Refrigeration system with closed circuit circulation

A refrigeration system having a closed circulating circuit filled with a refrigerant which on evaporation expands and gives rise to an increase in pressure in the whole or in parts of the circulating circuit, and which at ambient temperature has a saturation pressure that is higher than the maximum working pressure in the refrigeration circuit. A refrigeration of this kind may, for example, be carbon dioxide. By allowing vaporized refrigerant to condense against the surface of the refrigerant in liquid phase, contained in a container that is insulated and has adapted size and adapted liquid level, the pressure in the circulating circuit can be maintained below the maximum working pressure of the refrigeration circuit. Thus undesirable build-up of pressure in the event of, e.g., a period of inoperation or breakdown, is prevented, and the circulating circuit of the refrigeration system can be designed and made for a pressure which is below the saturation pressure at ambient temperature of the refrigerant used, and the refrigeration system can be made using conventional or at least virtually conventional elements, whereby the total system costs are reduced considerably in relation to a total system which is built to withstand higher pressure, e.g., the saturation pressure at room temperature of the refrigerant. Starting up after, e.g., a period of inoperation or breakdown is secured with valves which provide a controlled fall in pressure in an insulated container after an increase in pressure in the same container exceeding the maximum working pressure of the circuits.

The present invention relates to a refrigeration system having a closed 
circulating circuit filled with a refrigerant intended for heat transfer, 
which refrigerant at atmospheric pressure has a saturation pressure that 
is higher than maximum working pressure in the circulating circuit, which 
refrigeration system consists at least of one or more evaporators or heat 
exchangers, equipment for circulation of the refrigerant and one or more 
condensers, and also at least one container for the refrigerant in 
connection with the refrigeration circuit. 
In recent years concern for the environment has brought about a change in 
the use of refrigerants in refrigeration systems/heat pumps for, e.g. 
refrigerated cabinets in grocery shops, air cooling. refrigerated 
transport and refrigerated storage rooms. This change is primarily related 
to the fact that the vast majority of synthetic refrigerants which were 
used earlier (e.g., chlorofluorocarbons), if released, led to a depletion 
of the ozone layer in the stratosphere, and thus also increased 
ultraviolet radiation. The use and thus the emissions of these 
refrigerants have now been regulated through international agreements. and 
stringent national and international requirements mean that a great many 
synthetic refrigerants (CFC refrigerants) can no longer be used. 
To compare the different refrigerants and their environmental impact, it is 
essential to examine their ozone depletion potential (ODP) and greenhouse 
warming-up potential (GWP). An overview of refrigerants that have 
conventionally been used in refrigeration systems in e.g., grocery shops, 
is as follows: 
______________________________________ 
Greenhouse 
warming-up 
Ozone depletion 
potential (GWP) 
potential (ODP), 
(100 years), 
Refrigerants 
Not available after: 
(CFC11 = 1) (CO2 = 1) 
______________________________________ 
CFC - 12 
1995 1 7100 
CFC - 502 
1995 0.32 4300 
HCFC - 22 
2014 0.055 1600 
______________________________________ 
Halocarbons may be used to replace these refrigerants. These do not destroy 
the ozone layer, but still contribute to the greenhouse effect. Examples 
of some such refrigerants are: 
__________________________________________________________________________ 
Evap. Ozone Gr.house 
Based on 
temp. depletion 
warming- 
(% age) Temp. potential 
up pot. 
Refrigerants: 
Replace: 
Producer 
(other comm.) 
fluct. 
(ODP) (GWP) 
__________________________________________________________________________ 
HP 62 CFC 502 
DuPont 
HFC134a 4% 
-46.1.degree. C. 
0 2650 
HCF 404A 
HCFC 22 HFC125 44% 
0.7 
R-404A HFC143a 52% 
Klea 60 
CFC 502 
ICI HFC32 20% 
-42.2.degree. C. 
0 1575 
HCFC 22 HFC125 40% 
6.6 
R-407B HFC134a 40% 
Klea 61 
CFC 502 
ICI HFC32 10% 
-45.1.degree. C. 
0 2290 
HCFC 22 HFC125 70% 
4.4 
R-407B HFC134a 20% 
Genetron 
CFC 502 
Allied 
HFC125 50% 
-45.8.degree. C. 
0 2720 
AZ-50 HCFC 22 
Signal 
HFC143a 50% 
R-507 (Azeotrope) 
HCF 134a 
CFC12 All -26.5.degree. C. 
0 1200 
R-134A producers 
__________________________________________________________________________ 
In addition, natural refrigerants such as, e.g., ammonia (NH.sub.3), carbon 
dioxide (CO.sub.2) and propane (C.sub.3 H.sub.8) can be used. These 
refrigerants have virtually no ozone depletion potential and, with the 
exception of carbon dioxide, they have almost no greenhouse warming-up 
potential. However, the use of CO.sub.2 as a refrigerant cannot be looked 
upon as a contribution to the greenhouse effect as reutilisation is 
assumed. 
Of these naturally occurring refrigerants, ammonia and carbon dioxide are 
considered to be the most suitable and environmentally safe refrigerants 
that can be used. When using ammonia as a refrigerant, known technology is 
employed which is adapted to the individual use and system, but this 
medium is toxic and under certain circumstances it is flammable. This 
means that a brine should be used as a secondary agent for the individual 
applications in the refrigeration circuit. The same applies when using 
propane as a refrigerant. 
The use of carbon dioxide as a refrigerant is previously known, but when 
synthetic refrigerants were introduced, the use of carbon dioxide for this 
purpose was greatly reduced, a fact also attributable to a number of 
drawbacks connected to carbon dioxide as a refrigerant. 
These drawbacks include the fact that the temperature gap between the 
critical temperature and the so-called triple point is relatively small 
compared with traditional refrigerants. This means that when CO.sub.2 is 
used in an ordinary refrigeration process, the carbon dioxide will for the 
most part be used in a temperature range of from -50.degree. C. 
(evaporation) to about -5.degree. C. (condensation) with a reasonable 
coefficient of performance. This means that carbon dioxide is rather 
inflexible with respect to different applications (temperature levels). 
The individual system must therefore be adapted to the individual 
application. 
A further drawback when using CO.sub.2 as refrigerant compared with 
conventional refrigeration systems, is associated with the rise in 
pressure which occurs when the temperature of the refrigerant passes from 
working temperature to ambient temperature. At room temperature the 
saturation pressure of carbon dioxide is about 50 to 60 bar, and this is 
considerably higher than the working pressure in a conventional 
refrigeration system. This means that in the event of a breakdown, the 
saturation pressure will rise in the circulating circuit as the 
temperature rises, and if the circuit is to be capable of (withstanding 
saturation pressure at ambient temperature, the individual components in 
the refrigeration circuit must be designed for this high pressure, which 
means a sharp increase in costs compared with conventional refrigeration 
systems. 
In connection with this problem, it is previously known from. e.g., U.S. 
Pat. No. 5.042.262 that a refrigeration system using carbon dioxide as 
refrigerant, when the system is not operating, will maintain a pressure in 
the refrigeration circuit of less than about 17 bar by either a mechanical 
cooling of the refrigerant in the circulating circuit or by a pressure 
relief means which releases the vaporised carbon dioxide into the 
environment in order to adjust the pressure. In large systems, a 
mechanical cooling of the whole of or parts of the refrigeration circuit 
to reduce the pressure when the system is not in operation will result in 
a considerable rise in installation and maintenance costs. If the 
refrigerant is released through a pressure relief valve in order to 
maintain the pressure in the refrigeration circuit below the maximum 
working pressure, this will involve adding a new refrigerant when starting 
up the system, which involves costs, in addition to the indirect cost of 
the refrigeration system being inoperative pending a refill of 
refrigerant. 
Furthermore, from U.S. Pat. No. 4,693,737 it is known to use carbon dioxide 
as brine in a secondary circuit of a refrigeration system. In this case, 
the refrigerant in the secondary circuit is stored in a large tank in 
liquid form and the individual applications in the circuit are cooled by 
evaporation of liquid CO.sub.2. The tank is kept cooled by the primary 
circuit and on the return of vaporised CO.sub.2 in the secondary circuit 
it is condensed in the storage container. If the system is not in 
operation, the vaporised CO.sub.2 will condense against the surface of the 
contents in the container, but after some time the condensation will 
abate, with a subsequent increase in pressure which is limited by 
releasing vaporised CO.sub.2 from the secondary circuit. 
Moreover, U.S. Pat. No. 4,986,086 makes known a refrigeration system where 
a refrigerant, preferably carbon dioxide, is used, where the recommended 
maximum working pressure is about 35 bar. Evaporation which results in 
additional pressure is controlled by releasing CO.sub.2, from the system 
into the environment. This ventilation takes place chiefly from a 
container in the system which can accommodate a higher pressure than the 
working pressure in the rest of the refrigeration system. 
Another two-stage cooling process using carbon dioxide in the secondary 
circuit is described in GB 2 258 298 A. The secondary circuit in this 
system is described as having a maximum working pressure of about 34 bar, 
which is said to be higher than normal in a refrigeration system of this 
kind. This calls for a special design of the various elements in the 
refrigeration circuit in order to handle this high pressure. In the event 
of a breakdown or a period of non-operation, it is not stated how an 
additional increase in pressure as a result of the effect of temperature 
from the surroundings is dealt with. 
To maintain the temperature, and thus the pressure, in a container of 
carbon dioxide at a relatively low level when, e.g., transporting carbon 
dioxide, it is known from WO 88/04007 to insulate a container that is to 
hold carbon dioxide. In addition to insulation, it is known from WO 
93/23117 to provide a separate refrigeration unit in connection with a 
container that is to hold carbon dioxide with a view to maintaining the 
temperature, and thus the pressure, at a favourable level in relation to 
the maximum working temperature in the storage container. 
The use of carbon dioxide in a single application in connection with a 
refrigeration unit, where carbon dioxide is contained in an insulated 
tank, is also described in U.S. Pat. No. 4,129,432 and U.S. Pat. No. 
4,407,144. In these systems, carbon dioxide is released into the 
environment after evaporation. 
In the Nordic Refrigeration Journal ("Kulde-Skandinavia") No. 5/96, there 
is a discussion on pages 25 to 28 of the disadvantages and advantages 
which arise when using carbon dioxide as a refrigerant, and it is pointed 
out that carbon dioxide in refrigeration systems requires the system to 
have been built for especially high pressure, e.g., 120 to 140 bar, and 
even for a low temperature operation with a design pressure of 25 to 40 
bar, it is necessary to install supplementary equipment in order to cope 
with an inoperative situation. Similar problems are also presented in the 
article on pages 34 to 37 and page 60 in the Nordic Refrigeration Journal 
("Kulde-Skandinavia"), No. 4/96. Special attention is directed to the 
situation that arises when the system is not in operation, where the 
saturation pressure in the refrigerant exceeds maximum working pressure. 
SE 9202969 describes a cooling system where a container in a circulating 
circuit is located between a first and a second pressure reducing means. 
The purpose of the is container is to collect coolant in order to pass 
this into the screw compressor between the inlet and outlet of the 
compressor, in order to cool the screw compressor. Furthermore, a valve is 
installed which controls the flow of the gaseous coolant through the duct 
from the container to the screw compressor. A container is placed in the 
cooling circuit, but the pressure in parts of the cooling circuit is 
reduced further after the container by pressure reducing means and if the 
system stops operating, the coolant will be able to flow back to the 
container as it assumes ambient temperature and the pressure eventually 
increases, whereupon gaseous coolant will be able to condense against the 
surface of the liquid coolant in the container. However, this will not 
take place immediately from the parts of the system where the pressure is 
lower. i.e., after the pressure reduction valve. Furthermore, there is no 
disclosure of specific distinctive features of the container or the 
location of the pressure regulating means in connection therewith which 
enable the container to be a receptacle for vaporised coolant with the 
intention that this should to the greatest extent possible be condensed 
against the surface of the coolant in the container to be subsequently a 
storage container for coolant in a system that is not in operation. 
In DK 159894B, as in the aforementioned Swedish patent publication, a 
container is also located in a cooling circuit. The container is divided 
into two chambers and the purpose seems to be that a recirculation number 
greater than 1 is obtained, whereby the liquid and vapour circulate 
together in the cooling circuit, which gives better heat transfer in the 
evaporator. A valve system is provided in connection with the container, 
which helps to maintain the liquid levels in the separate chambers at the 
desired level, and also to contribute to a pressure equalisation between 
the chambers. Nor in this patent publication is the container designed for 
receiving coolant in vapour form in order that this should subsequently be 
condensed against the free surface of the coolant, and the container is 
thus not provided with the means which are necessary if the container is 
to have this function. 
One of the objects of the invention is to overcome the drawbacks that are 
associated with the prior art, and the refrigeration system is 
characterised according to the invention in that there is provided at 
least one insulated tank for the refrigerant in connection with the 
refrigeration circuit, which container is sufficiently proportioned and 
insulated and sufficiently filled with refrigerant in liquid phase so that 
at least parts of the vaporised refrigerant in the refrigeration circuit 
condense against the liquid surface in the container, and that the 
saturation pressure in the circuit essentially does not exceed maximum 
working pressure of the whole of or parts of the refrigeration circuit. 
Additional embodiments of the refrigeration system are set forth in the 
attached patent claims and in the following description with reference to 
appended drawings. 
The present invention provides a solution which enables a refrigeration 
system to be built primarily of conventional elements which require a 
maximum working pressure that is below the saturation pressure of the 
refrigerant used at ambient temperature. This will be the case, for 
example, when using carbon dioxide as refrigerant in most instances, as 
carbon dioxide at normal room temperature has a saturation pressure in the 
range of 50 to 60 bar which is higher than the normal maximum working 
pressure for a refrigeration system consisting of conventional elements. 
Furthermore, the present invention provides a solution where vaporised 
refrigerant, which will result in an increase in pressure in the 
refrigeration system, is not released through the pressure relief valve if 
the system is inoperative and affected by the temperature from the 
surroundings. This is to obviate the necessity of refilling the 
refrigeration system with refrigerant before it can be restarted. An ideal 
situation in this case would be that the refrigerant, in the event of a 
breakdown, is practically completely received in the container without the 
pressure exceeding maximum working pressure, so that the refrigeration 
system can be restarted without adding fresh refrigerant even if during 
the breakdown the refrigerant has reached a temperature that is 
considerably closer to the ambient temperature of the system than the 
working temperature of the refrigerant. Furthermore, the concept of the 
present invention will limit the build-up of pressure in the event of a 
breakdown, so that if the system is restarted after a relatively short 
time, this will happen without the refrigerant being released, or without 
the saturation pressure of the refrigerant having exceeded the maximum 
working pressure in the system. 
By arranging in the refrigeration circuit an insulated container which is 
adapted as regards size, insulation and rate of admission of the 
refrigerant in liquid phase, it will be possible, in the event of a 
breakdown, to maintain the temperature in the container at a level such 
that vaporised refrigerant returning to the container will condense 
against the surface of the liquid phase in the container and thus reduce 
the rise in pressure owing to evaporation in the circulating circuit. By 
designing the container so that wall thickness, insulation, magnitude of 
the liquid surface and size of the tank in other respects help to keep the 
temperature in the tank stable even in the event of a breakdown, it will 
be possible to obtain considerably lower increase of pressure per time 
unit in the circuit than by using an uninsulated container of the standard 
type. Furthermore, it will be possible to construct the container so that 
the whole of or parts of the quantity of fluid in the circulating circuit 
condense in the container before the saturation pressure exceeds maximum 
working pressure in the circuit if the system is not operating. 
As a result, a refrigeration system, for example, for grocery shops, may be 
produced using conventional elements for moderate working pressure which 
is considerably lower than the saturation pressure of the refrigerant at 
ambient temperature. In the event of a breakdown, according to the 
invention, it will be possible to condense vaporised refrigerant in the 
insulated container, thereby maintaining a pressure in the refrigeration 
system which does not exceed maximum working pressure. 
If, in addition, there are provided manual or automatic valves for closing 
the connections in/out of the container with a bypass of the valves, where 
there is provided a check valve, it will be possible to allow vaporised 
refrigerant to return to the insulated container and condense, in order 
thus to maintain a pressure in the circulating circuit which is lower than 
maximum working pressure. Safety valves may also be provided which, in the 
event of an undesirable build-up of pressure in the circulating circuit, 
release vaporised refrigerant into the surroundings. 
If the container is designed for a higher pressure, below, equal to or 
above the saturation pressure of the refrigerant, all of or parts of the 
refrigerant can be stored in the container after condensation for varying 
periods of time or indefinitely. 
Starting up after, e.g., a period of inoperation or a breakdown, is secured 
by valves which give a controlled fall in pressure in the insulated 
container after a rise in pressure in the same container above the maximum 
working pressure in the circuits.

FIG. 4 shows a system similar to that in FIG. 3, where the present 
invention is used in a secondary circuit, wherein an 
evaporator/condenser-device may be designed for lower pressure than the 
saturation pressure of the refrigerant at ambient temperature. 
FIG. 1 shows a refrigeration system having an insulated container 1 for the 
refrigerant in liquid phase and gas phase, and a circuit with intake 4 of 
the refrigerant in liquid phase, to evaporators 2 and then via a return 
pipe 5 to an insulated tank 1. From the tank 1 vaporised refrigerant then 
passes to the compressor 6 and then to the condenser 3 and then back via 
intake 7 to intake 4 via a heat exchanger in the insulated tank 1. On each 
of the pipe connections where the refrigerant is in the vaporised state 
there is arranged a safety valve 20 which, in the event of a build-up of 
pressure in the piping in excess of maximum working pressure, releases 
vaporised refrigerant into the surroundings. According to the invention, 
vaporised refrigerant in the return pipe 5 and the intake 8 will be 
capable of being conveyed back to the insulated tank 1 and, when the 
refrigeration system is inoperative, the vaporised refrigerant will be 
able to condense therein against the surface of the refrigerant in liquid 
form in order thus to maintain the saturation pressure in the refrigerant 
below the maximum working pressure of the refrigeration circuit without 
releasing vaporised refrigerant through the pressure relief valves or 
safety valves 20 to 22. In the event of a breakdown in the system, the 
valves 13 can be closed manually or automatically, and at bypass 14 there 
is arranged a check valve 15 which allows vaporised refrigerant to enter 
the insulated container 1 as the pressure rises in those parts of the 
refrigeration circuit where the temperature of the refrigerant rises as a 
result of the ambient temperature around the refrigeration system. 
The valves 40 and 41 allow for a controlled fall in pressure in the 
insulated tank 1 after an increase in pressure in the same tank above the 
maximum working pressure in the circuits owing to, e.g., a period of 
inoperation or a breakdown. The controlled fall in pressure is due to the 
operation of the refrigeration system or direct condensation in the 
condenser. During the fall in pressure it is important that the tank 50, 
condenser or associated pipe section have the necessary volume to 
accumulate condensed liquid during the fall in pressure. Moreover, 
evaporators 2 which, for example, may be freezer cabinets in a grocery 
shop or the like, are provided with valves etc. as in a normal 
conventional refrigeration circuit. 
FIG. 2 shows a refrigeration system essentially like that in FIG. 1 but 
where the intake 7 from the condenser 3 to the insulated tank 1 does not 
pass in a closed circuit with the intake 4 from the insulated tank 1 to 
evaporators 2. In this case, there is also provided on the intake 4 an 
automatic or manual valve 13 which can be closed if the refrigeration 
system breaks down. Moreover, a pump 9 may be provided for liquid 
transport of the refrigerant; alternatively the system may be based on 
self-circulation. This refrigeration system is also made in accordance 
with the inventive concept in that the container 1 is insulated and 
adapted in size and admission rate so that if the system breaks down, the 
refrigerant in the refrigeration circuit will be affected by the ambient 
temperature, whereby an increase in pressure will take place and vaporised 
refrigerant will be able to return to the insulated tank 1 via the pipes 5 
and 8. As the insulated tank 1 is made according to the invention, the 
vaporised refrigerant will condense in the tank against the surface of the 
refrigerant in liquid phase and pressure increase in the refrigeration 
system will be moderated. 
In FIG. 3 the present invention is used in a part of a secondary 
refrigeration circuit. In this case, the refrigeration circuit works in 
connection with a refrigeration system 30 through an evaporator/condenser 
device 31, 3 where the outflow 8 from the insulated tank 1 circulates 
through the condenser 3 and returns via the intake 7 to the insulated tank 
1. The circuit with evaporators 2 is in other respects the same as that in 
FIGS. 1 and 2, and in this system too it will be possible, in the event of 
a breakdown, for vaporised refrigerant to return to the insulated tank 1, 
whereby according to the invention it condenses against the surface of the 
refrigerant in liquid phase and the build-up of pressure in the 
refrigeration system is retarded considerably. 
In FIG. 4 the present invention is used in a part of a secondary 
refrigeration circuit as in FIG. 3. In this case, the refrigeration 
circuit works in connection with a refrigeration system 30 through an 
evaporator/condenser device 31, 3 where the outflow 8 from the insulated 
tank 1 circulates through the condenser 3 and returns via the intake 7 to 
the insulated tank 1. The valves between 3 and 7, 8 mean that the 
condenser device 3 can be designed for a lower pressure than the insulated 
tank 1. The circuit with evaporators 2 is in other respects the same as 
that in FIGS. 1, 2 and 3, and in this system too it will be possible, in 
the event of a breakdown, for vaporised refrigerant to return to the 
insulated tank 1, whereby according to the invention it condenses against 
the surface of the refrigerant in liquid phase and the build-up of 
pressure in the refrigeration system is retarded considerably. 
The container 1 will thus form a part of the circulating circuit as a low 
pressure receiver, optionally as a liquid container where the refrigerant 
is used as a secondary agent. 
By also designing the container 1 for a higher pressure and by providing it 
with the valves 13, 14 and 15 and also the valves 20, 21 and 22 adapted to 
the dimensioning of respectively the circulation system, container and 
optionally compressor/condenser, parts of or all of the refrigerant supply 
can be stored for varying lengths of time or indefinitely. 
When the refrigerant evaporates in the applications 2 and later condenses 
against the cold liquid surface in the tank 1, the relation between the 
condensation heat and the specific heat of the liquid will be crucial, and 
by insulating the tank 1 adequately and also ensuring there is a 
sufficient liquid volume, it will be possible to obtain an increase in 
pressure in the refrigeration system, for example, in the range of 2 bar 
per hour or less. Alternatively, all of or parts of the quantity of fluid 
in the circulating circuit will condense in the container or plurality of 
containers 1 before the saturation pressure in the refrigeration circuit 
exceeds maximum working pressure, even when the refrigeration circuit has 
reached approximately ambient temperature. If the breakdown is prolonged, 
the temperature in the insulated container 1 will rise so that the 
pressure here exceeds the maximum working pressure in the refrigeration 
circuit, but because of the valves 13 and the check valves 15, this rise 
in pressure will not spread to the rest of the refrigeration system, and 
if the pressure exceeds the maximum working pressure of the insulated 
tank, a pressure relief or safety valve 21 in association with the tank, 
located as shown on the outlet 8 from the tank 1 in FIGS. 1-4, will be 
able to release vaporised refrigerant and thus control the pressure in the 
container 1. This involves loss of refrigerant and when starting the 
refrigeration system after a breakdown, this loss must be replaced by 
adding fresh refrigerant. However, this situation can be greatly retarded 
or eliminated by using the present invention, and moreover refrigeration 
systems for the type of refrigerant discussed in connection with the 
present application, for example, carbon dioxide, can be designed and 
constructed for a considerably lower working pressure than the saturation 
pressure of the vaporised refrigerant at the ambient temperature of the 
refrigeration system. This reduces the costs of the refrigeration system 
considerably in that purpose-built elements are largely avoided and in 
that valves, pipes etc. will only take up a substantially lower load than 
would be the case if the system were to be designed for the saturation 
pressure of the refrigerant at ambient temperature.