U.S. patent application number 12/702386 was filed with the patent office on 2010-08-12 for refrigeration system.
This patent application is currently assigned to Star Refrigeration Limited. Invention is credited to Andrew Brash Pearson.
Application Number | 20100199707 12/702386 |
Document ID | / |
Family ID | 40527134 |
Filed Date | 2010-08-12 |
United States Patent
Application |
20100199707 |
Kind Code |
A1 |
Pearson; Andrew Brash |
August 12, 2010 |
REFRIGERATION SYSTEM
Abstract
A refrigerating system comprises a compressor (1A, 1B, 1C), a
heat rejection device (2), an expansion device (15), an evaporator
(5A, 5B, 5C) and a receiver (4), capable of operating with the
compressor discharge higher than the critical pressure of the
refrigerant; where the flow outlet from the heat rejection device
is regulated by a pressure control valve (7), the pressure
downstream of the pressure control valve is regulated by a gas vent
valve (8), the refrigerant flow to the evaporator is further
regulated by an automatic control device (41, 52, 14) at the inlet
to the evaporator, and the automatic control device being set to
permit intermittent flow of liquid refrigerant to the receiver
during normal operation of the system. The refrigerant may be
carbon dioxide, which may operate under transcritical pressures
e.g. 80 to 120 bar absolute. The receiver acts as a trap for any
liquid from the evaporator and ensures that the gas flow to the
compressor suction is dry.
Inventors: |
Pearson; Andrew Brash;
(Glasgow, GB) |
Correspondence
Address: |
GIFFORD, KRASS, SPRINKLE,ANDERSON & CITKOWSKI, P.C
PO BOX 7021
TROY
MI
48007-7021
US
|
Assignee: |
Star Refrigeration Limited
|
Family ID: |
40527134 |
Appl. No.: |
12/702386 |
Filed: |
February 9, 2010 |
Current U.S.
Class: |
62/434 ; 62/468;
62/498 |
Current CPC
Class: |
F25B 2309/061 20130101;
F25B 9/008 20130101; F25B 49/005 20130101; F25B 2600/2513 20130101;
F25B 2600/2525 20130101; F25B 2400/22 20130101; F25B 2400/051
20130101; F25B 43/006 20130101; F25B 2400/23 20130101; F25B 2400/13
20130101; F25B 2341/063 20130101; F25B 2400/075 20130101; F25B 5/02
20130101; F25B 2600/2501 20130101 |
Class at
Publication: |
62/434 ; 62/498;
62/468 |
International
Class: |
F25D 17/02 20060101
F25D017/02; F25B 1/00 20060101 F25B001/00; F25B 43/00 20060101
F25B043/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 11, 2009 |
GB |
0902192.4 |
Claims
1. A refrigerating system comprising a compressor, a heat rejection
device, an expansion device, an evaporator and a receiver, capable
of operating with the compressor discharge higher than the critical
pressure of the refrigerant; where the flow outlet from the heat
rejection device is regulated by a pressure control valve, the
pressure downstream of the pressure control valve is regulated by a
gas vent valve, the refrigerant flow to the evaporator is further
regulated by an automatic control device at the inlet to the
evaporator, and the automatic control device being set to permit
intermittent flow of liquid refrigerant to the receiver during
normal operation of the system.
2. A refrigerating system in accordance with claim 1 where the
refrigerant is carbon dioxide.
3. A refrigerating system in accordance with claim 1 where the
pressure between the pressure control valve and the expansion
device is permitted to rise to approximate the heat rejection
device operating pressure by driving the pressure control valve
fully open.
4. A refrigerating system in accordance with claim 1 where
lubricant, which has accumulated in the receiver, can be
automatically returned to the compressors through a sampling
device.
5. A refrigerating system in accordance with claim 1 where pressure
rise due to heat gain when the compressors are non-operational is
self-limiting by virtue of the size and disposition of the
refrigeration system components.
6. A refrigerating system in accordance with claim 1 where the
total refrigerant mass contained in the system can be accumulated
in the receiver.
7. A refrigeration system in accordance with claim 1 where the heat
rejection device rejects heat to a cooling fluid requiring to be
heated.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority of United Kingdom Patent
Application 0902192.4 filed Feb. 11, 2009.
TECHNICAL FIELD
[0002] The present invention relates to a refrigeration system,
especially utilising carbon dioxide as refrigerant.
BACKGROUND
[0003] The prohibition of chlorofluorocarbons as refrigerants,
which results from their deleterious effect on the ozone layer, has
led to a number of alternative strategies to provide cooling
without adverse environmental impact. The majority of systems have
used hydrofluorocarbons, which have no effect on stratospheric
ozone, but which in many cases have a high global warming potential
and are therefore known to contribute to climate change through
global warming. Carbon dioxide is a suitable fluid for use as a
refrigerant, and was one of the earliest fluids used when
mechanical refrigeration was developed in the raid 19.sup.th
century. Carbon dioxide has no effect on the ozone layer, and is
present in the atmosphere at a level of 380 ppm. This bulk
contributes to global warming, but the gas used in industrial
systems, including refrigeration plant, is recovered from
combustion processes and is therefore beneficially used in sealed
equipment.
[0004] Since 1990 several inventors have proposed novel ways of
using carbon dioxide for refrigeration, including Pearson in
GB2258298 (1991), Lorentzen et al in U.S. Pat. No. 5,245,836 (1993)
and Komatsubara et al in EP1790712 (2007). A comprehensive overview
of ideas for systems is given in Rieberer, R, Gassier, M and
Halozan, H, "Control of CO2 heat pumps" Proc IR Conf--4th Gustav
Lorentzen, Purdue (2000). The present invention extends the
application of these previous ideas and provides for additional
reliability in operation.
[0005] In a conventional refrigeration system heat is rejected by
condensation of refrigerant at a pressure below the critical point
of the fluid. When a thermostatically controlled expansion valve
(or electronic equivalent) is used to control the flow of
refrigerant the volume of liquid in the low pressure side of the
system, primarily the evaporator, is widely variable, depending on
the system load. The variation in liquid volume cannot be stored in
the low pressure side of such a system, and so a high pressure
liquid receiver is required at the outlet of the condenser to
accumulate the variation in distribution of refrigerant between the
high and the low side.
[0006] Transcritical operation of carbon dioxide refrigeration
systems has been widely reported, including Bergdoll in U.S. Pat.
No. 1,860,447 (1932), and many variant systems have been developed.
When operating in this transcritical mode all of the refrigerant on
the high pressure side of the system is in the form of gas. There
is therefore limited possibility for accumulation of refrigerant on
the high pressure side, as is required with the use of a
conventional expansion valve. Previous systems have used
intermediate pressure receivers to provide some liquid storage
volume for example Gernemann and Heinbokel in WO2006015741 (2006),
or have allowed the pressure in the high pressure side of the
system to vary, thereby accommodating variation in the refrigerant
mass by changing the gas density. Designs of the latter type are
suitable for very small refrigeration systems, typically
characterised by having a single evaporator, a single compressor
and a simple on/off control strategy. They are not well suited to
larger distributed systems where the refrigerating capacity is
variable, where there are multiple evaporators and where the
evaporators are large enough to be subject to liquid
maldistribution. In such larger systems application of these simple
designs is liable to lead to unreliability, lack of refrigerating
capacity under off-design conditions and impaired efficiency.
SUMMARY OF THE INVENTION
[0007] The present invention provides a refrigerating system
comprising a compressor, a heat rejection device, an expansion
device, an evaporator and a receiver, capable of operating with the
compressor discharge higher than the critical pressure of the
refrigerant; where
[0008] the flow outlet from the heat rejection device is regulated
by a pressure control valve,
[0009] the pressure downstream of the pressure control valve is
regulated by a gas vent valve,
[0010] the refrigerant flow to the evaporator is further regulated
by an automatic control device at the inlet to the evaporator,
and
[0011] the automatic control device being set to permit
intermittent flow of liquid refrigerant to the receiver during
normal operation of the system.
[0012] Thus, the present invention comprises a refrigeration
circuit comprising one or more refrigerant compressors capable of
raising the refrigerant pressure from the evaporating condition up
to a pressure suitable for rejection of heat to the atmosphere or a
cooling fluid, coupled with one or more heat rejection heat
exchangers, a flow control device and one or more heat extraction
heat exchangers. To achieve the novel mode of operation unique to
this invention the system also includes a receiver between the heat
extraction heat exchangers and the suction of the compressor(s),
preferably a gas separator, a gas vent valve from the separator to
the low pressure portion of the receiver and a set of flow control
devices in the inlet to each heat extraction heat exchanger.
[0013] The refrigerant is preferably capable of operating under
transcritical conditions, and carbon dioxide is particularly
preferred. The evaporating pressure can be anywhere from the triple
point of carbon dioxide (5.18 bar absolute) up to 60 bar absolute,
representing an evaporating temperature of 22.degree. C. When the
ambient temperature or cooling fluid temperature exceeds
approximately 25.degree. C. this requires the compressor to
discharge gas at a pressure above the critical point of carbon
dioxide (73.8 bar absolute). The preferred range for operation of
the compressor discharge under these conditions is between 80 bar
absolute and 120 bar absolute. When the ambient temperature or
cooling fluid temperature is lower than approximately 25.degree. C.
the compressor can run at lower discharge pressures enabling the
refrigerant to condense in the heat rejection device. Unlike
previous systems, the pressure in the refrigerant pipe which leads
from the heat rejection device to the evaporator is permitted under
these low ambient operating conditions to rise to the condensing
pressure of the refrigerant.
[0014] A preferred feature of the present invention is that the
pressure regulator in the outlet of the heat rejection device will
modulate when the system operates in the transcritical mode and
will be fully open in the subcritical mode, whereas the pressure
regulating valve in the vent line which leads from the gas
separator to the receiver will modulate when the system operates in
the transcritical mode and will be fully closed in the subcritical
mode, albeit capable of operation in the event that the liquid line
pressure rises above the set pressure level of the vent valve.
[0015] The pressure between the pressure control valve and the
expansion device is permitted to rise to approximate the heat
rejection device operating pressure by driving the pressure control
valve fully open.
[0016] Generally, the receiver is located downstream of the
pressure control valve and operates at a pressure intermediate the
compressor discharge pressure and the evaporating pressure. It acts
to receive liquid and gas refrigerant. Usually, an internal heat
exchanger within the receiver contains a flow of refrigerant from
the heat rejection device and to the evaporator(s). Low pressure
refrigerant from the evaporator(s), possibly containing liquid and
gaseous refrigerant is received into the low pressure portion of
the receiver and any liquid separated out. Dry refrigerant is
directed to the compressor inlet. The receiver is generally sized
to be able to accommodate the full charge of refrigerant in the
system.
[0017] A gas separator may be provided between the pressure control
valve and the receiver. This separates the gas refrigerant from the
liquid and passes the gas refrigerant into the low pressure portion
of the receiver regulated by the gas vent valve, whilst the liquid
passes through the internal heat exchanger within the receiver.
[0018] The evaporators are preferably equipped with individual
expansion devices to regulate the flow of the refrigerant into each
evaporator. It is a preferred feature of this invention that these
expansion devices can be similar in type to the traditional valves
used in conventional refrigerating systems, provided they can
withstand the higher pressure encountered when operating with
sub-critical carbon dioxide. The control of the expansion valves is
based upon refrigerant temperature and pressure, but set to a
relatively low value such that some liquid will, from time to time
during normal modulation of the valve, pass from the evaporator to
the receiver. In this respect, although the design and construction
of these valves is similar to the traditional valves used in
conventional systems, the control of the valves is novel. According
to another preferred aspect of the invention, this novel control
provides significant advantage in the event of a malfunction of one
of the individual expansion devices on the evaporators because the
excess liquid passed through the evaporator will be trapped by the
receiver and will not cause damage to any of the compressors in the
system.
[0019] A further preferred feature of this invention enables
lubricant, which has passed through the oil separator and
circulated through the refrigerant pipes to the evaporators and
hence to the receiver, to be returned to the compressor(s). Failure
to return this lubricant would result in a gradual accumulation in
the receiver, causing a loss of heat transfer performance of the
heat exchanger located in the receiver, and possibly also causing a
lack of lubricant at the compressors. The lubricant may be returned
by taking a sample of the liquid in the receiver, which may contain
a proportion of lubricant, and mixing this sample into the suction
gas to the compressor. By providing such lubricant return means to
each compressor it can be ensured that lubricant is only returned
to a machine which is operation at the time.
[0020] A preferred feature of the current invention is that the
components are configured in the system such that functional
failure of any of the control elements, comprising the flow control
valve in the outlet of the heat rejection device, the gas vent
pressure control valve or, as previously mentioned, the individual
expansion devices at the evaporators will not cause damage to the
rotating machinery incorporated into the circuit. Additionally, a
preferred feature of the current invention is that the receiver
shall be sufficiently large to contain the full mass of
refrigerant, in liquid form, required for correct normal operation
of the system.
[0021] A further object of the current invention is that, in the
event of total power loss to the system, the rise in pressure which
occurs due to heat infiltration through the pipe and vessel walls
shall not result in a significant loss of refrigerant through the
pressure relief devices, and shall not inhibit normal function of
the refrigerating system automatically when power is restored. This
feature of the design of the current invention is generally
achieved through the sizing and disposition of the main system
components. In the event that the system was correctly charged
prior to loss of power then the pressure rise due to heat gain may
be limited by the volume of liquid within the system in relation to
the total volume of the system, and should not reach or exceed the
set pressure of the safety relief device fitted to the receiver in
the compressor suction line. In the event that the system had been
overcharged and contained an excess of liquid, then it is possible
that the pressure would reach or exceed the set pressure of the
safety relief device fitted to the receiver in the compressor
suction line for a limited time. Such excess pressure may be vented
by the pressure regulating relief device or pressure relief device
fitted to the receiver, but only to the extent of the allowable
pressure of the system. In this manner, when power is restored and
the system restarts automatically there shall be sufficient
refrigerant contained within the system to resume normal operation
without any manual intervention.
[0022] The invention also relates to a method of conducting
refrigeration.
[0023] This invention can be applied to a wide range of
refrigerating systems including, but not limited to supermarket
refrigeration systems, air conditioning systems, IT cooling
systems, refrigerated warehouse systems and refrigerated food
processing systems.
[0024] A preferred embodiment of the invention will now be
described by way of example only.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0025] One embodiment of the invention is illustrated schematically
in FIG. 1.
[0026] In this system the refrigerant carbon dioxide is used for
cooling of chilled food display cases in a supermarket. A set of
one or more compressors (1A, 1B etc) pump refrigerant gas to a high
pressure condition at which it can reject heat to atmosphere or
some other cooling medium. The gas is conducted from the
compressor(s) through pipes (6B, 6C) to a heat rejection device
(2). From the heat rejection device the cooled refrigerant passes
through pipes (6D, 6F) to a heat exchanger located in the receiver
(4) and from there through pipes (6G) to a set of one or more
system evaporators (5A, 5B etc).
[0027] When the temperature of the heat sink (the zone to which
heat is rejected from heat rejection device (2)) is higher than
approximately 25.degree. C. then the compressors will be required
to raise the carbon dioxide gas to a supercritical pressure,
typically in the range 80 to 120 bar absolute. This can be
described as "transcritical mode of operation." Under these
conditions the pressure regulating valve (7) shall restrict the
flow of refrigerant under control of an upstream pressure
measurement device. This control may be to a fixed pressure
condition, or it may use an optimising algorithm to adjust the
control point. The pressure downstream of the regulating valve will
be at an intermediate pressure level somewhere between the
compressor discharge pressure and the evaporating pressure. The
expansion process in regulating valve (7) will reduce the pressure
from above the critical point to a level below the critical point,
and some of the gas will turn to liquid as it passes through the
pressure reduction valve. In order to ensure correct operation of
the expansion devices (15) at the evaporators (5A, 5B etc) during
this transcritical mode of operation it is necessary to vent the
gas remaining after the first expansion. This is done in a pipe
fitment (3) used as a gas separator. Unlike previous inventions
this pipe fitment is not a pressure vessel, but merely a particular
arrangement of the pipework configured to ensure that liquid flows
from the bottom of the fitment and gas, with perhaps a small
proportion of liquid, flows from the upper portion of the fitment.
The gas vented from the gas separator is passed through pipes (6E)
and a second pressure regulator (8) set to maintain a pressure on
the upstream side equivalent to the maximum normal operating
pressure for the liquid lines. In the preferred embodiment the
allowable pressure (design pressure) in the liquid line (6F, 6G)
and the suction line (6H) is 75 bar absolute and the operating
pressure of the vent regulator (8) is 65 bar absolute.
[0028] It may be appropriate to include a refrigerant filter and a
refrigerant drier in the refrigeration circuit. Item 11 in FIG. 1
illustrates one such embodiment of this feature whereby the
functions of drying and filtering the refrigerant are combined into
a single device, but it is recognised that there are many possible
alternative embodiments which achieve the same desired
function.
[0029] The flowrate of refrigerant which passes from the internal
heat exchanger within the receiver (4) to the expansion devices
(15) is regulated according to the evaporating pressure and the
evaporator outlet temperature to provide some liquid overfeed from
the evaporator to the receiver. This regulation is achieved by
sensors (S1, S2 etc) and a control element (14) connected to each
of the expansion devices. In the preferred embodiment, the control
setpoint for evaporator outlet superheat is 2K or less, and normal
system fluctuations result in liquid flowing out of the evaporator
intermittently. This liquid travels with the evaporated refrigerant
to the receiver vessel (4). The receiver acts as a trap for the
liquid, ensuring that the gas flow to the compressor suction is
dry.
[0030] When the temperature of the heat sink is lower than
approximately 25.degree. C. then the control of the pressure
regulating valve is over-ridden and the valve is opened to its
fullest extent. This can be described as "subcritical mode of
operation." Under these conditions the pressure in the liquid
refrigerant pipe from the heat rejection device (2) to the
evaporators, including the liquid in the gas separator, the
internal heat exchanger of the receiver (4) and the pipe feeding to
the evaporators shall be at the condensing pressure of the
refrigerant. The vent regulator valve (8) shall typically be closed
under these conditions because the condensing pressure shall be
less than 65 bar absolute.
[0031] The expansion valves at the evaporators (5A, 5B etc) shall
operate in the same manner as previously described, providing an
intermittent flow of liquid mixed in with the evaporator outlet gas
from the evaporator to the receiver.
[0032] Where the compressors use lubricating oil, it may be
appropriate to use an oil separator (12) in the discharge from the
compressors. In this case oil will return from the separator to the
compressors through a series of tubes (13). Whether or not an oil
separator is included in the system, the preferred embodiment of
the invention will include a method for returning oil from the
receiver (4) to the compressors (1A, 1B etc). This method will
preferably be fully automatic using a distillation system.
[0033] Protection against excess pressure for the system is
provided by pressure relief valve (9) connected to the receiver. In
addition a pressure regulating relief valve (10) is fitted to the
receiver to permit small quantities of refrigerant to be released
in a controlled manner if the pressure rises close to the setting
of the relief valve. It is a feature of the present invention that
the dimensions of the receiver vessel are carefully selected in
relation to the overall refrigerant content of the system to ensure
that this pressure rise is self-limiting. With this consideration
included in the design of the system the loss of refrigerant in the
event of a major power cut will be minimised.
[0034] The heat rejection device may take the form of a finned heat
exchanger in contact with atmospheric air, but in an alternative
embodiment of the invention will be a heat exchanger in contact
with a cooling fluid such as cooling tower water, process water
requiring to be heated or some other medium.
* * * * *