U.S. patent application number 13/014173 was filed with the patent office on 2012-07-26 for compressor motor preheat control.
This patent application is currently assigned to HAMILTON SUNDSTRAND CORPORATION. Invention is credited to Robert M. DeRoy, Joseph A. Giunta, Richard Rusich.
Application Number | 20120186283 13/014173 |
Document ID | / |
Family ID | 45421986 |
Filed Date | 2012-07-26 |
United States Patent
Application |
20120186283 |
Kind Code |
A1 |
Giunta; Joseph A. ; et
al. |
July 26, 2012 |
COMPRESSOR MOTOR PREHEAT CONTROL
Abstract
A method of controlling a vapor cycle system having first and
second compressors includes operating the first compressor to
compress refrigerant while the second compressor is idle, sensing a
motor temperature of the second compressor, activating a preheating
mode in which the motor of the second compressor is powered to
generate heat when the motor temperature of the second compressor
is below a lower temperature threshold, turning off the preheating
mode when the sensed motor temperature of the second compressor is
above an upper temperature threshold, and limiting the maximum
duration of the preheating mode to a preheating time period, such
that the preheating mode is turned off after being activated for
the preheating time period.
Inventors: |
Giunta; Joseph A.; (Windsor,
CT) ; DeRoy; Robert M.; (Tolland, CT) ;
Rusich; Richard; (Ellington, CT) |
Assignee: |
HAMILTON SUNDSTRAND
CORPORATION
Windsor Locks
CT
|
Family ID: |
45421986 |
Appl. No.: |
13/014173 |
Filed: |
January 26, 2011 |
Current U.S.
Class: |
62/115 ;
62/228.1 |
Current CPC
Class: |
F25B 2700/21156
20130101; F25B 49/025 20130101; F25B 2500/26 20130101; F25B
2400/075 20130101; F25B 2600/23 20130101; F25B 2500/28 20130101;
F25B 2500/31 20130101 |
Class at
Publication: |
62/115 ;
62/228.1 |
International
Class: |
F25B 1/00 20060101
F25B001/00; F25B 49/02 20060101 F25B049/02 |
Claims
1. A method of controlling a vapor cycle system having first and
second compressors, the method comprising: operating the first
compressor to compress refrigerant while the second compressor is
idle; sensing a motor temperature of the second compressor;
activating a preheating mode in which the motor of the second
compressor is powered to generate heat when the motor temperature
of the second compressor is below a lower temperature threshold;
turning off the preheating mode when the sensed motor temperature
of the second compressor is above an upper temperature threshold;
and limiting the maximum duration of the preheating mode to a
preheating time period, such that the preheating mode is turned off
after being activated for the preheating time period.
2. The method of claim 1, wherein the lower temperature threshold
is approximately 48.9.degree. C. (120.degree. F.), and wherein the
upper temperature threshold is approximately 54.4.degree. C.
(130.degree. F.).
3. The method of claim 1, wherein the motor temperature of the
second compressor is sensed by sensing resistance with a
thermistor.
4. The method of claim 1, wherein the preheating time period is
approximately 14 minutes.
5. The method of claim 1 and further comprising: maintaining the
second compressor preheat mode in an off condition for a delay
period; and reactivating the preheating mode for the second
compressor for the preheating time period following the delay
period.
6. The method of claim 5, wherein the delay period is approximately
30 minutes.
7. The method of claim 1 and further comprising: starting the
second compressor to compress refrigerant after the preheat mode is
turned off.
8. A vapor cycle system comprising: a first compressor having an
electric motor, a motor temperature sensor, and a motor controller;
a second compressor having an electric motor, a motor temperature
sensor, and a motor controller; a refrigerant circuit, wherein the
first compressor and the second compressor are connected in fluid
communication with the refrigerant circuit in parallel; and a
control module operatively connected to the motor controller of
each of the first and second compressors, wherein the control
module is configured to command the motor controllers of each of
the first and second compressors to operate only one of the first
and second compressors to compress refrigerant and to operate the
other, idle one of the first and second compressors in a preheat
mode, wherein the preheat mode is activated to generate heat with
the motor of the idle one of the first and second compressors to a
preheat temperature range, and wherein the control module is
further configured to limit a duration of the preheat mode to a
preheating time period.
9. The vapor cycle system of claim 8, wherein the preheat
temperature range is approximately 48.9.degree. C. (120.degree. F.)
to approximately 54.4.degree. C. (130.degree. F.).
10. The vapor cycle system of claim 8, wherein the motor
temperature sensors of each of the first and second compressors is
a thermistor.
11. The vapor cycle system of claim 8, wherein the control module
is further configured to maintain the preheat mode of the idle one
of the first and second compressors in an off condition for a delay
period and then reactivate the preheating mode for the idle one of
the first and second compressors for the preheating time period
following the delay period.
12. The vapor cycle system of claim 11, wherein the preheating time
period is approximately 14 minutes, and wherein the delay period is
approximately 30 minutes.
13. A method of operating a dual compressor vapor cycle system, the
method comprising: operating a first compressor to compress
refrigerant; selectively activating a preheating mode of a second
compressor connected in parallel with the first compressor to
reduce migration of refrigerant, lubricant or a combination thereof
from the first compressor to the second compressor, wherein the
preheating mode comprises supplying power to an electric motor of
the second compressor to generate heat, wherein the preheating mode
is activated when the second compressor is idle; controlling the
preheating mode of the second compressor according to sensed motor
temperature by default; and limiting a duration of the preheating
mode of the second compressor to a preheating time period to
provide failsafe protection against overheating.
14. The method of claim 13, wherein the preheating mode is
controlled to maintain a sensed motor temperature within a range of
approximately 48.9.degree. C. (120.degree. F.) to approximately
54.4.degree. C. (130.degree. F.).
15. The method of claim 13 and further comprising: maintaining the
second compressor preheat mode in an off condition for a delay
period; and reactivating the preheating mode for the second
compressor for the preheating time period following the delay
period.
16. The method of claim 15, wherein the preheating time period is
approximately 14 minutes, and wherein the delay period is
approximately 30 minutes.
17. A method of operating a vapor cycle system having a compressor
with an electric motor, the method comprising: sensing a motor
temperature of the electric motor when the compressor idle;
determining if the motor temperature is below a lower temperature
threshold; when the motor temperature is below the lower
temperature threshold, supplying current to the electric motor to
generate heat while the compressor remains idle; and ceasing to
generate heat with the electric motor when the sensed motor
temperature is above an upper temperature threshold.
18. The method of claim 17 and further comprising: activating the
compressor when the motor temperature is above the lower
temperature threshold and after ceasing to generate heat with the
electric motor.
19. The method of claim 17, wherein the lower temperature threshold
is approximately 29.4.degree. C. (85.degree. F.), and wherein the
upper temperature threshold is approximately 32.2.degree. C.
(90.degree. F.).
20. The method of claim 17, wherein the motor temperature is sensed
by sensing resistance with a thermistor.
Description
BACKGROUND
[0001] The present invention relates to vapor cycle systems having
a compressor, and more particularly to vapor cycle systems and
methods of operating the same to reduce flooded compressor startup
conditions and/or lubrication accumulation in the idle compressors
of multi-compressor systems.
[0002] Vapor cycle systems (VCSs) are known for providing cooling
in a variety of contexts. In general, a refrigerant is compressed
and expanded in a cyclical manner to transfer thermal energy and
help cool a given airspace or fluid. Some VCSs have a dual
compressor configuration, with two compressors operable within the
same overall refrigerant circuit.
[0003] It is desired to reduce fluid migration issues in dual
compressor VCSs, and to provide a cold-start procedure for VCSs
with any number of compressors.
SUMMARY
[0004] A method of controlling a vapor cycle system having first
and second compressors according to the present invention includes
operating the first compressor to compress refrigerant while the
second compressor is idle, sensing a motor temperature of the
second compressor, activating a preheating mode in which the motor
of the second compressor is powered to generate heat when the motor
temperature of the second compressor is below a lower temperature
threshold, turning off the preheating mode when the sensed motor
temperature of the second compressor is above an upper temperature
threshold, and limiting the maximum duration of the preheating mode
to a preheating time period, such that the preheating mode is
turned off after being activated for the preheating time
period.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1 is a block diagram schematically illustrating a dual
compressor vapor cycle system (VCS) according to the present
invention.
[0006] FIG. 2 is a flow chart illustrating an embodiment of a
method of controlling a VCS according to the present invention.
[0007] FIG. 3 is a flow chart illustrating an alternative
embodiment of a method of controlling a VCS according to the
present invention.
DETAILED DESCRIPTION
[0008] In general, one embodiment the present invention relates to
multi-compressor vapor cycle system (VCS) and associated control
logic. The present inventors have discovered a problem with dual
compressor VCSs when one compressor is idle, that is not operating
to compress refrigerant, whereby liquid state refrigerant and/or
lubricant can migrate to the idle compressor. Migrating fluid can
undesirably starve the active compressor of lubricant and
negatively impact system performance, resulting in reduced cooling
effectiveness as well as potential failure of the operating
compressor. Furthermore, the idle compressor flooded with liquid
refrigerant and/or lubricant can experience difficulties at startup
when requested to operate (called a "flooded start"), which can
lead to reliability issues. In one embodiment of a multi-compressor
VCS according to the present invention, the compressors are
connected in parallel, which allows fluid to pass between the
compressors and into the idle compressor. Each compressor can be
selectively activated based on desired cooling capacity, such that
both compressors or only one of them are operating simultaneously
to compress refrigerant. When only a single compressor is active,
the control logic provides heating to an idle compressor to reduce
or eliminate migration of refrigerant and/or lubricant in a liquid
state into the idle compressor. The idle compressor has an electric
motor that can be used to generate heat that reduces migration of
liquids and tends to drive out or evaporate any liquids that may be
present. By default, a preheat mode of the idle compressor
generates heat with windings of the idle compressor motor to a
given motor temperature range (e.g., 48.9.degree. C. (120.degree.
F.) to 54.4.degree. C. (130.degree. F.)), controlled by sensing the
motor temperature with a suitable sensor, such as a thermistor. The
system can further optionally provide a failsafe timer mechanism to
prevent overheating in the event of a failure of the temperature
sensor. The preheating mode of the idle compressor is cycled on
(i.e., operated) for a given time period (e.g., 14 minutes), and
can then by cycled off for a given delay period (e.g., 30 minutes)
before the preheating time period is reactivated, thereby providing
the optional failsafe time functionality. By heating the idle
compressor according to the present invention, an active (non-idle)
compressor is not starved of refrigerant during operation due to
migration of fluid to the idle compressor. Furthermore, the preheat
mode helps reduce a risk of a flooded start when the idle
compressor is activated to begin compressing refrigerant. In
another embodiment of the present invention, a preheating mode is
provided that is controlled based on sensed motor temperature, such
as using sensed resistance through a thermistor, and can be
implemented in single or multi-compressor VCSs. Other features and
benefits of the present invention will be appreciated in view of
the discussion that follows.
[0009] FIG. 1 is a block diagram schematically illustrating one
embodiment of a dual compressor VCS 10 that includes a vapor cycle
circuit 12, a compressor 14, a compressor 16, and a control module
18.
[0010] Each compressor 14 and 16 includes an electric motor 14-1
and 16-1, respectively, and can be of any suitable type (e.g.,
scroll, reciprocating, etc.). The electric motors 14-1 and 16-1 can
include suitable windings, etc., and can have a conventional
configuration. The compressors 14 and 16 can each be connected to a
common suction port manifold 20 and a common discharge port
manifold 26, with each manifold 20 and 26 connected in fluid
communication with the vapor cycle circuit 12. In the illustrated
embodiment, the compressors 14 and 16 are connected to the vapor
cycle circuit 12 in parallel. Common discharge and suction
manifolds can be omitted in alternative embodiments, with separate
discharge and suction ports for each compressor 14 and 16. A
temperature sensor 28 and 30 is provided for each compressor 14 and
16, respectively, to sense a motor temperature of the compressor 14
or 16. In one embodiment, the sensors 28 and 30 are thermistors
that allow temperature sensing by changing resistance as a function
of temperature. A motor controller 32 and 34 is provided from each
compressor 14 and 16, respectively. The motor controllers 32 and 34
each accept power, send operational commands to the motors 14-1 and
16-1 of the compressors 14 and 16, and receive temperature signals
from the sensors 28 and 30. Signals sent between the motor
controllers 32 and 34 and the compressors 14 and 16 can all be
analog. The sensors 28 and 30 and the motor controllers 32 and 34
can be part of a common assembly with the compressors 14 and 16,
respectively, in some embodiments.
[0011] The control module 18 is operatively connected to each motor
controller 32 and 34, and communication between the motor
controllers 32 and 34 and the control module 18 can be in the form
of digital signals. The control module 18 includes a data transfer
interface 36 and control logic 38, which can be implemented through
suitable circuitry and/or software (e.g., suitable processor(s),
computer-readable memory, etc.). The data transfer interface 36
facilitates communication between all of the motor controllers 32
and 34 and the control logic 38 of the control module 18. The
control logic 38 provides high level decision making for the VCS
10, while the motor controllers 32 and 34 handle lower-level tasks
such and providing operational commands to the motors 14-1 and 16-1
of the compressors 14 and 16.
[0012] A refrigerant 40 (e.g., R134A refrigerant) and a lubricant
42 are present within the VCS 10. In the illustrated embodiment,
which shows the compressor 14 in an active state and the compressor
16 in an idle state, portions of the refrigerant 40 and the
lubricant 42 in liquid phases are present in the compressor 16.
Vapor phase portions of the refrigerant 40 and the lubricant 42 are
also present in the vapor cycle circuit 12 and the compressors 14
and 16.
[0013] During operation, the VCS 10 can activate one or both of the
compressors 14 and 16 to move and compress fluid (e.g., the
refrigerant 40). Cooling can be provided by the VCS 10 in a
conventional manner. The basic operation of VCSs to provide cooling
is well-known, and will be understood by those of ordinary skill in
the art. Further discussion here is unnecessary. Depending upon
cooling demand, both compressors 14 and 16 can be operated
simultaneously to provide relatively high levels of cooling, or
only one of the compressors 14 or 16 can be operated while the
other compressor 14 or 16 remains idle (i.e., not moving or
compressing fluid) to provide a lower level of cooling. Operation
of only a single one of the compressors 14 and 16 can alternate
between the two, to provide relatively even usage of each
compressor 14 and 16 over time.
[0014] FIG. 2 is a flow chart illustrating one embodiment of a
method of controlling the VCS 10. Initially, the VCS 10 is operated
in a mode with only one compressor 14 or 16 active to move or
compress fluid (step 100). As discussed above, the VCS 10 can also
operate both compressors 14 and 16 active simultaneously, and,
further, in single compressor operation the particular compressor
14 or 16 that is active can alternate. By default, a motor
temperature of the motor 14-1 or 16-1 of the idle compressor 14 or
16 is sensed using the corresponding sensor 28 or 30 (step 102).
The temperature of fluid in the idle compressor 14 or 16 need not
be sensed directly, only the temperature of motor windings of the
motor 14-1 or 16-1 of the idle compressor 14 or 16. Temperature of
the motor windings can be sensed by delivering a current and
measuring resistance through a thermistor, or using other methods
as desired. The sensed temperature is communicated to the control
logic 38 of the control module 18. The control logic 38 analyzes
the sensed data and determines if the motor temperature of the idle
compressor 14 or 16 is above a lower threshold (also called a lower
temperature threshold) (step 104). In the illustrated embodiment
the lower threshold is approximately 48.9.degree. C. (120.degree.
F.), though other temperatures can be selected for the lower
threshold in alternative embodiments. For instance, the lower
threshold can be selected to be sufficiently high to ensure that
the refrigerant 40 is vaporized. If the motor temperature is above
the lower threshold, then it is okay to start (i.e., activate) the
idle compressor motor if desired (step 106). If the sensed motor
temperature is below the lower threshold, then a preheating mode
can be activated (step 108). The preheating mode can generate
thermal energy by delivering power from the motor controller 32 or
34 to windings of the motor 14-1 or 16-1 of the idle compressor 14
or 16, thereby using the idle motor windings as a heater, without
activating the motor 14-1 or 16-1 of the idle compressor 14 or 16
to generate a motive force to move or compress fluid. Thermal
energy created by motor windings can vaporize liquid, such as the
refrigerant 40 and/or the lubricant 42, that may be present in the
idle compressor 14 or 16, and can help drive out refrigerant 40 or
other liquid from the idle compressor 14 or 16 to help prevent
migration of fluid from the active compressor 14 or 16 through the
vapor cycle circuit 12 that may starve the active compressor 14 or
16 of such fluid used for operation.
[0015] It is desirable to maintain the motor temperature of the
idle compressor 14 or 16 within a given range between the lower
threshold and an upper threshold (also called an upper temperature
threshold). After the preheating mode has been activated at step
108, it is determined whether the sensed motor temperature of the
idle compressor 14 or 16 is above the upper threshold (step 110).
The upper threshold can be approximately 54.4.degree. C.
(130.degree. F.), or other temperatures in alternative embodiments.
If the motor temperature is above the upper threshold, then the
preheating mode of the idle compressor 14 or 16 can be turned off
(step 112), and then it is okay to start the motor of the idle
compressor 14 or 16 at step 106. In this way, sensed motor
temperature controls the preheat mode by default, and turns on or
off preheating of the idle compressor 14 or 16 as a function of
sensed motor temperature. If the motor temperature of the idle
compressor 14 or 16 is below the upper threshold, it is determined
whether the motor temperature is above the lower threshold and
there is a command to turn the idle compressor 14 or 16 on (i.e.,
activate the idle compressor 14 or 16) (step 114). If the motor
temperature is above the lower threshold and the idle compressor 14
or 16 is commanded on, then the preheat mode is turned off at step
112 and it is okay to start the motor 14-1 or 16-1 of the idle
compressor 14 or 16 at step 106.
[0016] If the motor temperature of the idle compressor 14 or 16 is
either below the lower threshold or the motor 14-1 or 16-1 of the
idle compressor 14 or 16 is not commanded on, then the preheat mode
is maintained on for a preheat time period (step 116). The preheat
time period can be 14 minutes, or another time period in
alternative embodiments. After the preheat time period, the preheat
mode is turned off (step 118) and thermal energy is no longer
generated by the motor of the idle compressor 14 or 16. The preheat
time period thus provides a failsafe to limit the maximum duration
of the preheating and the amount of time that heat is generated
with the motor of the idle compressor 14 or 16, which helps prevent
a risk of overheating in the event of a failure of the sensor 28 or
30. After the preheat time period has expired and the preheat mode
is turned off at step 118, then a determination is made as whether
the idle compressor 14 or 16 has been commanded on (step 120). If
there is a command to turn on the idle compressor after step 118,
then it is okay to start the motor at step 106. If the preheat time
period has lapsed, in effect overriding the default sensed
temperature control of the preheating mode, then the motor 14-1 or
16-1 of the idle compressor 14 or 16 can be started regardless of
the actual motor temperature. The preheat time period can selected
to help promote a suitable motor temperature of the idle compressor
14 or 16 even without the benefit of temperature sensing. If the
preheat time period has lapsed at step 118 but there is no command
to turn on the idle compressor 14 or 16, then a delay period is
triggered (step 122). In one embodiment the delay period can be 30
minutes, or other periods in alternative embodiments. During the
delay period the preheat mode of the idle compressor 14 or 16 is
maintained in an off condition. The delay period helps prevent
overheating and provides further failsafe protection. After
expiration of the delay period, the preheat mode of the idle
compressor 14 or 16 can be reactivated at step 116.
[0017] At any time it is determined it is okay to start the motor
14-1 or 16-1 of the idle compressor 14 or 16 at step 106, the idle
compressor 14 or 16 can be started (i.e., activated) to move or
compress fluid. After step 106, a determination can be made as to
whether single compressor mode is desired (step 124). At this
point, the single compressor mode using the same active compressor
14 or 16 can be maintained or the active and idle compressors 14
and 16 can alternate roles. If single compressor mode is desired,
then the method can proceed to step 100 and begin again. If single
compressor mode is not desired and all compressors 14 and 16 are
activated, then the method is finished because preheating is not
needed when all compressors 14 and 16 are active.
[0018] FIG. 3 is a flow chart illustrating an alternative
embodiment of a method of controlling a VCS to provide a "cold
start" routine. In the embodiment illustrated in FIG. 3, a time
fail-safe procedure, as provided in the embodiment illustrated in
FIG. 2, is omitted. It should be appreciated that the method
illustrated in FIG. 3 can readily be applied to single-compressor
VCSs, or to multi-compressor systems such as VCS 10 shown in FIG.
1.
[0019] Initially, a motor temperature of the motor 14-1 or 16-1 of
an idle compressor 14 or 16 is sensed using a corresponding sensor
28 or 30 (step 202). The temperature of fluid in the idle
compressor 14 or 16 need not be sensed directly, only the
temperature of the motor windings of the motor 14-1 or 16-1 of the
idle compressor 14 or 16. Temperature of the motor windings can be
sensed by delivering a current and measuring resistance through a
thermistor, or using other methods as desired. The sensed
temperature is communicated to the control logic 38 of the control
module 18. The control logic 38 analyzes the sensed data and
determines if the motor temperature of the idle compressor 14 or 16
is above a lower threshold (also called a lower temperature
threshold) (step 204). In the illustrated embodiment the lower
threshold is approximately 29.4.degree. C. (85.degree. F.), though
other temperatures can be selected for the lower threshold in
alternative embodiments. For instance, the lower threshold can be
selected to be sufficiently high to ensure that the refrigerant 40
is vaporized. If the motor temperature is above the lower
threshold, then it is okay to start (i.e., activate) the idle
compressor motor 14-1 or 16-1 if desired (step 206). If the sensed
motor temperature is below the lower threshold, then a preheating
mode can be activated (step 208). The preheating mode can generate
thermal energy by delivering power from the motor controller 32 or
34 to windings of the motor 14-1 or 16-1 of the idle compressor 14
or 16, thereby using the idle motor windings as a heater, without
activating the motor 14-1 or 16-1 of the idle compressor 14 or 16
to generate a motive force to move or compress fluid. Thermal
energy created by motor windings can vaporize liquid, such as the
refrigerant 40 and/or the lubricant 42, that may be present in the
idle compressor 14 or 16, and can help drive out refrigerant 40 or
other liquid from the idle compressor 14 or 16.
[0020] It is desirable to maintain the motor temperature of the
idle compressor 14 or 16 within a given range between the lower
threshold and an upper threshold (also called an upper temperature
threshold). After the preheating mode has been activated at step
208, it is determined whether the sensed motor temperature of the
idle compressor 14 or 16 is above the upper threshold (step 210).
The upper threshold can be approximately 32.2.degree. C.
(90.degree. F.), or other temperatures in alternative embodiments.
If the motor temperature is above the upper threshold, then the
preheating mode of the idle compressor 14 or 16 can be turned off
(step 212), and then it is okay to start the motor 14-1 or 16-1 of
the idle compressor 14 or 16 at step 206. In this way, sensed motor
temperature controls the preheat mode by default, and turns on or
off preheating of the idle compressor 14 or 16 as a function of
sensed motor temperature. If the motor temperature of the idle
compressor 14 or 16 is below the upper threshold, then it is
determined if there has been a motor temperature sensor failure
(step 215). This can be accomplished through built-in-testing
processes, or any other suitable method. If the sensor is operating
normally, preheating can continue at step 208. If the sensor has
failed, preheating is turned on for a time period (e.g., 18
minutes) (step 216), then preheating is turned off (step 218). At
this point it is determined if the idle compressor 14 or 16 is
commanded on (step 220). If the idle compressor 14 or 16 is
commanded on, then it is okay to start the motor 14-1 or 16-1 at
step 206. If there is no on command, then a delay period (e.g., 30
minutes) is begun (step 222). During the delay period, it is
determined if there is an on command (step 224). If there is an on
command any time during the delay period, then it is okay to start
the motor 14-1 or 16-1 at step 206. If there is no on command after
the expiration of the delay period, then a preheating period can be
cycled on for a new time period (e.g., 14 minutes) (step 226),
which returns to step 218 to continue the method.
[0021] At any time it is determined it is okay to start the motor
14-1 or 16-1 of the idle compressor 14 or 16 at step 206, the idle
compressor 14 or 16 can be started (i.e., activated) to move or
compress fluid.
[0022] It should be noted that the method illustrated in FIG. 3 can
be limited only to use in relatively cold environments or climates.
For instance, use of the method shown in FIG. 3 can be used only
when a sensed ambient temperature is below a suitable temperature
threshold.
[0023] While the invention has been described with reference to an
exemplary embodiment(s), it will be understood by those skilled in
the art that various changes may be made and equivalents may be
substituted for elements thereof without departing from the scope
of the invention. In addition, many modifications may be made to
adapt a particular situation or material to the teachings of the
invention without departing from the essential scope thereof.
Therefore, it is intended that the invention not be limited to the
particular embodiment(s) disclosed, but that the invention will
include all embodiments falling within the scope of the appended
claims. For example, although certain embodiments of the present
invention have been described with respect to a dual compressor
VCS, the present invention could be used with VCSs having a
different number of compressors. Moreover, temperature thresholds
for preheating control can vary as desired for particular
applications, such as to accommodate particular refrigerants or
other fluids used by a VCS.
* * * * *