U.S. patent application number 12/596885 was filed with the patent office on 2010-06-03 for transcritical refrigerant vapor compression system with charge management.
This patent application is currently assigned to CARRIER CORPORATION. Invention is credited to Yu H. Chen, Biswajit Mitra.
Application Number | 20100132399 12/596885 |
Document ID | / |
Family ID | 39875765 |
Filed Date | 2010-06-03 |
United States Patent
Application |
20100132399 |
Kind Code |
A1 |
Mitra; Biswajit ; et
al. |
June 3, 2010 |
TRANSCRITICAL REFRIGERANT VAPOR COMPRESSION SYSTEM WITH CHARGE
MANAGEMENT
Abstract
A refrigerant vapor compression system includes a
refrigerant-to-refrigerant heat exchanger economizer and a flash
tank disposed in series refrigerant flow relationship in the
refrigerant circuit intermediate a refrigerant heat rejection heat
exchanger and a refrigerant heat absorption heat exchanger. A
primary expansion valve is interdisposed in the refrigerant circuit
upstream of the refrigerant heat absorption heat exchanger and a
secondary expansion valve is interdisposed in the refrigerant
circuit upstream of the flash tank. The flash tank functions as a
refrigerant charge storage reservoir wherein refrigerant expanded
from a supercritical pressure to subcritical pressure separates
into liquid and vapor phases. A refrigerant vapor bypass line is
provided to return refrigerant vapor from the flash tank to the
refrigerant circuit downstream of the refrigerant heat absorption
heat exchanger. The primary expansion valve and a flow control
valve interdisposed in the refrigerant vapor bypass provide
refrigerant charge management.
Inventors: |
Mitra; Biswajit; (Charlotte,
NC) ; Chen; Yu H.; (Manlius, NY) |
Correspondence
Address: |
MARJAMA MULDOON BLASIAK & SULLIVAN LLP
250 SOUTH CLINTON STREET, SUITE 300
SYRACUSE
NY
13202
US
|
Assignee: |
CARRIER CORPORATION
Farmington
CT
|
Family ID: |
39875765 |
Appl. No.: |
12/596885 |
Filed: |
April 24, 2007 |
PCT Filed: |
April 24, 2007 |
PCT NO: |
PCT/US07/10066 |
371 Date: |
October 21, 2009 |
Current U.S.
Class: |
62/498 ; 62/512;
62/513 |
Current CPC
Class: |
F25B 2400/23 20130101;
F25B 2700/21151 20130101; F25B 2400/13 20130101; F25B 2309/061
20130101; F25B 2700/1931 20130101; F25B 2700/1933 20130101; F25B
1/10 20130101; F25B 2600/2509 20130101; F25B 2700/19 20130101; B60H
1/3228 20190501; F25B 2700/21152 20130101; F25B 9/008 20130101 |
Class at
Publication: |
62/498 ; 62/513;
62/512 |
International
Class: |
F25B 1/00 20060101
F25B001/00; F25B 41/00 20060101 F25B041/00; F25B 43/00 20060101
F25B043/00 |
Claims
1. A refrigerant vapor compression system comprising: a primary
refrigerant circuit including a refrigerant compression device, a
refrigerant cooling heat exchanger for passing refrigerant received
from said compression device at a high pressure in heat exchange
relationship with a cooling medium, a refrigerant heating heat
exchanger for passing refrigerant at a low pressure refrigerant in
heat exchange relationship with a heating medium, and a primary
expansion device interdisposed in the primary refrigerant circuit
downstream of said refrigerant cooling heat exchanger and upstream
of said refrigerant heating heat exchanger; a
refrigerant-to-refrigerant heat exchanger economizer having a first
refrigerant pass disposed in the primary refrigerant circuit
downstream of said refrigerant cooling heat exchanger and upstream
of said primary expansion device and a second bypass disposed in an
economizer circuit refrigerant line; a flash tank disposed in the
primary refrigerant circuit downstream of the first refrigerant
pass of said refrigerant-to-refrigerant exchanger and upstream of
said primary expansion device, said flash tank defining a
separation chamber wherein refrigerant in a liquid state collects
in a lower portion of said separation chamber and refrigerant in a
vapor state in a portion of said separation chamber above the
liquid refrigerant; a secondary expansion device disposed in the
primary refrigerant circuit in operative association with and
upstream with of said flash tank; an refrigerant vapor bypass line
establishing refrigerant flow communication between an upper
portion of said separation chamber of said flask tank and a suction
pressure portion of said primary refrigerant circuit downstream of
said refrigerant heat absorption heat exchanger; and a bypass flow
control valve interdisposed in said evaporator bypass line, said
bypass flow control valve having a first open position whereat
refrigerant vapor may pass through said evaporator bypass line and
a second closed position whereat refrigerant vapor is blocked from
passing through said evaporator bypass line.
2. A refrigerant vapor compression system as recited in claim 1
wherein said flow control valve comprises a solenoid valve having a
first open position and a second closed position.
3. A refrigerant vapor compression system as recited in claim 1
wherein said flow control valve comprises a pulse width modulated
solenoid valve.
4. A refrigerant vapor compression system as recited in claim 1
wherein said flow control valve comprises an electronic expansion
valve.
5. A refrigerant vapor compression system as recited in claim 1
wherein said primary expansion device comprises an electronic
expansion valve.
6. A refrigerant vapor compression system as recited in claim 1
wherein said primary expansion device comprises a thermostatic
expansion valve.
7. A refrigerant vapor compression system as recited in claim 1
wherein said secondary expansion device comprises an electronic
expansion valve.
8. A refrigerant vapor compression system as recited in claim 1
wherein said secondary expansion device comprises a fixed orifice
expansion device.
9. A refrigerant vapor compression system as recited in claim 1
wherein said economizer circuit refrigerant line extends in
refrigerant flow communication from said primary refrigerant
circuit to an intermediate pressure stage of said compression
device.
10. A refrigerant vapor compression system as recited in claim 9
further comprising an economizer circuit expansion device
interdisposed in said economizer circuit refrigerant line upstream
with respect to refrigerant flow of the second refrigerant pass of
said refrigerant-to-refrigerant heat exchanger economizer.
11. A refrigerant vapor compression system as recited in claim 10
wherein said economizer circuit expansion device comprises an
electronic expansion valve.
12. A refrigerant vapor compression system as recited in claim 10
wherein said economizer circuit expansion device comprises a
thermostatic expansion valve.
13. A refrigerant vapor compression system as recited in claim 1
wherein said compression device comprises a single compressor
having at least two compression stages.
14. A refrigerant vapor compression system as recited in claim 1
wherein said compression device comprises at least two compressors
disposed in the refrigerant circuit in a series relationship with
respect to refrigerant flow.
15. A refrigerant vapor compression system as recited in claim 1
wherein said compression device comprises a scroll compressor.
16. A refrigerant vapor compression system as recited in claim 1
wherein said compression device comprises a reciprocating
compressor.
17. A refrigerant vapor compression system as recited in claim 1
wherein said compression device comprises a screw compressor.
18. A refrigerant vapor compression system as recited in claim 1
wherein said system is incorporated in a transport refrigeration
system for conditioning a temperature controlled cargo storage
region.
19. A refrigerant vapor compression system as recited in claim 18
wherein said system operates in a transcritical cycle.
20. A refrigerant vapor compression system as recited in claim 19
wherein the refrigerant comprises carbon dioxide.
Description
FIELD OF THE INVENTION
[0001] This invention relates generally to refrigerant vapor
compression systems and, more particularly, to refrigerant charge
management in a refrigerant vapor compression system operating in a
transcritical cycle.
BACKGROUND OF THE INVENTION
[0002] Refrigerant vapor compression systems are well known in the
art and commonly 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 commonly used in
refrigerating air supplied to display cases, merchandisers, freezer
cabinets, cold rooms or other perishable/frozen product storage
area in commercial establishments.
[0003] Refrigerant vapor compression systems are also commonly used
in transport refrigeration systems for refrigerating air supplied
to a temperature controlled cargo 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 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 and refrigerant charge to rapidly pull down the
temperature of product loaded into the cargo space at ambient
temperature, but also operate efficiently at low load with excess
refrigerant charge when maintaining a stable product temperature
during transport. Additionally, transport refrigerant vapor
compression systems are subject to vibration and movements not
experienced by stationary refrigerant vapor compression systems.
Thus, the use of a conventional refrigerant accumulator in the
suction line upstream of the compressor suction inlet to store
excess refrigerant liquid would be subject to sloshing during
movement that could result in refrigerant liquid being undesirably
carried through the suction line into the compressor via the
suction inlet thereto.
[0004] Traditionally, most of these refrigerant vapor compression
systems operate at subcritical refrigerant pressures and typically
include a compressor, a condenser, and an evaporator, and expansion
device, commonly an expansion valve, disposed upstream, with
respect to refrigerant flow, of the evaporator and downstream of
the condenser. These basic refrigerant system components are
interconnected by refrigerant lines in a closed refrigerant
circuit, arranged in accord with known refrigerant vapor
compression cycles, and operated in the subcritical pressure range
for the particular refrigerant in use. Refrigerant vapor
compression systems operating in the subcritical range are commonly
charged with fluorocarbon refrigerants such as, but not limited to,
hydrochlorofluorocarbons (HCFCs), such as R22, and more commonly
hydrofluorocarbons (HFCs), such as R134a, R410A, R404A and
R407C.
[0005] In today's market, greater interest is being shown in
"natural" refrigerants, such as carbon dioxide, for use in air
conditioning and transport refrigeration systems instead of HFC
refrigerants. However, because carbon dioxide has a low critical
temperature, most refrigerant vapor compression systems charged
with carbon dioxide as the refrigerant are designed for operation
in the transcritical pressure regime. In refrigerant vapor
compression systems operating in a subcritical cycle, both the
condenser and the evaporator heat exchangers operate at refrigerant
temperatures and pressures below the refrigerant's critical point.
However, in refrigerant vapor compression systems operating in a
transcritical cycle, the heat rejection heat exchanger, which is a
gas cooler rather than a condenser, operates at a refrigerant
temperature and pressure in excess of the refrigerant's critical
point, while the evaporator operates at a refrigerant temperature
and pressure in the subcritical range. Thus, for a refrigerant
vapor compression system operating in a transcritical cycle, the
difference between the refrigerant pressure within the gas cooler
and refrigerant pressure within the evaporator is
characteristically substantially greater than the difference
between the refrigerant pressure within the condenser and the
refrigerant pressure within the evaporator for a refrigerant vapor
compression system operating in a subcritical cycle.
[0006] It is also common practice to incorporate an economizer into
the refrigerant circuit for increasing the capacity of the
refrigerant vapor compression system. For example, in some systems,
a refrigerant-to-refrigerant heat exchanger is incorporated into
the refrigerant circuit as an economizer. A first portion of the
refrigerant leaving the condenser passes through a first pass of
the heat exchanger in heat exchange with a second portion of the
refrigerant passing through the second pass of the heat exchanger.
The second portion of the refrigerant typically constitutes a
portion of the refrigerant leaving the condenser that is diverted
through an expansion device wherein this portion of the refrigerant
is expanded to a lower pressure and a lower temperature vapor or
vapor/liquid mixture refrigerant before this second portion of
refrigerant is passed through the second pass of the economizer
refrigerant-to-refrigerant heat exchanger. Having traversed the
second pass of the economizer heat exchanger, the second portion of
the refrigerant is thence directed into an intermediate pressure
change of the compression process. The refrigerant in the primary
refrigerant circuit passes through the first pass of the
refrigerant-to-refrigerant economizer heat exchanger and is thus
further cooled before it traverses the system's main expansion
device prior to entering the evaporator. U.S. Pat. No. 6,058,729
discloses a subcritical refrigerant vapor compression system for a
transport refrigeration unit incorporating a
refrigerant-to-refrigerant heat exchanger into the refrigerant
circuit as an economizer. U.S. Pat. No. 6,694,750 discloses a
subcritical refrigeration system that includes a first
refrigerant-to-refrigerant heat exchanger economizer and a second
refrigerant-to-refrigerant heat exchanger economizer disposed in
series in the refrigerant circuit between the condenser and the
evaporator.
[0007] In some systems, a flash tank economizer is incorporated
into the refrigerant circuit between the condenser and the
evaporator. In such case, the refrigerant leaving the condenser is
expanded through an expansion device, such as a thermostatic
expansion valve or an electronic expansion valve, prior to entering
the flash tank wherein the expanded refrigerant separates into a
liquid refrigerant component and a vapor refrigerant component. The
vapor component of the refrigerant is thence directed from the
flash tank into an intermediate pressure stage of the compression
process. The liquid component of the refrigerant is directed from
the flash tank through the system's main expansion valve prior to
entering the evaporator. U.S. Pat. No. 5,174,123 discloses a
subcritical vapor compression system incorporating a flash tank
economizer in the refrigerant circuit between the condenser and the
evaporator. U.S. Pat. No. 6,385,980 discloses a transcritical
refrigerant vapor compression system incorporating a flash tank
economizer in the refrigerant circuit between the gas cooler and
the evaporator.
SUMMARY OF THE INVENTION
[0008] A transcritical refrigerant vapor compression system having
improved refrigerant charge management includes a compression
device, a refrigerant heat rejection heat exchanger, a refrigerant
heat absorption heat exchanger, and a refrigerant-to-refrigerant
heat exchanger economizer and a flash tank disposed in a primary
refrigerant circuit in series refrigerant flow relationship
intermediate a refrigerant heat rejection heat exchanger and a
refrigerant heat absorption heat exchanger. A primary expansion
valve interdisposed in the refrigerant circuit in operative
association with and upstream of the refrigerant heat absorption
heat exchanger and a secondary expansion valve interdisposed in the
refrigerant circuit in operative association and upstream of the
flash tank. A refrigerant vapor bypass line establishes refrigerant
vapor flow communication between the flash tank and a suction
pressure portion of the primary refrigerant circuit downstream of
the refrigerant heat absorption heat exchanger. A bypass flow
control valve having an open position and a closed position is
interdisposed in the refrigerant vapor bypass line for controlling
the flow of refrigerant vapor through the refrigerant vapor bypass
line.
[0009] The refrigerant-to-refrigerant heat exchanger has a first
refrigerant pass disposed in the primary refrigerant circuit
downstream of the refrigerant cooling heat exchanger and upstream
of the primary expansion device and a second bypass disposed in an
economizer circuit refrigerant line that extends in refrigerant
flow communication from the primary refrigerant circuit to an
intermediate pressure stage of the compression device. An
economizer circuit expansion device is interdisposed in the
economizer circuit refrigerant line upstream with respect to
refrigerant flow of the second refrigerant pass of the
refrigerant-to-refrigerant heat exchanger economizer. The
economizer circuit expansion device may comprise an electronic
expansion valve or a thermostatic expansion valve.
[0010] In an embodiment, the bypass flow control valve may comprise
a two-position solenoid valve, a pulse width modulated solenoid
valve or an electronic expansion valve. In an embodiment, the
primary expansion valve may comprise an electronic expansion valve
or a thermostatic expansion valve. In an embodiment, the secondary
expansion valve may comprise an electronic expansion valve or a
fixed orifice expansion device.
[0011] In an embodiment, the compression device may be single
compressor having at least a first compression stage and a second
compression stage. In an embodiment, the compression device may be
a first compressor and a second compressor disposed in the
refrigerant circuit in series refrigerant flow relationship with a
discharge outlet of the first compressor in refrigerant flow
communication with a suction inlet of the second compressor. In
either the single compressor arrangement or the dual compressor
arrangement, each compressor may be a scroll compressor, a
reciprocating compressor or a screw compressor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] For a further understanding of the invention, reference will
be made to the following detailed description of the invention
which is to be read in connection with the accompanying drawing,
where:
[0013] FIG. 1 is a schematic diagram illustrating an exemplary
embodiment of a refrigerant vapor compression system in accord with
the invention;
[0014] FIG. 2 is a graph illustrating the pressure to enthalpy
relationship for the exemplary embodiment of the refrigerant vapor
compression system of the invention illustrated in FIG. 1 operating
in a transcritical cycle;
[0015] FIG. 3 is a graph illustrating the pressure to enthalpy
relationship for a prior art refrigerant vapor compression system
operating in a transcritical cycle with a single
refrigerant-to-refrigerant heat exchanger economizer; and
[0016] FIG. 4 is a graph illustrating the pressure to enthalpy
relationship for a prior art refrigerant vapor compression system
operating in a transcritical cycle with a single flash tank
economizer.
DETAILED DESCRIPTION OF THE INVENTION
[0017] Referring now to FIG. 1, there is depicted an exemplary
embodiment of a transcritical refrigerant vapor compression system
10 suitable for use in a transport refrigeration system for
refrigerating air supplied to a temperature controlled cargo space
of a truck, trailer, container or the like for transporting
perishable and frozen goods. The refrigerant vapor compression
system 10 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 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.
[0018] The transcritical refrigerant vapor compression system 10
includes a multi-stage compression device 20, a refrigerant heat
rejection heat exchanger 40, also referred to herein as a gas
cooler, a refrigerant heat absorption heat exchanger 50, also
referred to herein as an evaporator, and a primary expansion device
55, such as for example an electronic expansion valve or a
thermostatic expansion valve, operatively associated with the
evaporator 50, with various refrigerant lines 2, 4 and 6 connecting
the aforementioned components in a primary refrigerant circuit.
[0019] The compression device 20 functions to compress the
refrigerant and to circulate refrigerant through the primary
refrigerant circuit as will be discussed in further detail
hereinafter. The compression device 20 may comprise a single,
multiple-stage refrigerant compressor, for example a reciprocating
compressor, having a first compression stage 20a and a second stage
20b, or a single compressor, for example a scroll compressor or a
screw compressor, adapted in a conventional manner for injection of
refrigerant, for example via an injection port, into an
intermediate pressure point of the compression chamber of the
compressor, whereby the first compression stage 20a is upstream of
the intermediate pressure point and the second compression stage
20b is downstream of the intermediate pressure point. The first and
second compression stages 20a and 20b are disposed in series
refrigerant flow relationship with the refrigerant leaving the
first compression stage passing directly to the second compression
stage for further compression. The compression device 20 may also
comprise a pair of compressors 20a and 20b, connected in series
refrigerant flow relationship in the primary refrigerant circuit
via a refrigerant line connecting the discharge outlet port of the
first compressor 20a in refrigerant flow communication with the
suction inlet port of the second compressor 20b. The compressors
20a and 20b may be scroll compressors, screw compressors,
reciprocating compressors, rotary compressors or any other type of
compressor or a combination of any such compressors.
[0020] The refrigerant heat rejecting heat exchanger 40 may
comprise a finned tube heat exchanger 42 through which hot, high
pressure refrigerant passes in heat exchange relationship with a
cooling medium, most commonly ambient air drawn through the heat
exchanger 42 by the condenser 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.
[0021] Additionally, the refrigerant vapor compression system 10 of
the invention includes a refrigerant-to-refrigerant heat exchanger
economizer 60 and a flash tank 70 interdisposed in series
refrigerant flow relationship in refrigerant line 4 of the primary
refrigerant circuit downstream with respect to refrigerant flow of
the gas cooler 40 and upstream with respect to refrigerant flow of
the evaporator 50. The refrigerant-to-refrigerant heat exchanger
economizer 60 is disposed in refrigerant line 4 of the primary
refrigerant circuit downstream with respect to refrigerant flow of
the gas cooler 40 and upstream with respect to refrigerant flow of
the flash tank 70. Additionally, a secondary expansion device 75,
such as for example, an electronic expansion valve or a fixed
orifice device, is interdisposed in the primary refrigerant circuit
intermediate the refrigerant-to-refrigerant heat exchanger
economizer 60 and the flash tank 70.
[0022] 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 refrigerant line 12 and forms part of an
economizer circuit. The economizer circuit refrigerant line 12
connects in refrigerant flow communication with an intermediate
pressure stage of the compression process. In the exemplary
embodiment depicted in FIG. 1, the economizer circuit refrigerant
line 12 connect to refrigerant line 4 of the primary refrigerant
circuit either upstream with respect to refrigerant flow of the
first pass 62 of the refrigerant-to-refrigerant heat exchanger
economizer 60 and establishes refrigerant flow Alternatively, the
economizer circuit refrigerant line may connect to refrigerant line
4 of the primary circuit downstream with respect to refrigerant
flow of the first pass 62 of the refrigerant-to-refrigerant heat
exchanger economizer 60. 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 parallel flow heat
exchange relationship or in a counter flow heat exchange
relationship, as desired. The refrigerant-to-refrigerant heat
exchanger 60 may be 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] An economizer circuit expansion device 65 is disposed in the
economizer circuit refrigerant line 12 upstream with respect to
refrigerant flow of the second pass 64 of the
refrigerant-to-refrigerant heat exchanger economizer 60. The
economizer circuit expansion device 65 meters the refrigerant flow
that passes through the refrigerant line 12 and the second pass 64
of the refrigerant-to-refrigerant heat exchanger economizer 60 in
heat exchange relationship with the refrigerant passing through the
first pass of the heat exchanger economizer 60 to maintain a
desired level of superheat in the refrigerant vapor leaving the
second pass 64 of the heat exchanger economizer 60 to ensure that
no liquid is present therein. The expansion valve 65 may be an
electronic expansion valve, for example as depicted in FIGS. 1-3,
in which case the expansion valve 65 meters refrigerant flow in
response to a signal from a controller 100 to maintain a desired
refrigerant temperature or pressure in refrigerant line 12. The
expansion device 65 may also be a thermostatic expansion valve, in
which case the expansion valve 65 meters refrigerant flow in
response to a signal indicative of the refrigerant temperature or
pressure sensed by the sensing device (not shown) which may be a
conventional temperature sensing element, such as a bulb or
thermocouple mounted to the refrigerant line 12 downstream of the
second pass of the heat exchanger economizer 60. The refrigerant
vapor passing through the economizer circuit refrigerant line 12 is
injected into the compression device 20 at an intermediate pressure
point of the compression process. For example, if the compression
device 20 is a multi-stage reciprocating compressor, refrigerant
line 12 directs refrigerant vapor directly into an intermediate
pressure stage of the reciprocating compressor between the first
compression stage 20a and the second compression stage 20b. If the
compression device 20 is a single scroll compressor or a single
screw compressor, the refrigerant line 12 directs refrigerant vapor
into an injection port of the compression device opening to the
compression chamber of the compression device at an intermediate
pressure of the compression process. If the compression device 20
is a pair of compressors 20a, 20b, for example a pair of scroll
compressors, or screw compressors, or reciprocating compressors,
connected in series, or a single reciprocating compressor having a
first bank and a second bank of cylinders, the second economizer
circuit refrigerant line 12 directs refrigerant vapor into a
refrigerant line that connects the discharge outlet port of the
first compressor 20a in refrigerant flow communication with the
suction inlet port of the second compressor 20b.
[0024] The flash tank 70 is interdisposed in refrigerant line 4 of
the primary refrigerant circuit downstream with respect to
refrigerant flow of the first pass 62 of the
refrigerant-to-refrigerant heat exchanger economizer 60 and
upstream with respect to refrigerant flow of the evaporator 50 to
receive the refrigerant flowing through refrigerant line 4. A
secondary expansion device 75 is interdisposed in refrigerant line
4 of the primary refrigerant circuit downstream with respect to
refrigerant flow of the first refrigerant pass 62 of the
refrigerant-to-refrigerant heat exchanger economizer 60 and
upstream with respect to refrigerant flow of the inlet to the flash
tank 70. High pressure refrigerant vapor passing through
refrigerant line 4 is expanded as it traverses the secondary
expansion device 75 to a subcritical pressure and temperature
before the refrigerant passes into the flash tank 70. The secondary
expansion device 75 may be an electronic expansion valve, such as
illustrated in FIG. 1, in which case the secondary expansion valve
75 meters refrigerant flow in response to a signal from a
controller 100 to maintain a desired refrigerant pressure in
refrigerant line 4 upstream with respect to refrigerant flow of the
secondary expansion device 75. The secondary expansion device 75
may also simply be a fixed orifice expansion device, in which case
the refrigerant pressure in refrigerant line 4 upstream with
respect to refrigerant flow of the secondary expansion device 75
will fluctuate depending upon ambient conditions and the
refrigerant flow will be inherently metered in accord with the
magnitude of the pressure differential across the fixed
orifice.
[0025] The flash tank 70 defines a separation chamber 72 into which
the expanded refrigerant flows at a subcritical pressure and
separates into a liquid refrigerant portion that collects in the
lower portion of the flash tank 70 and into a vapor portion that
collects in the upper portion of the flash tank 70 above the liquid
level within the flash tank 70. Thus, the flash tank 70 functions
as a receiver for storing liquid refrigerant whenever the
refrigerant vapor compression system is operating at a capacity
that does not require the system's full refrigerant charge.
[0026] Additionally, the refrigerant vapor compression system
includes a refrigerant line 14 that establishes refrigerant flow
communication between the flash tank 70 and refrigerant line 6 of
the primary refrigerant circuit at a point downstream with respect
to refrigerant flow of the outlet of the evaporator 50 and upstream
with respect to refrigerant flow of the suction inlet to the
compression device 20. Refrigerant vapor collecting in the portion
of the flash tank 70 above the liquid level therein passes from the
flash tank 70 through refrigerant line 14 to enter the primary
refrigerant circuit to return to the compression device 20. A flow
control valve 85 is interdisposed in refrigerant line 14 to
restrict the flow of refrigerant vapor through refrigerant line 14
as necessary to maintain the separation chamber 72 of the flash
tank 70 at a refrigerant pressure higher than suction pressure. In
an embodiment, the flow control valve 85 comprises a solenoid valve
having a first open position and a second closed position, such as
for example, but not limited to, a pulse width modulated solenoid
valve. In an embodiment, the flow control valve 85 may comprise an
electronic expansion valve.
[0027] Liquid refrigerant collecting in the lower portion of the
flash tank economizer 70 passes therefrom through refrigerant line
4 and traverses the primary refrigerant circuit expansion valve 55,
which may be an electronic expansion valve or a conventional
thermostatic expansion valve, disposed in refrigerant line 4
upstream with respect to refrigerant flow of the evaporator 50. As
this liquid refrigerant traverses the first expansion device 55, it
expands to a lower pressure and temperature before entering the
evaporator 50. As the liquid refrigerant passes through the
evaporator 50, the liquid refrigerant passes in heat exchange
relationship with a heating medium whereby the liquid refrigerant
is vaporized and typically superheated and the heating medium is
cooled. In an embodiment, the evaporator 50 constitutes a finned
tube coil heat exchanger 52, such as a fin and round tube heat
exchanger or a fin and flat, mini-channel tube heat exchanger. The
heating fluid passed in heat exchange relationship with the
refrigerant in the evaporator 50 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. The low pressure refrigerant vapor leaving the
evaporator 50 returns through refrigerant line 6 to the suction
inlet of the compression device 20.
[0028] As in conventional practice, the primary expansion valve 55
meters the refrigerant flow through the refrigerant line 4 to
maintain a desired level of superheat in the refrigerant vapor
leaving the evaporator 50 and passing through refrigerant line 6 to
ensure that no liquid is present in the refrigerant leaving the
evaporator. As noted before, the primary expansion valve 55 may be
an electronic expansion valve, in which case the expansion valve 55
meters refrigerant flow in response to a signal from a controller
100 to maintain a desired suction temperature or suction pressure
in refrigerant line 6 on the suction side of the compression device
20. The primary expansion device 55 may also be a thermostatic
expansion valve in which case the expansion valve 55 meters
refrigerant flow in response to a signal indicative of the
refrigerant temperature or pressure sensed by the sensing device,
which may be a conventional temperature sensing element, such as a
bulb or thermocouple mounted to the refrigerant line 6 in the
vicinity of the evaporator outlet.
[0029] In the exemplary embodiment of the refrigerant vapor
compression system 10 depicted in FIG. 1, operation of the
refrigerant vapor compression system is controlled by a control
system that includes a controller 100 operatively associated with
the flow control valve 85 interdisposed in refrigerant line 14 and
the economizer circuit expansion device 65 interdisposed in
refrigerant line 12. The controller 100 also may control operation
of the electronic expansion valves 55 and 65, the compression
device 20, and the fans 44 and 54. As in conventional practice, in
addition to monitoring ambient conditions, the controller 100 may
also monitors various operating parameters by means of various
sensors operatively associated with the controller 100 and disposed
at selected locations throughout the system. For example, in the
exemplary embodiments depicted in FIG. 1, a pressure sensor 102 may
be disposed in operative association with the flash tank 70 to
sense the pressure within the flash tank 70, a temperature sensor
103 and a pressure sensor 104 may be provided to sense the
refrigerant suction temperature and pressure, respectively, and a
temperature sensor 105 and a pressure sensor 106 may be provided to
sense refrigerant discharge temperature and pressure, respectively.
The pressure sensors 102, 104, 106 may be conventional pressure
sensors, such as for example, pressure transducers, and the
temperature sensors 103 and 105 may be conventional temperature
sensors, such as for example, thermocouples or thermistors.
[0030] The refrigerant vapor compression system of the invention is
particularly adapted for operation in a transcritical cycle with a
lower critical point refrigerant such as carbon dioxide, but may
also be operated in a subcritical cycle with a conventional higher
critical point refrigerant. When the refrigerant vapor compression
system 10 is operating in an economized mode, the controller 100
controls the economizer circuit expansion device 65 to meter the
flow of refrigerant vapor from refrigerant line 4 through the
economizer circuit refrigerant line 12 in response to system
operating conditions and capacity requirements. When the system is
operating in a non-economized mode, the controller 100 closes the
economizer circuit expansion valve 65 so that all of the
refrigerant passing from the gas cooler 40 through refrigerant line
4 passes through the secondary expansion device 75 and thence into
the flash tank 70. In either the economized or non-economized
modes, the controller 100 controls the primary expansion valve 55
to meter the correct amount of refrigerant liquid out of the flash
tank 70 in response to the sensed system operating parameters, for
example compressor discharge temperature, to match the refrigerant
charge demand of the system.
[0031] Additionally, the controller 100 controls positioning of the
flow control valve 85 interdisposed in refrigerant line 14 to
restrict the flow of refrigerant vapor from the flash tank 70 in
response to the sensed pressure within the separation chamber flash
tank 70 so as to maintain a desired subcritical flash tank
pressure. As the ratio of refrigerant liquid to refrigerant vapor
present in the flash tank will depend upon the subcritical pressure
level within the separation chamber, the flash tank pressure may be
controlled through positioning of the flow control valve 85 so as
to produce a selected refrigerant quality upon expansion. If the
flow control valve 85 is continuously closed, the pressure within
the flash tank with rise to an upper limit of the gas cooler
pressure. If the flow control valve 85 is continuously open, the
pressure within the flash tank 70 will fall to a lower pressure,
but above the suction pressure. The actual pressure differential
between the pressure within flash tank and suction pressure when
the flow control valve is fully open will be governed by the size
of the orifice in the particular flow control valve used. The
controlled discharge of refrigerant vapor from the flash tank 70
through refrigerant line 14 to suction pressure is essential for
maintaining a low pressure within the flash tank. Therefore, the
controller 100 may also continuously cycle the flow control valve
85 between its open and closed positions to selectively control the
flash tank pressure. This manipulation of the primary expansion
valve 55 and the flow control valve 85 provides the controller 100
with the ability to effectively manage refrigerant charge over a
wide range of operating conditions even when the refrigerant vapor
compression system 10 is operating in a transcritical mode.
Additionally, separating the refrigerant into its liquid and vapor
phases in the flash tank 70 and sending only the liquid refrigerant
through the evaporator, while diverting the vapor refrigerant to a
point downstream of the evaporator, improves the effectiveness of
heat exchange in the evaporator.
[0032] A comparison of the pressure to enthalpy relationship
presented in FIG. 2, which represents a characteristic pressure to
enthalpy relationship for the refrigerant vapor compression system
10 of FIG. 1, to either of the pressure to enthalpy relationships
representative of conventional refrigerant vapor compression
systems presented in FIG. 3 or FIG. 4, illustrates the capacity
improvement associated with the refrigerant vapor compression
system of the invention. FIG. 3 presents a characteristic pressure
to enthalpy relationship for a conventional prior art transcritical
refrigerant vapor compression having a single
refrigerant-to-refrigerant heat exchanger economizer. FIG. 4
presents a characteristic pressure to enthalpy relationship for a
conventional prior art transcritical refrigerant vapor compression
having a single flash tank economizer. In each of FIGS. 2-4, AB
represents the gas heat rejection process within gas cooler 40 and
DE represents the gas heat absorption process within the evaporator
50. In FIG. 2, KG represents the process within the
refrigerant-to-refrigerant heat exchanger economizer circuit and MN
represents the process within the flash tank-to-suction evaporator
bypass circuit. In FIG. 3, KG represents the process within the
refrigerant-to-refrigerant heat exchanger economizer circuit. In
FIG. 4, JL represents the process within a flash tank economizer
circuit. The evaporator line DE in FIG. 1 is longer than the
respective evaporator lines associated with either of the prior art
single economizer systems, indicating the increased evaporator
effectiveness associated with the refrigerant vapor compression
system of the invention.
[0033] Those skilled in the art will recognize that many variations
may be made to the particular exemplary embodiments described
herein. While the present invention has been particularly shown and
described with reference to the exemplary embodiment as illustrated
in the drawings, it will be understood by one skilled in the art
that various changes in detail may be effected therein without
departing from the spirit and scope of the invention as defined by
the claims.
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