U.S. patent application number 12/864783 was filed with the patent office on 2010-12-09 for rapid compressor cycling.
This patent application is currently assigned to CARRIER CORPORATION. Invention is credited to Alexander Lifson, Michael F. Taras.
Application Number | 20100307177 12/864783 |
Document ID | / |
Family ID | 40913100 |
Filed Date | 2010-12-09 |
United States Patent
Application |
20100307177 |
Kind Code |
A1 |
Lifson; Alexander ; et
al. |
December 9, 2010 |
RAPID COMPRESSOR CYCLING
Abstract
A single speed compressor is provided with a switching device
and control for turning the compressor drive motor ON and OFF in
repeated succession at a selected ON time/OFF time ratio within a
selected cycle time interval. The ON/OFF ratio and cycle time
interval are selected to maintain desired temperature and/or
humidity control within a conditioned space.
Inventors: |
Lifson; Alexander; (Manlius,
NY) ; Taras; Michael F.; (Fayetteville, NY) |
Correspondence
Address: |
MARJAMA MULDOON BLASIAK & SULLIVAN LLP
250 SOUTH CLINTON STREET, SUITE 300
SYRACUSE
NY
13202
US
|
Assignee: |
CARRIER CORPORATION
Farmington
CT
|
Family ID: |
40913100 |
Appl. No.: |
12/864783 |
Filed: |
January 31, 2008 |
PCT Filed: |
January 31, 2008 |
PCT NO: |
PCT/US08/52571 |
371 Date: |
July 27, 2010 |
Current U.S.
Class: |
62/115 ;
62/228.1; 62/507; 62/510 |
Current CPC
Class: |
F25B 2600/0251 20130101;
F25B 49/022 20130101; Y02B 30/70 20130101; Y02B 30/743 20130101;
F25B 2600/112 20130101; F25B 2600/111 20130101; F25B 2400/075
20130101 |
Class at
Publication: |
62/115 ; 62/510;
62/507; 62/228.1 |
International
Class: |
F25B 1/00 20060101
F25B001/00; F25B 1/10 20060101 F25B001/10; F25B 39/04 20060101
F25B039/04; F25B 49/02 20060101 F25B049/02 |
Claims
1. A method of controlling the temperature and/or humidity within a
space being conditioned by a refrigerant system including in serial
flow relationship a single speed compressor, a heat rejection heat
exchanger, an expansion device and an evaporator, comprising the
steps of: providing a switching device between a power source and a
drive motor for said single speed compressor; providing a control
for selectively operating said switching device between ON and OFF
positions; and using said control to turn said switching device ON
and OFF in repeated succession at a selected ON time/OFF time ratio
within a selected cycle time interval.
2. A method as set forth in claim 1 wherein said selected cycle
time interval is in the range of 20 seconds to 1.5 minutes
3. A method as set forth in claim 2 wherein said selected time
interval is in the range of 10 seconds to 20 seconds.
4. A method as set forth in claim 1 wherein said selected cycle
time is in the range of 1.5 minutes to 4 minutes.
5. A method as set forth in claim 1 wherein said refrigerant system
includes another single speed compressor operably connected in
parallel with said single speed compressor, with respect to
refrigerant flow.
6. A method as set forth in claim 1 wherein said heat rejection
heat exchanger and said evaporator have associated fans, and
including the further step of turning at least one of said fans ON
and OFF in coordination with the turning ON and OFF said switching
device.
7. A method as set forth in claim 6 wherein the turning ON and OFF
said at least one fan is selectively timed so as to either lead or
lag the turning ON and OFF said switching device.
8. A method as set forth in claim 6 wherein the selected cycle time
interval of said at least one fan is larger or smaller than the
selected cycle time interval of said switching device.
9. A method as set forth in claim 6 wherein at least one of said
fans is operated on a continuous basis.
10. A method as set forth in claim 1 and including the further step
of closing said expansion device when said switching device is
turned OFF.
11. A method as set forth in claim 10 wherein the closing of said
expansion device is selectively timed to either lead or lag the
time of turning said switching device OFF.
12. A method as set forth in claim 10 wherein the cycle time
interval of said expansion device is larger or smaller than the
selected cycle time interval of said switching device.
13. Control apparatus for a drive motor of a single speed
compressor in a refrigerant system which includes in serial flow
relationship, the single speed compressor, a heat rejection heat
exchanger, an expansion device and an evaporator, comprising: a
drive motor for said single speed compressor; a switching device
connected between a power source and said drive motor; and a
control for selectively operating said switching device been ON and
OFF positions in repeated succession at a selected ON time/OFF time
ratio within a selected cycle time interval to control the
temperature and/or humidity within a conditioned space.
14. A control apparatus as set forth in claim 13 wherein said
selected cycle time interval is in the range of 20 seconds to 1.5
minutes.
15. A control apparatus as set forth in claim 13 wherein said
selected cycle time interval is in the range of 10 seconds to 20
seconds.
16. A control apparatus as set forth in claim 13 wherein said
selected cycle time interval is in the range of 1.5 minutes to 4
minutes.
17. A control apparatus as set forth in claim 13 wherein said
refrigerant system includes another single speed compressor
operably connected in parallel with said single speed compressor,
with respect to refrigerant flow.
18. A control apparatus as set forth n claim 13 wherein said heat
rejection heat exchanger and said evaporator have associated fans,
and further wherein said control is adapted to turn at least one of
said fans ON and OFF in coordination with the turning ON and OFF
said switching device.
19. A control apparatus as set forth in claim 18 wherein the
turning ON and OFF said at least one fan is selectively timed so as
to either lead or lag the turning ON and OFF said switching
device.
20. A control apparatus as set forth in claim 18 wherein the
selected cycle time interval of said at least one fan is larger or
smaller than the selected cycle time interval of said switching
device.
21. A control apparatus as set forth in claim 18 wherein at least
one of said fans is adapted to be operated on a continuous
basis.
22. A control apparatus as set forth in claim 13 wherein said
control is operative to close said expansion device when said
switching device is in said OFF position.
23. A control apparatus as set forth in claim 22 wherein the
closing of said expansion device is selectively timed to either
lead or lag the time of turning said switching device OFF.
24. A control apparatus as set forth in claim 22 wherein the cycle
time interval of said expansion device is larger or smaller than
the selected cycle time interval of said switching device.
Description
TECHNICAL FIELD
[0001] This invention relates generally to rapid cycling of single
speed compressors, and more particularly, to a precise control of
temperature and humidity in a conditioned space by utilizing such
rapidly cycled single speed compressors.
BACKGROUND OF THE INVENTION
[0002] During operation of air conditioning and refrigeration
systems, it is desirable to vary the capacity of the compressor in
an effort to match thermal load demands in a climate-controlled
space to provide tight temperature and/or humidity control. This is
frequently done by way of a variable speed drive (VSD) which
controls the supply voltage and frequency by utilizing an inverter.
While a variable speed drive permits continuously varying
compressor speed, which in turn allows for a continuous adjustment
of the capacity delivered by the variable speed compressor, thereby
giving the advantage of smooth operation and better part-load
performance, it is relatively expensive. The cost of variable speed
drives is on the order of that of compressors. Variable speed
drives also tend to be bulky and present reliability problems.
Furthermore, variable speed drives themselves have associated
unavoidable efficiency losses.
[0003] There are also other problems associated with the use of
variable speed drives to control the capacity of compressors. For
example, quite often a compressor cannot operate below a certain
speed threshold, for substantial periods of time, due to problems
associated with oil delivery for the lubrication of internal
compressor components. Also, some compressors have internal design
features which would cause the compressor to operate improperly if
the speed is reduced below a certain level. For example, in the
case of scroll compressors, where the fixed and orbiting scrolls
have a certain radial compliance requirement, the operation below a
certain speed would cause the two scrolls to separate beyond the
allowable limit or oil delivery to bearings would be
interrupted.
[0004] The other method of maintaining temperature and/or humidity
in a climate-controlled space is turning the compressor ON and OFF
for extended periods of time (normally in the order of at least
several minutes, with the common practice in the industry having at
least five minute intervals between subsequent compressor
startups). This method has its major drawbacks as the temperature
and/or humidity in the conditioned space cannot be precisely
controlled. Also, it creates a discomfort as the temperature and/or
humidity of the delivered air varies, especially in the cases when
they fall outside the comfort zone defined by the industry
standards. For example, in an air conditioning mode of operation,
the temperature and humidity are lower when the compressor is ON
and higher when the compressor is OFF. If the evaporator fan is
also shut off for prolonged periods of time when the compressor is
OFF, it creates additional discomfort as the amount of air
delivered to the conditioned space is changing from no flow to full
amount of flow. Further, condensate accumulated on the evaporator
external heat transfer surfaces would re-evaporate during the
compressor shutdowns and eventually re-enter into the conditioned
environment in the form of moist air, which is obviously
undesirable.
[0005] The problem would represent itself in a similar fashion if
the unit is operating in the heat pump mode, when the conditioned
air passes over the heat rejection heat exchanger and then
delivered to the conditioned (heated in this case) environment.
[0006] There is therefore a need for a cost effective, reliable and
efficient manner and apparatus for varying the capacity of a
compressor.
DISCLOSURE OF THE INVENTION
[0007] A compressor is provided with a switching device and an
associated control for selectively sequencing the electrical power
to a single speed compressor drive motor to rapidly alternate the
compressor between ON and OFF positions. The compressor is cycled
in a rapid fashion such that the relatively large thermal inertia
of the refrigerant system would not allow the refrigerant system to
respond in a timely fashion to changes introduced by refrigerant
mass flow variations as compressor alternates between ON to OFF
positions. In other words, the time constant of the refrigerant
system thermal inertia is higher or at least the same order of
magnitude as the time period of the compressor ON/OFF cycle. In
this case, the temperature of the delivered air essentially stays
the same regardless whether the compressor is in ON or OFF
positions as the evaporator surfaces and heat rejection heat
exchanger surfaces do not have enough time to change its
temperature during the compressor ON/OFF cycles. Thus, the
temperature of the air being flown over these heat exchange
surfaces and supplied to the conditioned environment would
essentially remain constant during the period of time when the
compressor is either in ON or OFF positions. Therefore, the
temperature within the environment can be very tightly controlled,
while the humidity can be maintained at an appropriate level as
well. Consequently, the occupants of the conditioned space would be
more comfortable as the temperature and humidity of the delivered
conditioned air would remain the same and the flow of this air
would not be interrupted. In the context of this invention, the
heat rejection heat exchanger can be of a condenser type, if the
transition from a single phase refrigerant vapor into a two-phase
vapor/liquid mixture takes place within the heat exchanger, or it
can be of a gas cooler type, where the refrigerant gas is simply
cooled in the heat exchanger without any transition into a two
phase region. The gas coolers are typically associated with
refrigerant cycles where the refrigerant on the high side is at the
pressure which exceeds the critical pressure. This operation is
normally called a transcritical operation and would be typical for
the refrigerants such as R744 refrigerant, commonly known as
CO.sub.2.
[0008] It should be noted that in the prior art systems instead of
allowing the compressor to be rapidly cycled, the compressor
cycling, even on an intermittent and infrequent basis, was strictly
avoided. The thermostats, for example, were equipped with
electronic boards which prevented the compressor from being rapidly
cycled (so-called thermostat "jiggling"). In this case the
compressor must have been shut off, for instance, for at least five
minutes before starting again. In this invention, the compressor is
actually expected and controlled to continuously cycle, with the
cycle time normally being less than one minute. In this case, the
compressor is expected to accumulate several million of cycles
during its life time.
[0009] In the drawings as hereinafter described, a preferred
embodiment is depicted; however, various other modifications and
alternate constructions can be made thereto without departing from
the spirit and scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a schematic illustration of a basic vapor
compression system with the present invention incorporated
therein.
[0011] FIG. 2 is a graphic illustration of the compressor speed as
a function of time in accordance with the present invention.
[0012] FIG. 3 is a graphic illustration of the capacity delivered
by a compressor as a function of the ratio of ON-time/OFF-time for
the compressor drive motor.
[0013] FIG. 3A is a graphic illustration of the conditioned space
temperature as a function of time, in accordance with the present
invention.
[0014] FIG. 4 is a schematic illustration of an alternative
embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0015] Shown in FIG. 1 is a basic vapor compression system which
includes a compressor assembly 11 a heat rejection heat exchanger
(a condenser or a gas cooler) 12, an expansion device 13 and an
evaporator 14. In a conventional manner, the refrigerant vapor is
compressed in the compressor assembly after which it passes to the
heat rejection heat exchanger 12 where the heat is transferred from
the refrigerant to a secondary loop fluid such as air, water or
glycol. The refrigerant is expanded in the expansion device 13, and
the expanded refrigerant that is at lower pressure and temperature
then passes to the evaporator 14 where it assimilates heat from an
environment, with the resultant refrigerant vapor then passing back
to the compressor assembly 11.
[0016] The compressor assembly 11 as shown comprises a pair of
compressors 16 and 17 operating in parallel or a so-called tandem
configuration. In this regard, it should be understood that the
present invention is equally applicable to use in a single
compressor configuration, as shown in FIG. 4. Further, the
compressors 16 and 17 could be of any suitable type such as a
rotary compressor, a reciprocating compressor, a scroll compressor,
or a screw compressor. The compressors 16 and 17 are driven by
single speed motors 18 and 19, respectively.
[0017] With a dual, parallel, compressor arrangement as shown,
additional levels of the capacity for the refrigerant system 11 can
be achieved as compared to a single compressor. For example, one of
the compressors could be operated continuously, while the other
compressor would be operated in the ON/OFF fashion. In this case,
the system capacity can be adjusted between the two levels, the
first low level provided by a single compressor being operated
continuously, and the second level with both compressors operated
continuously and simultaneously. By turning one compressor ON and
OFF, the capacity of the refrigerant system 11 can be adjusted
between the first and the second levels. While it is shown that the
compressor 16 is the compressor that is operating in a rapid ON/OFF
cycling manner, it should be understood that any of the two
compressors 16 and 17, can be operated continuously while the other
compressor can be operated in the ON/OFF fashion.
[0018] Electrical power to each of the compressor drive motors 18
and 19 is provided by way of an electrical power source 21 via an
electrical line 22 to a control 23 and then to electrical lines 24
and 26 as shown. Electrical power to the drive motor 19 is provided
by the control 23 in a conventional manner to turn the compressor
on when operation of the system is desired and to turn it off when
it is not desired. Electrical power to the drive motor 18, on the
other hand, is provided through the electrical switching device 27
in a controlled manner, with the switching device 27 being
sequentially and rapidly turned ON and OFF by the control 23
operating through an electrical line 28. The method in which this
power flow is controlled will now be described.
[0019] As shown in FIG. 2, the single speed drive motor 18 may be
operated either at full speed when it is turned on, or at zero
speed when it is turned off. That is, by operation of the switching
device 27 as controlled by the control 23 via the electric line 28,
the drive motor 18 is turned on at time t.sub.0 and remains at full
speed until time t.sub.1, at which the switching device 27 is
turned off and the speed of the drive motor 18 drops to zero. The
drive motor 18 remains off until time t.sub.2, when it is turned on
and resumes operation at full speed until time t.sub.3, when the
drive motor 18 is turned off again and the motor speed drops to
zero. This mode of operation continues as long as the capacity of
the compressor 16 is desired to be at less then full capacity. In
accordance with the present invention, the respective ON-time and
OFF-time intervals can be varied as desired. As shown in FIG. 3,
for the system configuration depicted in FIG. 4, where only one
compressor is present in the system, the greater the ratio of
ON-time to OFF-time, the greater the delivered capacity, Q, on an
average basis. Thus, if full capacity Q.sub.1 is desired, the
switching device 27 will remain ON and the drive motor 18 will
continue to operate at full speed. If a lower capacity, Q.sub.2, is
desired, the switching device 27 will be operated in such a manner
that the ratio of ON-time to OFF-time would be kept at a value to
provide this desired capacity and could be equal, for instance, 0.5
as shown in the FIG. 3. In this manner, any capacity level between
nominal capacity and full capacity can be obtained by selectively
varying the ON-time/OFF-time ratio as shown in FIG. 3. It should be
understood that this ratio could be anywhere between zero and
infinity. It is equal zero when the compressor is continuously in
the OFF position and it is equal to infinity when the compressor is
continuously in the ON position. When the compressor is cycling
between ON and OFF positions, this ratio could be of any value
between zero and infinity. For example, if the compressor is being
operated in ON and OFF positions for equal amount of time, then
this ratio would be equal to unity.
[0020] If there is only one compressor present within the
refrigerant system, such as in FIG. 4, then the nominal capacity
would be equal to zero, which corresponds to the ON/OFF ratio being
equal to zero with this one compressor being shut off. If there are
two compressors present within the refrigerant system, such as in
FIG. 1, and the other compressor 17 is operating continuously, then
the nominal capacity would be equal to the capacity delivered by
the compressor 17 when it is running continuously.
[0021] The ratio of ON/OFF time intervals, and therefore the
capacity of the compressor 16 would be controlled such that the
desired average temperature within the conditioned environment is
maintained. As shown in FIG. 2, the ON/OFF cycle time interval
(which is the time interval equal to the sum to of the time the
compressor is ON and the time the compressor is OFF, during one
cycle) is selected to be small enough to assure that that there are
minimal temperature fluctuations from the average temperature value
within the conditioned space. In other words, the time interval
corresponding to this ON/OFF cycle should be sufficiently low such
that the thermal inertia time constant of the refrigerant system
would be higher than this cycle time period. As in the prior art,
the average temperature within the conditioned space is controlled
by the ON/OFF ratio. However, in this invention, the cycle time
would control how much the temperature would fluctuate from the
average value. This is illustrated in FIG. 3A, wherein the
temperature fluctuations are shown by a curve A when the compressor
is not allowed to cycle on a frequent basis, such as in the prior
art, as explained above, as compared to a curve B wherein there are
substantially reduced temperature fluctuations in the conditioned
space when the compressor is rapidly cycled, as described by this
invention.
[0022] The change in the cycle time would also control the humidity
within the conditioned space. The humidity control may be executed
simultaneously with the temperature control, as known in the art,
while the refrigerant system is operated in the conventional
cooling mode or any of known dehumidification modes (not shown). As
stated above, similarly to the temperature fluctuations, the
humidity fluctuations would be tightly controlled as well, due to a
wide range and flexibility in the compressor ON/OFF cycle time
ratio. For typical refrigerant systems, the cycle time would be
between 10 seconds and 2 minutes. Most typically, the cycle time
would be selected between 20 seconds and 1.5 minute. However, under
some circumstances, when the requirements on the conditioned air
temperature and humidity control can be relaxed, the ON/OFF
compressor cycle time period can be extended to the range of 1.5 to
3 minutes.
[0023] Such fast cycling on a regular basis, as compared to the
prior art systems, where the system control prohibited even
intermittent cycling below 5 minutes, has been made technically
feasible and reliable in view of recent developments such as solid
state motor controls, which allow motors to be switched ON and OFF
for millions of cycles with no damage, as well as control
techniques to limit the starting torque on a start-up and provide
for gradual shutdowns (measured in milliseconds) by gradually
reducing voltage to the motor on a shutdown, and more robust
mechanical designs of compressors permitting frequent cycling.
[0024] Referring now to FIG. 4, an alternative embodiment is shown,
wherein a single compressor 16 and its drive motor 18 are shown to
be controlled in the manner described hereinabove. However, an
additional feature is also shown, wherein the control 23 is
connected by an electrical line 29 to the expansion device 13, such
that, during periods in which the drive motor 18 is turned off, the
expansion device 13 is also to be closed. This allows for the
refrigerant system to maintain pressure differential between high
discharge side and low suction side during the OFF time periods of
operation when the drive motor 18 is in an OFF position. This in
turn will reduce cycling losses and improve refrigerant system
efficiency. In addition, the control 23 can control the operation
of an evaporator fan 31 and heat rejection heat exchanger fan 32 by
way of electrical lines 33 and 34, respectively. The control 23 can
either operate one or both of the fans continuously or turn the
fans ON and OFF on an intermittent basis. Each of the fans 31 and
32 can either lead or lag the compressor ON operation. Each of the
fans 31 and 32 can also be ON for the time period that is shorter
or longer than the compressor ON operation. In the context of this
invention, the fans 31 and 32 can be substituted by liquid pumps.
For example, instead of using a fan that is blowing ambient air to
cool the refrigerant in the heat rejection heat exchanger 12, a
liquid pump carrying cooling fluid can be utilized.
[0025] This invention can be applied to various types of
refrigerant systems, which for example include container and
truck-trailer applications, supermarket refrigeration
installations, and residential and commercial air conditioning and
heat pump applications. It can be applied to a variety of
refrigerants including, but not limited to, R410A, R134a, R22,
R407c, R404A, R422D, R422A, R744 refrigerants. This invention also
applies to various types of compressors including, for example,
screw compressors, scroll compressors, rotary compressors, and
reciprocating compressors.
[0026] Although the present invention has been particularly shown
and described with reference to preferred and modified embodiments
as illustrated by the drawings, it will be understood by one
skilled in the art that various changes in detail may be made
thereto without departing from the spirit and scope of the
invention as defined by the claims.
* * * * *