DE102007025319A1 - Refrigerating device comprises a first heat exchanger operating as a gas cooler and a regulating valve connected to the first heat exchanger in the flow direction of the coolant to control the pressure in the first heat exchanger - Google Patents

Abstract

Refrigerating device comprises a first heat exchanger (6) operating as a gas cooler and a regulating valve (18) connected to the first heat exchanger in the flow direction of the coolant to control the pressure in the first heat exchanger. An independent claim is also included for a method for operating the refrigerating device. Preferred Features: The first heat exchanger and the regulating valve form one component unit. A first sensor determines the outlet temperature of the coolant at the outlet of the first heat exchanger to control the valve depending on the outlet temperature. A second sensor determines an ambient temperature of the first heat exchanger to control the valve depending on the ambient temperature.

Description

Background of the invention

The The present invention relates to a refrigeration system in which a refrigerant circulated in a circuit, according to the preamble of the claim 1. It also concerns Method for operating a refrigeration system according to the preambles of claims 10 and 15. Finally The invention relates to a method for operating a refrigeration system according to the generic term of claim 16.

State of the art

Out the prior art are refrigeration systems known in a thermodynamic cycle with supply of Work cold produce. It works a refrigerant in a cycle involving a first heat exchanger, an expansion medium, a second heat exchanger and a compressor. In the first heat exchanger is the refrigerant of one compared to the refrigerant cooler Environment heat withdrawn. Subsequently becomes the refrigerant expanded with the help of an expansion agent approximately adiabatically and thereby brought to a lower temperature. That expanded refrigerant goes through a second heat exchanger, in which there is heat from one to be cooled Environment absorbs. Subsequently becomes the refrigerant compressed in a compressor while at a higher temperature finally brought in the first heat exchanger the Heat again to the environment of this heat exchanger give, compared to the temperature of the compressed refrigerant cooler is.

In conventional refrigeration systems, the first heat exchanger is operated as a condenser in such a way that the refrigerant flows in the gaseous state in the first heat exchanger and is liquefied there by the cooling. During expansion in the expansion medium, the liquid refrigerant partially reverts to the gas phase. So operated refrigeration systems achieve a particularly favorable thermal efficiency (coefficient of performance, COP), wherein the thermal efficiency by the ratio of cooling capacity Q. _o Applied technical power W. is determined: COP = Q. _o / w ..

at many widely used refrigerants, z. As the hydrofluorocarbons (HFC), may be disadvantageous that they have a significant greenhouse effect (global warming potential, GWP) when released from the refrigeration system and in the atmosphere arrive, z. B. by leaks in the system or during their dismantling at the end of their life. The refrigerant Carbon dioxide, on the other hand, belongs to the refrigerants with a comparatively low GWP. However, with carbon dioxide as a refrigerant be disadvantageous that it above a temperature of about 31 ° C (critical Temperature) no longer liquefied can be, but always present in the so-called fluid phase. A refrigeration system, at the carbon dioxide as a refrigerant is used, therefore, in cases where in the first heat exchanger a condensation temperature below the critical temperature can not be reached, for. B. because of a high ambient temperature be called Summer days, no longer be operated in efficient liquefaction operation.

The invention is based task

Of the Invention is based on the object, an improved refrigeration system provide. Furthermore its task is to improve the way it operates a refrigeration system provide.

Inventive solution

to solution The object of the present invention teaches a refrigeration system with the features of claim 1. It also teaches methods of operation a refrigeration system with the features of the claims 10 and 15 and a method of operating a refrigeration system with the features of claim 16.

in the In the context of the present invention, "gas cooling operation" means that the first heat exchanger as a gas cooler is operated in the sense that in the first heat exchanger no liquefaction of the refrigerant takes place. In gas cooling mode is the refrigerant in the first heat exchanger preferably transcritical, d. H. in the fluid phase. "Liquefaction operation" means that the first heat exchanger as a liquefier is operated, d. h., that the refrigerant in the first heat exchanger at least partially from gaseous in the liquid State of aggregation passes.

The fact that the refrigeration system according to the invention can be operated in gas cooling operation is achievable that it can not be achieved even in cases where in the first heat exchanger, a condensation temperature below the critical temperature of the refrigerant can not be achieved, for. B. on warm summer days, can be used. The pressure p in the gas cooler can be controlled by the downstream of the first heat exchanger accumulation valve. By controlling the pressure p as a function of an outlet temperature T _{Aus of} the refrigerant from the first heat exchanger can be achieved that the refrigeration system is operated in gas cooling operation with a favorable thermal efficiency. In this case, the invention utilizes the knowledge of the inventor that the thermal effect is a function of both the outlet temperature T _{off of} the refrigerant at the first heat exchanger operated as a gas cooler and the pressure p of the refrigerant in the first heat exchanger. Next, the inventor has found that at a given outlet temperature T _out each of a pressure or pressure range exists in which the thermal efficiency is particularly favorable. Characterized in that according to the invention, the pressure p in the heat exchanger in dependence on the existing outlet temperature T _{off is} controlled, the first heat exchanger can be operated at a pressure or in a pressure range, which is characterized by this particularly favorable thermal efficiency.

at the method according to the invention for operating a refrigeration system according to claim 11, the refrigeration system depending on the determined temperature between the gas cooling operation and a liquefaction operation be switched back and forth. So is achievable that of the gas cooling operation to the liquefaction plant can be switched if the operating conditions, eg. B. the Ambient temperature of the first heat exchanger, to change so that a liquefaction temperature can be reached below the critical temperature of the refrigerant, and conversely, also from the liquefaction operation in the gas cooling operation can be switched if a condensing temperature below the critical temperature of the refrigerant no longer or only conditionally can be achieved. Especially is achievable that the refrigeration system operated in such a way that, if both modes are possible, preferably operated in liquefaction operation becomes. The liquefaction operation is generally in terms of thermal efficiency better as the gas cooling operation.

Preferred embodiments of the invention

advantageous Training and further education, which individually and in combination with each other can be used are the subject of the dependent Claims.

In a preferred embodiment The invention is the control of the pressure p of the refrigerant in gas cooling mode with the help of one in the first heat exchanger downstream accumulation valve. The preferred damming valve can the flow of the refrigerant throttle, in a conceivable embodiment also essentially Stop completely to the pressure p of the refrigerant in the first heat exchanger increase, or it can facilitate the flow to lower the pressure p. In this way, the pressure in the first heat exchanger on a preferred Value to be set. The damming valve is preferred for this purpose equipped with a pressure sensor, which is particularly preferably integrated in the backup valve.

Preferably, the refrigeration system also includes a first temperature sensor to determine the outlet temperature of the refrigerant at the exit from the first heat exchanger. The first temperature sensor is preferably in functional connection with the damming valve in order to control the latter depending on the outlet temperature T _Aus . The first temperature sensor is preferably arranged at the outlet of the first heat exchanger. However, it is also conceivable to arrange a temperature sensor elsewhere and to control the damming valve on the basis of the temperature determined there, if this temperature is in a known relationship with the outlet temperature T _Off . Thus, in particular instead of the first temperature sensor (or in addition to it), a second temperature sensor for determining the ambient temperature T may _be provided on the _outside .

In order to control the damming valve on the basis of the determined temperature values in the gas cooling operation, preferably a computing means is provided, for example an electronic circuit with a microprocessor. In particular, if the backup valve is not based on the outlet temperature T _Aus but z. B. is _externally controlled on the basis of the ambient temperature T, heat transfer properties of the first heat exchanger are preferably stored as parameters in the computing means to close on the basis of these heat transfer properties of the / the measured temperature (s) to the set pressure p of the refrigerant. The computing means preferably forms a structural unit with the first heat exchanger, particularly preferably with the first heat exchanger and the backup valve.

Of the first heat exchanger and the backup valve preferably form a structural unit. Particularly preferably they are housed in a common housing. Particularly preferably, the common structural unit includes beside the first heat exchanger and the damming valve also the first temperature sensor. The first temperature sensor is preferably between the first heat exchanger and the back pressure valve in the cycle of the refrigeration system arranged, more preferably directly adjacent to the first Heat exchanger.

In a preferred embodiment of the invention, in the gas cooling operation, the pressure p of the refrigerant in the first heat exchanger is controlled at least within a predetermined range of the pressure p so that it depends continuously on the outlet temperature T _Aus , preferably substantially linearly: p = a × T _off + b. Here, a and b are predetermined sizes.

Preferably, the pressure p of the refrigerant in the first heat exchanger at least within a predetermined range of the outlet temperature T _out is controlled so that it above p _min = 2.775 bar / ° C × T _off - 18.2917 bar, more preferably above p _min = 2.775 bar / ° C × T _off - 13.2917 bar; particularly preferably above p _min = 2.775 bar / ° C × T _off - 10.7917 bar, more preferably above p _min = 2.775 bar / ° C × T _off - 9.5417 bar. Preferably, the pressure p of the refrigerant in the first heat exchanger at least within a predetermined range of the outlet temperature T _out is controlled to be below p _max = 2.775 bar / ° C × T _out + 12.2917 bar, more preferably below p _min = 2.775 bar / ° C × T _off + 2.2917 bar, more preferably below p _max = 2.775 bar / ° C × T _off - 3.2917 bar particularly preferably below p _max = 2.775 bar / ° C × T _off - 5.7917 bar is located. Particularly preferably, the pressure is within the predetermined range in the vicinity of p _opt = 2.775 bar / ° C × T _off - 8.2917 bar.

Of the mentioned above predetermined range of the pressure p is preferably an upward open area whose lower limit is above the critical pressure of the refrigerant is, especially preferably over 31 ° C, z. Between 31 and 34 ° C, z. At 32 ° C. The pressure p in the first heat exchanger is in gas cooling mode preferably controlled so that it does not fall below the lower limit, to make sure the refrigerant in the first heat exchanger remains transcritical. With this embodiment of the invention may be advantageous ensure that the refrigeration system according to the invention stable in gas cooler operation is working.

In the liquefaction operation, the pressure p of the refrigerant in the first heat exchanger is preferably controlled by means of a surge valve connected downstream of the first heat exchanger so that the pressure p does not fall below a minimum condensing pressure p _min . With this embodiment of the invention can be advantageously achieved that the pressure p is adjustable so that proper functioning of the refrigeration system, in particular the expansion valve and the compressor of the refrigeration system is ensured. These usually require a minimum pressure difference between the condensation pressures and the evaporation pressure in a second heat exchanger in which heat is supplied to the coolant from the environment.

This embodiment of the invention utilizes the fact that the heat transfer characteristics of the first heat exchanger may be deteriorated by damming up of the refrigerant, preferably by reducing the standing to available heat transfer surface, thus increasing the condensing temperature T increases _feature and thus the pressure p. The minimum condensing pressure P _min is preferably selected as low as possible but as high as necessary to ensure the proper functioning of the refrigeration system. It is preferably 1 to 10 bar, more preferably between 2 and 5 bar above the evaporation pressure.

The pressure p can be done using a pressure sensor or it may be _features of the condensing temperature T and / or at least a temperature which is related to the outlet temperature T _out in a known relationship derived. Thus, instead of the first temperature sensor (or in addition to it), a second temperature sensor for determining the ambient temperature T can _be provided _externally . It is also conceivable that a further temperature sensor is provided, preferably for measuring an evaporation temperature in the second heat exchanger, and the evaporation temperature also used to control the backup valve.

The control of the backup valve on the basis of the determined temperature values in the liquefaction operation is preferably also done by the computing means. In particular, if the backup valve is not based on the outlet temperature T _Aus but z. B. is controlled _externally on the basis of the ambient temperature T, heat transfer properties of the first heat exchanger are preferably stored as parameters in the computing means to close on the basis of these heat transfer properties of the / the measured temperature (s) to the set pressure p of the refrigerant.

The preferred refrigeration system is operable both in gas cooling operation and in liquefaction operation. Preferably, in the liquefaction operation of the refrigeration system, the liquefaction or condensation temperature T is determined _feature of the refrigerant and the refrigeration system is switched on gas cooling operation, when the liquefaction or condensation temperature T _features specified liquefaction limit temperature T exceeds _{switch feature.} The switching from the liquefaction operation to the gas cooling operation is preferably carried out by accumulating the refrigerant at the accumulation valve, preferably in order to achieve a pressure p in the first heat exchanger above the critical pressure of the refrigerant. For damming the damming valve is preferably at least partially closed. In one embodiment of the invention, a bridging of the accumulation valve is interrupted.

For determining the liquefaction or condensation temperature, preferably the temperature of the refrigerant downstream of the first heat exchanger, particularly preferably at the outlet of the first heat exchanger, particularly preferably measured before the back pressure, preferably with a temperature sensor, for. B. the first temperature sensor. Exceeding the liquefaction limit temperature is preferably an override for the duration of a predetermined period of time, e.g. B. 5 min. However, embodiments of the invention are also conceivable in which the refrigeration system switches immediately to the gas cooling operation when a liquefaction or condensation temperature T _verf above the liquefaction limit temperature T _{switch is} detected. The liquefaction _limit temperature T _switch is preferably below the critical temperature of the refrigerant, particularly preferably below 31 ° C., particularly preferably below 30 ° C., particularly preferably below 29 ° C. It is also preferably above 0 ° C, more preferably above 10 ° C, more preferably above 20 ° C, particularly preferably above 25 ° C, z. At 27 ° C.

During operation of the refrigeration system in gas cooling operation, an outside temperature T is preferably determined _outside an environment of the first heat exchanger, particularly preferably by means of a second temperature sensor. The second temperature sensor preferably forms a structural unit with the first heat exchanger, particularly preferably with the first heat exchanger and the backup valve. Particularly preferably, the second temperature sensor for measuring the outside temperature T is accommodated _outside in a common housing with the first heat exchanger, particularly preferably with the first heat exchanger and the backup valve. Preferably, the refrigeration system is switched from the gas cooling operation in the liquefaction operation when the determined ambient temperature T _outside the first heat exchanger falls below a predetermined ambient limit temperature T _{switch outside} . The preferred ambient limit temperature T _outside is preferably below the critical temperature of the refrigerant, particularly preferably below 31 ° C., particularly preferably below 25 ° C., particularly preferably below 20 ° C., particularly preferably below 18 ° C. It is preferably above 0 ° C, more preferably above 8 ° C, more preferably above 12 ° C, particularly preferably above 14 ° C, z. At 16 ° C. The switching from the gas cooling operation to the liquefaction operation is preferably carried out by opening or bridging the backup valve.

The preferred refrigerant is carbon dioxide. But there are also other refrigerants conceivable.

A preferred refrigeration system also includes a the damming valve in the flow direction of the refrigerant downstream expansion means, preferably an expansion valve, to the refrigerant to expand, preferably substantially adiabatic. In liquefaction operation goes the liquefied refrigerant during expansion, preferably at least partially into the gas phase.

Preferably includes the refrigeration system according to the invention a the expansion agent in the flow direction of the refrigerant downstream second heat exchanger, to supply heat to the expanded refrigerant, preferably from a to be cooled Environment of the second heat exchanger, z. B. a refrigerator. In liquefaction operation go liquid Proportions of the refrigerant in the second heat exchanger preferably completely into the gas phase over.

In a preferred embodiment The invention includes the refrigeration system Furthermore one in the flow direction of the refrigerant the second heat exchanger downstream and the first heat exchanger upstream compressor, to the refrigerant to condense. The preferred compressor is a compressor, for. B. a rolling piston, a scroll compressor, a reciprocating compressor, a screw compressor or a turbocompressor. But they are too versions the invention conceivable in which the compressor is a thermal Compressor is how he z. B. from absorption refrigeration systems is known.

In a possible execution includes the refrigeration system according to the invention a heat exchanger, the one heat transfer from in the first heat exchanger cooled compressed refrigerant to the expanded refrigerant before the compression. For this, the warm side is preferably in the flow direction of the refrigerant downstream of the first heat exchanger and the compressor. In this embodiment of the invention can be achieved that the refrigerant Cooled down before expansion is going to be on the second heat exchanger yet to reach lower temperatures. The invention also includes embodiments with several heat exchangers, Expansion means and compressors that are parallel to each other or can be connected in series. For several compressors connected in series, the cold can Side of the heat exchanger in refrigerant circuit be arranged between the two compressors. The refrigeration system according to the invention can also comprise several refrigerant circuits, which are connected in parallel or cascade-like manner.

Brief description of the figures

The The invention will be described below with reference to schematic drawings embodiments in more detail explained.

It demonstrate:

1 schematically shows the cycle of a refrigeration system according to the invention in the liquefaction operation, shown in the phase diagram of the refrigerant carbon dioxide;

2 schematically shows the cycle of a refrigeration system according to the invention in the transcritical gas cooling operation, shown in the phase diagram of the refrigerant carbon dioxide;

3 : the thermal efficiency (COP) of a refrigeration system according to the invention in the transcritical gas cooling operation with the refrigerant carbon dioxide as a function of the pressure at different outlet temperatures of the refrigerant;

4 : the refrigerant pressure p in the first heat exchanger with the best thermal efficiency and this thermal efficiency (COP) as a function of the outlet temperature T _{Aus of} the refrigerant from the first heat exchanger in transcritical gas cooling operation;

5 : a schematic representation of a refrigeration system according to the invention; and

6 : the sorted annual temperature profiles of the ambient temperature of the first heat exchanger, the refrigerant temperature at the outlet of the first heat exchanger, the cooling capacity and the fan speed of a refrigeration system according to the invention.

Detailed description of a execution Beipiels

To explain the operation of the refrigeration system 1 is in 1 the phase diagram of the refrigerant carbon dioxide in the plane defined by the gas enthalpy (horizontal axis) and gas pressure (vertical axis) plane. The phase boundary line 2 includes the mixing zone in which the carbon dioxide is partly liquid and partly gaseous. On the left side of the mixing zone, the carbon dioxide is only liquid, on the right side only gaseous. In addition, in the phase diagram are isotherms 3 specified. At pressures or temperatures above the critical point 4 Carbon dioxide can no longer be liquefied, but is present in the transcritical so-called fluid phase.

The cycle during the liquefaction operation of the refrigeration plant 1 begins with the withdrawal 5 heat from the refrigerant under substantially isothermal conditions and the transition of the refrigerant from the gaseous to the liquid state. The process takes place in a first heat exchanger 6 the refrigeration system 1 instead (see 5 ). This is followed by an approximate adiabatic expansion 7 of the refrigerant through expansion valves 8th the refrigeration system. In this case, the refrigerant continues to cool and also partially reverts to the gaseous state of matter. In the next step 9 is the refrigerant in the second heat exchanger 10 Heat is supplied from an environment to be cooled, wherein the refrigerant is now completely in the gaseous state. Finally, the refrigerant is in compressors 11 compacted from the outside while providing mechanical energy 12 so that both pressure and temperature increase significantly. The compressed refrigerant then becomes the withdrawal of heat 5 the first heat exchanger 6 fed.

2 shows in the same phase diagram of the refrigerant carbon dioxide, the cycle of trans-critical operation of the refrigeration system 1 , The cycle begins with the transcritical CO ₂ in the first heat exchanger 6 deprived of heat under essentially isobaric conditions 13 becomes. Subsequently, the carbon dioxide is subjected to essentially adiabatic conditions through the expansion valve 8th expands 14 , As a result, it becomes subcritical, partially gaseous, partially liquid carbon dioxide, in the second heat exchanger 10 Heat is supplied under essentially isothermal and isobaric conditions 15 is until the refrigerant has passed substantially completely into the gas phase. After that it will be in the compressor 11 compressed under supply of mechanical work 16 and goes back into the transcritical state. In 2 are also the heat dissipated per cycle amount of heat Q _o and the mechanical work W spent thereon whose quotient COP = Q _o / W is the thermal efficiency.

3 shows the thermal efficiency (COP) of the refrigeration system 1 as a function of the pressure p in the first heat exchanger at different temperatures of the transcritical CO ,. It can be seen that at any temperature an optimum pressure is possible at which the thermal efficiency is highest. Out 4 showing the pressure p in the first heat exchanger with the best thermal efficiency and this thermal efficiency (COP) as a function of the outlet temperature T _Aus , it can be seen that the optimum pressure with the carbon dioxide temperature in the first heat exchanger 6 linear 17 related.

This context uses the in 5 illustrated refrigeration system 1 in transcritical operation by holding the back pressure valve 18 depending on the temperature sensor at the output of the first heat exchanger 6 measured temperature of the transcritical carbon dioxide pressure p in the first heat exchanger 6 controls so that it essentially corresponds to the optimum pressure. The refrigeration system in 5 Also includes another refrigerant circuit cascade-linked to the first one and the refrigerant before expansion through the expansion valves 19 through the warm side of a heat exchanger 20 is led to white ter to cool, z. B. to cool a freezer compartment. After this refrigerant the second heat exchanger 21 has undergone it first in compressors 22 then compressed on the cold side of the heat exchanger 20 To absorb heat and together with the refrigerant from the previously described cycle to the compressors 11 to be guided.

6 shows the ambient temperature refueling 23 , the refrigerant _temperature T _verf 24 (in liquefaction mode) or T _Off 25 (in transcritical gas cooling operation) at the outlet of the first heat exchanger 6 , the cooling capacity 26 and the fan speed 27 the refrigeration system 1 as a function of the sorted annual temperature history. For the most part 28 In their operation, the refrigeration system is running 1 in the liquefaction operation. In this operation, the ambient temperature is below about 17 ° C and it liquefaction temperatures T _verf 21 of less than 26 ° C. During liquefaction operation, an in 5 not shown pressure sensor the pressure p in the first heat exchanger 6 , If the pressure p at low ambient temperatures threatens to drop below the minimum condensing pressure p _min which is necessary for the proper functioning of the refrigeration system, the backup valve becomes 18 operated to maintain the pressure p at the minimum condensing pressure p _min . This corresponds to a liquefaction temperature, which in 6 on the right with a horizontal curve of the condensing temperature T _verf 24 at about 5 ° C is shown.

If the condensing temperature rises above T _switch = 26 ° C, the refrigeration system is switched to gas cooling mode with transcritical carbon dioxide. That happens by the damming valve 18 is actuated to adjust the pressure p in the first heat exchanger to a pressure above the critical pressure of the refrigerant. In this gas cooling mode, the pressure in the first heat exchanger 6 according to the in 4 shown relationship at outlet temperatures T _{off of} the refrigerant from the first heat exchanger above 32 ° C adjusted so that the thermal efficiency of the refrigeration system 1 is maximum. At the same time, the pressure p is kept above 82 bar, so that the refrigeration system operates stably in the transcritical range. During gas cooling operation, an in 5 not shown temperature sensor, the ambient temperature T _{outside of} the first heat exchanger 6 , If this drops below about T _{switch outside} = 17 ° C, the backup valve will be 18 opened to switch from the gas cooling operation back into the liquefaction, because from this ambient temperature T _outside a liquefaction temperature below the critical temperature of the refrigerant in the first heat exchanger 6 is possible.

The in the foregoing description, claims and drawings Features can both individually and in any combination for the realization the invention in its various embodiments of importance be.

Claims (18)

refrigeration plant 1 in which a refrigerant circulates in a circuit and the first heat exchanger 6 comprises, to extract heat from the refrigerant, characterized in that the first heat exchanger 6 is operated as a gas cooler and the refrigeration system 1 a in the direction of flow of the refrigerant to the first heat exchanger 6 downstream damming valve 18 includes the pressure in the first heat exchanger 6 can control. refrigeration plant 1 according to claim 1 or 2, characterized in that the first heat exchanger 6 and the damming valve 18 form a structural unit. refrigeration plant 1 according to one of the preceding claims, characterized in that it comprises a first temperature sensor for determining an outlet temperature T _{Aus of} the refrigerant at the outlet of the first heat exchanger 6 includes, depending on the outlet temperature T _out the accumulation valve 18 head for. refrigeration plant 1 according to one of the preceding claims, characterized in that it comprises a second temperature sensor for determining an ambient temperature T _{outside of} the first heat exchanger 6 includes, depending on this ambient temperature T _outside the back pressure valve 18 head for. refrigeration plant 1 according to claim 3 or 4, characterized in that the accumulation valve 18 is in operative connection with the first and / or the second temperature sensor such that a pressure p of the refrigerant in the first heat exchanger 6 at least within a predetermined range of the pressure p is controllable so that it depends substantially continuously on the outlet temperature T _Aus . refrigeration plant 1 according to one of the preceding claims, characterized in that it comprises a pressure sensor to the pressure p of the refrigerant in the gas cooler 6 to investigate. refrigeration plant 1 according to one of claims 3 to 6, characterized in that the accumulation valve 18 are in operative connection with the first and / or the second temperature sensor and / or the pressure sensor such that the pressure p of the refrigerant in the first heat exchanger 18 is so controllable, that it does not fall below a minimum condensing pressure p _min . refrigeration plant 1 according to claim 5 or 7, characterized in that the refrigeration system comprises computing means to the back pressure valve 18 to drive and the computing means with the heat exchanger 6 and / or the back pressure valve 18 forming a structural unit. refrigeration plant 1 according to one of the preceding claims, characterized in that the refrigerant is carbon dioxide. Method for operating a refrigeration system 1 in which a refrigerant circulates in a circuit and the first heat exchanger 6 includes, which is operated in the gas cooling operation to extract heat from the refrigerant, comprising the steps of: - Determining an outlet temperature T _{out of} the refrigerant when exiting the first heat exchanger 6 and / or at least one temperature which is in known relation to the outlet temperature T _Aus , and - controlling a pressure p of the refrigerant in the first heat exchanger 6 depending on the determined temperature (s). A method according to claim 10, characterized in that the determination of the outlet temperature T _Aus by means of a between the first heat exchanger 6 and the damming valve 18 arranged first temperature sensor is carried out and the control of the pressure p by means of a first heat exchanger 6 downstream accumulation valve 18 he follows. A method according to claim 10 or 11, characterized in that the pressure p of the refrigerant in the first heat exchanger 6 is controlled at least within a predetermined range of the pressure p so that it depends continuously on the outlet temperature T _Aus . Method according to one of claims 10 to 12, characterized in that the pressure p of the refrigerant in the first heat exchanger 6 at least within a predetermined range of the outlet temperature T _Aus is controlled so that it above p _min = 2.775 bar / ° C × T _Off - 18.2917 bar and / or below p _max = 2.775 bar / ° C × T _Aus + 12, 2917 cash. Method according to claim 12 or 13, characterized that the predetermined range of the pressure p is preferably an after upper open area whose lower limit is above the critical pressure of the refrigerant lies. Method for operating a refrigeration system 1 in which a refrigerant circulates in a circuit and the first heat exchanger 6 which is operated in the liquefaction operation to extract heat from the refrigerant, wherein the pressure p of the refrigerant in the first heat exchanger 6 by means of a first heat exchanger 6 downstream accumulation valve 18 is controlled so that the pressure p does not fall below a minimum condensing pressure p _min . Method for operating a refrigeration system 1 in which a refrigerant circulates in a circuit and the first heat exchanger 6 to extract heat from the refrigerant, comprising the steps of: - operating the refrigeration system 1 in one of the two operating modes condensing operation and gas cooling operation; - determining a temperature; and - comparing the determined temperature with a predetermined limit temperature to decide, based on the comparison, whether the refrigeration system 1 is switched to another of the two operating modes condensing operation, gas cooling operation. A method according to claim 16, characterized in that during operation of the refrigeration system 1 in the liquefaction operation, a liquefaction or condensation temperature T _{ref of} the refrigerant is determined and the refrigeration system switches over to gas cooling operation when the liquefaction or condensation temperature T _{verf of} the refrigerant exceeds a predetermined liquefaction limit temperature T _switch . A method according to claim 16 or 17, characterized in that during operation of the refrigeration system 1 in the gas cooling operation, an outside temperature T _outside an environment of the first heat exchanger 6 is determined and the refrigeration system 1 switched to liquefaction operation when the ambient temperature T _{outside of} the first heat exchanger 6 _falls below a predetermined ambient _limit temperature T _{switch outside} .