Frequency Drive (VFD) 64a is
connected to the first motor 38a and the controller 20 to variably
control the rotation speed of the first motor 38a in accordance
with FIG. 6, and the second Variable Frequency Drive (VFD) 64b is
connected to the second motor 38b and the controller 20 to variably
control the rotation speed of the second motor 38b in accordance
with FIG. 6.
[0062] The start/repeat point of FIG. 6 is at step S1, while the
finish/repeat point is at step S14. Steps S2-S4 are steps used to
calculate current efficiency and determine if the first stage
compression mechanism 23a is operating at a most efficient point
(See for Example FIG. 9B). If it is determined that the first stage
compression mechanism 23a is operating at a most efficient point at
step S4, the controller 20 proceeds to step S5. If not, the
controller 20 proceeds to step S8. In steps S8-S10, the controller
20 adjusts the inlet guide vane 32a and the first VFD speed to
improve the efficiency of the first compression mechanism 23a (See
for example FIG. 10B). If it is determined that the first stage
compression mechanism 23a is operating at a most efficient point
after these changes at step S10, the controller 20 proceeds to step
S5. If not, the controller 20 returns to *A to repeat the above
determinations and controls.
[0063] When the controller 20 has proceeded to step S5, the same
logic as the preceding paragraph is repeated for the second stage
compressions mechanism. Steps S5-S7 are steps used to calculate
current efficiency and determine if the second stage compression
mechanism 23b is operating at a most efficient point (See for
Example FIG. 9C). If it is determined that the second stage
compression mechanism 23b is operating at a most efficient point at
step S7, the controller 20 proceeds to step S14. If not, the
controller 20 proceeds to step S11. In steps S11-S13, the
controller 20 adjusts the inlet guide vane 32b and the second VFD
speed to improve the efficiency of the second compression mechanism
23b (See for example FIG. 10C). If it is determined that the second
stage compression mechanism 23b is operating at a most efficient
point after these changes at step S13, the controller 20 proceeds
to step S14. If not, the controller 20 returns to *B to repeat the
above determinations and controls.
[0064] In addition to the control illustrated in FIG. 6, the
rotation speeds of the VFDs 64a and 64b can be controlled to
maintain the first and second compression mechanisms 23a and 23b in
their operating ranges. However, this will not be illustrated in a
flow chart since the logic of FIG. 6 can be used, except
"efficiency" is replaced with "operating range." This type of
control can also be understood from FIGS. 7A-8C, which will be
explained below.
[0065] More details of operation of the centrifugal compressor in
accordance with the present invention will now be discussed. In the
illustrated embodiment, the two stages are separate so that each
stage (each impeller) operates by independent speed control. By
separately varying the speed of each impeller the operation range
of each impeller can be maintained within its boundary limits as
mentioned in the preceding paragraph. In addition varying
economizer flow over wide range of operating conditions can be
possible when speeds of the impellers are independently variable as
disclosed herein. Furthermore, because each stage (each impeller)
is operable by independent speed control; by separately varying the
speed of each impeller the boundary limits can be adjusted for
better matching of the each of the stages, and an increase in the
operating range of the two stage compressor; independent speed
control allows for better balancing of mass flow and work input
between the each stages, especially when considering varying
economizer flow over wide range of operating conditions.
[0066] The most restrictive operating boundary limit from either
stage becomes the limit of the 2-stage compressor; so impeller
matching (selection of compatible impellers) can become important
for a configurable product that can be sold to many different
customers with many different operating conditions; poor matching
results in useless operating range (it works very well at or near
the design point but cannot operate well away from the design
point, or can operate but the efficiency & cost is not
competitive against single-stage design); even best case scenario
for impeller matching could show improved operating range by
applying this new concept; across a wide range of operating
conditions, the side flow from economizer vapor into the inlet of
the second stage creates a significant design challenge to find an
"optimal" design because the second stage impeller mass flow varies
quite a lot.
[0067] There is relationship between operating range and rotational
speed in each impeller. Since current technology (a normal 2 stage
compressor with one motor and two impellers) only allows each
impeller to rotate at the same speed, the compressor will be
impossible to operate when either impeller operates at outside the
range. In addition, with current technology (a normal 2 stage
compressor with one motor and two impellers) once either impeller
does not operate at the point designed, the efficiency of
compressor will be dropped.
[0068] The illustrated embodiment technology can improve
compressor's operating range and efficiency because new structure
allows each compressor to rotate in different speed. Specifically,
by rotating each impeller in different rotational speed, impellers
would not be operated at outside of the operating range. Also,
efficiency of 1.sup.st and 2.sup.nd stage impeller's rotational
speed will be adjusted to increase their efficiency, and it will
improve overall compressor efficiency.
[0069] FIG. 7A is a graph illustrating operating range of a two
stage compressor (overall compressor operation), with A
representing an overall operating point outside the overall
operating range. FIG. 7B is a graph illustrating operating range of
the first stage impeller, with A1 representing a first stage
operating point outside the first stage operating range. FIG. 7C is
a graph illustrating operating range of the second stage impeller,
with A2 representing a second stage operating point inside the
second stage operating range;
[0070] For current technology's operation (a normal 2 stage
compressor with one motor and two impellers), each compressor's
operating range will be dominated by impeller's operating range.
Therefore, compressor cannot be operated when either impeller
operates at outside of the range. As FIGS. 7A-7C show, the 2.sup.nd
stage impeller can be operated at A2 (FIG. 7C), but 1.sup.st stage
impeller cannot be operated at A1 (FIG. 7B). As a result,
compressor will not be operated at A (FIG. 7A).
[0071] FIG. 8A is a graph illustrating operating range of a two
stage compressor (overall compressor operation), with A
representing an overall operating point outside the overall
operating range (like FIG. 7A) and with B representing a shifted
operating point within the overall operating range in accordance
with the present invention. In FIG. 8B, A1 represents a first stage
operating point outside the first stage operating range (like FIG.
7B) and B1 represents a shifted first operating point by decreasing
the rotation speed of the first stage impeller in accordance with
the present invention. FIG. 8C is like FIG. 7C where A2 represents
a second stage operating point inside the second stage operating
range.
[0072] By rotating each impeller at a different speed, the
impellers can both be operated inside the range. As FIG. 8B shows
the 1.sup.st stage impeller's operating point will be moved from A1
to B1 by decreasing the rotational speed. As a result, the overall
compressor's operating point will be moved from A to B in FIG. 8A.
The 2.sup.nd stage impeller's operating point will not be changed
since it operates at inside the range already.
[0073] FIG. 9A is a graph illustrating efficiency of a two stage
compressor (overall compressor efficiency), with E representing a
designed highest efficiency point and with D and E representing
shifted lower efficiency operating points. In FIG. 9B, E1
represents a designed highest efficiency point of the first stage
and D1 and E1 represent shifted lower efficiency operating points
of the first stage. Likewise, in FIG. 9C, with E2 represents a
designed highest efficiency point of the second stage and D2 and E2
represent shifted lower efficiency operating points of the second
stage.
[0074] For current technology's operation (a normal 2 stage
compressor with one motor and two impellers), compressor efficiency
will be dropped if either impeller does not operate at designed
point. The reason for this is due to the change of head coefficient
and flow coefficient. Once these values are changed, the compressor
cannot operate at designed (Highest efficiency) point. See FIGS.
9A-9C.
[0075] FIG. 10A is a graph illustrating efficiency of a two stage
compressor (overall compressor efficiency) like FIG. 9A, with E
representing a designed highest efficiency point and with D and E
representing shifted lower efficiency operating points. In FIG.
10B, E1 represents a designed highest efficiency point of the first
stage and D1 and F1 represent shifted lower efficiency operating
points of the first stage. The arrows illustrate how the efficiency
can be increased from points D1 or F1 by reducing or increasing the
first impeller speed, respectively. FIG. 10C is the same as FIG.
10b but for the second stage.
[0076] By rotating each impeller in different speed, each impeller
can be operated at the point close to the designed point. When flow
coefficient is low (point like D1 or D2), the impeller speed will
be decreased to get higher efficiency. On the other hand, when flow
coefficient is high (point like F1 and F2), the impeller speed will
be increased to get higher efficiency. The overall compressor
efficiency will be close to the highest efficiency. The change of
rotation speed will require the change of IGV position as well.
This is because the flow coefficient and head coefficient has been
changed due to the rotational speed change. Specifically, if RPM of
an impeller increases then the IGV should close to reduce inlet
flow. On the other hand, if RPM of an impeller decreases then the
IGV should open to increase inlet flow.
[0077] Referring to FIGS. 1-6, the chiller controller 20 may
include numerous control sections programmed to control the
conventional parts in a conventional manner. For example,
conventional magnetic bearing control sections, conventional
compressor variable frequency drives, a conventional compressor
motor control sections, conventional inlet guide vane control
sections, and conventional expansion valve control sections. These
sections can be separate or combined sections.
[0078] In the illustrated embodiment, the control sections are
sections of the chiller controller 20 programmed to execute the
control of the parts of the chiller 10 in accordance with FIG. 6
and as described and illustrated herein. 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.
[0079] 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 as disclosed herein. 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, except as explained herein, and thus, will not be
discussed in further detail herein, except as needed to understand
the embodiment(s).
General Interpretation of Terms
[0080] 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.
[0081] 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.
[0082] 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.
[0083] 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.
[0084] 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.
* * * * *