U.S. patent application number 11/674505 was filed with the patent office on 2007-08-16 for multi-stage compression system and method of operating the same.
This patent application is currently assigned to INGERSOLL-RAND COMPANY. Invention is credited to Britton D. Dinsdale, Michael O. Muller, James L. Robb.
Application Number | 20070189905 11/674505 |
Document ID | / |
Family ID | 38371859 |
Filed Date | 2007-08-16 |
United States Patent
Application |
20070189905 |
Kind Code |
A1 |
Dinsdale; Britton D. ; et
al. |
August 16, 2007 |
MULTI-STAGE COMPRESSION SYSTEM AND METHOD OF OPERATING THE SAME
Abstract
A multi-stage compression system includes a plurality of
centrifugal compression stages. Each stage includes an impeller and
a variable speed motor coupled to the impeller. Each variable speed
motor is operable at a speed between a first speed and a second
speed. The multi-stage compression system also includes a control
system that is connected to each of the variable speed motors and
is operable to vary the speed of each motor. The speed of each
motor is varied simultaneously such that a ratio of the speed of
any two variable speed motors remains constant.
Inventors: |
Dinsdale; Britton D.;
(Mooresville, NC) ; Muller; Michael O.;
(Charlotte, NC) ; Robb; James L.; (Rockwell,
NC) |
Correspondence
Address: |
MICHAEL BEST & FRIEDRICH, LLP
100 E WISCONSIN AVENUE, Suite 3300
MILWAUKEE
WI
53202
US
|
Assignee: |
INGERSOLL-RAND COMPANY
Montvale
NJ
|
Family ID: |
38371859 |
Appl. No.: |
11/674505 |
Filed: |
February 13, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60772715 |
Feb 13, 2006 |
|
|
|
Current U.S.
Class: |
417/2 |
Current CPC
Class: |
F04D 25/06 20130101;
F04D 25/16 20130101; F04D 27/004 20130101; F04D 27/0261 20130101;
Y02B 30/70 20130101; F04D 29/286 20130101; F04D 17/12 20130101;
F04D 29/5826 20130101; F04D 29/4206 20130101 |
Class at
Publication: |
417/2 |
International
Class: |
F04B 41/06 20060101
F04B041/06 |
Claims
1. A multi-stage compression system comprising: a plurality of
centrifugal compression stages, each stage including an impeller
and a variable speed motor coupled to the impeller, each variable
speed motor operable at a speed between a first speed and a second
speed; and a control system connected to each of the variable speed
motors and operable to vary the speed of each motor, the speed of
each motor being varied simultaneously such that a ratio of the
speed of any two variable speed motors remains constant.
2. The multi-stage compression system of claim 1, wherein a fluid
flow passes through the plurality of centrifugal compression stages
sequentially.
3. The multi-stage compression system of claim 1, wherein the
plurality of centrifugal compression stages are mechanically
de-coupled from each other.
4. The multi-stage compression system of claim 1, further
comprising at least one heat exchanger, the at least one heat
exchanger positioned to receive a fluid flow from at least one of
the plurality of centrifugal compression stages.
5. The multi-stage compression system of claim 1, wherein the
control system includes at least one sensor positioned to measure a
parameter of a fluid flow passing through the plurality of
centrifugal compression stages.
6. The multi-stage compression system of claim 5, further
comprising a valve disposed between the plurality of centrifugal
compression stages and a gas utilization system, the valve operable
to discharge a portion of the fluid flow from the multi-stage
compression system.
7. The multi-stage compression system of claim 6, wherein the
control system varies the speed of the variable speed motor and
operates the valve at least partially in response to the measured
parameter.
8. The multi-stage compression system of claim 5, wherein the
parameter is at least one of a pressure, a temperature, and a mass
flow rate.
9. The multi-stage compression system of claim 1, wherein a mass
flow rate of the plurality of centrifugal compression stages is
substantially constant.
10. The multi-stage compression system of claim 1, wherein a mass
flow rate of the plurality of centrifugal compression stages is
variable in response to use of a fluid flow at a gas utilization
system.
11. The multi-stage compression system of claim 1, wherein a first
variable speed motor operates at a different speed than a second of
the variable speed motors.
12. A multi-stage compression system comprising: a first
centrifugal compressor stage including a first impeller and a first
variable speed motor, the first variable speed motor operable at a
first speed between a low speed and a high speed; a second
centrifugal compressor stage including a second impeller and a
second variable speed motor, the second variable speed motor
operable at a second speed between a low speed and a high speed,
the first speed and the second speed defining a first ratio; a
third centrifugal compressor stage including a third impeller and a
third variable speed motor, the third variable speed motor operable
at a third speed between a low speed and a high speed, the first
speed and the third speed defining a second ratio and the second
speed and the third speed defining a third ratio; and a control
system operable to synchronously vary the first speed, the second
speed, and the third speed such that the first ratio, the second
ratio, and the third ratio remain constant.
13. The multi-stage compression system of claim 12, wherein the
first centrifugal compressor stage, the second centrifugal
compressor stage, and the third centrifugal compressor stage are
arranged in series.
14. The multi-stage compression system of claim 12, wherein the
centrifugal compressor stages are mechanically de-coupled from each
other.
15. The multi-stage compression system of claim 12, further
comprising a first heat exchanger positioned between the first
centrifugal compression stage and the second centrifugal
compression stage, a second heat exchanger positioned between the
second centrifugal compression stage and the third centrifugal
compression stage, and a third heat exchanger positioned between
the third centrifugal compression stage and an air utilization
system.
16. The multi-stage compression system of claim 12, wherein the
control system includes at least one sensor positioned to measure a
parameter of a fluid flow passing through the centrifugal
compression stages.
17. The multi-stage compression system of claim 16, further
comprising a valve disposed between the third centrifugal
compression stage and an air utilization system, the valve operable
to discharge a portion of the fluid flow from the multi-stage
compression system.
18. The multi-stage compression system of claim 17, wherein the
control system varies the speed of the first variable speed motor,
the second variable speed motor, and the third variable speed motor
and operates the valve in response to the measured parameter.
19. The multi-stage compression system of claim 16, wherein the
parameter is at least one of a temperature, a pressure, and a mass
flow rate.
20. The multi-stage compression system of claim 12, wherein a mass
flow rate of the centrifugal compression stages is substantially
constant.
21. The multi-stage compression system of claim 12, wherein a mass
flow rate of the centrifugal compression stages is variable in
response to use of a fluid flow at a gas utilization system.
22. The multi-stage compression system of claim 12, wherein the
first speed does not equal the second speed and the third
speed.
23. A method of controlling a multi-stage compression system to
deliver gas to a gas utilization system that uses the gas at a
variable rate, the method comprising: operating a first centrifugal
compressor stage at a first speed to produce a fluid flow;
directing the fluid flow to a second centrifugal compressor stage;
operating the second centrifugal compressor stage at a second
speed; and varying the first speed and the second speed in response
to the variable rate and in unison such that a ratio between the
first speed and the second speed remains constant.
24. The method of claim 23, further comprising directing the fluid
flow from the second centrifugal compressor stage to a third
centrifugal compressor stage, and operating the third centrifugal
compressor stage at a third speed.
25. The method of claim 24, further comprising varying the third
speed in unison with the first speed and the second speed such that
a second ratio between the first speed and the third speed and a
third ratio between the second speed and the third speed remain
constant.
26. The method of claim 23, further comprising monitoring a
property of the fluid flow, and varying the first speed and the
second speed in unison in response to the monitored property.
27. The method of claim 23, wherein the first speed does not equal
the second speed.
Description
RELATED APPLICATION DATA
[0001] This application claims benefit under 35 U.S.C. Section
119(e) of co-pending U.S. Provisional Application No. 60/772,715,
filed Feb. 13, 2006, which is fully incorporated herein by
reference.
BACKGROUND
[0002] The present invention relates to a system and method to
control a centrifugal compression system. In particular, the
invention relates to a control system and method that varies the
speed of multiple compression stages while maintaining constant
speed ratios.
[0003] Compressors, and more particularly centrifugal compressors,
operate across a wide range of operating parameters. Variation of
some of these parameters may produce undesirable efficiency and
capacity variations.
[0004] Compression of a gas in centrifugal compressors, also known
as dynamic compressors, is based on the transfer of energy from a
set of rotating blades to a gas such as air. The rotating blades
impart energy by changing the momentum and pressure of the gas. The
gas momentum, which is related to kinetic energy, is then converted
into pressure energy by decreasing the velocity of the gas in
stationary diffusers and downstream collecting systems.
[0005] The performance of a multi-stage centrifugal compression
system is impacted by conditions of the gas at the inlet, such as
pressure, temperature, and relative humidity, and by the operating
speed of the compressor stages. Specifically, for a given
rotational speed of a stage, variations in pressure, temperature,
and relative humidity of the gas at the inlet of that compression
stage will alter the compressor discharge head and capacity.
Additionally, a change in the operating rotational speed of a stage
of compression also results in a change in the performance
parameters of the overall compressor in terms of discharge head,
capacity, and thermodynamic efficiency. It is relevant to note that
in dynamic compressors there is a dependent relationship between
capacity and compression ratio, defined as the discharge pressure
divided by the inlet pressure (in consistent units). Accordingly, a
change in gas capacity is always accompanied by a change in the
compression ratio. As the operating point of the compressor
changes, the efficiency of the compression thermodynamic process
will also change.
SUMMARY
[0006] In one construction, the invention provides a multi-stage
compression system including a plurality of centrifugal compression
stages. Each stage includes an impeller and a variable speed motor
coupled to the impeller. Each variable speed motor is operable at a
speed between a first speed and a second speed. The multi-stage
compression system also includes a control system that is connected
to each of the variable speed motors and is operable to vary the
speed of each motor. The speed of each motor is varied
simultaneously such that a ratio of the speed of any two variable
speed motors remains constant.
[0007] In another construction, the invention provides a
multi-stage compression system including a first centrifugal
compressor stage having a first impeller and a first variable speed
motor. The first variable speed motor is operable at a first speed
between a low speed and a high speed. The multi-stage compression
system also includes a second centrifugal compressor stage having a
second impeller and a second variable speed motor. The second
variable speed motor is operable at a second speed between a low
speed and a high speed. The first speed and the second speed define
a first ratio. The multi-stage compression system further includes
a third centrifugal compressor stage having a third impeller and a
third variable speed motor. The third variable speed motor is
operable at a third speed between a low speed and a high speed. The
first speed and the third speed define a second ratio and the
second speed and the third speed define a third ratio. The
multi-stage compression system also includes a control system
operable to synchronously vary the first speed, the second speed,
and the third speed such that the first ratio, the second ratio,
and the third ratio remain constant.
[0008] In yet another construction, the invention provides a method
of controlling a multi-stage compression system to deliver a gas to
a gas utilization system that uses the gas at a variable rate. The
method includes operating a first centrifugal compressor stage at a
first speed to produce a fluid flow, directing the fluid flow to a
second centrifugal compressor stage, and operating the second
centrifugal compressor stage at a second speed. The method also
includes varying the first speed and the second speed in unison
such that a ratio between the first speed and the second speed
remains constant.
BRIEF DESCRIPTION OF DRAWINGS
[0009] FIG. 1 is a cross-sectional view of a centrifugal
compression stage;
[0010] FIG. 2 is a schematic of a multi-stage compression system
including three centrifugal compression stages of FIG. 1; and
[0011] FIG. 3 is a performance map of the centrifugal variable
speed compression stage of FIG. 1.
DETAILED DESCRIPTION
[0012] Before any embodiments of the invention are explained in
detail, it is to be understood that the invention is not limited in
its application to the details of construction and the arrangement
of components set forth in the following description or illustrated
in the following drawings. The invention is capable of other
embodiments and of being practiced or of being carried out in
various ways. Also, it is to be understood that the phraseology and
terminology used herein are for the purpose of description and
should not be regarded as limiting. The use of "including,"
"comprising," or "having" and variations thereof herein is meant to
encompass the items listed thereafter and equivalents thereof as
well as additional items. Unless specified or limited otherwise,
the terms "mounted," "connected," "supported," and "coupled" and
variations thereof are used broadly and encompass direct and
indirect mountings, connections, supports, and couplings. Further,
"connected" and "coupled" are not restricted to physical or
mechanical connections or couplings.
[0013] FIG. 1 illustrates a fluid compression module 10 (sometimes
referred to as a compression stage or a compression unit) that
includes a prime mover coupled to a compressor 20 and that is
operable to produce a compressed fluid. In the illustrated
construction, an electric motor 15 is employed as the prime mover.
However, other constructions may employ other prime movers such as
but not limited to internal combustion engines, diesel engines,
combustion turbines, etc.
[0014] The electric motor 15 includes a rotor 25 and a stator 30
that defines a stator bore 35. The rotor 25 is supported for
rotation on a shaft 40 and is positioned substantially within the
stator bore 35. The illustrated rotor 25 includes permanent magnets
45 that interact with a magnetic field produced by the stator 30 to
produce rotation of the rotor 25 and the shaft 40. In one
construction, the rotor 25 is operable between about 0 RPM and
50,000 RPM, with faster speeds also being possible. Before
proceeding, it should be noted that the word "between" as used
herein in conjunction with discussions of operating speeds of the
motors or compression stages means that the motor or compression
stage is operable at any speed between the defined end points
(e.g., 0 RPM, 50,000 RPM) and including the end points. Thus, a
two-speed motor (i.e., one operable at 0 RPM and 50,000 RPM) is not
operable between 0 RPM and 50,000 RPM. Rather, it is operable at 0
RPM and it is operable at 50,000 RPM. A motor operable between two
speeds is operable at those two speeds as well as any intermediate
speed between the end points. The magnetic field of the stator 30
can be varied to vary the speed of rotation of the shaft 40. Of
course, other constructions may employ other types of electric
motors (e.g., synchronous, induction, brushed DC motors, etc.) if
desired.
[0015] The motor 15 is positioned within a housing 50 which
provides both support and protection for the motor 15. A bearing 55
is positioned on either end of the housing 50 and is directly or
indirectly supported by the housing 50. The bearings 55 in turn
support the shaft 40 for rotation. In the illustrated construction,
magnetic bearings 55 are employed with other bearings (e.g.,
roller, ball, needle, etc.) also suitable for use. In the
construction illustrated in FIG. 1, secondary bearings 60 are
employed to provide shaft support in the event one or both of the
magnetic bearings 55 fail.
[0016] In some constructions, an outer jacket 65 surrounds a
portion of the housing 50 and defines cooling paths 70
therebetween. A liquid (e.g., glycol, refrigerant, etc.) or gas
(e.g., air, carbon dioxide, etc.) coolant flows through the cooling
paths 70 to cool the motor 15 during operation.
[0017] An electrical cabinet 75 may be positioned at one end of the
housing 50 to enclose various items such as a motor controller,
breakers, switches, and the like. The motor shaft 40 extends beyond
the opposite end of the housing 50 to allow the shaft to be coupled
to the compressor 20.
[0018] The compressor 20 includes an intake housing 80 or intake
ring, an impeller 85, a diffuser 90, and a volute 95. The volute 95
includes a first portion 100 and a second portion 105. The first
portion 100 attaches to the housing 50 to couple the stationary
portion of the compressor 20 to the stationary portion of the motor
15. The second portion 105 attaches to the first portion 100 to
define an inlet channel 110 and a collecting channel 115. The
second portion 105 also defines a discharge portion 120 that
includes a discharge channel 125 that is in fluid communication
with the collecting channel 115 to discharge the compressed fluid
from the compressor 20.
[0019] In the illustrated construction, the first portion 100 of
the volute 95 includes a leg 130 that provides support for the
compressor 20 and the motor 15. In other constructions, other
components are used to support the compressor 20 and the motor 15
in the horizontal position. In still other constructions, one or
more legs, or other means are employed to support the motor 15 and
compressor 20 in a vertical orientation or any other desired
orientation.
[0020] The diffuser 90 is positioned radially inward of the
collecting channel 115 such that fluid flowing from the impeller 85
passes through the diffuser 90 before entering the volute 95. The
diffuser 90 includes aerodynamic surfaces (e.g., blades, vanes,
fins, etc.) arranged to reduce the flow velocity and increase the
pressure of the fluid as it passes through the diffuser 90.
[0021] The impeller 85 is coupled to the rotor shaft 40 such that
the impeller 85 rotates with the motor rotor 25. In the illustrated
construction, a rod 140 threadably engages the shaft 40 and a nut
145 threadably engages the rod 140 to fixedly attach the impeller
85 to the shaft 40. The impeller 85 extends beyond the bearing 55
that supports the motor shaft 40 and, as such is supported in a
cantilever fashion. Other constructions may employ other attachment
schemes to attach the impeller 85 to the shaft 40 and other support
schemes to support the impeller 85. As such, the invention should
not be limited to the construction illustrated in FIG. 1.
Furthermore, while the illustrated construction includes a motor 15
that is directly coupled to the impeller 85, other constructions
may employ a speed increaser such as a gear box to allow the motor
15 to operate at a lower speed than the impeller 85.
[0022] The impeller 85 includes a plurality of aerodynamic surfaces
or blades that are arranged to define an inducer portion 155 and an
exducer portion 160. The inducer portion 155 is positioned at a
first end of the impeller 85 and is operable to draw fluid into the
impeller 85 in a substantially axial direction. The blades
accelerate the fluid and direct it toward the exducer portion 160
located near the opposite end of the impeller 85. The fluid is
discharged from the exducer portion 160 in at least partially
radial directions that extend 360 degrees around the impeller
85.
[0023] The intake housing 80, sometimes referred to as the intake
ring, is connected to the volute 95 and includes a flow passage 165
that leads to the impeller 85. Fluid to be compressed is drawn by
the impeller 85 down the flow passage 165 and into the inducer
portion 155 of the impeller 85. The flow passage 165 includes an
impeller interface portion 170 that is positioned near the blades
of the impeller 85 to reduce leakage of fluid over the top of the
blades. Thus, the impeller 85 and the intake housing 80 cooperate
to define a plurality of substantially closed flow passages.
[0024] In the illustrated construction, the intake housing 80 also
includes a flange 180 that facilitates the attachment of a pipe or
other flow conducting or holding component. For example, a filter
assembly can be connected to the flange 180 and employed to filter
the fluid to be compressed before it is directed to the impeller
85. A pipe may lead from the filter assembly to the flange 180 to
substantially seal the system after the filter and inhibit the
entry of unwanted fluids or contaminates. In other embodiments, a
pipe leads from the outlet of one stage to the inlet of a second
stage. The pipe connects at the flange 180.
[0025] FIG. 2 illustrates three compression stages 10a, 10b, 10c
arranged in series to define a multi-stage compression system 205.
In the illustrated construction, each compression stage 10a, 10b,
10c is similar to the compression module 10 of FIG. 1. However,
other constructions may use different compression modules
altogether, or may include a combination of different compression
module types.
[0026] For purposes of description, FIG. 2 will be described using
air as the fluid being compressed. Of course one of ordinary skill
in the art will realize that many other fluids can be compressed
using the present system. The first stage 10a draws in a flow of
air 210 in an uncompressed state and discharges a flow of
partially-compressed air 215.
[0027] The partially compressed air flow 215 passes to an
inter-stage heat exchanger or cooler 220 that cools the partially
compressed air flow 215 to improve the overall compression system
efficiency. In the illustrated, construction, a cooling fluid 225
(e.g., cool air, water, glycol, refrigerant, etc.) flows through
the heat exchanger 220 to cool the air 215.
[0028] A cooled partially compressed air flow 230 passes into the
inlet of the second stage 10b of the multi-stage compression system
205. The second stage 10b further compresses the air and discharges
a second flow of partially compressed air 235.
[0029] The second flow of partially compressed air 235 flows
through a second inter-stage heat exchanger 240 where the air is
again cooled by a coolant 245 that flows through the heat exchanger
240. After passing through the second inter-stage heat exchanger
240, a cooled partially compressed air flow 250 proceeds to the
third compression stage 10c.
[0030] The third stage 10c receives the partially compressed air
flow 250 at the inlet and is operable to further compress the air
to the final desired output pressure. A compressed air flow 255 is
discharged from the third stage 10c at the desired output
pressure.
[0031] A final inter-stage cooler 260 may be employed following the
final compression stage 10c to cool the air before being directed
to additional systems (e.g., valves, filters, dryers, etc.) or to a
compressed gas utilization system 265. As with the other heat
exchangers 220, 240 a flow of coolant 270 is used to cool the air
before the air is discharged as a final flow of compressed air
275.
[0032] The heat exchangers 220, 240, 260 may be, for example,
counter-flow heat exchangers, cross-flow heat exchangers, or
parallel flow heat exchangers. In some constructions, the heat
exchangers 220, 240, 260 may be plate-fin heat exchangers, shell
and tube heat exchangers, or any other type operable to exchange
heat between the compressed air flow 210, 235, 255 and the cooling
fluids 225, 245, 270, respectively. In addition, one or more of the
heat exchangers 220, 240, 260 may be configured as a regenerative
heat exchanger, using a compressed air flow as the cooling
fluid.
[0033] The multi-stage compression system 205 also includes a check
valve 280 and a blow-off valve 285, both located downstream of the
last compression stage 10c. The check valve 280 isolates the
compression system 205 from the compressed gas utilization system
265 during periods when the compression system 205 is not running
or when the exit pressure at the third stage 10c is lower than the
operating pressure of the utilization system 265. The blow-off
valve 285 allows a portion of compressed gas to be discharged from
the compression system 205 when the compressed gas generated
exceeds the demand of the utilization system 265.
[0034] While FIG. 2 illustrates a three-stage compression system
205 employing a single compressor 20 at each stage 10a, 10b, 10c,
other systems may employ fewer or more than three stages to meet
the requirements of the gas utilization system 265. In addition,
some arrangements may include multiple compressors at one or more
of the stages (i.e., parallel operation) to increase the capacity
of the system 205. As such, the invention should not be limited to
a three-stage compression system that employs only a single
compressor at each stage with the stages arranged in series.
[0035] As illustrated in FIG. 2, the multi-stage compression system
205 includes three stages 10a, 10b, 10c that are mechanically
de-coupled from one another with each driven by a high-speed,
variable speed electric motor 15a, 15b, 15c. As mentioned above,
the compression stages 10a, 10b, 10c are arranged in series such
that air passes sequentially through each stage 10a, 10b, 10c
before exiting the last compression stage 10c and flowing to the
compressed gas utilization system 265.
[0036] The control system 290 varies the rotational speeds of the
stages 10a, 10b, 10c simultaneously in response to a variety of
inputs. For example, in one construction, the control system 290
monitors the volumetric flow rate exiting the final stage 10c as
well as the pressure of the flow exiting the final stage 10c and
uses this data to control the speed of the individual motors 15a,
15b, 15c. In other constructions, the control system 290 monitors
the pressure at the gas utilization system 265 and/or the rate of
use at the gas utilization system 265.
[0037] The control system 290 may include sensors for sensing
pressure, temperature, and flow rate at various points within the
system 205. Additionally, the control system 290 may include
control logic algorithms and associated hardware to run the
algorithms and monitor the system parameters. Furthermore, the
control system 290 may include supplemental hardware for powering
the electric motors 15a, 15b, 15c and regulating their speeds in
response to the measured system parameters. The control system 290
is designed to regulate the performance of the centrifugal
compression stages 10a, 10b, 10c without the use of an inlet
throttling valve. However, in some constructions, a throttling
valve may still be provided at the inlet of the first compression
stage 10a. In one control system 290 sensors are employed to
measure the power (kW) consumed by one or more of the motors. This
power consumption is then used to control the speed of the
motors.
[0038] While each stage 10a, 10b, 10c is mechanically decoupled
from the other stages, preferred constructions employ a control
system 290 that varies the speed of the motors 15a, 15b, 15c
synchronously. For example, in one construction, the first motor
15a operates at a first speed between a low speed and a high speed,
the second motor 15b operates at a second speed between a low speed
and a high speed, and the third motor 15c operates at a third speed
between a low speed and a high speed. In this example, the speed of
each motor 15a, 15b, 15c is varied by the control system 290 at the
same time such that the ratio of the speeds of any two motors 15a,
15b, 15c remains constant. Thus, if the speed of the first motor
15a is doubled, the speeds of the second motor 15b and the third
motor 15c are also doubled. In one construction, the first speed,
the second speed, and the third speed are substantially the same.
However, different speeds could be employed if desired.
[0039] During the compression process, each stage 10a, 10b, 10c
imparts energy to the air in the form of pressure and motion. While
the mass flow through the compression system 205 remains
practically constant, with the exception of some amount of air that
escapes through various seals, the volume of the air is reduced
from stage to stage as the pressure increases. Customarily, the gas
leakage flow is limited to between 1 percent and 2 percent of the
volumetric inlet capacity of the compressor 20.
[0040] A typical centrifugal compressor performance map 300 is
shown in FIG. 3. FIG. 3 can be interpreted as the characteristic
performance map of a multi-stage centrifugal compression system, or
as the characteristic performance map of a single stage of
compression, although the specific values for each of the curves
will be different depending on which map it is. From FIG. 3, it can
be readily deduced that if the process demand departs from an
operating point O.sub.o, a variation in the operating speed of the
multi-stage compression system 205 may satisfy the new process
demand while maintaining a high level of operating efficiency.
[0041] For a given fixed rotational speed (shown in FIG. 3 as
constant speed lines 305) of a centrifugal compressor stage, a
relationship exists between capacity and stage pressure ratio. FIG.
3 suggests that an increase in capacity is accompanied by a
decrease in pressure ratio. Conversely, a decrease in capacity
corresponds to an increase in the pressure ratio of the compression
stage. When multiple stages are connected in series the pressure of
the gas is incrementally increased. If the inlet of an intermediate
stage coincides with the discharge of the immediately upstream
stage, and the discharge of any stage coincides with the inlet of
the following stage, the pressure ratio of each stage accounts for
interstage losses. The pressure ratio of a complete multi-stage
compressor can be expressed as the product of the pressure ratios,
R.sub.i, of the individual stages. The discharge pressure from the
last stage of compression, P.sub.disch, would then be equal to the
product of the compressor pressure ratio, R.sub.comp, times the
inlet pressure, P.sub.inlet, to the compressor, as indicated by the
following equation:
P.sub.disch=(R.sub.1.times.R.sub.2.times., . . . ,
.times.R.sub.N).times.P.sub.inlet=R.sub.comp.times.P.sub.inlet
(1)
[0042] The demand of the downstream compressed gas utilization
system 265 dictates the flow and pressure conditions at which the
compression system 205 operates. An increase in system pressure
indicates a supply of compressed gas that exceeds the demand of the
receiving system (i.e., compressed gas is being delivered faster
than it is being used). Similarly, a decrease in receiving system
pressure triggers a request of a greater capacity from the
compression system 205 to maintain, in the case of a constant
pressure process, the target operating pressure (i.e., gas is being
used faster than it is being delivered). When Equation (1) is
applied to a constant speed compressor, with performance
corresponding to one speed line 305 in the diagram of FIG. 3, and
the condition of a constant discharge pressure is imposed, i.e.
P.sub.disch=Constant, then a required capacity change to
accommodate the process demand will affect the pressure ratio of
the compression system 205 since the pressure ratio of each stage
10a, 10b, 10c is uniquely linked to a given capacity. It follows
that since the term on the left-hand side of Equation (1) is
constant, and the pressure ratio of the compression system 205
physically changes, then the inlet pressure to the first
compression stage 10a assumes a value so that the product in
Equation (1) remains, in fact, constant.
[0043] As an example, if the capacity handled by the compression
system 205 is reduced in order to counteract a contingent increase
in system pressure, the pressure ratio of the compression system
205 increases because, based on the performance characteristics of
centrifugal compressors, a decrease in capacity corresponds to an
increase in compressor pressure ratio. Accordingly, to maintain
constant pressure at the discharge of the compression system 205,
the inlet pressure to the compression system 205,
P.sub.inlet.sup.(New), assumes a new value equal to:
P disch R comp ( New ) = P inlet ( New ) ( 2 ) ##EQU00001##
where: [0044] R.sub.comp.sup.(New)=Compressor pressure ratio
corresponding to the required flow [0045]
P.sub.inlet.sup.(New)=Updated, new inlet pressure at the first
stage of compression to maintain the required constant discharge
pressure
[0046] The change in the inlet pressure of a first stage of
compression, in the case of an open to atmosphere compression
system, is typically obtained in prior art systems by interposing a
throttling valve between the atmospheric environment and the inlet
of the first stage of compression. Following the example at hand, a
calibrated closure of the inlet throttling valve introduces a
pressure drop, thus reducing the pressure at the inlet of the first
compression stage. The new reduced inlet pressure that satisfies
Equation (2), when multiplied by the increased pressure ratio
required to achieve a smaller compressor capacity, yields the
required constant pressure at the discharge of the compression
system. This is the most common regulation system for fixed speed
compressors, and while it properly controls the capacity of a
centrifugal compressor, it also introduces an energy loss because
of the presence of the inlet throttling valve.
[0047] However, an unaltered pressure ratio of the compression
stages 10a, 10b, 10c, under a changing capacity requirement, can
also be satisfied by modifying the operating speed of the stages
10a, 10b, 10c. Accordingly, the overall pressure ratio of the
compression system 205 can be maintained as the capacity is
adjusted to the demand of the utilization system 265. Since the
discharge pressure of the compression system 205 is maintained,
under changing capacity requirements, on the basis of the
relationship that is uniquely related to the pressure ratio, the
capacity, and the rotational speed of the compression stages 10a,
10b, 10c, there is no requirement to alter the inlet pressure to
the first stage 10a with a regulating inlet throttling valve. In
fact, with reference to Equation (2), if the compressor pressure
ratio is maintained constant, then the inlet pressure to the first
stage 10a does not need to be changed to maintain a value for the
discharge pressure. Because of the change in capacity, the inlet
pressure of the compression system 205 may actually change because
of changes in the pressure losses that are linked to the contingent
capacity. Nonetheless, in this context, it is emphasized that the
control system 290 does not require the first stage inlet pressure
to be varied by means of a throttling valve, and accordingly there
is no throttling valve. It should be noted that the removal of the
inlet throttling valve directly improves the efficiency of the
first stage 10a, reduces control software complexity, eliminates
hardware, and improves costs as compared to fixed speed systems
that include the throttling valve.
[0048] Each stage 10a, 10b, 10c in the multi-stage compression
system 205 is designed to handle fluid having a certain inlet
pressure range, but particularly a defined intake volume rate
range. Because the stages 10a, 10b, 10c are in series, each stage
10a, 10b, 10c is flow-matched to the others so as to allow for the
proper transfer of fluid from one stage to the next. Accordingly,
if for some reason one stage is not in the condition to accept the
flow delivered by an upstream stage, or does not deliver the flow
at the intended pressure and volume to the next stage, a disruption
in the operation of the multi-stage compression system 205 may
occur, thereby resulting in an upset in the performance of all the
stages 10a, 10b, 10c in the compression system 205. Because of the
gas dynamics accompanying the behavior of centrifugal compressors,
a condition of flow instability may quickly develop which requires
the unloading of the compression system 205 to properly handle and
suppress the unstable situation.
[0049] The performance of the compression system 205 is
particularly sensitive to the variation in rotational speed of the
stages 10a, 10b, 10c. Compressor stage capacity varies linearly
with speed, head changes vary proportionally to the square of the
speed, and power varies as the cube of the stage speed. Because of
the aerodynamic coupling between compressor stages 10a, 10b, 10c in
series, the approach of independently changing the speed of each
stage 10a, 10b, 10c, when the stages 10a, 10b, 10c are mechanically
de-coupled, requires a significant level of sophistication and
control logic complexity. The operating conditions of each
compression stage 10a, 10b, 10c is monitored in terms of gas
pressure, temperature, and flow, so as to predict the speed change
for one stage in conjunction with the speed of the other stages,
while not upsetting the internal operation of each compression
stage 10a, 10b, 10c and while satisfying the demand of the
utilization system 265.
[0050] The rate of change in fluid flow demand of the utilization
system 265 adds complications to an approach of independently
regulating the speeds of the stages 10a, 10b, 10c because
background calculations are performed before attempting the
adaptation of the compression system 205 to the variation in the
operating conditions of the utilization system 265. A significant
time delay may be introduced between the acknowledgement that the
process conditions are changing and an action is taken to satisfy
the process demand. Moreover, a predetermined databank must be
available to the computational control algorithms so as to
reasonably predict the appropriate speed regulation of the
independently driven centrifugal compressor stages 10a, 10b, 10c
present in the compression system 205.
[0051] An explorative type approach may be employed, where changes
are made in a semi-static compressor operating condition so as to
prevent transient driven upsets within the compressor system while
continuing to satisfy the utilization system demand in terms of
discharge pressure and flow rate. Such an approach may not satisfy
required rapid changes in the operating conditions of the
compression system 205 to respond to the demand of the compressed
gas utilization system 265.
[0052] The constructions discussed herein approach the speed
regulation of the compression system 205 by adjusting the
contingent operating speed of each stage 10a, 10b, 10c
synchronously, while maintaining a predetermined constant ratio
between the speeds of each stage 10a, 10b, 10c. This means that the
speed of each stage 10a, 10b, 10c is modified at the same time and
the ratio between the rotational speeds is kept constant at all
times. In one construction, a predetermined value of the speed
ratio is equal to one. In this case, a speed ratio of one
corresponds to the condition where two referenced stages operate at
the same rotational speed. In other embodiments, the ratios are
equal to values other than one, meaning the two referenced stages
are operating at different rotational speeds.
[0053] During the act of regulation, as the stage speeds are
synchronously changed, the ratio between the speeds will remain
constant as predetermined by the individual characteristics of the
compressor stages. Nonetheless, in the control algorithms, the
speed ratios can be defined as variables that can be changed from
one compressor configuration to another. For example, one set of
simultaneous constraint equations that characterize the control
approach can be summarized symbolically as follows: Equations (3a)
impose the condition that the speed ratio, and consequently the
rotational speed change of one stage with respect to all the other
stages, is constant during the compressor control action.
.DELTA. N 1 .DELTA. N 2 = K 1 - 2 .DELTA. N 1 .DELTA. N 3 = K 1 - 3
.DELTA. N 1 .DELTA. N n = K 1 - n .DELTA. N n - 1 .DELTA. N n = K (
n - 1 ) - n ( 3 a ) .DELTA. N 1 = .DELTA. N 1 .delta. .DELTA. N 2 =
.DELTA. N 2 .delta. .DELTA. N n = .DELTA. N n .delta. ( 3 b )
.delta. = 0 , 1 ( 3 c ) Abs ( P disch - P demand ) .ltoreq. P (
Constant Pressure Control ) ( 3 d ) Abs ( Q disch - Q demand )
.ltoreq. Q ( Constant Delivered Capacity Control ) ( 3 e )
##EQU00002##
where: [0054] .DELTA.N.sub.i=Speed change of the ith stage [0055]
K.sub.i-j=Speed ratio of the ith stage with respect to the jth
stage. The speed ratio is a real number in a mathematical sense.
[0056] .delta.=a Kronecker delta function which can be interpreted
as a control decision variable suggesting that stage speed
variations must occur synchronously. [0057] .epsilon..sub.P=Error
related to pressure [0058] .epsilon..sub.Q=Error related to
capacity [0059] P=Pressure [0060] Q=Capacity
[0061] Equations (3b) identify the synchronous occurrence of the
rotational speed change with variable .delta. assuming a value of
zero when no speed regulation is in process, and assuming a value
of one when compressor control, with reference to stage speed
changes, is required. Equations (3d) and (3e) impose the condition
that the speed change satisfies the demand of the compressed gas
utilization system 265 within a certain tolerance.
[0062] The compressor control scheme is enhanced by an aerodynamic
design of the compression stages 10a, 10b, 10c that accommodates a
synchronous variation of the rotational speed of the stages 10a,
10b, 10c while insuring stable changes in compressor head, pressure
ratio, and capacity. Such aerodynamic design elements include
evenly distributed impeller blade loading, advanced 3-D impeller
blade shapes, and a 3-D diffuser design. Thus, the illustrated
construction operates in a way that avoids boundary conditions at
the inlet or at the discharge of a stage that may cause the surge
limit 310 or choke limit 315, shown in FIG. 3, to be active
constraints.
[0063] FIG. 3 also shows paths 320 along which the efficiency of
the compression process is constant. The highest efficiency of the
stages 10a, 10b, 10c is planned at the nominal operating conditions
and, in the figure, is identifiable by the region at the center of
the constant efficiency paths 320. The deterioration in efficiency
can be kept to a minimum as the operating point O.sub.o departs
from the design point, such that, for dynamically similar stages, a
synchronous stage speed change, while maintaining a constant ratio
between the speed of each stage, does not significantly deteriorate
the efficiency of the multi-stage compression system 205.
[0064] Also, the design of dynamically similar stages, in
aerodynamic terms, allows stable operation during speed variations,
without incurring transient instability issues. The aerodynamic
design of the stages 10a, 10b, 10c augments the performance of the
control system 290 by resulting in a modest deterioration of the
design optimal efficiency within a significant speed range.
[0065] Further, a control inlet throttling valve is not required in
the multi-stage compression system 205, thus reducing the cost and
complexity of the system 205.
[0066] Finally, compressor unload can be achieved by synchronously
reducing the speeds of the stages 10a, 10b, 10c while maintaining a
constant relative speed ratio. Opening the blow-off valve 285
accommodates the transient unload event of the compression system
205, as the check valve 280 isolates the compression system 205
from the utilization system 265. The control system 290 may also
allow the complete shut-down of the compression system 205 in the
event that the utilization system 265 does not require the
operation of the compression system 205.
[0067] Various features and advantages of the invention are set
forth in the following claims.
* * * * *