U.S. patent application number 13/501552 was filed with the patent office on 2012-08-09 for dehumidification control in refrigerant vapor compression systems.
This patent application is currently assigned to CARRIER CORPORATION. Invention is credited to Martin O. Johnson, Swee M. Ng, Raymond L. Senf, JR..
Application Number | 20120198867 13/501552 |
Document ID | / |
Family ID | 43876801 |
Filed Date | 2012-08-09 |
United States Patent
Application |
20120198867 |
Kind Code |
A1 |
Ng; Swee M. ; et
al. |
August 9, 2012 |
DEHUMIDIFICATION CONTROL IN REFRIGERANT VAPOR COMPRESSION
SYSTEMS
Abstract
A method and apparatus are disclosed for controlling
dehumidification of an airflow to be conditioned for supply to a
climate controlled space. The airflow to be conditioned is passed
over a plurality of refrigerant conveying conduits of an evaporator
of a refrigerant vapor compression thereby cooling the airflow. A
controller operates the refrigeration vapor compression system to
maintain the airflow at a set point air temperature indicative of a
desired temperature within the climate controlled space. The
controller also adjusts an evaporator expansion device for
controlling the flow of refrigerant through the evaporator so as to
reduce the temperature of the refrigerant within the refrigerant
conveying conduits of the evaporator whenever further
dehumidification of the air flow to be conditioned is desired.
Inventors: |
Ng; Swee M.; (Singapore,
SG) ; Johnson; Martin O.; (Jamesville, NY) ;
Senf, JR.; Raymond L.; (Central Square, NY) |
Assignee: |
CARRIER CORPORATION
Farmington
CT
|
Family ID: |
43876801 |
Appl. No.: |
13/501552 |
Filed: |
October 7, 2010 |
PCT Filed: |
October 7, 2010 |
PCT NO: |
PCT/US10/51802 |
371 Date: |
April 16, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61251462 |
Oct 14, 2009 |
|
|
|
Current U.S.
Class: |
62/93 ;
62/180 |
Current CPC
Class: |
F25B 2600/21 20130101;
F24F 2140/20 20180101; F24F 11/0008 20130101; F25B 41/062 20130101;
F25B 2600/2513 20130101; F25B 2700/1931 20130101; F24F 3/14
20130101; F24F 2140/12 20180101 |
Class at
Publication: |
62/93 ;
62/180 |
International
Class: |
F25D 17/04 20060101
F25D017/04; F25D 17/06 20060101 F25D017/06 |
Claims
1. A method for controlling dehumidification of an airflow to be
conditioned for supply to a climate controlled space, said method
comprising the steps of: passing the airflow to be conditioned over
a plurality of refrigerant conveying conduits of an evaporator of a
refrigerant vapor compression system thereby cooling the airflow;
operating the refrigeration vapor compression system to maintain
the airflow at a set point air temperature indicative of a desired
temperature within the climate controlled space; and adjusting the
evaporator expansion device so as to reduce the temperature of the
refrigerant within the refrigerant conveying conduits of the
evaporator whenever further dehumidification of the air flow to be
conditioned is desired.
2. A method as recited in claim 1 wherein the step of operating the
refrigerant vapor compression system to maintain said airflow at a
set point air temperature indicative of a desired temperature
within the climate controlled space includes the step of reheating
said airflow having passed over the plurality of refrigerant
conveying conduits of the evaporator prior to supplying said
airflow to the climate controlled space.
3. The method as recited in claim 1 wherein the set point air
temperature comprises a temperature in the range from a preselected
air temperature minus a preselected tolerance to the preselected
air temperature plus the preselected tolerance, inclusive of the
end point air temperatures.
4. The method as recited in claim 1 wherein the step of adjusting
the evaporator expansion device so as to reduce the temperature of
the refrigerant within the refrigerant conveying conduits of the
evaporator is implemented if a sensed relative humidity reflective
of the relative humidity within the climate controlled space
exceeds a desired set point relative humidity.
5. The method as recited in claim 4 wherein the set point relative
humidity comprises a relative humidity in the range from a
preselected relative humidity minus a preselected tolerance to the
preselected relative humidity plus the preselected tolerance,
inclusive of the end point relative humidities.
6. The method as recited in claim 1 wherein the expansion device
comprises an expansion device having a selectively flow area
opening therethrough.
7. The method as recited in claim 1 wherein the expansion device is
an electronic expansion valve (EXV).
8. The method as recited in claim 6 wherein the step of adjusting
the expansion device so as to reduce the temperature of the
refrigerant within the refrigerant conveying conduits of the
evaporator comprises reducing a refrigerant flow area through the
expansion device.
9. The method as recited in claim 6 wherein the step of adjusting
the expansion device so as to reduce the temperature of the
refrigerant within the refrigerant conveying conduits of the
evaporator comprises increasing a pressure drop in the refrigerant
passing through the expansion device.
10. The method as recited in claim 6 wherein the step of adjusting
the expansion device so as to reduce the temperature of the
refrigerant within the refrigerant conveying conduits of the
evaporator comprises decreasing a flow rate of refrigerant passing
through the expansion device.
11. The method as recited in claim 7 wherein the step of adjusting
the expansion device so as to reduce the temperature of the
refrigerant within the refrigerant conveying conduits of the
evaporator comprises setting a superheat setting of the expansion
device to a maximum superheat setting.
12. A method for controlling dehumidification of air within a
climate controlled space, said method comprising the steps of:
drawing a flow of air from the climate controlled space; passing
said airflow over a plurality of refrigerant conveying conduits of
an evaporator of a refrigerant vapor compression system having a
compression device, a refrigerant heat rejection heat exchanger, an
expansion device and the evaporator arranged in series refrigerant
flow relationship in accordance with a refrigerant vapor
compression cycle, thereby cooling said airflow; adjusting a
superheat level of the expansion device so as to further reduce the
temperature of the refrigerant within the refrigerant conveying
conduits of the evaporator thereby increasing condensation of
moisture from said airflow; and reheating said airflow having
passed over the plurality of refrigerant conveying conduits of the
evaporator prior to supplying said airflow to the climate
controlled space.
13. The method as recited in claim 12 wherein the step of adjusting
a superheat level the expansion device so as to further reduce the
temperature of the refrigerant within the refrigerant conveying
conduits of the evaporator comprises setting the superheat setting
of the expansion device to a maximum superheat setting.
14. The method as recited in claim 13 wherein the step of adjusting
a superheat level the expansion device so as to further reduce the
temperature of the refrigerant within the refrigerant conveying
conduits of the evaporator further comprises the steps of: sensing
a relative humidity of said airflow prior to passing said airflow
over a plurality of refrigerant conveying conduits of the
evaporator; comparing the sensed relative humidity of said airflow
to a set point relative humidity of a desired temperature to be
maintained within the climate controlled space; and maintaining the
superheat level of the expansion device at the maximum superheat
setting so long as the sensed relative humidity of said airflow
exceeds said set point relative humidity.
15. The method as recited in claim 14 wherein the set point
relative humidity comprises a relative humidity in the range from a
preselected relative humidity minus a preselected tolerance to the
preselected relative humidity plus the preselected tolerance,
inclusive of the end point relative humidities.
16. A method as recited in claim 12 wherein the step of reheating
said airflow having passed over a plurality of refrigerant
conveying conduits of the evaporator prior to supplying said
airflow to the climate controlled space includes the steps of:
sensing a temperature of said airflow having passed over a
plurality of refrigerant conveying conduits of the evaporator;
comparing the sensed temperature of said airflow to a set point
temperature indicative of a desired temperature to be maintained
within the climate controlled space; and reheating said airflow to
the set point temperature prior to supplying the airflow to the
climate controlled space.
17. The method as recited in claim 16 wherein the step of reheating
said airflow to the set point temperature comprises reheating said
airflow to a temperature in the range from a preselected air
temperature minus a preselected tolerance to the preselected air
temperature plus the preselected tolerance, inclusive of the end
point air temperatures of the range.
18. A method as recited in claim 12 further comprising the steps
of: providing a variable speed fan for drawing a flow of air from
the climate controlled space and passing said airflow over a
plurality of refrigerant conveying conduits of an evaporator; and
operating said variable speed fan in a low speed mode during
dehumidification of said airflow.
19. A method as recited in claim 12 further comprising the step of
bypassing a portion of said airflow around the evaporator whereby
the bypass portion does not pass in heat exchange relationship with
the refrigerant flowing through the refrigerant conveying conduits
of the evaporator.
20. A method as recited in claim 12 further comprising the steps
of: providing the evaporator with a first set and second set of
independent refrigerant conveying conduits; and selectively closing
one of the first set and the second set of refrigerant convening
conduits to enhance dehumidification.
21. An apparatus for supplying conditioned air to a climate
controlled space comprising: a refrigerant vapor compression system
including a compression device, a refrigerant heat rejection heat
exchanger, an electronic expansion valve, and an evaporator heat
exchanger arranged in series refrigerant flow relationship in
accordance with a refrigerant vapor compression cycle; an
evaporator fan for passing a flow of air to be conditioned over a
plurality of refrigerant conveying conduits of the evaporator heat
exchanger in heat exchange relationship with the refrigerant
thereby cooling the airflow flow of air to be conditioned; and a
controller operatively associated with the refrigerant vapor
compression system, said controller configured to operate the
refrigeration vapor compression system to maintain the airflow at a
set point air temperature indicative of a desired temperature
within the climate controlled space, and to adjust the electronic
expansion valve so as to reduce the temperature of the refrigerant
within the refrigerant conveying conduits of the evaporator
whenever further dehumidification of the air flow to be conditioned
is desired.
22. The apparatus as recited in claim 21 wherein the controller
adjusts a superheat setting of the electronic expansion valve to a
maximum superheat setting so as to reduce the temperature of the
refrigerant within the refrigerant conveying conduits of the
evaporator whenever further dehumidification of the air flow to be
conditioned is desired.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] Reference is made to and this application claims priority
from and the benefit of U.S. Provisional Application Ser. No.
61/251,462, filed Oct. 14, 2009, and entitled DEHUMIDIFICATION
CONTROL IN REFRIGERANT VAPOR COMPRESSION SYSTEMS, which application
is incorporated herein in its entirety by reference.
FIELD OF THE INVENTION
[0002] This invention relates generally to refrigerant vapor
compression systems, such as refrigeration systems and air
conditioning systems. More particularly, this invention relates to
controlling the dehumidification of air or an air-gas mixture being
conditioned for supply to a climate controlled space.
BACKGROUND OF THE INVENTION
[0003] Refrigerant vapor compression systems are well known in the
art and are commonly used for conditioning air to be supplied to a
climate controlled space. For example, refrigerant vapor
compression systems are used for conditioning air to be supplied to
a climate controlled comfort zone within a residence, office
building, hospital, school, restaurant or other facility.
Refrigerant vapor compression systems are also used in
refrigerating air supplied to display cases, merchandisers, freezer
cabinets, cold rooms or other perishable/frozen product storage
area in commercial establishments.
[0004] Refrigerant vapor compression systems are also commonly used
in transport refrigeration systems for refrigerating air or an
air-gas mixture supplied to a cargo storage space of a truck,
trailer, container or the like for transporting perishable/frozen
items by truck, rail, ship or intermodally. 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 and relative humidity within the cargo space at
which the cargo needs to be controlled can also vary over a wide
range depending on the nature of cargo to be preserved.
Consequently, in transport refrigeration systems, it is desirable
that the refrigerant vapor compression system be capable of
controlling relative humidity to a desired level for the perishable
product being shipped, typically in the range of from about 50%
relative humidity to about 75% RH, over a wide range of product
transport temperatures, for example from -5.degree. C. (23.degree.
F.) to 25.degree. C. (77.degree. C.). At the high end of
temperature range, the control of relative humidity to the desired
low levels becomes more challenging.
SUMMARY OF THE INVENTION
[0005] A method is provided for controlling dehumidification of an
airflow to be conditioned for supply to a climate controlled space.
The method includes the steps of: passing the airflow to be
conditioned over a plurality of refrigerant conveying conduits of
an evaporator of a refrigerant vapor compression thereby cooling
the airflow; operating the refrigeration vapor compression system
to maintain the airflow at a set point air temperature indicative
of a desired temperature within the climate controlled space; and
adjusting the evaporator expansion device so as to reduce the
temperature of the refrigerant within the refrigerant conveying
conduits of the evaporator whenever further dehumidification of the
air flow to be conditioned is desired. The step of operating the
refrigerant vapor compression system to maintain said airflow at a
set point air temperature indicative of a desired temperature
within the climate controlled space may include the step of
reheating said airflow having passed over the plurality of
refrigerant conveying conduits of the evaporator prior to supplying
said airflow to the climate controlled space. The step of adjusting
the evaporator expansion device so as to reduce the temperature of
the refrigerant within the refrigerant conveying conduits of the
evaporator may be implemented if a sensed relative humidity
reflective of the relative humidity within the climate controlled
space exceeds a desired set point relative humidity. The set point
relative humidity may lie in the range from a preselected relative
humidity minus a preselected tolerance to the preselected relative
humidity plus the preselected tolerance, inclusive of the end point
relative humidities.
[0006] The step of adjusting the evaporator expansion device so as
to reduce the temperature of the refrigerant within the refrigerant
conveying conduits of the evaporator may include reducing a
refrigerant flow area through the expansion device.
[0007] The step of adjusting the expansion device so as to reduce
the temperature of the refrigerant within the refrigerant conveying
conduits of the evaporator may include increasing a pressure drop
in the refrigerant passing through the expansion device.
[0008] The step of adjusting the expansion device so as to reduce
the temperature of the refrigerant within the refrigerant conveying
conduits of the evaporator may include decreasing a flow rate of
refrigerant passing through the expansion.
[0009] The step of adjusting the expansion device so as to reduce
the temperature of the refrigerant within the refrigerant conveying
conduits of the evaporator may include setting a superheat setting
of the expansion device to a maximum superheat setting.
[0010] An apparatus for supplying conditioned air to a climate
controlled space includes a refrigerant vapor compression system
including a compression device, a refrigerant heat rejection heat
exchanger, an electronic expansion valve, and an evaporator heat
exchanger arranged in series refrigerant flow relationship in
accordance with a refrigerant vapor compression cycle; an
evaporator fan for passing a flow of air to be conditioned over a
plurality of refrigerant conveying conduits of the evaporator heat
exchanger in heat exchange relationship with the refrigerant
thereby cooling the airflow flow of air to be conditioned; and a
controller operatively associated with the refrigerant vapor
compression system. The controller is configured to operate the
refrigeration vapor compression system to maintain the airflow at a
set point air temperature indicative of a desired temperature
within the climate controlled space, and to adjust the electronic
expansion valve so as to reduce the temperature of the refrigerant
within the refrigerant conveying conduits of the evaporator
whenever further dehumidification of the air flow to be conditioned
is desired. In an embodiment, the controller adjusts a superheat
setting of the electronic expansion valve to a maximum superheat
setting so as to reduce the temperature of the refrigerant within
the refrigerant conveying conduits of the evaporator.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] 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:
[0012] FIG. 1 is a schematic diagram illustrating an exemplary
embodiment of a refrigerant vapor compression system operable in a
low humidity control mode in accordance with the method of the
invention;
[0013] FIG. 2 is a process flow chart illustrating an exemplary
embodiment of a method of controlling dehumidification in
accordance with the invention;
[0014] FIG. 3 is a schematic diagram illustrating an embodiment of
an air flow bypass around the evaporator of the refrigerant vapor
compression system of FIG. 1; and
[0015] FIG. 4 is a schematic diagram of an embodiment of the
evaporator of FIG. 1 wherein the evaporator heat exchanger includes
a pair of independent refrigerant flow circuits.
DETAILED DESCRIPTION OF THE INVENTION
[0016] Referring initially to FIG. 1 of the drawing, there is
depicted an exemplary embodiment of a refrigerant vapor compression
system 100 for supplying conditioned air to a climate controlled
space 110. Although the invention will be described herein with
reference to the climate controlled space being the cargo storage
space, commonly referred to as the "cargo box", of a truck,
trailer, container or the like for transporting perishable/frozen
items by truck, rail, ship or intermodally container, it is to be
understood that the invention is also applicable to and be used in
connection with dehumidification control in connection with
refrigerant vapor compression systems on air conditioning systems
used for conditioning air to be supplied to a climate controlled
comfort zone within a residence, office building, hospital, school,
restaurant or other facility, and with refrigerant vapor
compression systems on commercial refrigeration systems for
refrigerating air within the product storage spaces of display
cases, merchandisers, freezer cabinets, cold rooms or other
perishable/frozen product storage areas in commercial
establishments.
[0017] Additionally, as used throughout this application, the terms
"air" and "airflow" are intended to include and refer to not only
air, but also air-gas mixtures. For example, without limiting the
scope of air-gas mixture, the terms "air" and "airflow" includes
air-gas mixtures containing air and one or more of carbon dioxide
gas, nitrogen gas, and ethylene gas. Such air-gas mixtures may be
used in connection with establishing a desired controlled
atmosphere within the cargo storage space of a container or the
like for transporting certain perishable/frozen items by truck,
rail, ship or intermodally container.
[0018] The refrigerant vapor compression system 100 includes a
compressor 20, a condenser 30 including a condenser heat exchanger
32 and associated condenser fan(s) 34, an evaporator 40 including
an evaporator heat exchanger 42 and associated evaporator fan(s)
44, and an expansion device 50 associated with the evaporator 40,
connected in a refrigeration cycle in a conventional manner by
refrigerant lines 2, 4 and 6, which collectively define a
refrigerant flow circuit. As in conventional practice, refrigerant
line 2 connects the discharge outlet of the compressor 20 in
refrigerant flow communication with the inlet to the condenser heat
exchanger 32, refrigerant line 4 connects the outlet of the
condenser heat exchanger 32 in refrigerant flow communication with
the inlet to the evaporator heat exchanger 42, and refrigerant line
6 connects the outlet of the evaporator heat exchanger 42 in
refrigerant flow communication with the suction inlet of the
compressor 20, thereby completing the primary refrigerant flow
circuit. The expansion device 50 is disposed in refrigerant line 4
in operative association with and upstream with respect to
refrigerant flow of the inlet to the evaporator heat exchanger 42.
The expansion device 50 comprises an expansion device having a
selectively variable flow area therethrough, such as an electronic
expansion valve (EXV), whereby the amount of flow of refrigerant
through the evaporator heat exchanger 42 may be controlled by
selectively varying the flow area through the expansion device
50.
[0019] The compressor 20 is depicted in the exemplary embodiment as
a scroll compressor. However, the compressor 20 may be a
reciprocating compressor or other type of compressor, the
particular type of compressor used not being germane to or limiting
of the invention. In the depicted embodiment, each of the condenser
heat exchanger 32 and the evaporator heat exchanger 42 comprises
one or more refrigerant-conveying heat exchange tube coils formed
of conventional round tubes. However, it is to be understood that
either or both of the condenser heat exchanger 32 and the
evaporator heat exchanger 42 may comprise other forms of heat
exchangers such as for example a tube bank heat exchanger formed of
one or more banks of flat, multiple flow channel,
refrigerant-conveying tubes extending between an inlet header and
an outlet header. The particular type of heat exchanger used for
either of the condenser heat exchanger 32 or the evaporator heat
exchanger 42 is not germane to or limiting of the invention.
[0020] As in conventional practice, the refrigerant vapor
compression system 100 may also include a compressor unloader valve
25 disposed in a compressor unload refrigerant line 22 that
connects the discharge chamber of the compressor 20 in refrigerant
flow communication with the refrigerant line 6 between the outlet
of the evaporator heat exchanger 42 and the suction inlet to the
compressor 20. In addition, the refrigerant vapor compression
system 100 may include other components such as, but not limited
to, a filter/drier 16 and a receiver 18 disposed in refrigerant
line 4 upstream with respect to refrigerant flow of the economizer
60.
[0021] The refrigerant vapor compression system 100 may, if
desired, as in the embodiment depicted in FIG. 1, include an
economizer circuit having an economizer 60, such as a
refrigerant-to-refrigerant heat exchanger, an economizer circuit
flow control valve 63, such as for example an open/closed solenoid
valve, and an economizer expansion device 65, such as a
thermostatic expansion valve (TXV) with associated refrigerant
temperature sensing blub 67, as depicted in FIG. 1, or an
electronic expansion valve (EXV) or a fixed orifice device. The
refrigerant-to-refrigerant heat exchanger economizer 60 includes a
first refrigerant pass 62 and a second refrigerant pass 64 arranged
in heat transfer relationship. The first refrigerant pass 62 is
interdisposed in refrigerant line 4 and forms part of the primary
refrigerant circuit. The second refrigerant pass 64 is
interdisposed in the economizer circuit refrigerant line 8 and
forms part of an economizer circuit. The economizer circuit
refrigerant line 8 taps into refrigerant line 4 of the primary
refrigerant circuit upstream with respect to refrigerant flow of
the economizer 60 and connects in refrigerant flow communication to
an intermediate pressure stage of the compression process within
the compressor 20. In the depicted embodiment, wherein the
compressor 20 is a scroll compressor, the economizer line 8
connects to an intermediate pressure port on the scroll compressor
20 that opens into the compression chamber within the scroll
compressor 20 whereby refrigerant passing through the economizer
circuit refrigerant line 8 is injected directly into an
intermediate compression stage of the compression process.
[0022] When the economizer circuit flow control valve 63 is open,
refrigerant having traversed the condenser heat exchanger 32 passes
from refrigerant line 4 into refrigerant line 8, thence through the
economizer expansion device 65 wherein the refrigerant is expand to
a lower pressure, lower temperature before passing through the
second pass 64 of the refrigerant-to-refrigerant economizer heat
exchanger 60. In traversing the second pass 64, the lower
temperature refrigerant passes in heat exchange relationship with
the higher temperature refrigerant passing through the first pass
62, whereby the refrigerant passing through the first pass 62 is
cooled before passing on through refrigerant line 4 to the
evaporator expansion device 50 and the refrigerant passing through
the second pass 64 is heated and typically at least partially, if
not fully, evaporated, before continuing through the economizer
circuit refrigerant line 8 to be injected into an intermediate
stage of the compression process. The first refrigerant pass 62 and
the second refrigerant pass 64 of the refrigerant-to-refrigerant
heat exchanger economizer 60 may be arranged in a counter flow heat
exchange relationship, as depicted in FIG. 1, or in a parallel flow
heat exchange relationship, as desired. The
refrigerant-to-refrigerant heat exchanger 60 may be, for example
but not limited to, a brazed plate heat exchanger, a tube-in-tube
heat exchanger, a tube-on-tube heat exchanger or a shell-and-tube
heat exchanger.
[0023] The refrigeration unit includes a controller 150 that is
operatively associated with and forms part of the refrigerant vapor
compression system 100. As shown in the depicted embodiment, the
controller 150 may include a microprocessor 154 and its associated
memory 156, as well as an input/output interface 152 with an
associated analog-to-digital converter 158. The memory 156 of
controller 150 may contain operator or owner preselected, desired
values for various operating parameters within the system,
including, but not limited to temperature set points for various
parameters within the system 100 or the climate controlled space
110, pressure limits, current limits, and any variety of other
desired operating parameter set points or limits associated with
the refrigeration unit or its operation. The input/output interface
152 includes various drive circuits or field effect transistors
("FETs") and relays which, under the command of the controller 150,
communicate with and/or manipulate various devices in the
refrigerant vapor compression system 100, including without
limitation: drives motors (not shown) operatively associated with
the compressor 20, the condenser fan(s) 34 associated with the
condenser heat exchanger coil 32, and the evaporator fan(s) 44
associated with the evaporator heat exchanger coil 42; various
valves, such as the compressor unloader valve 25, the economizer
expansion device 50, and the economizer solenoid valve 63; and
various pressure sensors, for example pressure transducers, and
temperature sensors such as a compressor discharge pressure
transducer 101, a compressor suction pressure transducer 103, an
evaporator pressure transducer 105, a compressor discharge
temperature sensor 107, an evaporator outlet refrigerant
temperature sensor 109, a box air temperature sensor 113, a
humidity sensor 115, an ambient air sensor 117, a return air sensor
119 and such other sensors as desired.
[0024] The controller 150 is configured to operate the refrigerant
vapor compression system 100 to maintain a temperature and humidity
controlled environment within the climate controlled space 110,
which in the embodiment described herein constitutes the cargo box
of the truck, trailer or container wherein the product is stored.
As in conventional practice, the controller 150 maintains a
predetermined box temperature, T.sub.SPACE, by selectively
controlling the operation of the compressor 20, the condenser
fan(s) 34 associated with the condenser heat exchanger coil 32, and
the evaporator fan(s) 44 associated with the evaporator heat
exchanger coil 42, as well as selectively opening or closing the
compressor unloader valve 25 and the economizer solenoid valve
63.
[0025] In operation, when the temperature of the product within the
space 110 is above a predetermined temperature, for example when
the product has been loaded into the box in a "hot" condition, such
as produce loaded directly from the field, the controller 150 will
operate the refrigerant vapor compression system in a pull-down
mode so as to provide maximum or near maximum cooling capacity to
rapidly pull-down the temperature, T.sub.SPACE, of
climate-controlled space 110 to a temperature within a preset
tolerance, .DELTA.T, typically about 0.25.degree. C. (0.45.degree.
C.), of a desired set point temperature, T.sub.SP, that represents
the desired temperature at which the product loaded into the box is
to be transported. Typically, in the pull-down mode, the controller
150 will energize the economizer solenoid valve 63 to position the
solenoid valve 63 in its open position, thereby permitting
refrigerant to flow from refrigerant line 4 through refrigerant
line 8 pass through the second pass 64 of the economizer 60 so as
to increase the cooling capacity of the system. The controller 150
monitors the temperature, T.sub.SPACE, of climate-controlled space
110 by means of one or more temperature sensors 113 disposed within
the climate controlled space 110 at one or more locations. The
temperature sensors 113 may be thermostat type sensors, thermister
type sensors or other types of temperature sensing devices.
[0026] When the sensed temperature indicates that the temperature,
T.sub.SPACE, within the enclosed space 110 has been pulled down to
the set point temperature, T.sub.SP, the controller 150 will switch
operation of the refrigerant vapor compression system 100 from the
"pull-down" mode" into an "equilibrium" mode to maintain the
temperature, T.sub.SPACE, within the enclosed space 110 stable at
the set point temperature, T.sub.SP, that is at a temperature
within a temperature range from T.sub.SP-.DELTA.T to
T.sub.SP+.DELTA.T, inclusive of the end points. In this mode, the
controller 150 maintains the predetermined box temperature,
T.sub.SPACE, by selectively controlling the operation of the
compressor 20, the condenser fan(s) 34 associated with the
condenser heat exchanger coil 32, and the evaporator fan(s) 44
associated with the evaporator heat exchanger coil 42, as well as
selectively opening or closing the compressor unloader valve 25 and
the economizer solenoid valve 63, in various schemes.
[0027] When the refrigerant vapor compression system is operating
in the "equilibrium" mode with the temperature, T.sub.SPACE, lying
within the equilibrium range of equal to or greater than
T.sub.SP-.DELTA.T and equal to or less than T.sub.SP+.DELTA.T, the
controller 150 will enter a humidity control mode in which the
controller 150 monitors the relative humidity, RH.sub.SPACE, by
means of a humidity sensor 115, such as for example a humidistat or
other type of humidity sensing device, disposed to sense a relative
humidity reflective of the relative humidity with the climate
controlled space 110. The humidistat 115 may be disposed in the
flow of supply air being delivered to the climate controlled space
110 downstream of the airside outlet of the evaporator heat
exchanger 42 as depicted in FIG. 1, or one or more humidity sensors
may be disposed at selected locations within the climate-controlled
space 110 and connected in operative communication with the
controller 150 to transmit a signal indicative of the relative
humidity in the vicinity of the humidity sensor. The controller 150
will compare the sensed relative humidity, RH.sub.SPACE, to a
desired set point relative humidity, RH.sub.SP. The controller 150
may be programmed to make the relative humidity comparison
periodically at a specified time interval. The controller 150
selects the desired set point relatively humidity, RH.sub.SP, based
upon the set point temperature, T.sub.SP. For example, the
controller 150 may determine the desired set point relative
humidity using an algorithm preprogrammed into the microprocessor
of the controller 150 that calculates the optimal relative humidity
at the set point temperature for the particular product or class of
the particular product loaded into the space 110. Alternatively,
the controller 150 may select the desired relative humidity set
point from one or more maps of relative humidity versus temperature
preprogrammed into the memory bank of the controller 150.
[0028] Referring now to FIG. 2, after the controller 150 initiates
operation of the refrigeration unit in the humidity control mode at
step 200, the controller 150 will, at step 205, compare the sensed
relative humidity, RH.sub.SPACE, to the desired relative humidity
set point RH.sub.SP, to determine whether the sensed relative
humidity, RH.sub.SPACER, is greater than the desired relative
humidity set point, RH.sub.SP. If not, the controller 150 will
check, at step 235, whether the system 100 is to remain in the
humidity control mode. If the controller 150 verifies that the
system is to remain in the humidity control mode, the controller
will periodically check, at step 225, so long as the system 100
remains operating in the humidity control mode, whether the sensed
relative humidity, RH.sub.SPACE, is greater than or equal to the
desired relative humidity set point plus a relative humidity
tolerance, that is RH.sub.SP+.DELTA.RH, where .DELTA.RH is the
relative humidity tolerance, a preselected value typically, for
example but not limited to, of about 2 degrees. If the controller
150 finds, at step 235, that the system is to no longer remain in
the humidity control mode, the controller will simply exit the
humidity control mode at step 240.
[0029] However, if, at step 205, the sensed relative humidity
within the climate controlled space 110 is indeed greater than the
desired relative humidity set point, the controller 150 will enter
a dehumidification mode. Upon entering the dehumidification mode,
at step 210, the controller 150 will generate and transmit a
command signal to the expansion device 50, disposed in refrigerant
line 4 downstream with respect to refrigerant flow of the
economizer 60 and upstream with respect to the refrigerant inlet to
the evaporator heat exchanger 42 of the evaporator 40, to raise the
superheat level of the refrigerant vapor leaving the evaporator
heat exchanger 42 by causing the refrigerant flow area through the
electric expansion valve 50 to further close. For example, the
controller 150 will, at step 210, transit a command signal to the
expansion device 50 setting the superheat level to its maximum
value, thereby closing the flow area through the expansion device
50 to a minimum open area. Reducing the flow area opening within
the expansion device 50 causes a further restricting of the flow of
refrigerant through the expansion device 50 which results in a
lower refrigerant flow and a lower refrigerant pressure within the
evaporator heat exchanger 42.
[0030] As a result of the drop in refrigerant pressure, the
temperature of the refrigerant passing through the evaporator heat
exchanger 42 also drops to a lower temperature. As a result of the
lower temperature of the refrigerant passing through the tubes of
the evaporator heat exchanger 42, the surface temperature of the
outer surface of the tubes of the evaporator heat exchanger 42 also
decreases. Because the outer surface temperature of the refrigerant
conveying tubes of the evaporator heat exchanger 42 is now colder
and the temperature difference between the airflow circulating from
the climate controlled space 110, which remains at the set point
temperature, T.sub.SP, is now greater, more moisture contained in
that airflow will condense out of the airflow onto the outer
surface of the tubes as the airflow passes over the outer surface
of the tubes of the evaporator heat exchanger 42, thereby removing
moisture from the airflow. As a result, the supply airflow, that is
the airflow being supplied back to the climate controlled space 110
after having traversed the evaporator 40, will have a lower
moisture content than the return airflow, that is the airflow drawn
from the climate controlled space to be passed through the
evaporator 40.
[0031] Additionally, the supply airflow leaving the airside outlet
of the evaporator 40 will have a lower temperature due to the
increased heat transfer from the air flow to the colder refrigerant
as the air flow traverses the evaporator heat exchanger 42 in heat
exchange relationship with the colder refrigerant. Therefore, at
step 210, the controller 150 will also initiate reheat of the
supply air flow leaving the air side outlet of the evaporator 40 by
activating an air reheater 70 operatively associated with the
evaporator 40. The air reheater 70 heats the supply airflow having
traversed the evaporator heat exchanger 42 prior to delivery back
to the climate controlled space 110. The air reheater 70 may
comprise an electric heater, a hot refrigerant air heater coil, or
other type of means for adding sensible heat to the air flow. The
air reheater 70 may be disposed at the air side outlet of the
evaporator 40 downstream of the evaporator heat exchanger 42
whereby the supply air flow having traversed the evaporator heat
exchanger 42 is heated prior to entering the climate controlled
space 110.
[0032] Without reheating the airflow during the dehumidification
mode, the amount of dehumidification attainable is limited in that
temperature of the airflow traversing the evaporator heat exchanger
42 could not be reduced lower than the set point temperature.
However, with reheating available, the controller 150 can allow the
temperature of the airflow having traversed the evaporator heat
exchanger 42 to drop below the set point temperature so as to
achieve increased condensation of moisture from the airflow since
the reheater 70 may be operated to raise the temperature of the
airflow up to the set point temperature prior to the airflow
passing back into the climate controlled space. Thus, the
controller 150 may maintain the superheat setting of the electronic
expansion valve 50 at the maximum superheat setting without
worrying that the temperature of the airflow has dropped below the
set point temperature. Rather, the controller 150 may maintain the
superheat setting of the electronic expansion valve 50 at the
maximum superheat setting so long as the temperature of the
refrigerant passing through the evaporator heat exchanger 42 drops
so low that the temperature of the surface of the evaporator heat
exchanger 42 exposed to condensation of moisture from the airflow
is so low as to result in the rapid build-up of frost on the
exposed surface of the evaporator heat exchanger 42. Therefore,
lower levels of relative humidity may be attained when desired,
such as when the climate controlled space is the cargo box of a
truck, trailer or container wherein a cargo such as flower bulbs is
being transported.
[0033] Over a period of time of operation in this enhanced
dehumidification mode, the relative humidity will necessarily drop
to the set point relative humidity, RH.sub.SP. Therefore, at
selected time intervals during operation in the enhanced
dehumidification mode, the controller 150 will, at step 215,
repeatedly compare the sensed relative humidity, RH.sub.SPACE, to
the relative humidity set point, RH.sub.SP, minus the preselected
relative humidity tolerance, .DELTA.RH, to determine whether the
sensed relative humidity, RH.sub.SPACE, has dropped to a level that
is equal to or less than the desired relative humidity set point
minus the relative humidity tolerance, that is,
RH.sub.SP-.DELTA.RH. If not, the controller 150 continues operation
in the dehumidification mode with the reheater 70 activated and the
superheat setting of the expansion device 50 set at maximum
superheat.
[0034] If the sensed relative humidity, RH.sub.SPACE, has indeed
dropped to a level that is equal to or less than the desired
relative humidity set point minus the relative humidity tolerance,
that is, RH.sub.SP-.DELTA.RH, the controller 150, at step 220,
terminates reheat by deactivating the reheater 70 and resetting the
superheat setting of the electronic expansion valve 50 to a
preprogrammed default value, and thence proceeds to step 225. At
this point, the controller 150 periodically checks, so long as the
system remains operating in the humidity control mode, whether the
sensed relative humidity, RH.sub.SPACE, is greater than or equal to
the desired relative humidity set point plus a relative humidity
tolerance, that is RH.sub.SP+.DELTA.RH. If no, and the controller
150 checks, at step 235, whether the system remains operating in a
humidity control mode. If no, the controller 150 will simply exit
the dehumidification mode at step 240. If yes, the controller 150
will continue operation in the dehumidification mode by repeating
steps 220 and 235, in succession, and return to the
dehumidification cycle at step 210 if and when the sensed relative
humidity, RH.sub.SPACE, again exceeds the desired relative humidity
set point plus a relative humidity tolerance, that is
RH.sub.SP+.DELTA.RH.
[0035] As noted previously, if the temperature of the refrigerant
passing through the evaporator heat exchanger 42 drops too low, the
temperature of the surface of the evaporator heat exchanger coil
exposed to condensation of moisture from the airflow, the rapid
build-up of frost could occur. Therefore, during operation in the
dehumidification mode, the controller 150 will, at regular
intervals, check the evaporator outlet pressure sensed by the
evaporator outlet pressure sensor 105 against a low limit pressure
value to ensure that the temperature of the evaporator heat
exchanger 42 does not drop so low as to cause rapid build-up of
frost on the surface of the evaporator heat exchanger 42 exposed to
the airflow. In the event that the sensed evaporator outlet
temperature does drop below the lower limit pressure value, the
controller 150 will gradually reduce the superheat setting of the
evaporator expansion valve 50 from its maximum superheat setting
value until the controller 150 determines that the sensed
evaporator outlet pressure again rises above the lower limit
pressure value. The controller 150 will maintain the superheat
setting of the electronic expansion valve 50 at this reduced
superheat setting valve for the remainder of the dehumidification
mode so long as the sensed evaporator outlet pressure does not
again drop below the lower limit pressure value.
[0036] In an embodiment, the evaporator fans 44 may be variable
speed fans having a variable speed motor/drive that is capable of
operating at a lower speed setting and at a higher speed setting.
The controller 150 controls the supply of electrical power to the
fan motor/drive (not shown) associated with each evaporator fan 44
in a conventional manner, for example by controlling the frequency
or the current output of the electrical power supply, to
selectively operate the evaporator fan or fans 44 at either the
lower speed setting or the higher speed setting. At the lower speed
setting, the evaporator fan or fans 44 draw airflow from the
climate controlled space 110 and pass the airflow over the
refrigerant conveying conduits 45 of the evaporator heat exchanger
44 at a lower volume flow rate lower than the volume flow rate
produced when the evaporator fan or fans 44 are operating at the
higher speed setting. To further enhance the dehumidification
process, the controller 150 may selectively operate the variable
speed evaporator fan(s) 44 at the lower speed setting to reduce the
volume flow rate of the airflow passed through the evaporator 40 in
heat exchange relationship with the refrigerant passing through the
refrigerant conveying conduits 45 of the evaporator heat exchanger
42. By operating at a lower volume flow rate of airflow through the
evaporator 40, the range of dehumidification will potentially be
extended as the airflow will take more time to traverse the
evaporator heat exchanger 42, allowing more moisture to condense.
Similarly, the capability to reheat the airflow by means of the
reheater 70 is also potentially enhanced as the airflow will take
more time to traverse the reheater 70.
[0037] Referring now to FIG. 3, the dehumidification of the airflow
passing through the evaporator heat exchanger 42 may also be
enhanced by bypassing a portion of the air drawn by the evaporator
fan(s) 44 from the climate controlled space 110 through bypass duct
46 around the evaporator 40 and reintroducing that bypassed airflow
into the airflow having passed through the evaporator 40 downstream
of the airside outlet of the evaporator heat exchanger 42. A bypass
damper 48, whose positioning may be selectively controlled by the
controller 150, may be provided in the bypass duct 46 to control
the portion of airflow bypassing the evaporator 40. Due to the
bypassing of a portion of the airflow, a lower volume of airflow
passes at a lower flow rate through the evaporator 40 in heat
exchange relationship with the refrigerant passing through the
refrigerant conveying conduits 45 of the evaporator heat exchanger
42, therefore more moisture can condense from that airflow and the
dehumidification of that airflow is increased. Additionally,
because the bypassed airflow does not pass in heat exchange
relationship with the refrigerant, the bypassed airflow remains at
the return air temperature. Therefore, when reintroduced into the
airflow having traversed the evaporator heat exchanger 42 assists
in reheating the airflow having traversed the evaporator heat
exchanger 42.
[0038] Referring now to FIG. 4, the dehumidification of the airflow
passing through the evaporator heat exchanger 42 may also be
enhanced by segmenting the evaporator heat exchanger into two (or
more, if desired) separate sets 45A and 45B of refrigerant
conveying conduits and selectively controlling the flow of
refrigerant through the sets 45A and 45B of refrigerant conveying
conduits independently of each other. As depicted in FIG. 4, the
refrigerant flow having traversed the evaporator expansion device
50 and passing through refrigerant line 4 is split into two
portions, one of which flows via branch line 4A through the first
set 45A of refrigerant conveying conduits and the other of which
flows via branch line 4B through the second set 45B of refrigerant
conveying conduits. Branch lines 4A and 4B rejoin at the
refrigerant side outlet of the evaporator 40 and the refrigerant
passes into refrigerant line 6. Flow control valves 47A and 47B,
which may be simple two position, open/closed solenoid valves, are
interdisposed in branch lines 4A and 4B, respectively, upward of
the sets 45A and 45B, respectively, of refrigerant conveying
conduits. Each of the flow control valves 47A and 47B are
operatively associated with and independently selectively opened or
closed by the controller 150. When operating in a dehumidification
mode, to enhance dehumidification, the controller 150 may
selectively close one of the flow control valves 47A, 47B, thereby
causing all of the refrigerant flow having traversed the evaporator
expansion device 50 to flow through only one of the sets 45A or 45B
of the refrigerant conveying conduits. In effect, the closing to
refrigerant flow of one of the sets of refrigerant conveying
conduits reduces the effective heat transfer surface of the
evaporator heat exchanger which results in reducing the heat
transfer capability of the evaporator heat exchanger. This in turn
results in operation of the heat exchanger at a lower refrigerant
saturation temperature and thereby increases condensation of
moisture from the airflow passing through the evaporator, thus
increasing dehumidification.
[0039] 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. 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. 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.
[0040] 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.
* * * * *