U.S. patent application number 13/818457 was filed with the patent office on 2013-06-13 for multi-compressor refrigeration system and method for operating it.
This patent application is currently assigned to CARRIER CORPORATION. The applicant listed for this patent is Lucy Y. Liu. Invention is credited to Lucy Y. Liu.
Application Number | 20130145781 13/818457 |
Document ID | / |
Family ID | 46125506 |
Filed Date | 2013-06-13 |
United States Patent
Application |
20130145781 |
Kind Code |
A1 |
Liu; Lucy Y. |
June 13, 2013 |
Multi-Compressor Refrigeration System and Method for Operating
It
Abstract
A refrigeration system (20) has a first compressor (24) and a
second compressor (26). The second compressor has at least a first
condition t least partially in parallel with the first compressor
along a refrigerant flowpath. A heat rejection heat exchanger (50)
is downstream of the first and second compressors along the
refrigerant flowpath. An expansion device (54) is downstream of the
heat rejection heat exchanger along the refrigerant flowpath. A
heat absorption heat exchanger (56) is downstream of the expansion
device along the refrigerant flowpath. The first compressor is a
variable speed compressor coupled to a variable speed drive (32).
The second compressor is a fixed speed compressor.
Inventors: |
Liu; Lucy Y.; (Fayetteville,
NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Liu; Lucy Y. |
Fayetteville |
NY |
US |
|
|
Assignee: |
CARRIER CORPORATION
Farmington
CT
|
Family ID: |
46125506 |
Appl. No.: |
13/818457 |
Filed: |
April 27, 2012 |
PCT Filed: |
April 27, 2012 |
PCT NO: |
PCT/US2012/035416 |
371 Date: |
February 22, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61486496 |
May 16, 2011 |
|
|
|
Current U.S.
Class: |
62/115 ;
62/510 |
Current CPC
Class: |
F25B 1/02 20130101; F25B
49/022 20130101; Y02B 30/70 20130101; F25B 2600/021 20130101; Y02B
30/741 20130101; F25B 2400/075 20130101; F25B 9/008 20130101; F25B
2600/024 20130101; F25B 2400/0751 20130101; F25B 1/00 20130101 |
Class at
Publication: |
62/115 ;
62/510 |
International
Class: |
F25B 1/00 20060101
F25B001/00 |
Claims
1. A refrigeration system (20) comprising: a first compressor (24);
a second compressor (26) having at least a first condition at least
partially in parallel with the first compressor along a refrigerant
flowpath; a heat rejection heat exchanger (50) downstream of the
first compressor and second compressor along the refrigerant
flowpath; an expansion device (54) downstream of the heat rejection
heat exchanger along the refrigerant flowpath; and a heat
absorption heat exchanger (56) downstream of the expansion device
along the refrigerant flowpath, wherein: the first compressor is a
variable speed compressor coupled to a variable speed drive (32)
and the second compressor is a fixed speed compressor; and the
second compressor is larger than the first compressor.
2. (canceled)
3. The system of claim 1 wherein: the second compressor has a
larger displacement per revolution than a displacement per
revolution of the first compressor.
4. The system of claim 1 wherein: the first compressor and the
second compressor are reciprocating compressors.
5. The system of claim 1 in operational condition with the second
compressor connected directly to a line voltage (34) and the first
compressor connected to the line voltage via its variable speed
drive.
6. A transport system (200) comprising: the refrigeration system
(20) of claim 1; and a refrigerated container (201) having an
interior (202) containing or in air flow communication with the
heat absorption heat exchanger.
7. (canceled)
8. The system of claim 1 wherein: a displacement per revolution of
the second compressor is 110-350% of a displacement per revolution
of the first compressor.
9. The system of claim 1 wherein: the first compressor has an
induction motor or a permanent magnet motor; and the second
compressor has an induction motor.
10. The system of claim 1 further comprising a controller
configured to: at high required capacity (upper range), operate
(310) both the first compressor and the second compressor, the
second compressor being operated at a fixed speed; and in low
required capacity (lower range), operate (318) only the first
compressor, over at least a portion of said lower capacity range
the operating being with variable speed.
11. The system of claim 10 wherein the controller is configured to
in no part of a normal operational range operate the second
compressor alone.
12. A method for operating the system of claim 1, the method
comprising: at high required capacity (upper range), operating
(310) both the first compressor and the second compressor, the
second compressor being operated at a fixed speed; and in low
required capacity (lower range), operating (318) only the first
compressor, over at least a portion of said lower capacity range
the operating being with variable speed.
13. The method of claim 12 wherein: the lower range meets the upper
range.
14. (canceled)
15. The method of claim 12 wherein: the operation of the first
compressor in an uppermost portion of the lower capacity range is
at a power frequency in excess of a line power frequency.
16. The method of claim 12 wherein: a cooldown phase comprises the
operation in the high capacity range; and a post-cooldown phase
comprises the operation in the lower capacity range.
17. The method of claim 12 wherein: the control is responsive to a
sensed air temperature of a controlled space.
18. A method for operating a refrigeration system, the
refrigeration system comprising: a first compressor (24); a second
compressor (26) having at least a first condition at least
partially in parallel with the first compressor along a refrigerant
flowpath; a heat rejection heat exchanger (50) downstream of the
first compressor and second compressor along the refrigerant
flowpath; an expansion device (54) downstream of the heat rejection
heat exchanger along the refrigerant flowpath; and a heat
absorption heat exchanger (56) downstream of the expansion device
along the refrigerant flowpath, wherein: the first compressor is a
variable speed compressor coupled to a variable speed drive (32)
and the second compressor is a fixed speed compressor, the method
comprising: controlling operation of the compressor responsive to a
sensed air temperature of the controlled space.
19. The method of claim 18 wherein: the sensed air temperature is
used to control transitions between operation of only one of the
compressors and both of the compressors.
20. A method for operating a refrigeration system, the
refrigeration system comprising: a first compressor (24); a second
compressor (26) having at least a first condition at least
partially in parallel with the first compressor along a refrigerant
flowpath; a heat rejection heat exchanger (50) downstream of the
first compressor and second compressor along the refrigerant
flowpath; an expansion device (54) downstream of the heat rejection
heat exchanger along the refrigerant flowpath; and a heat
absorption heat exchanger (56) downstream of the expansion device
along the refrigerant flowpath, wherein: the first compressor is a
variable speed compressor coupled to a variable speed drive (32)
and the second compressor is a fixed speed compressor, the method
comprising: at high required capacity (upper range), operating
(310) both the first compressor and the second compressor, the
second compressor being operated at a fixed speed; and in low
required capacity (lower range), operating (318) only the first
compressor, over at least a portion of said lower capacity range
the operating being with variable speed, wherein: the lower range
comprises a lower sub-range wherein the first compressor is
operated in a cyclic mode with essentially fixed speed when
operating and, an upper sub-range operated continuously with speed
increasing with required capacity.
21. The method of claim 20 wherein: the lower range meets the upper
range.
22. The method of claim 20 wherein: the operation of the first
compressor in an uppermost portion of the lower capacity range is
at a power frequency in excess of a line power frequency.
23. The method of claim 20 wherein: a cooldown phase comprises the
operation in the high capacity range; and a post-cooldown phase
comprises the operation in the lower capacity range.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] Benefit is claimed of U.S. patent application Ser. No.
61/486,496, filed May 16, 2011, and entitled "Multi-Compressor
Refrigeration System", the disclosure of which is incorporated by
reference herein in its entirety as if set forth at length.
BACKGROUND
[0002] The disclosure relates to refrigeration. More particularly,
the disclosure relates to refrigerated transport containers using
CO.sub.2-based refrigerant.
[0003] CO.sub.2-based refrigerant such as R744 has drawn increasing
attention for use in refrigerated transport containers. Exemplary
refrigerated transport containers include shipping containers and
containers integral with trucks, trailers, or rail cars. Such
containers, especially shipping containers, may be subject to a
wide variety of operating conditions. The operating conditions
reflect both the external/environmental temperature and the
interior temperature. Interior temperature varies based upon the
nature of the goods being transported, with low temperatures being
required for frozen goods and higher temperatures being required
for non-frozen refrigerated perishable goods. Exemplary systems
include an electrically-powered compressor for driving refrigerant
along a circuit/flowpath through an exterior heat rejection heat
exchanger and an interior heat absorption heat exchanger.
SUMMARY
[0004] One aspect of the disclosure involves a refrigeration system
having a first compressor and a second compressor. The second
compressor has at least a first condition at least partially in
parallel with the first compressor along a refrigerant flowpath. A
heat rejection heat exchanger is downstream of the first and second
compressors along the refrigerant flowpath. An expansion device is
downstream of the heat rejection heat exchanger along the
refrigerant flowpath. A heat absorption heat exchanger is
downstream of the expansion device along the refrigerant flowpath.
The first compressor is a variable speed compressor coupled to a
variable speed drive. The second compressor is a fixed speed
compressor.
[0005] In various implementations, the fixed speed compressor may
have a larger displacement than the variable speed compressor. The
compressors may be reciprocating compressors. This may be in an
operational condition with the fixed speed compressor connected
directly to a line voltage and the variable speed compressor
connected to the line voltage via its variable speed drive.
[0006] The details of one or more embodiments are set forth in the
accompanying drawings and the description below. Other features,
objects, and advantages will be apparent from the description and
drawings, and from the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is a schematic view of a refrigeration system.
[0008] FIG. 2 is a view of a refrigerated container.
[0009] FIG. 3 is a control flowchart for the system of FIG. 1.
[0010] Like reference numbers and designations in the various
drawings indicate like elements.
DETAILED DESCRIPTION
[0011] FIG. 1 shows a vapor compression system 20 having a
compressor subsystem 22. The exemplary compressor subsystem 22
includes a first compressor 24 and a second compressor 26 at least
partially in parallel with the first compressor along a refrigerant
flowpath 500. The exemplary first and second compressors are both
reciprocating compressors. The exemplary compressors have
respective electric motors 28 and 30. The exemplary motor 28 is
powered by a variable frequency drive (variable speed drive (VSD))
32 in turn powered by line power 34. The exemplary motor 30 is
directly powered by the line power 34. Exemplary line power is 50
Hz or 60 Hz providing three-phase power to the motor 30 and VSD 32.
For example, in the case of a cargo container drawing line power
from a generator on the ship carrying the container, the exemplary
voltage is 460V.
[0012] The exemplary motors 28 and 30 are hermetic induction
motors. An alternate motor 28 is a permanent magnet motor. The
exemplary motor 28 is rated for adjustable speed duty (ASD) under
applicable industry standards. An ASD-rated motor will likely have
more robust winding insulation than a non-ASD-rated motor. The line
voltage may represent a maximum of the VSD output voltage, but the
VSD may be configured to provide a higher than line frequency at
such maximum voltage. The exemplary VSD is capable of running the
motor 28 across a frequency range spanning the line frequency. An
exemplary low end of the range is 15-20 Hz. An exemplary high end
of the range is at least 110 Hz or 120 HZ. For example, with an
exemplary line power of 460 V and 60 Hz, at a VSD output frequency
of 120 Hz, the VSD output voltage may be that maximum 460 V. With a
linear V/f curve, the VSD output voltage is 230 V@ 60 Hz and 77V@20
Hz (an exemplary lowest operating frequency of the motor 28 noted
above and further discussed below).
[0013] Downstream from the compressor discharge ports, the
refrigerant flowpath sequentially passes through a first heat
exchanger 50, a receiver 52, an expansion device 54, and a second
heat exchanger 56. In the first mode, the heat exchanger 50 is a
heat rejection heat exchanger (e.g., a gas cooler or condenser) and
the second heat exchanger 56 is a heat absorption heat exchanger
(e.g., an evaporator). The exemplary heat exchangers 50 and 56 are
refrigerant-air heat exchangers wherein electric-powered fans 60
and 62, respectively, drive air flows 64 and 66 across the transfer
elements (e.g., coils).
[0014] A controller 70 may be coupled to various controllable
system components (e.g., the compressor motors, the fans, the
expansion device or any other control valves, and the like). The
controller may be coupled to receive inputs from sensors (e.g.,
pressure and/or temperature sensors). Exemplary sensors include a
supply air sensor 72 positioned to measure the temperature of the
flow 66 exiting the heat exchanger 56, a return air temperature
sensor 74 positioned to measure the temperature of the air 66
entering the heat exchanger 56, and an ambient/external temperature
sensor 76 (e.g., positioned to measure ambient/external air
temperature of the flow 64 entering the heat exchanger 50).
[0015] The exemplary system 20 is used in a refrigerated transport
system 200 (FIG. 2). An exemplary system is shown as a shipping
container 201 having a refrigerated compartment 202. An equipment
compartment 204 is located at one end of the container and contains
the components of the system 20. The evaporator 56 in the
refrigerated compartment 202 (or in air flow communication with the
refrigerated compartment 202 via the recirculating air flow 66).
Other similar refrigerated transport systems include trucks and
trailers as described above. The exemplary shipping container draws
power from an external source (e.g., the generator of a ship).
However, truck and trailer systems are more likely to include
electrical generators (e.g., diesel engine-powered electrical
generators). Such electrical generators may be in housings external
to the main box of the truck or trailer (e.g., along with the
compressor and heat rejection heat exchanger).
[0016] In configuring the system, total cost of ownership (TCO) is
an important consideration. Common industry practices have arisen
involving measuring cost at specific operating conditions (the TCO
points) which may be associated with specific users (e.g., at which
such users spend majority of time). Each TCO point is characterized
by an ambient temperature and a refrigerated compartment
temperature. If a single compressor were used and sized to meet the
full load pulldown capacity requirement, it would have substantial
extra capacity at partial load conditions which provide the
majority of TCO points. Such excess capacity would involve
inefficient operation. Accordingly, the presence of multiple
compressors may allow sizing to provide better efficiency at the
lower load TCO points and meet the full load pulldown capacity
requirement (even if use of multiple compressors provides lower
efficiency during pulldown).
[0017] In an exemplary implementation, the variable speed
compressor is run alone over a load range from minimal load to an
intermediate load. The variable speed compressor is thus sized to
provide maximum efficiency over this range which includes the
majority of TCO points. Once the variable speed compressor has been
sized, the fixed speed compressor may be sized to make up for the
difference between the maximum capacity of the variable speed
compressor and the maximum required system capacity. The maximum
required system capacity is usually defined by an extreme of
anticipated need to provide cool down (pulldown) in an initial
operating condition. Steady state operating condition is typically
at a substantially lower capacity. The exemplary fixed speed
compressor is larger than the exemplary variable speed compressor.
An exemplary size measurement is displacement per revolution. The
exemplary fixed speed compressor is 110 to 350% the size of the
variable speed compressor, more narrowly, 125-350%. Yet, more
narrowly, 125-250%. Even though the variable speed compressor has a
smaller displacement, the ability of the VSD to output frequencies
greater than line frequency allows the smaller displacement
variable speed compressor to be run at capacities which may exceed
the capacity of the fixed speed compressor run at line power. This
allows the variable speed compressor to be operated alone at low
loads without having a gap in load handling. More particularly, the
peak capacity of the variable speed compressor may meet or exceed
the combination of the capacity of the fixed speed compressor and
the minimum capacity of the variable speed compressor. This allows
a smooth handover between operations in: a mode where only the
variable speed compressor runs; and a mode where both compressors
run (without a gap between available capacities of those two
modes).
[0018] FIG. 3 shows a basic control algorithm 300. There is a
start-up 302. The relationship between a measured or otherwise
determined system temperature and a target temperature is then
determined. For example, the measured temperature may be the
temperature T.sub.C(S) of the supply air measured by the sensor 72
or T.sub.C(R) of the return air measured by the sensor 74. The
desired temperature may be a user-entered temperature set point.
The relationship may involve determining 304 whether the measured
temperature exceeds the set point by at least a given threshold
(DT). Exemplary DT may be preset based upon the nature of the use
(e.g., refrigerating frozen goods versus refrigerating non-frozen
perishable goods) and the particular measured temperature may be
determined by such use. For example, in a frozen goods scenario, a
main focus of operation may be to avoid temperature high enough to
melt the goods. The return temperature T.sub.C(R) may be measured
to ensure that it does not exceed a user-entered temperature
setpoint. An exemplary target return temperature may be relatively
low (e.g., less than 14.4 F). For non-frozen perishable goods, it
may be more important to measure the supply temperature to ensure
that the supply temperature is not so low as to freeze the goods
that the air initially comes in contact with. Exemplary supply
temperature is thus in excess of 14.4 F. For frozen goods, there
may be more flexibility in DT than for non-frozen perishable goods.
Thus, an exemplary DT for frozen goods is approximately 4 F whereas
a DT for non-frozen perishable goods is approximately 0.5 F. This
effectively determines whether the system is in a high capacity
situation or a low capacity situation. If yes (high capacity
situation), then the compressors are run simultaneously 310 with
the variable speed compressor at a temperature-dependent speed up
to its maximum speed. This provides the fastest possible cool
down/pull down.
[0019] If not in the high capacity situation, then it is determined
312 whether any cooling is required (e.g., the measured temperature
is greater than the set temperature by less than the DT) (low
capacity situation). If no, then both compressors are shut put in
their off conditions 314 and the cycle can repeat. If yes, then it
is determined 316 whether the fixed speed compressor has been off
or it has run for its minimum time. This determination 316 helps
avoid short cycling of the fixed speed compressor. If yes at 316,
then only the variable speed compressor is run 318. It is run at a
speed appropriate for the needed capacity. If no, then there is
simultaneous operation 310 to avoid short cycling of the fixed
speed compressor. If the variable speed compressor only is run at
318, then it is determined 320 whether it has been at its maximum
speed for a predetermined time. Yes indicates that the variable
speed compressor alone is not effective to bring down the
temperature quickly enough. Thus, if yes, then simultaneous
operation is resumed at 310. If no, however, there is a minimum
capacity which the variable speed compressor may provide in
continuous operation. It is therefore determined 322 whether
required speed has reduced to this minimum speed for a
predetermined time. If no, the control cycle merely repeats at step
304. If yes, then the variable speed compressor is shut off at
314.
[0020] Once in simultaneous operation at 310, there is then a
determination 330 of whether the variable speed compressor is being
operated within a certain proximity of its minimum speed. if no,
then the cycle repeats at 304. If yes, then the system shifts to
variable speed compressor only operation at 318.
[0021] In the foregoing example, the controller is configured to in
no part of a normal operational range operate the fixed speed
compressor alone. However, this may be done in abnormal situations
such as a failure of the variable speed compressor or associated
components, a service mode, or a manual override mode where the
user commands shut down of the variable speed compressor.
[0022] Although an embodiment is described above in detail, such
description is not intended for limiting the scope of the present
disclosure. It will be understood that various modifications may be
made without departing from the spirit and scope of the disclosure.
For example, when applied to the reengineering of the configuration
of an existing system, details of the existing system may influence
details of any particular implementation. Accordingly, other
embodiments are within the scope of the following claims.
* * * * *