U.S. patent application number 14/423329 was filed with the patent office on 2015-10-22 for stage transition in transcritical refrigerant vapor compression system.
The applicant listed for this patent is Carrier Corporation. Invention is credited to Mingfei Gan, Lucy Yi Liu, Aryn Shapiro, Jian Sun.
Application Number | 20150300713 14/423329 |
Document ID | / |
Family ID | 49085213 |
Filed Date | 2015-10-22 |
United States Patent
Application |
20150300713 |
Kind Code |
A1 |
Sun; Jian ; et al. |
October 22, 2015 |
STAGE TRANSITION IN TRANSCRITICAL REFRIGERANT VAPOR COMPRESSION
SYSTEM
Abstract
Operation of a transcritical refrigerant vapor compression
system for supplying temperature conditioned air to a temperature
controlled space is controlled when staging up or staging down to
avoid undesirable overshoot and undershoot of the narrow
temperature band bounding the control temperature set point for the
temperature within the temperature controlled space.
Inventors: |
Sun; Jian; (Fayetteville,
NY) ; Shapiro; Aryn; (Syracuse, NY) ; Liu;
Lucy Yi; (Fayetteville, NY) ; Gan; Mingfei;
(Manlius, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Carrier Corporation |
Farmington |
CT |
US |
|
|
Family ID: |
49085213 |
Appl. No.: |
14/423329 |
Filed: |
August 21, 2013 |
PCT Filed: |
August 21, 2013 |
PCT NO: |
PCT/US2013/055904 |
371 Date: |
February 23, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61692837 |
Aug 24, 2012 |
|
|
|
Current U.S.
Class: |
62/115 ;
62/157 |
Current CPC
Class: |
F25B 2400/13 20130101;
F25B 1/10 20130101; F25B 2600/111 20130101; F25B 49/022 20130101;
F25B 49/02 20130101; Y02B 30/70 20130101; B60H 1/3228 20190501;
F25B 2309/061 20130101; F25B 2600/0253 20130101; F25B 2600/2509
20130101 |
International
Class: |
F25B 49/02 20060101
F25B049/02 |
Claims
1. A method for operation of a transcritical refrigerant vapor
compression system for supplying conditioned air to a temperature
controlled space comprising operating the refrigerant vapor
compression system in at least one transition mode when staging up
or staging down between a higher capacity mode and a lower capacity
mode.
2. The method as recited in claim 1 wherein operation in the
transition mode comprises: monitoring a first parameter and a
second parameter; and staging out of the transition mode in
response to a determination that at least one of a first condition
related to said first parameter and a second condition related to
said second parameter has been established or has been established
for a particular period of time.
3. The method as recited in claim 1 wherein operation in the
transition mode comprises: monitoring a temperature control
differential, TSETPT-TCRTL, between a control temperature set
point, TSETPT, and a sensed control temperature, TCRTL; and
establishing a trend in a change of the temperature control
differential over an elapsed time.
4. The method as recited in claim 3 wherein establishing a trend in
a change of the temperature control differential over an elapsed
time comprises calculating an average derivative of the temperature
control differential over the elapsed time.
5. The method as recited in claim 4 further comprising: entering a
stage down transition mode for staging down from operation in a
high capacity mode to operation in one of a first low capacity mode
and a second lower capacity mode; determining whether the average
derivative of the temperature control differential has been
positive for an elapsed time greater than a preset first period of
time; and if the average derivative of the temperature control
differential has been positive for an elapsed time greater than the
preset first period of time, staging down to the second lower
capacity mode and bypassing the first low capacity mode.
6. The method as recited in claim 5 further comprising: if the
average derivative of the temperature control differential has not
been positive for an elapsed time greater than the preset first
period of time, determining whether the temperature control
differential is currently less than zero; and if the temperature
control differential is currently less than zero, staging down to
the first low capacity mode.
7. The method as recited in claim 6 further comprising: if the
temperature control differential is not currently less than zero,
determining whether the temperature control differential has been
greater than zero for a preset second period of time; and if the
temperature control differential has been greater than zero for the
second preset period of time, staging down to the second lower
capacity mode and bypassing the first low capacity mode.
8. The method as recited in claim 7 further comprising: if the
temperature control differential has not been greater than zero for
the second preset period of time, continuing operation in the high
capacity mode.
9. The method as recited in claim 4 further comprising: entering a
stage up transition mode for staging up from operation in a low
capacity mode to operation in one of a first high capacity mode and
a second higher capacity mode; determining whether the average
derivative of the temperature control differential has been
negative for an elapsed time greater than a preset third period of
time; and if the average derivative of the temperature control
differential has been negative for an elapsed time greater than the
preset third period of time, staging up to the second higher
capacity mode and bypassing the first high capacity mode.
10. The method as recited in claim 9 further comprising: if the
average derivative of the temperature control differential has not
been negative for an elapsed time greater than the preset third
period of time, determining whether the temperature control
differential is currently greater than zero; and if the temperature
control differential is currently greater than zero, staging up to
the first high capacity mode.
11. The method as recited in claim 10 further comprising: if the
temperature control differential is not currently greater than
zero, determining whether the temperature control differential has
been less than zero for a preset fourth period of time; and if the
temperature control differential has been less than zero for the
fourth preset period of time, staging up to the second higher
capacity mode and bypassing the first high capacity mode.
12. The method as recited in claim 11 further comprising: if the
temperature control differential has not been less than zero for
the fourth preset period of time, continuing operation in the low
capacity mode.
13. A refrigerant vapor compression system comprising: a
compression device for compressing a refrigerant vapor from a
suction pressure to a discharge pressure, a refrigerant heat
rejection heat exchanger and a refrigerant heat absorption heat
exchanger arranged in serial refrigerant flow relationship in a
transcritical cycle closed-loop primary refrigerant circuit, the
refrigerant heat rejection heat exchanger functioning as a
refrigerant gas cooler and the refrigerant heat absorption heat
exchanger functioning as a refrigerant evaporator; and a controller
operatively associated with the refrigerant vapor compression
system, the controller configured to control operation of the
refrigerant vapor compression system in a transition mode when
staging up or staging down between a higher capacity mode and a
lower capacity mode, the controller further configured to monitor a
temperature control differential, TSETPT-TCRTL, between a control
temperature set point, TSETPT, and a sensed control temperature,
TCRTL, during operation in the transition mode; and to calculate an
average derivative of the temperature control differential over an
elapsed time during operation in the transition mode.
14. The refrigerant vapor compression system as recited in claim
13, wherein the controller is further configured to: enter a stage
down transition mode when staging down from operation in a high
capacity mode to operation in one of a first low capacity mode and
a second lower capacity mode; determine whether the average
derivative of the temperature control differential has been
positive for an elapsed time greater than a preset first period of
time; and if the average derivative of the temperature control
differential has been positive for an elapsed time greater than the
preset first period of time, stage down to the second lower
capacity mode bypassing the first low capacity mode.
15. The refrigerant vapor compression system as recited in claim
14, wherein the controller is further configured to: if the average
derivative of the temperature control differential has not been
positive for an elapsed time greater than the preset first period
of time, determine whether the temperature control differential is
currently less than zero; and if the temperature control
differential is currently less than zero, stage down to the first
low capacity mode.
16. The refrigerant vapor compression system as recited in claim
15, wherein the controller is further configured to: if the
temperature control differential is not currently less than zero,
determine whether the temperature control differential has been
greater than zero for a preset second period of time; and if the
temperature control differential has been greater than zero for the
second preset period of time, stage down to the second lower
capacity mode bypassing the first low capacity mode.
17. The refrigerant vapor compression system as recited in claim
13, wherein the controller is further configured to: enter a stage
up transition mode for staging up from operation in a low capacity
mode to operation in one of a first high capacity mode and a second
higher capacity mode; determine whether the average derivative of
the temperature control differential has been negative for an
elapsed time greater than a preset third period of time; and if the
average derivative of the temperature control differential has been
negative for an elapsed time greater than the preset third period
of time, stage up to the second higher capacity mode bypassing the
first high capacity mode.
18. The refrigerant vapor compression system as recited in claim
17, wherein the controller is further configured to: if the average
derivative of the temperature control differential has not been
negative for an elapsed time greater than the preset third period
of time, determine whether the temperature control differential is
currently greater than zero; and if the temperature control
differential is currently greater than zero, stage up to the first
high capacity mode.
19. The refrigerant vapor compression system as recited in claim
18, wherein the controller is further configured to: if the
temperature control differential is not currently greater than
zero, determine whether the temperature control differential has
been less than zero for a preset fourth period of time; and if the
temperature control differential has been less than zero for the
fourth preset period of time, stage up to the second higher
capacity mode bypassing the first high capacity mode.
20. A method for controlling low capacity operation of a
transcritical refrigerant vapor compression system for supplying
conditioned air to a temperature controlled space, the refrigerant
vapor compression system having a primary refrigerant flow circuit
including a compression device driven by a variable speed motor
driven by a variable speed drive, a refrigerant gas cooler, a high
pressure expansion device, a flash tank, an evaporator expansion
device, and an evaporator disposed in serial refrigerant flow
arrangement in the primary refrigerant flow circuit and a
compressor unload circuit including an unload valve selectively
positionable to open or close the compressor unload circuit, the
method comprising: opening the unload valve to allow refrigerant to
pass from the compressor through the compressor unload circuit to a
suction pressure portion of the primary refrigerant flow circuit;
opening the high pressure expansion valve to a full open position;
operating the compressor drive motor at a frequency to drive the
compressor at a low/minimum speed; and controlling operation of at
least one of an air moving device associated with the gas cooler or
a heating device associated with the evaporator for heating a flow
of air drawn from the temperature controlled space.
21. The method as recited in claim 20 wherein operation of at least
one of an air moving device associated with the gas cooler or a
heating device associated with the evaporator for heating a flow of
air drawn from the temperature controlled space is controlled in
such a way as to preserve the temperature control sensitivity of
the variable speed drive driving the variable speed motor driving
the compression device.
22. The method as recited in claim 20 wherein controlling operation
of the air moving device associated with the gas cooler comprises
cycling the gas cooler air moving device through a plurality of
on/off duty cycles wherein in each duty cycle the gas cooler air
moving device is on for a first time period and off for a second
time period.
23. The method as recited in claim 22 wherein the first time period
is increased and the second time period is increased when advancing
from a duty cycle to a next duty cycle when cycling the air moving
device through a plurality of on/off duty cycles.
24. The method as recited in claim 20 wherein controlling operation
of the air moving device associated with the gas cooler comprises
selectively varying the speed of the air moving device.
25. The method as recited in claim 20 wherein controlling operation
of a heating device associated with the evaporator comprises
selectively cycling the heating device associated with the
evaporator through a plurality of on/off duty cycles for repeatedly
heating the air flow for a first time period and not heating the
air flow for a second time period.
26. The method as recited in claim 20 wherein controlling operation
of a heating device associated with the evaporator comprises
selectively varying the amount of electric current supplied to an
electric heater associated with the evaporator.
27. A method for operation of a transcritical refrigerant vapor
compression system for supplying conditioned air to a temperature
controlled space comprising: operating the refrigerant vapor
compression system in one of an economized mode, a non-economized
mode, and an unloaded mode; and operating the refrigerant vapor
compression system in a transition stage when staging down of
staging up between any two operating modes of said economized mode,
said non-economized mode, and said unloaded mode.
28. The method as recited in claim 27 wherein said transition stage
comprises a first transition down stage for staging down from
operation of the refrigerant vapor compression system in the
economized mode to operation in one of either the non-economized
mode or the unloaded mode.
29. The method as recited in claim 28 wherein said transition stage
comprises a second transition down stage for staging down from
operation of the refrigerant vapor compression system in the
non-economized mode to operation in the unloaded mode.
30. The method as recited in claim 27 wherein said transition stage
comprises a first transition up stage for staging up from operation
of the refrigerant vapor compression system in the unloaded mode to
operation in the non-economized mode.
31. The method as recited in claim 30 wherein said transition stage
comprises a second transition up stage for staging up operation of
the refrigerant vapor compression system in the non-economized mode
to operation in the economized mode.
Description
BACKGROUND OF THE DISCLOSURE
[0001] This disclosure relates generally to refrigerant vapor
compression systems and, more particularly, to improving the energy
efficiency and/or low cooling capacity operation of a transcritical
refrigerant vapor compression system.
[0002] Refrigerant vapor compression systems used in connection
with transport refrigeration systems are generally subject to more
stringent operating conditions due to the wide range of operating
load conditions and the wide range of outdoor ambient conditions
over which the refrigerant vapor compression system must operate to
maintain product within the cargo space at a desired temperature.
The desired temperature at which the cargo needs to be controlled
can also vary over a wide range depending on the nature of cargo to
be preserved. The refrigerant vapor compression system must not
only have sufficient capacity to rapidly pull down the temperature
of product loaded into the cargo space at ambient temperature, but
also should operate energy efficiently over the entire load range,
including at low cooling capacity when maintaining a stable product
temperature during transport.
[0003] During temperature maintenance, the object is to maintain
the temperature within the temperature controlled space, e.g. the
cargo box, within a narrow temperature band bounding a control
temperature set point. If the refrigerant vapor compression system
outputs too much cooling capacity, the temperature within the
temperature controlled space will drop below the narrow
temperature. Conversely, if the refrigerant vapor compression
system outputs too little cooling capacity, the temperature within
the temperature controlled space will rise above the narrow
temperature band. In conventional practice, it is common to cycle
the refrigerant compressor on and off during low capacity
operation. However, doing so can result in undesirable repeated
overshoot and undershoot of the narrow temperature band bounding
the control temperature set point.
SUMMARY OF THE DISCLOSURE
[0004] Operation of a transcritical refrigerant vapor compression
system for supplying temperature conditioned air to a temperature
controlled space is controlled when staging up or staging down to
avoid undesirable overshoot and undershoot of the narrow
temperature band bounding the control temperature set point for the
temperature within the temperature controlled space.
[0005] In an aspect, a method for operation of a transcritical
refrigerant vapor compression system for supplying conditioned air
to a temperature controlled space includes operating the
refrigerant vapor compression system in a transition mode when
staging up or staging down between a higher capacity mode and a
lower capacity mode. Operation in a transition mode may include
monitoring a first parameter and a second parameter, and staging
out of the transition mode in response to a determination that at
least one of a first condition related to the first parameter and a
second condition related to the second parameter has been
established or has been established for a particular period of
time. In an embodiment, operation in the transition mode includes:
monitoring a temperature control differential, TSETPT-TCRTL,
between a control temperature set point, TSETPT, and a sensed
control temperature, TCRTL; and establishing a trend in a change of
the temperature control differential over an elapsed time, which
may include calculating an average derivative of the temperature
control differential over the elapsed time.
[0006] In an embodiment, the method includes: entering a stage down
transition mode for staging down from operation in a high capacity
mode to operation in one of a first low capacity mode and a second
lower capacity mode; determining whether the average derivative of
the temperature control differential has been positive for an
elapsed time greater than a preset first period of time; and if the
average derivative of the temperature control differential has been
positive for an elapsed time greater than the preset first period
of time, staging down to the second lower capacity mode and
bypassing the first low capacity mode. In this embodiment, the
method further includes: if the average derivative of the
temperature control differential has not been positive for an
elapsed time greater than the preset first period of time,
determining whether the temperature control differential is
currently less than zero; and if the temperature control
differential is currently less than zero, staging down to the first
low capacity mode. In this embodiment, the method further includes:
if the temperature control differential is not currently less than
zero, determining whether the temperature control differential has
been greater than zero for a preset second period of time; and if
the temperature control differential has been greater than zero for
the second preset period of time, staging down to the second lower
capacity mode and bypassing the first low capacity mode.
[0007] In an embodiment, the method includes: entering a stage up
transition mode for staging up from operation in a low capacity
mode to operation in one of a first high capacity mode and a second
higher capacity mode; determining whether the average derivative of
the temperature control differential has been negative for an
elapsed time greater than a preset third period of time; and if the
average derivative of the temperature control differential has been
negative for an elapsed time greater than the preset third period
of time, staging up to the second higher capacity mode and
bypassing the first high capacity mode. In this embodiment the
method further includes: if the average derivative of the
temperature control differential has not been negative for an
elapsed time greater than the preset third period of time,
determining whether the temperature control differential is
currently greater than zero; and if the temperature control
differential is currently greater than zero, staging up to the
first high capacity mode. In this embodiment, the method may
further include: if the temperature control differential is not
currently greater than zero, determining whether the temperature
control differential has been less than zero for a preset fourth
period of time; and if the temperature control differential has
been less than zero for the fourth preset period of time, staging
up to the second higher capacity mode and bypassing the first high
capacity mode.
[0008] In an aspect, a refrigerant vapor compression system
includes: a compression device for compressing a refrigerant vapor
from a suction pressure to a discharge pressure, a refrigerant heat
rejection heat exchanger and a refrigerant heat absorption heat
exchanger arranged in serial refrigerant flow relationship in a
transcritical cycle closed-loop primary refrigerant circuit, the
refrigerant heat rejection heat exchanger functioning as a
refrigerant gas cooler and the refrigerant heat absorption heat
exchanger functioning as a refrigerant evaporator; and a controller
operatively associated with the refrigerant vapor compression
system, the controller configured to control operation of the
refrigerant vapor compression system in a transition mode when
staging up or staging down between a higher capacity mode and a
lower capacity mode, the controller further configured to monitor a
temperature control differential, TSETPT-TCRTL, between a control
temperature set point, TSETPT, and a sensed control temperature,
TCRTL, during operation in the transition mode; and to calculate an
average derivative of the temperature control differential over an
elapsed time during operation in the transition mode.
[0009] In an embodiment, the controller is further configured to:
enter a stage down transition mode when staging down from operation
in a high capacity mode to operation in one of a first low capacity
mode and a second lower capacity mode; determine whether the
average derivative of the temperature control differential has been
positive for an elapsed time greater than a preset first period of
time; and if the average derivative of the temperature control
differential has been positive for an elapsed time greater than the
preset first period of time, stage down to the second lower
capacity mode and bypassing the first low capacity mode. In this
embodiment, the controller may be further configured to: if the
average derivative of the temperature control differential has not
been positive for an elapsed time greater than the preset first
period of time, determine whether the temperature control
differential is currently less than zero; and if the temperature
control differential is currently less than zero, stage down to the
first low capacity mode. The controller may be further configured
to: if the temperature control differential is not currently less
than zero, determine whether the temperature control differential
has been greater than zero for a preset second period of time; and
if the temperature control differential has been greater than zero
for the second preset period of time, stage down to the second
lower capacity mode and bypassing the first low capacity mode.
[0010] In an embodiment, the controller is further configured to:
enter a stage up transition mode for staging up from operation in a
low capacity mode to operation in one of a first high capacity mode
and a second higher capacity mode; determine whether the average
derivative of the temperature control differential has been
negative for an elapsed time greater than a preset third period of
time; and if the average derivative of the temperature control
differential has been negative for an elapsed time greater than the
preset third period of time, stage up to the second higher capacity
mode and bypassing the first high capacity mode. The controller may
be further configured to: if the average derivative of the
temperature control differential has not been negative for an
elapsed time greater than the preset third period of time,
determine whether the temperature control differential is currently
greater than zero; and if the temperature control differential is
currently greater than zero, stage up to the first high capacity
mode. The controller may be further configured to: if the
temperature control differential is not currently greater than
zero, determine whether the temperature control differential has
been less than zero for a preset fourth period of time; and if the
temperature control differential has been less than zero for the
fourth preset period of time, stage up to the second higher
capacity mode and bypassing the first high capacity mode.
[0011] In an aspect, a method is provided for controlling low
capacity operation of a transcritical refrigerant vapor compression
system for supplying conditioned air to a temperature controlled
space, the refrigerant vapor compression system having a primary
refrigerant flow circuit including a variable speed compressor
driven by a variable frequency motor, a refrigerant gas cooler, a
high pressure expansion device, a flash tank, an evaporator
expansion device, and an evaporator disposed in serial refrigerant
flow arrangement in the primary refrigerant flow circuit and a
compressor unload circuit including an unload valve selectively
positionable to open or close the compressor unload circuit. The
method includes: opening the unload valve to allow refrigerant to
pass from the compressor through the compressor unload circuit to a
suction pressure portion of the primary refrigerant flow circuit;
opening the high pressure expansion valve to a full open position;
operating the compressor drive motor at a minimum frequency to
drive the compressor at a minimum speed; and selectively cycling an
air moving device associated with the gas cooler through an on/off
duty cycle.
[0012] In an aspect, a method is provided for controlling low
capacity operation of a transcritical vapor compression system for
supplying conditioned air to a temperature controlled space
includes controlling operation of at least one of an air moving
device associated with the gas cooler or a heating device
associated with the evaporator for heating a flow of air drawn from
the temperature controlled space. In an embodiment of the method,
the air moving device associated with the gas cooler or the heating
device associated with the evaporator is controlled in such a way
as to preserve the temperature control sensitivity of the variable
speed drive driving the variable speed motor driving the
compression device.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] For a further understanding of the disclosure, reference
will be made to the following detailed description which is to be
read in connection with the accompanying drawing, wherein:
[0014] FIG. 1 is perspective view of a refrigerated container
equipped with a transport refrigeration unit;
[0015] FIG. 2 is a schematic illustration of an embodiment of the
refrigerant vapor compression system of the transport refrigeration
unit in accord with an aspect of the invention;
[0016] FIG. 3 is a block diagram process flow chart illustrating
staging down and staging up a refrigerant vapor compression system
in accordance with an embodiment of the method disclosed
herein;
[0017] FIG. 4 is a block diagram process flow chart illustrating an
exemplary logic for staging down a refrigerant vapor compression
system from an economized mode to an unloaded mode in accordance
with an embodiment of the method disclosed herein.
DETAILED DESCRIPTION OF THE INVENTION
[0018] There is depicted in FIG. 1 an exemplary embodiment of a
refrigerated container 10 having a temperature controlled cargo
space 12 the atmosphere of which is refrigerated by operation of a
transport refrigeration unit 14 associated with the cargo space 12.
In the depicted embodiment of the refrigerated container 10, the
transport refrigeration unit 14 is mounted in a wall of the
refrigerated container 10, typically in the front wall 18 in
conventional practice. However, the refrigeration unit 14 may be
mounted in the roof, floor or other walls of the refrigerated
container 10. Additionally, the refrigerated container 10 has at
least one access door 16 through which perishable goods, such as,
for example, fresh or frozen food products, may be loaded into and
removed from the cargo space 12 of the refrigerated container
10.
[0019] Referring now to FIG. 2, there is depicted schematically an
embodiment of a refrigerant vapor compression system 20 suitable
for use in the transport refrigeration unit 14 for refrigerating
air drawn from and supplied back to the temperature controlled
cargo space 12. Although the refrigerant vapor compression system
20 will be described herein in connection with a refrigerated
container 10 of the type commonly used for transporting perishable
goods by ship, by rail, by land or intermodally, it is to be
understood that the refrigerant vapor compression system 20 may
also be used in transport refrigeration units for refrigerating the
cargo space of a truck, a trailer or the like for transporting
perishable fresh or frozen goods. The refrigerant vapor compression
system 20 is also suitable for use in conditioning air to be
supplied to a climate controlled comfort zone within a residence,
office building, hospital, school, restaurant or other facility.
The refrigerant vapor compression system 20 could also be employed
in refrigerating air supplied to display cases, merchandisers,
freezer cabinets, cold rooms or other perishable and frozen product
storage areas in commercial establishments.
[0020] The refrigerant vapor compression system 20 includes a
multi-stage compression device 30, a refrigerant heat rejection
heat exchanger 40, a flash tank 60, and a refrigerant heat
absorption heat exchanger 50, also referred to herein as an
evaporator, with refrigerant lines 22, 24 and 26 connecting the
aforementioned components in serial refrigerant flow order in a
primary refrigerant circuit. A high pressure expansion device
(HPXV) 45, such as for example an electronic expansion valve, is
disposed in the refrigerant line 24 upstream of the flash tank 60
and downstream of refrigerant heat rejection heat exchanger 40. An
evaporator expansion device (EVXV) 55, such as for example an
electronic expansion valve, operatively associated with the
evaporator 50, is disposed in the refrigerant line 24 downstream of
the flash tank 60 and upstream of the evaporator 50.
[0021] The compression device 30 compresses the refrigerant and
circulates the refrigerant through the primary refrigerant circuit
as will be discussed in further detail hereinafter. The compression
device 30 may comprise a single, multiple-stage refrigerant
compressor, for example a reciprocating compressor or a scroll
compressor, having a first compression stage 30a and a second stage
30b, wherein the refrigerant discharging from the first compression
stage 30a passes to the second compression stage 30b for further
compression. Alternatively, the compression device 30 may comprise
a pair of individual compressors, one of which constitutes the
first compression stage 30a and other of which constitutes the
second compression stage 30b, connected in series refrigerant flow
relationship in the primary refrigerant circuit via a refrigerant
line connecting the discharge outlet port of the compressor
constituting the first compression stage 30a in refrigerant flow
communication with the suction inlet port of the compressor
constituting the second compression stage 30b for further
compression. In a two compressor embodiment, the compressors may be
scroll compressors, screw compressors, reciprocating compressors,
rotary compressors or any other type of compressor or a combination
of any such compressors. In both embodiments, in the first
compression stage 30a, the refrigerant vapor is compressed from a
lower pressure to an intermediate pressure and in the second
compression stage 30b, the refrigerant vapor is compressed from an
intermediate pressure to higher pressure.
[0022] In the embodiment of the refrigerant vapor compression
system 20 depicted in FIG. 2, the compression device 30 is driven
by a variable speed compressor motor 32 powered by electric current
delivered through a variable frequency drive 34. The electric
current may be supplied to the variable speed drive 34 from an
external power source (not shown), such as for example a ship board
power plant, or from a fuel-powered engine drawn generator unit,
such as a diesel engine driven generator set, attached to the front
of the container. The speed of the variable speed compressor 30 may
be varied by varying the frequency of the current output by the
variable frequency drive 34 to the compressor drive motor 32. It is
to be understood, however, that the compression device 30 could in
other embodiments comprise a fixed speed compressor.
[0023] The refrigerant heat rejection heat exchanger 40 may
comprise a finned tube heat exchanger 42 through which hot, high
pressure refrigerant discharged from the second compression stage
30b (i.e. the final compression charge) passes in heat exchange
relationship with a secondary fluid, most commonly ambient air
drawn through the heat exchanger 42 by the fan(s) 44. The finned
tube heat exchanger 42 may comprise, for example, a fin and round
tube heat exchange coil or a fin and flat mini-channel tube heat
exchanger. In the depicted embodiment, a variable speed fan motor
46 powered by a variable frequency drive drives the fan(s) 44
associated with the heat rejection heat exchanger 40. The variable
speed fan motor 46 may be powered by the same variable frequency
drive 34 that powers the compressor motor or by a separate
dedicated variable frequency drive 48, as depicted in FIG. 2.
[0024] When the refrigerant vapor compression system 20 operates in
a transcritical cycle, the pressure of the refrigerant discharging
from the second compression stage 30b and passing through the
refrigerant heat rejection heat exchanger 40, referred to herein as
the high side pressure, exceeds the critical point of the
refrigerant, and the refrigerant heat rejection heat exchanger 40
functions as a gas cooler. However, it should be understood that if
the refrigerant vapor compression system 20 operates solely in the
subcritical cycle, the pressure of the refrigerant discharging from
the compressor and passing through the refrigerant heat rejection
heat exchanger 40 is below the critical point of the refrigerant,
and the refrigerant heat rejection heat exchanger 40 functions as a
condenser. As the method of operation disclosed herein pertains to
operation of the refrigerant vapor compression system 20 in a
transcritical cycle, the refrigerant heat rejection heat exchanger
will also be referred to herein as gas cooler 40.
[0025] The refrigerant heat absorption heat exchanger 50 may also
comprise a finned tube coil heat exchanger 52, such as a fin and
round tube heat exchanger or a fin and flat, micro-channel or
mini-channel tube heat exchanger. Whether the refrigerant vapor
compression system is operating in a transcritical cycle or a
subcritical cycle, the refrigerant heat absorption heat exchanger
50 functions as a refrigerant evaporator. Before entering the
evaporator 50, the refrigerant passing through refrigerant line 24
traverses the evaporator expansion device 55, such as, for example,
an electronic expansion valve or a thermostatic expansion valve,
and expands to a lower pressure and a lower temperature to enter
heat exchanger 52. As the refrigerant traverses the heat exchanger
52, the refrigerant, typically a two-phase (liquid/vapor mix)
refrigerant, passes in heat exchange relationship with a heating
fluid whereby the liquid refrigerant is evaporated to a vapor and
the vapor typically superheated to a desired degree. The low
pressure vapor refrigerant leaving the heat exchanger 52 passes
through the refrigerant line 26 to the suction inlet of the first
compression stage 30a. The heating fluid may be air drawn by an
associated fan(s) 54 from a climate controlled environment, such as
a perishable/frozen cargo storage zone associated with a transport
refrigeration unit, or a food display or storage area of a
commercial establishment, or a building comfort zone associated
with an air conditioning system, to be cooled, and generally also
dehumidified, and thence returned to a climate controlled
environment.
[0026] The flash tank 60, which is disposed in the refrigerant line
24 between the gas cooler 40 and the evaporator 50, upstream of the
evaporator expansion valve 55 and downstream of the high pressure
expansion device 45, functions as an economizer and a receiver. The
flash tank 60 defines a chamber 62 into which expanded refrigerant
having traversed the high pressure expansion device 45 enters and
separates into a liquid refrigerant portion and a vapor refrigerant
portion. The liquid refrigerant collects in the chamber 62 and is
metered therefrom through the downstream leg of the refrigerant
line 24 by the evaporator expansion device 55 to flow through the
evaporator 50.
[0027] The vapor refrigerant collects in the chamber 62 above the
liquid refrigerant and may pass therefrom through economizer vapor
line 64 for injection of refrigerant vapor into an intermediate
stage of the compression process. An economizer flow control device
65, such as, for example, a solenoid valve (ESV) having an open
position and a closed position, is interposed in the economizer
vapor line 64. When the refrigerant vapor compression system 20 is
operating in a economized mode, the economizer flow control device,
ESV, 65 is opened thereby allowing refrigerant vapor to pass
through the economizer vapor line 64 from the flash tank 60 into an
intermediate stage of the compression process. When the refrigerant
vapor compression system 20 is operating in a standard,
non-economized mode, the economizer flow control device, ESV, 65 is
closed thereby preventing refrigerant vapor to pass through the
economizer vapor line 64 from the flash tank 60 into an
intermediate stage of the compression process.
[0028] In an embodiment where the compression device 30 has two
compressors connected in serial flow relationship by a refrigerant
line, one being a first compression stage 30a and the other being a
second compression stage 30b, the vapor injection line 64
communicates with the refrigerant line interconnecting the outlet
of the first compression stage 30a to the inlet of the second
compression stage 30b. In an embodiment where the compression
device 30 comprises a single compressor having a first compression
stage 30a feeding a second compression stage 30b, the refrigerant
vapor injection line 64 can open directly into an intermediate
stage of the compression process through a dedicated port opening
into the compression chamber.
[0029] The refrigerant vapor compression system 20 also includes a
controller 100 operatively associated with the plurality of flow
control devices 45, 55, 65 and 75 interdisposed in various
refrigerant lines as previously described for selectively
controlling the opening, the closing, and, as applicable, the
openness. The controller 100 also monitors the ambient air
temperature, T.sub.AMAIR, supply air temperature, T.sub.SBAIR, and
return air temperature, T.sub.RBAIR. In the embodiment of the
refrigerant vapor compression system 20 depicted in FIG. 2, a
temperature sensor 102 may be provided for sensing the ambient air
temperature and transmitting a signal indicative of the ambient air
temperature to the controller 100. In an embodiment, the ambient
air temperature air sensor 102 may be disposed to sense the
temperature of the ambient air being delivered to the refrigerant
heat rejection heat exchanger 40 upstream of the heat exchanger
coil 42. A temperature sensor 104 is disposed in association with
the evaporator 50 for sensing the temperature of the supply air
having traversed the evaporator heat exchange coil 52 and passing
back to the temperature controlled space 12 and for transmitting a
signal indicative of the supply air temperature, T.sub.SBAIR, to
the controller 100. A temperature sensor 106 is disposed in
association with the evaporator 50 for sensing the temperature of
the return air being drawn from the temperature controlled space 12
to pass over the evaporator heat exchange coil 52 and for
transmitting a signal indicative of the return air temperature,
T.sub.RBAIR, to the controller 100. The temperature sensors 102,
104 and 106 may be conventional temperature sensors, such as for
example, thermometers, thermocouples or thermistors. The controller
100 may also monitor various pressures and temperatures and
operating parameters by various sensors operatively associated with
the controller 100 and disposed at selected locations throughout
the refrigerant vapor compression system 20.
[0030] The term "controller" as used herein refers to any method or
system for controlling and should be understood to encompass
microprocessors, microcontrollers, programmed digital signal
processors, integrated circuits, computer hardware, computer
software, electrical circuits, application specific integrated
circuits, programmable logic devices, programmable gate arrays,
programmable array logic, personal computers, chips, and any other
combination of discrete analog, digital, or programmable
components, or other devices capable of providing processing
functions.
[0031] The controller 100 is configured to control operation of the
refrigerant vapor compression system in various operational modes,
including several capacity modes and an unloaded mode. A capacity
mode is a system operating mode wherein a refrigeration load is
imposed on the system requiring the compressor to run in a loaded
condition to meet the cooling demand. In an unloaded mode, the
refrigeration load imposed upon the system is so low that
sufficient cooling capacity may be generated to meet the cooling
demand with the compressor running in an unloaded condition. The
controller 100 is also configured to control the variable speed
drive 34 to vary the frequency of electric current delivered to the
compressor drive motor so as to vary the speed of the compression
device 30 to vary the capacity output of the compression device in
response to cooling demand.
[0032] As noted previously, in transport refrigeration
applications, the refrigerant vapor compression system 20 must be
capable of operating at high capacity to rapidly pull-down the
temperature within the cargo box upon loading and must be capable
of operating at extremely low capacity during maintenance of the
box temperature within a very narrow band, such as for example as
little as +/-0.25.degree. C. (+/-0.45.degree. F.), during
transport. Depending upon the particular cargo being shipped, the
required box air temperature may range from as low as -34.4.degree.
C. (-30.degree. F.) up to 30.degree. C. (86.degree. F.). Thus, the
controller 100 will selectively operate the refrigerant vapor
compression system in a loaded mode (high refrigeration capacity
mode) in response to a high cooling demand, such as during initial
pull-down and recovery pull downs, in a standard economized mode or
a standard non-economized mode.
[0033] Once the box air temperature has been pulled down, that is
reduced, to a temperature within a narrow band around a set point
box temperature, the controller 100 will also selectively operate
the refrigerant vapor compression system 20 in an unload mode (low
refrigeration capacity mode) when maintaining the box temperature
in the narrow band around a set point box temperature. The narrow
temperature band has an upper temperature equal to the control
temperature set point, TSETPT, plus an upper bound temperature
differential, .DELTA.TUPPER, and a lower bound temperature
differential, .DELTA.TLOWER. Typically, the box temperature is
controlled indirectly through monitoring and set point control of
either one of the temperature, T.sub.SBAIR, of the supply box air,
i.e. the air leaving the evaporator 50, and the temperature,
T.sub.RBAIR, of the return box air, i.e. the box air entering the
evaporator 50.
[0034] In response to the cooling load imposed on the refrigerant
vapor compression system 20, the controller 100 is configured to
selectively operate the refrigerant vapor compression in one of
following operating modes: at least one high capacity economized
mode, a moderate capacity non-economized standard mode, and a
low/minimum capacity unloaded mode. In the depicted refrigerant
vapor compression system 20, wherein the gas cooler fan 44 is
driven by a multiple speed or variable speed motor 46, the
economized capacity mode includes a maximum capacity economized
mode, as well as a high capacity economized mode. In the maximum
capacity economized mode, also referred to herein as stage 0, the
economizer solenoid valve 65 is open, the unload solenoid valve 75
is closed and the gas cooler fan 44 is operated at a high speed. In
the high capacity economized mode, also referred to herein as stage
1, the economizer solenoid valve 65 is open, the unloaded solenoid
valve 75 is closed and the gas cooler fan 44 is operated at a low
speed. In the moderate capacity non-economized standard mode, also
referred to herein as stage 2, the economizer solenoid valve 65 is
closed, the unload solenoid valve 75 is closed and the gas cooler
fan 44 is operated at a low speed. In the low/minimum capacity
unload mode, also referred to herein as stage 3, the economizer
solenoid valve 65 is closed, the unload solenoid valve 75 is open
and the gas cooler fan 44 is operated in a duty cycle as will be
explained in further detail later herein.
[0035] The controller 100 also selectively operates the refrigerant
vapor compression system 20 in various transition modes when
staging down or staging up the refrigerant vapor compression
system. For example, when staging down from the economized
operation in stage 1, the controller 100 operates the refrigerant
vapor compression system for a limited period of time in a first
transition down mode referred to as stage 12. When staging down
from the non-economized stage 2 to the unloaded operation in stage
3, the controller 100 may also operate the refrigerant vapor
compression system 20 for a limited period of time in a second
transition down mode referred to as stage 23. Similarly, when
staging up from the unloaded mode, stage 3, to the non-economized
mode of stage 2, the controller 100 may operate the refrigerant
vapor compression system 20 for a limited period of time in a first
transition up mode, referred to a stage 32. When staging up from
operation in the non-economized mode, stage 2, to operation in the
economized mode, stage 1, the controller 100 may operate the
refrigerant vapor compression system 20 for a limited period of
time in a second transition up mode referred to as stage 21.
[0036] The frequency of the current output from the variable speed
drive 34 to the compressor motor 32 is varied in stages 0, 1, 2 and
3 for varying the speed of the compression device 30, and
maintained at a respective predetermined frequency that may be
determined in accordance with a frequency map in each of the
respective transition stages 12, 23, 32 and 21 (FIG. 3). The
frequency map comprises an algorithm for determining the optimal
VFD output frequency in real time as a function of the sensed
ambient air temperature, the sensed return air temperature and the
sensed supply air temperature.
[0037] The controller 100 controls operation of the refrigerant
vapor compression system 20 during transition in such a manner as
to reduce, if not eliminate, overshoot and undershoot of the narrow
temperature band bounding the box control temperature set point.
Referring now to FIG. 3, there is depicted a block diagram process
flow chart illustrating an exemplary embodiment of the method
disclosed herein for staging down and staging up of the refrigerant
vapor compression system 20. In staging down, the controller 100
selectively reduces system capacity, moving from the maximum
capacity economized mode, stage 0, at block 200, to the high
capacity economized mode, stage 1, at block 220. When the cooling
capacity of the compression device 30 is sufficient to meet the
cooling demand without use of the economizer circuit and the
control temperature has been brought down into the narrow
temperature bounding the control temperature set point, the
controller 100 will transition the refrigerant vapor compression
system 20 down from the economized mode to the unloaded mode. In
doing so, the controller 100 implements the first transition down
stage 12 at block 212, from which capacity is reduced either
directly to the unloaded mode, stage 3, at block 230, or first to
the non-economized mode, stage 2, at block 220, and thence through
the second transition down stage 23, at block 223, to the unloaded
mode, stage 3, at block 230. In staging up, the controller 100
selectively increases system capacity, moving from the unloaded
mode, stage 3, at block 230, through the first transition up stage
32, at block 232, to the non-economized mode, stage 2, at block
220, thence through the second transition up stage 21, at block
221, to the high capacity economized mode, stage 1, at block 210,
and to the maximum capacity economized mode, stage 0, at block
200.
[0038] During operation in the first transition down stage 12, the
economizer solenoid valve 65 is open, the unload solenoid valve 75
is closed, the gas cooler fan 44 is operated at a low speed, and
the variable speed drive 34 outputs electric current a first
predetermined frequency to drive the compression device 30. During
operation in the second transition down stage 23, the economizer
solenoid valve 65 is closed, the unload solenoid valve 75 is open,
the gas cooler fan 44 is operated at a low speed, and the variable
speed drive 34 outputs electric current at a second predetermined
frequency to drive the compression device 30. During operation in
the first transition up stage 32, the economizer solenoid valve 65
is closed, the unload solenoid valve 75 is closed, the gas cooler
fan 44 is operated at a low speed, and the variable speed drive 34
outputs electric current at a third predetermined frequency to
drive the compression device 30. During operation in the second
transition up stage 21, the economizer solenoid valve 65 is open,
the unload solenoid valve 75 is closed, the gas cooler fan 44 is
operated at low speed, and the variable speed drive 34 outputs
electric current at a fourth predetermined frequency to drive the
compression device 30. The first, second, third and fourth
predetermined frequencies may be determined through use of a
frequency map as previously mentioned.
[0039] In the transition stages, the controller 100 may be
configured to monitor a first parameter and a second parameter and
stage out of the transition mode in response to a determination
that one or both of a first condition related to the first
parameter and a second condition related to the second parameter
has been established or established for a particular period of
time. It is to be understood that the controller 100 may be
configured to use similar logic processes when the refrigerant
vapor compression system 20 is operating in any transition down or
transition up stage.
[0040] For example, in the first transition down stage 12, the
controller 100 may selectively transition down from the economized
mode, stage 1, at block 210, to the unloaded mode, at block 230,
through either of two routes, that is either directly into the
unloaded mode, stage 3, at block 230, or first to the
non-economized mode, stage 2, at block 220, and thence through the
second transition down mode, stage 23, at block 232, to the
unloaded mode, stage 3, at block 230. Referring now to the logic
process diagram illustrated in FIG. 4, upon entering the first
transition down stage 12, the controller 100 sets a timer to zero
and monitors a first condition and a second condition. If the first
condition has been established for a time period greater than a
preset time period, tc1, t(first condition)>tc1, controller 100
transitions the refrigerant vapor compression system 20 down
directly to the unloaded stage 3, at block 230. However, the first
condition has not been established for a period of time greater
than the preset time period tc1, t(first condition) not >tc1,
controller 100 checks whether a second condition is currently
satisfied. If so, the controller 100 transitions the refrigerant
vapor compression system 20 down to the non-economized loaded stage
2, at block 220, rather than directly into the unloaded stage
3.
[0041] For clarity purposes, the nomenclature used herein will be
explained before further discussing the method disclosed herein.
The expression "t(condition)" translates: the time that the
condition within the parentheses has existed. For example,
"t(TEMP1>TEMP2)" would be read as the time that temperature 1
has been greater than temperature 2. "TCTRL" is the control
temperature. "TSETPT" is the control temperature set point
temperature. "TEMP_TREND" is the average derivative, that is the
rate of change, of the temperature differential "TSETPT-TCTRL" over
a specified period of time, for example, for purposes of
illustration but not limitation, ten seconds.
[0042] In an embodiment of the method disclosed herein, when
operating in the first transition down mode 12, the controller 100
monitors the rate of change, TEMP_TREND, as the first condition,
and the temperature differential TSETPT-TCTRL as the second
condition, and repeatedly executes the control loop 300,
illustrated in FIG. 4, to determine when to stage down from the
first transition stage 12 into one of the non-economized mode,
stage 2 at block 220, or the unloaded mode, stage 3 at block 230,
and to determine into which of these two modes to transition. In an
embodiment, the controller 100 continuously executes the control
loop 300 at a loop rate of one Hertz.
[0043] At block 310, the controller 100 compares an elapsed time at
which the TEMP_TREND has been greater than zero to the preset time
period, tc1. If the TEMP_TREND has been greater than 0, that is
positive, for an elapsed time greater than the preset time period,
t1, then the controller 100 stages down the refrigerant vapor
compression system 20 to operation in the unloaded mode, stage 3 at
block 230. However, if the TEMP_TREND has not been greater than 0,
that is not positive, for an elapsed time greater than the preset
time period, tc1, then the controller 100 proceeds to block
320.
[0044] At block 320, the controller 100 determines whether the
temperature differential TSETPT-TCTRL is less than zero, which
means that the sensed control temperature, typically either the
current sensed T.sub.RBAIR or T.sub.SBAIR, is greater than the
control temperature set point. If the temperature differential
TSETPT-TCTRL is less than zero, the controller 100 stages down the
refrigerant vapor compression system 20 to operation in the
non-economized mode, stage 2 at block 220. However, if the
temperature differential TSETPT-TCTRL is not less than zero, the
controller 100 continues to execute the control logic 300 until one
of the first and second conditions satisfies the control logic or a
preset time out limit has been reached. In the event the time out
limit has been reached, the controller 100 will transition down the
refrigerant vapor compression system to operation in the unloaded
mode, stage 3.
[0045] In an embodiment, the controller 100 may be configured to
stage down from operation in the non-economized stage 2 to
operation in the unloaded stage 3 when, during operation in the
second transition down stage 23, the following two conditions are
met simultaneously: the output frequency of the variable speed
drive 34 powering the compression device 30 has been less than or
equal to a lower frequency level that is a preset frequency margin
above the minimum output frequency of the variable speed drive for
an elapsed time greater than a preset period of time, t5; and the
TEMP_TREND has been positive for an elapsed time greater than a
preset period of time, t6. In this embodiment, if these two
conditions are not met simultaneously, operation continues in the
second transition down stage 23 until these conditions are
simultaneously met.
[0046] In an embodiment, the controller 100 may be configured to
stage down from the non-economized stage 2 to the unloaded stage 3
when, during operation in the second transition down stage 23, the
temperature differential TSETPT-TCTRL has been negative and has had
an absolute value greater than a preset temperature differential
for an elapsed time greater than a preset period of time, t7. In
this embodiment, operation continues in the unloaded low capacity
stage 2 until this condition is met. It is to be understood,
however, that the transitioning method discussed herein and
illustrated in FIG. 3, may be used in connection with transitioning
from the non-economized mode, stage 2, at block 220, to the
unloaded mode, stage 3, at block 230, irrespective of the specific
criteria used in determining when to stage down from stage 2 to
stage 3.
[0047] When the refrigerant vapor compression system 20 is
operating in the unloaded mode, stage 3, the controller 100 may be
configured to control operation of the gas cooler fan 44 in such a
way as to preserve the temperature control sensitivity of the
variable speed drive 34 driving the variable speed compressor motor
32. For example, the controller 100 may be configured to pulse the
gas cooler fan 44 by on/off cycling. Initially in stage 3, the gas
cooler fan 44 will be on and operating at low speed. However, as
the variable speed drive 34 cycles down to control temperature, the
controller 100 is configured to further reduce the effective speed
on the gas cooler fan 44 by selectively powering the gas cooler fan
on for a first period of time and then powering the gas cooler fan
off for a second period of time and then repeat the cycle. The
first and second periods of time constitute a duty cycle. The
controller 100 may be configured to adjust the first and second
periods of time relative to each other to vary the time the gas
cooler fan 44 is on and the fan is off within a duty cycle. In an
embodiment, the controller 100 may be configured to selectively
operate the gas cooler fan 44 to incrementally further reduce the
effective speed of the gas cooler fan 44. For example, in an
embodiment, the controller 100 may be configured to operate the gas
cooler fan 44 through a series of duty cycles wherein the fan on
portion of the duty cycle is decreased incrementally. For example,
in an embodiment, the fan on portion of duty cycle decreases in 20%
increments: 100% on/0% off; 80% on/20% off; 60% on/40% off; 40%
on/60% off; 20% on/80% off; 0% on/100% off. It is be to understood
that in other embodiments the fan on portion of the duty cycle may
decrease in other increments, such as for example, but not limited
to increments of 10%. Each duty cycle may span a preselected time
period, such as, for example, 40 seconds. Upon completion of the
series of duty cycles, the gas cooler fan 44 will remain powered
off.
[0048] When the refrigerant vapor compression system 20 is
operating in the unloaded mode, stage 3, the controller 100 may
also be configured to selectively operate electric heaters 56 (FIG.
2) associated with the evaporator coil 52 in such a way as to
preserve the temperature control sensitivity of the variable speed
drive 34 driving the variable speed compressor motor 32. In the
unloaded mode, stage 3, the electric heaters are initially off, and
will remain off, unless selectively powered on by the controller
100. For example, in an embodiment, as the variable frequency drive
34 cycles down to control temperature, the controller 100 may be
configured to operate the electric heaters through a series of duty
cycles wherein in each duty cycle the electric heater is powered on
for a first portion of the duty cycle and powered off for a second
period of the duty cycle, i.e. the remainder of the duty cycle,
with the powered on portion increasing by 5% from one duty cycle to
the next duty cycle, until a predetermined maximum power on limit
is reached. Each duty cycle may span a preselected time period,
such as, for example, 60 seconds. However, the controller 100 may
also be configured to operate the electric heaters associated with
the evaporator coil 52 at 100% on, if at any point during operation
in the unloaded mode, stage 3, the control temperature, TCTRL,
falls below the set point temperature, TSETPT, by an amount greater
than .DELTA.TLOWER, i.e. drops out of the temperature control band.
Upon return of the control temperature, TCTRL, into the control
temperature band, the controller 100 may return to pulsing of the
electric heaters. The controller 100 may be configured to implement
the pulsing of the gas cooler fan 44 and the pulsing of the
electric heaters associated with the evaporator coil 52 in such a
way as to prevent overlapping of the gas cooler fan cycling and the
electric heater cycling.
[0049] Referring now to the staging up process illustrated in FIG.
3, the controller 100 is configured to stage up the operation of
the refrigerant vapor compression system 20 from the unloaded mode,
stage 3, at block 230 to the non-economized mode, stage 2, through
the first transition up stage 32 when the cooling demand on the
refrigerant vapor compression system 20 exceeds the refrigeration
capacity of the refrigerant vapor compression system 20 when
operating in the unloaded mode, stage 3. When operating in the
first transition up stage 32, at block 232, the controller 100
monitors two conditions and when each condition has been
established for a respective predetermined period of time, the
controller 100 shifts operation of the refrigerant vapor
compression system 20 into the non-economized mode, stage 2, at
block 220. In an embodiment, the controller 100 monitors the
operating frequency of the variable frequency drive 34, FreqVFD,
and the rate of change of the temperature differential, TEMP_TREND.
When the output frequency of the variable speed drive 34 powering
the compression device 30 has been greater than or equal to a limit
frequency, determined from the frequency map, for an elapsed time
greater than a preset period of time, t13
(t(FreqVFD>Freq3)>t13); and the TEMP_TREND has been negative
for an elapsed time greater than a preset period of time, t6
(t(TEMP_TREND)<0)>t14), the controller 100 shifts operation
of the refrigerant vapor compression system 20 into the
non-economized mode, stage 2. In this embodiment, if these two
conditions are not met simultaneously, operation continues in the
first transition up stage 32, repeatedly executing a control loop,
for example at a loop rate of 1 Hertz, until these conditions are
simultaneously met.
[0050] When still additional refrigeration capacity is needed to
met the cooling demand, the controller 100 is configured of shift
operation of the refrigerant vapor compression system 20 from the
non-economized mode, stage 2, through the second transition up
stage 21 and into operation in one of the economized modes, stage 1
and stage 0. When operating in the second transition up stage 21,
the controller 100 again monitors two conditions. In stage 21, the
monitored conditions are the rate of change of the temperature
differential, TEMP_TREND, and the temperature differential
TSETPT-TCTRL. If the rate of change of the temperature differential
has been negative for a period of time greater than a present time
period t10 (t(TEMP_TREND)<0)>t10), the controller, at block
221, is configured to shift operation of the refrigerant vapor
compression system 20 into the maximum capacity economized mode,
stage 0, block 200. However, if the rate of change of the
temperature differential has not been negative for a period of time
greater than a present time period t10
(t(TEMP_TREND)<0)>t10), and the temperature differential
TSETPT-TCTRL is positive ((TSETPT-TCTRL)>0), the controller 100,
at block 221, is configured to shift operation of the refrigerant
vapor compression system 20 into the high capacity economized mode,
stage 1, at block 210, rather than into the maximum capacity
economized mode, stage 0.
[0051] In an embodiment, the controller 100 may be configured to
stage up within the economized mode from the high capacity
economized mode, stage 1 to the maximum capacity economized mode,
stage 0, when, during operation in stage 1, the following two
conditions are met simultaneously: the output frequency of the
variable speed drive 34 powering the compression device 30 has been
greater than a frequency level, FREQ2, for an elapsed time greater
than a preset period of time, t8 ((t(TEMP_TREND)<0)>t10));
and the TEMP_TREND has been negative for an elapsed time greater
than a preset period of time, t9, (t(TEMP_TREND)<0)>t9)). If
these two conditions are simultaneously met, the configured is
configured to shift operation of the refrigeration vapor
compression system 20 into the maximum capacity economized mode,
stage 0. In this embodiment, if these two conditions are not met
simultaneously, operation continues in the high capacity economized
mode, stage 1, until these conditions are simultaneously met.
[0052] In an embodiment, the controller 100 may select the various
stage transition frequencies FREQ1, FREQ2 and FREQ3 from a
pre-defined frequency map wherein the stage transition frequency is
defined as a function of selected operating parameters indicative
of cooling demand. For example, the stage transition frequencies
may be calculated as a function of the ambient air temperature,
T.sub.AMBAIR, the supply air temperature, T.sub.SBAIR, and the
return air temperature, T.sub.RBAIR.
[0053] The terminology used herein is for the purpose of
description, not limitation. Specific structural and functional
details disclosed herein are not to be interpreted as limiting, but
merely as basis for teaching one skilled in the art to employ the
present invention. Those skilled in the art will also recognize the
equivalents that may be substituted for elements described with
reference to the exemplary embodiments disclosed herein without
departing from the scope of the present invention.
[0054] While the present invention has been particularly shown and
described with reference to the exemplary embodiments as
illustrated in the drawing, it will be recognized by those skilled
in the art that various modifications may be made without departing
from the spirit and scope of the invention. For example, the
refrigerant vapor compression system 20 may further include an
intercooler heat exchanger (not shown) disposed in the primary
refrigerant circuit between the discharge outlet of the first
compression stage 30a and the inlet to the second compression stage
30b whereby the partially compressed (intermediate pressure)
refrigerant vapor (gas) passing from the discharge outlet of the
first compression stage to the inlet to the second compression
stage passes in heat exchange relationship with a flow of cooling
media, such as, for example, but not limited to the cooling air
flow generated by the gas cooler fan.
[0055] Therefore, it is intended that the present disclosure not be
limited to the particular embodiment(s) disclosed as, but that the
disclosure will include all embodiments falling within the scope of
the appended claims.
* * * * *