U.S. patent application number 12/672742 was filed with the patent office on 2011-12-29 for tadem compressor operation.
This patent application is currently assigned to CARRIER CORPORATION. Invention is credited to Alexander Lifson, Michael F. Taras.
Application Number | 20110314845 12/672742 |
Document ID | / |
Family ID | 40549443 |
Filed Date | 2011-12-29 |
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United States Patent
Application |
20110314845 |
Kind Code |
A1 |
Lifson; Alexander ; et
al. |
December 29, 2011 |
TADEM COMPRESSOR OPERATION
Abstract
A refrigerant system operates with tandem compressors. As is
known, one or more of the tandem compressors can be shut down at
part-load operating conditions when lower refrigerant system
capacity is required. A control periodically engages at least one
shut down tandem compressor for a short period of time. This
increases the refrigerant mass flow of refrigerant throughout the
refrigerant system to ensure that oil travels through the system
and is not retained in the system.
Inventors: |
Lifson; Alexander; (Manlius,
NY) ; Taras; Michael F.; (Fayetteville, NY) |
Assignee: |
CARRIER CORPORATION
Syracuse
NY
|
Family ID: |
40549443 |
Appl. No.: |
12/672742 |
Filed: |
October 10, 2007 |
PCT Filed: |
October 10, 2007 |
PCT NO: |
PCT/US2007/080873 |
371 Date: |
February 9, 2010 |
Current U.S.
Class: |
62/115 ;
62/228.1; 62/468 |
Current CPC
Class: |
F25B 2400/075 20130101;
F25B 2600/0251 20130101; F25B 49/022 20130101; F25B 2400/13
20130101; F25B 2600/23 20130101 |
Class at
Publication: |
62/115 ; 62/468;
62/228.1 |
International
Class: |
F25B 49/02 20060101
F25B049/02; F25B 1/00 20060101 F25B001/00; F25B 41/00 20060101
F25B041/00 |
Claims
1. A refrigerant system having a compressor unit comprising: a
plurality of tandem compressors, said plurality of tandem
compressors utilized for compressing a refrigerant, and delivering
the refrigerant throughout the refrigerant system, and to at least
one heat rejection heat exchanger, an expansion device and at least
one evaporator; and a control for operating said refrigerant system
having capability to shut down at least one of said plurality of
tandem compressors to provide unloading of the refrigerant system,
and said control periodically moving into an oil return mode by
engaging at least one shutdown compressor for at least one short
period of time to increase refrigerant mass flow rate and
facilitate oil return.
2. The refrigerant system as set forth in claim 1, wherein said
plurality of tandem compressors have both a common suction manifold
and a common discharge manifold.
3. The refrigerant system as set forth in claim 1, wherein said
plurality of tandem compressors have at least one of a common
suction manifold and a common discharge manifold.
4. The refrigerant system as set forth in claim 1, wherein the
compressor unit incorporates at least one economized tandem
compressor.
5. The refrigerant system as set forth in claim 1, wherein the
compressor unit incorporates at least two economized tandem
compressors also having a common intermediate manifold.
6. The refrigerant system as set forth in claim 1, wherein the
compressor engagement is controlled based on a safety device or
sensor feedback.
7. The refrigerant system as set forth in claim 6, wherein the
sensor is an oil level sensor.
8. The refrigerant system as set forth in claim 1, wherein
compressor engagement is based on at least one environmental or
operating control parameter.
9. The refrigerant system as set forth in claim 8, wherein the
control parameter being a shutdown time for at least one of the
shutdown compressors.
10. The refrigerant system as set forth in claim 9, wherein an
additional control parameter is included, and is at least one of a
saturation suction temperature, suction pressure, evaporator
secondary fluid temperature, oil temperature, and suction
temperature.
11. The refrigerant system as set forth in claim 1, wherein the
short period of time is predetermined.
12. The refrigerant system as set forth in claim 1, wherein the
short period of time is defined by at least one of the following
control parameters: saturation suction temperature, suction
pressure, evaporator secondary fluid temperature, oil temperature,
suction temperature, temperature variations in the conditioned
environment, humidity variation in the conditioned environment.
13. The refrigerant system as set forth in claim 1, wherein said
control engages a shutdown compressor after a predetermined period
of time has elapsed with said at least one compressor being shut
down.
14. The refrigerant system as set forth in claim 13, wherein the
elapsed shutdown period of time is between 30 minutes and 12
hours.
15. The refrigerant system as set forth in claim 1, wherein the
short period of time is between 10 seconds and 10 minutes.
16. The refrigerant system as set forth in claim 13, wherein said
at least one compressor being engaged for a plurality of short
periods of time in sequence.
17. The refrigerant system as set forth in claim 1, wherein said
engaged at least one compressor is again shut down after it has
been engaged for a short period of time.
18. The refrigerant system as set forth in claim 1, wherein
multiple compressors are brought online in sequence.
19. The refrigerant system as set forth in claim 1, wherein
multiple compressors are brought online simultaneously.
20. The refrigerant system as set forth in claim 1, wherein
multiple compressors are shutdown in sequence.
21. The refrigerant system as set forth in claim 1, wherein
multiple compressors are shutdown simultaneously.
22. A method of operating a refrigerant system including the steps
of: providing a compressor unit comprising a plurality of tandem
compressors, said plurality of tandem compressors compressing a
refrigerant, and delivering the refrigerant throughout the
refrigerant system, and to at least one heat rejection heat
exchanger, an expansion device and at least one evaporator; and
controlling said refrigerant system to shut down at least one of
said plurality of tandem compressors to provide unloading of the
refrigerant system, and periodically moving into an oil return mode
by engaging at least one shutdown compressor for at least one short
period of time to increase refrigerant mass flow rate and
facilitate oil return.
23. The method as set forth in claim 22, wherein the compressor
engagement is controlled based on a safety device or sensor
feedback.
24. The method as set forth in claim 22, wherein compressor
engagement is based on at least one environmental or operating
control parameter.
25. The method as set forth in claim 22, wherein the short period
of time is defined by at least one of the following control
parameters: saturation suction temperature, suction pressure,
evaporator secondary fluid temperature, oil temperature, suction
temperature, temperature variations in the conditioned environment,
humidity variation in the conditioned environment.
Description
BACKGROUND OF THE INVENTION
[0001] This application relates to a refrigerant system having
tandem compressors, wherein at least one compressor that has been
shut down for a prolonged period of time, is periodically engaged
for a short period of time to ensure proper oil return throughout
the refrigerant system back to the tandem compressors.
[0002] Heating, ventilation, air conditioning and refrigeration
(HVAC&R) systems are utilized to condition various
environments. The HVAC&R systems typically use a refrigerant
circulating throughout a closed-loop refrigerant circuit and are
applied as air conditioners, heat pumps, refrigeration units, etc.
Various enhancement techniques and system configurations are known
and implemented to provide a required performance over a wide
spectrum of environmental conditions to satisfy diverse thermal
load demands.
[0003] In a very basic refrigerant system, a compressor compresses
a refrigerant and delivers it downstream to a heat rejection heat
exchanger, which is a condenser for subcritical applications and a
gas cooler for transcritical applications. Refrigerant passes from
the condenser to an expansion device, and from the expansion device
to an evaporator. From the evaporator, refrigerant returns to the
compressor. This basic refrigerant system is typically supplemented
and enhanced by a number of different options and features to
satisfy application requirements.
[0004] One such enhancement is the use of tandem compressors.
Tandem compressors include a plurality of compressors operating in
parallel each typically receiving refrigerant from a common suction
manifold, each separately compressing the refrigerant and typically
delivering the refrigerant to a common discharge manifold. Each of
these compressors may be independently turned on or off to vary
refrigerant system capacity at part-load operation. In this manner,
the capacity provided by the tandem compressor subsystem to the
overall refrigerant system can be tailored to the thermal load
demands in the conditioned space as well as environmental
conditions. Quite often, tandem compressor configurations include
oil and/or vapor equalization lines connecting tandem compressors
for functionality and reliability enhancement. Also, tandem
compressors having only one suction or discharge common manifold
are known in the art. Further, tandem compressor configurations may
include at least some economized compressors that may have a common
economizer manifold.
[0005] One concern with tandem compressors occurs when at least one
of the tandem compressors is shut off under system part-load
conditions. As one compressor is shut down, the refrigerant mass
flow through the refrigerant system is substantially reduced, which
can often lead to problems associated with lubrication oil not
being returned back to the compressors. This occurs as reduced
refrigerant mass flow causes oil to be retained throughout the
refrigerant system, and in the evaporator in particular. This
problem can become especially acute in refrigerant systems equipped
with microchannel heat exchangers, since the size of the
refrigerant channels in these heat exchangers is particularly
small, and refrigerant flow may not have enough momentum to
overcome viscous drag forces and to carry oil with it. The cause
for the oil holdup and poor oil return back to the compressor unit
is that the refrigerant mass flow throughout the refrigerant system
and its components could be drastically reduced when some of the
tandem compressors are shut off. Additionally, lubrication oil
logged in the refrigerant system heat exchangers causes performance
degradation for the heat exchanger and entire refrigerant
system.
SUMMARY OF THE INVENTION
[0006] When a refrigerant system is operating with at least one
tandem compressor turned off for a prolonged period of time, that
one tandem compressor is periodically engaged for a short period of
time. By engaging the shutdown compressor, the refrigerant mass
flow through the system is increased instantaneously. In some
applications, several or all of the tandem compressors can be
engaged simultaneously or in sequence during this short period of
time. When tandem compressors are activated in sequence, they can
be deactivated in sequence as well or shut off simultaneously.
[0007] These and other features of the present invention can be
best understood from the following specification and drawings, the
following of which is a brief description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 shows a schematic view of a refrigerant system
incorporating tandem compressors having common suction and common
discharge manifolds.
[0009] FIG. 2A shows a schematic view of a refrigerant system
incorporating tandem compressors having a common suction
manifold.
[0010] FIG. 2B shows a schematic view of a refrigerant system
incorporating tandem compressors having a common discharge
manifold.
[0011] FIG. 3 shows a schematic view of an economized refrigerant
system incorporating tandem economized and non-economized
compressors.
[0012] FIG. 4 is a control logic flow chart.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0013] FIG. 1 shows a refrigerant system 20 incorporating tandem
compressors 24 and 26. The tandem compressors 24 and 26 compress a
refrigerant and deliver it to a common discharge manifold 27 then
through a heat rejection heat exchanger 28, to an expansion device
30, to an evaporator 32, and to a common suction manifold 33
leading back to the tandem compressors 24 and 26. The tandem
compressors 24 and 26 are often connected by an oil equalization
line 25 and may be connected by an optional vapor equalization line
(not shown). Also, all or some of the tandem compressors may have
optional discharge shutoff valves (not shown) to prevent
refrigerant backflow when these compressors are shut down, while
the other compressors are still operating. As is known, at
part-load operation, to satisfy reduced thermal load demands in the
conditioned space and/or provide operation at a wide spectrum of
environmental conditions, a control for the refrigerant system 20
(not shown) may shut down either of the compressors 24 or 26 for a
prolonged period of time. When this occurs, the mass flow of
refrigerant passing through the refrigerant system 20 is
significantly reduced. Lubrication oil that is entrained in the
refrigerant may be retained in the components of the refrigerant
system 20, and in the evaporator 32 in particular, due to this low
refrigerant mass flow circulating throughout the system. This is
undesirable in that the retained oil degrades performance of the
heat exchangers 28 and 32 of the refrigerant system 20, and in
particular, performance of the evaporator 32. Moreover, if oil is
pumped out from the tandem compressors 24 and 26 and is not
returned back to the compressors, this may result in less oil than
would be desired for a reliable operation of the tandem compressors
24 and 26. Low lubrication oil amounts returned to the tandem
compressors 24 and 26 may cause severe damage of compressor
components, prolonged maintenance intervals and expensive repairs.
Of course, the tandem compressors 24 and 26 may be of different
sizes (capacities), and there may be more than two compressors
connected in tandem within the refrigerant system 20.
[0014] FIGS. 2A exhibits a refrigerant system 40 with tandem
compressors 44 and 46 having a common suction manifold 33 and
separate discharge lines 27A and 27B leading to separate heat
rejection heat exchangers 28A and 28B, typically operating at
different temperature levels. In all other aspects of design and
operation the FIG. 2A embodiment 40 is similar to the refrigerant
system 20.
[0015] FIGS. 2B exhibits a refrigerant system 60 with tandem
compressors 64 and 66 having a common discharge manifold 27 and
separate suction lines 33A and 33B leading from separate
evaporators 32A and 32B, typically operating at different
temperature levels. In all other aspects of design and operation
the FIG. 2B embodiment 60 is similar to the refrigerant system
20.
[0016] FIG. 3 shows a refrigerant system 80 with economized tandem
compressors 84 and 86 and non-economized tandem compressor 88
having common suction manifold 83 and common discharge manifold 87.
Tandem economized compressors 84 and 86 also have a common
intermediate pressure manifold 85. As known, economized compressors
may be represented either by sequential compression stages or
separate compressors connected in sequence.
[0017] The oil return problem can become especially acute when the
economized compressor is shutdown for a prolonged period of time,
as substantial amount of oil can be retained in the non-operating
economizer heat exchanger and the economizer lines leading to the
compressors.
[0018] FIG. 4 shows exemplary control logic for an improved
operation of the tandem compressors. For instance, a control for
the refrigerant system 20, 40, 60 or 80 determines whether at least
one of the tandem compressors has been shut down. If that
compressor has been shut down longer than a particular period of
time t.sub.1 for a particular set of operating conditions "c", then
at least one shutdown tandem compressor is engaged for a short
period of time t.sub.2. Normally, the decision to engage the
shutdown compressor may be made if the shutdown period of time is
between 30 minutes and 12 hours. As an example, the operating
conditions "c" can be represented by a saturation suction
temperature or suction pressure. In this manner, the mass flow of
refrigerant through the refrigerant system is instantaneously
increased, and the lubrication oil retained within the refrigerant
system components will be driven out and returned to the tandem
compressors assuring their reliable operation and improved
performance of the refrigerant system.
[0019] The engagement of the shutdown compressors need not be for
any undue length of time, simply, it should be long enough to
ensure the proper oil return and short enough not to disrupt the
controlled parameters in the conditioned space. The duration of
compressor engagement can normally vary from 10 seconds to 10
minutes. As explained above, the time intervals t.sub.2 and t.sub.1
can be adjusted based upon environmental conditions and/or system
operational characteristics, and depend on a particular
configuration of the refrigerant system and tandem compressor unit.
For instance, if one of two equally sized tandem compressors of the
basic refrigerant system was shutdown for more than 2 hours at a
saturated suction temperature of 45.degree. F., this compressor can
be activated for 2 minutes and then shut down again. To enhance oil
return even further, a shutdown tandem compressor can be activated
several times in sequence within a short period of time. This will
create intermittent refrigerant flow that will facilitate the oil
return process. For instance, for the example cited above, the
shutdown tandem compressor can be activated 3 times for 1 minute
intervals with 30 second 2 shutdown periods in between these
intervals.
[0020] Furthermore, the shutdown tandem compressor engagement may
be based upon a compressor safety device or a sensor. For instance,
an oil level sensor may be installed in the compressor sump. If the
oil level falls below a predetermined level, then at least one of
the shutdown tandem compressors is turned on by the refrigerant
system controller that had received a feedback from the oil level
sensor.
[0021] If a refrigerant system has multiple (more than two) tandem
compressors, such as shown in FIG. 3 embodiment, the shutdown
tandem compressors can be engaged and disengaged individually,
simultaneously or in sequence. Also, the tandem compression systems
may have a nested structure, where each of the subsystem configured
for tandem operation may have an internal structure itself
consisting of tandem compressors. Further, each of the tandem
compressors may comprise a number of sequential compression stages
or several compressors connected in sequence.
[0022] The present invention augments time-averaged refrigerant
system performance (capacity and efficiency) by removing the
unwanted excessive oil from the internal surfaces of the heat
rejection heat exchanger and evaporator, improving heat transfer
and pressure drop characteristics for these heat exchangers. Also,
system reliability is improved by ensuring the compressor has
adequate lubrication oil amount. This application is especially
beneficial for microchannel heat exchangers, where the oil return
problem becomes especially prevalent. No new hardware is required
for achieving this invention, and only the control logic for
operation of the refrigerant systems having tandem compressors need
to be enhanced. Further, this invention can be applied to newly
built refrigerant systems and even to those systems that are now
installed in the field. Also, it has to be recognized that the
refrigerant system may be operated in the oil return mode during
normal occupancy hours of the conditioned space or may be
preferably controlled in this mode during unoccupied hours.
[0023] It should be pointed out that many different compressor
types could be used in this invention. For example, scroll, screw,
rotary, or reciprocating compressors can be employed.
[0024] The refrigerant systems that utilize this invention can be
used in many different applications, including, but not limited to,
air conditioning systems, heat pump systems, marine container
units, refrigeration truck-trailer units, and supermarket
refrigeration systems.
[0025] Although the embodiments of this invention have been
disclosed, a worker of ordinary skill in this art would recognize
that certain modifications would come within the scope of this
invention. For that reason, the following claims should be studied
to determine the true scope and content of this invention.
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