Cooling device, method for operating a cooling device and test chamber

Description

[0001] The invention relates to a method for operating a cooling device, a cooling device and a test chamber with a cooling device, wherein by means of the cooling device with a cooling circuit with a refrigerant, a heat exchanger, an internal heat exchanger, a compressor, a condenser and a controllable expansion element of the cooling device, a temperature of at least -80 °C or lower is formed at the heat exchanger, wherein the cooling device is designed as a compression refrigeration system, wherein the refrigerant undergoes a phase change in the heat exchanger, wherein the refrigerant of a high-pressure side of the cooling circuit is cooled by means of the internal heat exchanger, wherein the cooling of the refrigerant of the high-pressure side by means of the internal heat exchanger is used to lower an evaporation temperature at the expansion element,where a zeotropic refrigerant is used as the refrigerant, where the refrigerant is non-flammable and has a relative CO2 equivalent, based on 100 years, of < 2500.,

[0002] Such processes and cooling devices are regularly used in so-called test chambers, which are used to test the physical and / or chemical properties of objects, in particular devices. Temperature test cabinets or climatic test cabinets are known, within which temperatures can be set in a range of -70 °C to +180 °C. In climatic test cabinets, additional desired climatic conditions can be set, to which the device or test object is then exposed for a defined period of time. The temperature of a test chamber containing the test object to be tested is regularly controlled in a recirculation duct within the test chamber. The recirculation duct forms an air treatment chamber in the test chamber, in which heat exchangers are arranged to heat or cool the air flowing through the recirculation duct or the test chamber. A fan orA fan draws in the air in the test chamber and directs it through the recirculation duct to the respective heat exchangers. The test specimen can thus be tempered or subjected to a defined temperature change. During a test interval, for example, the temperature in the test chamber can fluctuate between a maximum and a minimum temperature. Such a test chamber is known, for example, from EP 0 344 397 A2.

[0003] The refrigerant used in a cooling circuit should have a relatively low CO2 equivalent, i.e. a relative greenhouse potential or global warming potential (GWP) should be as low as possible in order to avoid indirect damage to the environment caused by the refrigerant if it is released. The GWP indicates how much a specified mass of a greenhouse gas contributes to global warming, with carbon dioxide serving as a reference value. The value describes the average warming effect over a certain period of time, with 100 years being specified here for the sake of comparability. For the definition of the relative CO2 equivalent or GWP, reference is made to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), Assessment Report, Appendix 8.A, Table 8.A.1. Carbon dioxide (CO2) or carbon dioxide is also known to be used as a pure refrigerant.Carbon dioxide is inexpensive, non-flammable, and, with a global warming potential (GWP) of 1, essentially environmentally neutral. Carbon dioxide has a freezing temperature, or triple point, of -56.6 °C, which makes it impossible to achieve lower temperatures with carbon dioxide. Furthermore, a refrigerant should be non-flammable to avoid, among other things, complicating the filling, shipping, and operation of a refrigeration circuit due to potential safety regulations. The use of a flammable refrigerant also increases the cost of manufacturing a refrigeration circuit due to the necessary design measures. Flammability refers to the refrigerant's ability to react with ambient oxygen, releasing heat.A refrigerant is flammable in particular if it falls into fire class C according to the European standard EN2 or DIN 378 classes A2, A2L and A3 in the version valid before the priority date of the application.

[0004] In a zeotropic refrigerant mixture, a phase transition occurs over a temperature range, the so-called temperature glide. The temperature glide is defined as the difference between the boiling point and the dew point at constant pressure. Zeotropic refrigerant mixtures usually contain a high mass fraction of a non-flammable component in the refrigerant mixture, although this component is characterized by a comparatively high GWP. Refrigerants should also be as easy to use as possible, i.e., not require complex technical modifications to a cooling system. Particularly for refrigerants with a temperature glide > 3 K, it is necessary to adapt an expansion device and a heat exchanger or evaporator in the relevant cooling circuit to the evaporation temperature of the refrigerant and to provide appropriate control.

[0005] A further distinction must be made between refrigerants designed for static operation of a cooling device, i.e. a cooling device with an essentially constant temperature at the heat exchanger or evaporator over a longer period of time, and a dynamic cooling device with a comparatively rapid temperature change at the heat exchanger. Such dynamic cooling devices are installed in test chambers, among other places, so that the refrigerant used must be usable within a wide temperature range. Furthermore, cooling devices are known in which a zeotropic refrigerant mixture is successively evaporated. This means that material components of the refrigerant are evaporated one after the other via an expansion device. Such cooling devices are also referred to as mixture cascade systems and are suitable for creating an essentially static low temperature.

[0006] US Pat. No. 3,872,682 A discloses a refrigeration circuit comprising a compressor, a condenser, an internal heat exchanger, an expansion valve, and a heat exchanger. The refrigeration circuit uses a zeotropic refrigerant mixture of R744 and R12, with the two refrigerant components being mixed in a homogenizer. After expansion at the expansion valve, the R744 freezes and subsequently sublimes. Furthermore, subcooling of the refrigerant on a high-pressure side is provided by the internal heat exchanger.

[0007] The document SOBIERAJ MICHAL ET AL: "High phase-separation efficiency..." also discloses such a refrigeration cycle in an experimental setup. Here, a refrigerant mixture of R744 and R600A is used. In particular, the temperature of the refrigerant after an expansion device drops below the triple point of CO2 or R744. The CO2 is then at least partially in the solid phase.

[0008] WO 2019 / 048250 A1 describes a test chamber with an additional cooling circuit containing a refrigerant mixture.

[0009] The present invention is therefore based on the object of proposing a method for operating a cooling device, a cooling device and a test chamber with a cooling device, with which a temperature of at least - 80 °C can be achieved in an environmentally friendly and safe manner using simple means.

[0010] This object is achieved by a method having the features of claim 1, a cooling device having the features of claim 12 and a test chamber having the features of claim 17.

[0011] In the method according to the invention for operating a cooling device, a temperature of at least -80°C or lower is achieved at the heat exchanger by means of the cooling device having a cooling circuit with a refrigerant, a heat exchanger, an internal heat exchanger, a compressor, a condenser, and a controllable expansion element of the cooling device, wherein the cooling device is designed as a compression refrigeration system, wherein the refrigerant undergoes a phase change in the heat exchanger, wherein the refrigerant of a high-pressure side of the cooling circuit is cooled by means of the internal heat exchanger, wherein the cooling of the refrigerant of the high-pressure side by means of the internal heat exchanger is used to lower an evaporation temperature at the expansion element, wherein a zeotropic refrigerant is used as the refrigerant, wherein the refrigerant is non-flammable and has a relative CO2 equivalent, based on 100 years, of < 2500,wherein the cooling device is operated with the refrigerant in a liquid and vapor phase, optionally in a first operating state, in which the carbon dioxide is present exclusively in the liquid phase even during expansion, or in a second operating state as a sublimation refrigeration system with the refrigerant in a partially solid phase, wherein in the second operating state the expansion element is controlled by means of a control device of the cooling device in such a way that the refrigerant partially freezes at the expansion element during expansion.

[0012] In the method according to the invention, the heat exchanger is connected to the cooling circuit or integrated into it in such a way that refrigerant circulating in the cooling circuit flows through the heat exchanger. The heat exchanger of the cooling circuit can, for example, be arranged within a test space of a test chamber or in an air treatment space of the test chamber, so that air is conditioned or tempered via the heat exchanger. The internal heat exchanger is also integrated into the cooling circuit and is arranged downstream of the condenser and upstream of the expansion element in the direction of refrigerant flow. In the internal heat exchanger, the refrigerant is subcooled, thus lowering its evaporation temperature. This makes it possible to develop comparatively low temperatures at the heat exchanger when the refrigerant evaporates in the heat exchanger after expansion at the expansion element.The expansion element can be controlled by means of the control device of the cooling device such that a defined amount of refrigerant flows over the expansion element in a period of time, whereby the subcooled refrigerant is always liquid or in a liquid phase. After expansion at the expansion element, the state of aggregation of the refrigerant changes from liquid to gaseous depending on the evaporation temperature at the expansion element. The liquid refrigerant gradually evaporates within a temperature range or glide, so that in the cooling circuit or heat exchanger the refrigerant is present in both a liquid and a gaseous phase in sections. The cooling device is then operated as a so-called cold vapor compression refrigeration system. According to the invention, the expansion element is controlled by means of the control device such that the refrigerant partially freezes during expansion at the expansion element.This means that downstream of the expansion element in the cooling circuit, the refrigerant is at least partially in the solid phase. It is crucial that the refrigerant does not completely freeze, so that the still liquid or gaseous components of the refrigerant can transport the solid components of the refrigerant in the cooling circuit, preventing any potential blockage of the cooling circuit by solid components. The partially frozen or solid components of the refrigerant then sublimate, or change from the solid to the gaseous phase. This makes it possible to utilize the refrigerant's sublimation enthalpy, which is the sum of the enthalpy of fusion and the enthalpy of vaporization. Consequently, it is also possible to utilize the enthalpy of fusion to generate cold.Since the sublimation process of the refrigerant takes place at lower temperatures than the evaporation process of the refrigerant, it is possible to achieve an even lower temperature with the refrigerant, as would be possible with the refrigerant in a cold vapor process. The components of the refrigerant that freeze therefore have the highest freezing point of all the components of the refrigerant. When a component of the refrigerant freezes, solid particles are formed within the liquid phase of the refrigerant and the phases are intensively mixed, which ensures the flow of the refrigerant. Overall, it is therefore possible to partially freeze the refrigerant using only an adapted control device for the cooling system or a suitable control of the expansion device, whereby even lower temperatures can be achieved at the heat exchanger.A refrigerant can be used which is also used for a cold steam process and is environmentally friendly and safe to use.

[0013] According to the invention, the cooling device is designed as a compression cooling system and is operated as such with the refrigerant in a liquid and vapor phase optionally in a first operating state or in a second operating state as a sublimation cooling system with the refrigerant in a partially solid phase.

[0014] As a result, the refrigerant in the heat exchanger can partially sublimate. In the process, the frozen portion of the refrigerant in the heat exchanger, or the particles in the liquid phase, sublimates. This lowers the triple point of the liquid phase, and conventional evaporation occurs due to the low triple point. In addition to expanding or reducing the temperature range of the cooling device, the performance of the cooling device is also increased. Depending on requirements, the cooling device can now be operated with a partially sublimating refrigerant or with a fully evaporating or liquid refrigerant. The method therefore makes it possible to operate the cooling device with an essentially conventional cooling circuit. In this way, the compressor orThe compressor sucks in the superheated refrigerant in the conventional manner and continuously pumps it through the cooling circuit without any modifications to the cooling circuit being necessary.

[0015] In the cooling circuit, the refrigerant can be formed into a homogeneous mixture of a liquid and / or gaseous phase with particles of a solid phase in a flow direction downstream of the expansion device and upstream of the heat exchanger. In particular, the fact that a largely homogeneous mixture of the liquid phase or wet vapor with the particles of the solid phase can be formed prevents clogging or blockage of the cooling circuit by particles of the solid phase. Furthermore, it can be ensured that no separation of refrigerant components occurs. The mixing ratio of the components or a refrigerant mixture is therefore always essentially constant or consistent along the cooling circuit.

[0016] The control device can be used to regulate the expansion element in such a way that a pressure of 1 bar or less can be developed on a low-pressure side of the cooling circuit. This makes it possible to set a sublimation temperature or a sublimation pressure in the cooling circuit even below an ambient pressure of 1 bar. This pressure can be set or regulated particularly easily using the control device or by the expansion element. No additional modifications to the cooling circuit or components are required for this purpose. For example, the pressure on the low-pressure side can easily be developed by the control device by throttling the expansion element while the compressor is operating. The sublimation pressure can then be selected depending on the refrigerant used or the components contained in the refrigerant and their respective sublimation temperatures.The control device can therefore be adjusted or programmed depending on the refrigerant used.

[0017] The refrigerant can be a refrigerant mixture consisting of a mass fraction of carbon dioxide and a mass fraction of at least one other component. The refrigerant mixture can consist of carbon dioxide and one or more fluorinated refrigerants that have a low GWP and are non-flammable or only flammable to a limited extent. The proportion of carbon dioxide must be as low as possible, otherwise the freezing point of the refrigerant mixture increases with an increasing mass fraction of carbon dioxide. However, a lower mass fraction of carbon dioxide reduces the GWP-reducing effect of carbon dioxide. Partially fluorinated refrigerants have a significantly higher GWP than carbon dioxide, but they also have an improved fire-retardant effect. Furthermore, the refrigerant can have a temperature glide of ≥ 10 K, preferably ≥ 15 K, particularly preferably ≥ 18 K.The temperature glide of the refrigerant should not be > 20 K so that a cooling system can be operated effectively.

[0018] The ratio of the refrigerant components can always remain constant in the cooling circuit. Here, it is explicitly intended that the refrigerant components are passed through the expansion device in the refrigerant mixing ratio.

[0019] The other components can have a freezing point that is lower than the freezing point of carbon dioxide at a pressure developed in the cooling circuit. The carbon dioxide is then the component of the refrigerant that freezes partially or completely at the expansion element. Subcooling of the refrigerant in the internal heat exchanger can occur close to or below a triple point temperature of the component with the highest melting point or of the carbon dioxide. During expansion of the refrigerant at the expansion element, the carbon dioxide can then partially or completely freeze, depending on the subcooling of the refrigerant. If the cooling device is operated conventionally via the control device, the carbon dioxide can initially only be present in the liquid phase during expansion. The other components of the refrigerant mixture have lower freezing points than the refrigerant mixture orthe carbon dioxide and then form a liquid phase during expansion at the expansion element, which acts as a carrier medium for the carbon dioxide present in the solid phase or particles.

[0020] The mass fraction of carbon dioxide can be 10 to 50 mass percent, preferably 30 to 50 mass percent. With such a mass fraction, a sufficiently low GWP can be achieved. At the same time, clogging of the cooling circuit, which could occur if the mass fraction of carbon dioxide were comparatively high, can be prevented.

[0021] The additional component can be pentafluoroethane and / or difluoromethane. Pentafluoroethane and difluoromethane in particular contain a significant amount of fluorine atoms, which leads to an undesirably high GWP. Surprisingly, however, it has been found that a sufficiently low GWP, i.e., for example, < 400, can be achieved using a refrigerant mixture with a carbon dioxide mass fraction of 30 to 40 mass percent using pentafluoroethane and / or difluoromethane. By selecting additional components, low GWP values, for example, < 150, can be achieved. It has also been found that the fire-retardant effect of pentafluoroethane is comparatively greater than that of carbon dioxide. By adding difluoromethane as the third component to the refrigerant mixture, the negative properties of pentafluoroethane and carbon dioxide can be further reduced.For example, a refrigerant mixture containing pentafluoroethane and difluoromethane is classified as non-flammable. At the same time, difluoromethane with carbon dioxide has a lower freezing point than with pentafluoroethane. Consequently, a mixture of pentafluoroethane, difluoromethane, and carbon dioxide can achieve a lower freezing point than with pentafluoroethane and carbon dioxide alone. Difluoromethane thus significantly lowers the freezing point of the refrigerant mixture, although a certain mass fraction of carbon dioxide is required for the refrigerant mixture to be non-flammable. At the same time, however, difluoromethane produces a high discharge temperature, which is why difluoromethane is only suitable to a limited extent as a sole blending partner for carbon dioxide.Pentafluoroethane cannot lower the freezing point of the refrigerant mixture as much as difluoromethane, but has a higher flame retardant effect compared to carbon dioxide, which is advantageous.

[0022] In a further embodiment, the refrigerant can have a mass fraction of carbon dioxide of 33 to 38, preferably 35 mass percent, a mass fraction of pentafluoroethane of 33.5 to 31, preferably 32.5 mass percent, and a mass fraction of difluoromethane of 33.5 to 31, preferably 32.5 mass percent. The refrigerant mixture can therefore consist of three components alone. The mass fraction of the components difluoromethane and pentafluoroethane is then equal. As already described above, a mixture of carbon dioxide with pentafluoroethane and difluoromethane has proven particularly advantageous. This refrigerant mixture can have a temperature glide > 7 K at evaporation pressures around 1 bar. Furthermore, this refrigerant mixture leads to a reduction in the freezing point, which is concentration-dependent.Therefore, if the mass proportions deviate from the specified mass proportions, flammable and non-flammable refrigerant mixtures may result for different temperature applications.

[0023] Carbon dioxide (CO 2 ) is also known as a refrigerant or component under the designation R744, pentafluoroethane (C 2 HF 5 ) under the designation R125 and difluoromethane (CH 2 F 2 ) under the designation R32 according to DIN 8960 in the last valid version before the priority date of the application.

[0024] The refrigerant can partially freeze at temperatures below -75 °C and be liquid and / or gaseous at temperatures above -75 °C. Consequently, the refrigerant can be used for operation of the cooling system solely with refrigerant in the liquid and vapor phases, and optionally with refrigerant in the liquid phase, vapor phase, and solid phase.

[0025] In the process, a temperature of at least -90 °C, preferably -100 °C or lower, can be achieved at the heat exchanger. The process then enables a significant reduction in the temperature at the heat exchanger, which would not be possible with conventional operation of the cooling device with the refrigerant.

[0026] The refrigerant can have a relative CO2 equivalent, based on 100 years, of < 1500, preferably < 500. Consequently, the refrigerant can be less harmful to the environment. If the refrigerant is non-flammable, it is possible to design the cooling circuit and, in particular, a test chamber more cost-effectively, since no special safety precautions regarding the flammability of the refrigerant need to be observed. The refrigerant can then at least not be assigned to fire class C and / or refrigerant safety group A1. Furthermore, shipping or transport of the cooling circuit is simplified, since the cooling circuit can be filled with the refrigerant prior to transport, regardless of the mode of transport. With flammable refrigerants, filling may only be possible during commissioning at the installation site. Furthermore, the non-flammable refrigerant can be used if ignition sources are present.

[0027] The cooling device according to the invention serves to condition air, wherein the cooling device has a cooling circuit with a refrigerant, a heat exchanger, an internal heat exchanger, a compressor, a condenser and a controllable expansion element, wherein the cooling device is designed as a compression refrigeration system, wherein by means of the cooling device, a temperature of at least -80 °C or lower can be formed at the heat exchanger, wherein the refrigerant can undergo a phase change in the heat exchanger, wherein the internal heat exchanger can be used to cool the refrigerant on the high-pressure side of the cooling circuit and to lower an evaporation temperature at the expansion element, wherein the refrigerant is a zeotropic refrigerant, wherein the refrigerant is non-flammable and has a relative CO2 equivalent, based on 100 years, of < 2500, wherein the cooling device has a control device,by means of which the expansion element is controllable, wherein the cooling device can be operated with the refrigerant in a liquid and vapor phase, optionally in a first operating state, in which the carbon dioxide is present exclusively in the liquid phase even during expansion, or in a second operating state as a sublimation refrigeration system with the refrigerant in a partially solid phase, wherein the expansion element can be controlled by means of the control device in such a way that the refrigerant can partially freeze at the expansion element during expansion. For the advantages of the cooling device according to the invention, reference is made to the description of the advantages of the method according to the invention.

[0028] The internal heat exchanger can be connected to the high-pressure side of the cooling circuit in a flow direction upstream of the expansion element and downstream of the condenser, and to the low-pressure side of the cooling circuit in a flow direction upstream of the compressor and downstream of the heat exchanger. By using the internal heat exchanger and the resulting cooling of the liquefied refrigerant on the high-pressure side, temperatures below -56 °C can easily be achieved. The evaporation temperature of the refrigerant cooled by the internal heat exchanger at the expansion element can be lowered relative to the evaporation temperature of an uncooled refrigerant. The cooling capacity transferred from the low-pressure side to the high-pressure side via the internal heat exchanger can thus be used at least partially, preferably exclusively, to lower the evaporation temperature of the refrigerant at the expansion element.Furthermore, it is now possible to use a zeotropic refrigerant with a temperature glide, as the location of the dew point temperature of the refrigerant, or rather the dew point of the refrigerant, can then be shifted into the internal heat exchanger. Due to the temperature glide of the zeotropic refrigerant, the dew point temperature achieved can be comparatively high, thus preventing further cooling of the heat exchanger. In this way, the so-called effective temperature glide, i.e. the temperature difference that exists in the heat exchanger during isobaric partial evaporation, can be specifically controlled. Consequently, precise temperature adjustment is possible even with refrigerants with a high temperature glide > 7 K. The internal heat exchanger can be designed as a subcooling section or a heat exchanger, in particular a plate heat exchanger.The subcooling section can already be formed by two adjacent pipe sections of the cooling circuit.

[0029] The control device can have at least one pressure sensor and / or at least one temperature sensor on the cooling circuit, wherein the expansion element can be actuated by means of the control device depending on a measured temperature or pressure. The expansion element can have a throttle element and a solenoid valve, wherein refrigerant can be metered via the throttle element and the solenoid valve. The throttle element can be an adjustable valve or a capillary through which refrigerant is then passed via the solenoid valve. The solenoid valve, in turn, can be actuated by means of the control device. The control device can comprise data processing means that process data sets from sensors and control the solenoid valves. Control of a function of the cooling device can then also be adapted to the refrigerant used, for example via an appropriate computer program.Furthermore, the control device can signal an operating fault and, if necessary, initiate a shutdown of the cooling device in order to protect the cooling device from damage due to critical or undesirable operating conditions.

[0030] In a further embodiment, the condenser can be designed as a cascade heat exchanger of a further cooling circuit of the cooling device. Accordingly, the cooling device can then have at least two cooling circuits, wherein the cooling circuit can form a second stage of the cooling device, and a further cooling circuit, which is then arranged upstream of the cooling circuit, can form a first stage of the cooling device. The condenser then serves as a cascade heat exchanger or heat exchanger for the further cooling circuit. This embodiment makes it possible to achieve particularly low temperatures at the heat exchanger in the test chamber.

[0031] A first bypass with at least one controllable second expansion element can be formed in the cooling circuit. The first bypass can be connected to the cooling circuit upstream of the internal heat exchanger and downstream of the condenser in one flow direction, and upstream of the internal heat exchanger and downstream of the heat exchanger in the flow direction. The first bypass can be designed as a controllable internal supplementary cooling and a re-injection device for refrigerant. Accordingly, refrigerant can be supplied from the controllable second expansion element to the internal heat exchanger on the low-pressure side. The first bypass can then be connected to the low-pressure side of the cooling circuit upstream of the internal heat exchanger and downstream of the heat exchanger in one flow direction.The refrigerant cooled or lowered in temperature by the second expansion element can then be passed through the internal heat exchanger, increasing the cooling of the refrigerant on the high-pressure side of the internal heat exchanger. This also allows the cooling capacity of the internal heat exchanger to be controlled even more precisely.

[0032] The test chamber according to the invention has a cooling device according to the invention, wherein the test chamber for conditioning air comprises a temperature-insulated test chamber that can be closed off from the environment and accommodates test material, and a temperature control device comprising the cooling device for temperature control of the test chamber, wherein by means of the temperature control device, a temperature in a temperature range of -80 °C to +180 °C, preferably -90 °C to +180 °C, particularly preferably -100 °C to +180 °C, can be formed within the test chamber. The temperature control device can have a heating device with a heater and a heating heat exchanger in the test chamber. The heating device can, for example, be an electrical resistance heater that heats the heating heat exchanger in such a way that a temperature increase in the test chamber is enabled via the heating heat exchanger.If the heat exchanger and the heating / heat exchanger can be specifically controlled by a control device for cooling or heating the air circulated during the test, a temperature within the temperature range specified above can be achieved within the test chamber using the temperature control device. Alternatively, a high temperature in the test chamber can also be achieved using waste heat from the test material.

[0033] Further embodiments of a test chamber emerge from the feature descriptions of the subclaims referring back to device claim 13.

[0034] When using a zeotropic refrigerant consisting of a refrigerant mixture of a mass fraction of carbon dioxide and a mass fraction of at least one further component in a cooling circuit of a cooling device, wherein the further component has a freezing point that is below a freezing point of the carbon dioxide at a pressure developed in the cooling circuit, wherein the further component is in a liquid and / or gaseous phase and the carbon dioxide is at least partially present in a solid phase, the further component forms a carrier medium for the carbon dioxide in the solid phase. For the advantages of the use, reference is made to the description of the advantages of the method according to the invention. Further embodiments of a use emerge from the descriptions of the features of the subclaims relating to method claim 1 and device claim 13.

[0035] Preferred embodiments of the invention are explained in more detail below with reference to the accompanying drawings.

[0036] They show: Fig. 1 a pressure-enthalpy diagram for a refrigerant; Fig. 2 a schematic representation of a first embodiment of a cooling device; Fig. 3 a schematic representation of a second embodiment of a cooling device; Fig. 4 a schematic representation of a third embodiment of a cooling device; Fig. 5 a schematic representation of a fourth embodiment of a cooling device; Fig. 6 a schematic representation of a fifth embodiment of a cooling device.

[0037] The Fig. 2 shows a first embodiment of a cooling device 10 of a test chamber (not shown in detail here). The cooling device 10 comprises a cooling circuit 11 with a refrigerant, a heat exchanger 12, a compressor 13, a condenser 14, and an expansion element 15. The condenser 14 is cooled here by a further cooling circuit 16. The heat exchanger 12 is arranged in a test space (not shown here) of the test chamber. Furthermore, the cooling circuit 11 has a high-pressure side 17 and a low-pressure side 18, to which an internal heat exchanger 19 is connected.

[0038] The Fig. 1 shows a pressure-enthalpy diagram (log p / h diagram) for the refrigerant circulating in the cooling circuit 11, which is a zeotropic refrigerant. The pressure-enthalpy diagram of the refrigerant shows various aggregate states of the refrigerant. In particular, there is a region 48 with the refrigerant in a gaseous phase, a region 49 with the refrigerant in a liquid phase, a region 50 with the refrigerant in a solid phase, a region 51 with the refrigerant in a solid and liquid phase, a region 52 with the refrigerant in a liquid and gaseous phase, and a region 53 with the refrigerant in a solid, liquid, and gaseous phase. Since the composition of the liquid phase changes during expansion, the refrigerant can also exist in equilibrium with three coexisting phases.A transition between the respective aggregate states occurs at boundary lines 54 between areas 48 to 53.

[0039] Starting from position A, after a review of the Fig. 1 und 2 the refrigerant is sucked in upstream of the compressor 13 and compressed so that a pressure corresponding to position B in the flow direction after the compressor 13 is achieved. The refrigerant is compressed by the compressor 13 and subsequently liquefied in the condenser 14 according to position C. The refrigerant passes through the internal heat exchanger 19 on the high-pressure side 17 and is further cooled therein so that position C' is reached upstream of the expansion element 15 in the flow direction. With the help of the internal heat exchanger 19, the part of the wet vapor region not usable in the heat exchanger 12 (positions E to E') can be used to further reduce the temperature of the refrigerant (positions C' to C). At the expansion element 15, the refrigerant is expanded (positions C' to D') and partially liquefied in the heat exchanger 12 (positions D' to E).The refrigerant partially freezes depending on the subcooling of the refrigerant at the internal heat exchanger 19 or a control of the expansion element 15. The wet vapor of the refrigerant then enters the internal heat exchanger 19 on the low-pressure side 18, where the refrigerant is subsequently evaporated down to the dew point temperature or the dew point of the refrigerant at position E'. A first section 20 of an evaporation section 22 of the refrigerant therefore runs through the heat exchanger 12, with a second section 21 of the evaporation section 22 running through the internal heat exchanger 19. What is important here is that on the evaporation section 22, a suction pressure of the compressor 13 on the low-pressure side 18 is kept constant even when the evaporation temperature at the expansion element 15 changes.

[0040] The refrigerant is a refrigerant mixture consisting of a mass fraction of carbon dioxide of 30 to 50 mass percent and a mass fraction of at least one other component, the other component being pentafluoroethane and / or difluoromethane. In principle, it is also possible to use other suitable refrigerants in the cooling circuit 11 and the cooling circuits described below.

[0041] The Fig. 3 shows a schematic representation of a simplest embodiment of a cooling device 23, wherein the cooling device 23 is designed to be self-regulating. The cooling device comprises a cooling circuit 24 with a heat exchanger 25, a compressor 26, a condenser 27, an expansion element 28, and an internal heat exchanger 29. Depending on the temperature at the heat exchanger 25, incompletely evaporated refrigerant escapes from the heat exchanger 25 because the temperature at the heat exchanger 25 or in a test chamber not shown here is no longer sufficient to generate a phase change. In this case, liquid refrigerant is further evaporated in the internal heat exchanger 29, since the temperature difference here must always be greater than at the heat exchanger 25.As soon as the temperature of the liquid refrigerant in front of the expansion element 28 has been reduced by the heat exchange in the internal heat exchanger 29, the energy density and the resulting achievable temperature difference at the heat exchanger 25 increases. In the cooling device 23, complex control with sensors etc. is not required.

[0042] The Fig. 4 shows a cooling device 30 which, in contrast to the cooling device from Fig. 3 with a first bypass 31 and a second bypass 32. A controllable second expansion element 33 is arranged in the first bypass 31, the first bypass 31 being designed as an internal supplementary cooling system 34. The first bypass 31 is connected to the cooling circuit 24 immediately downstream of the condenser 27 in the direction of flow, upstream of the internal heat exchanger 29, and downstream of the heat exchanger 25 and upstream of the internal heat exchanger 29 in the direction of flow. The first bypass 31 thus bridges the expansion element 28 with the heat exchanger 25, wherein evaporating refrigerant can be supplied to the internal heat exchanger 29 via the second expansion element 33. A suction gas mass flow that is fed into the internal heat exchanger 29 can be additionally cooled with the help of the first bypass 31 at high suction gas temperatures, which can arise due to the heat exchanger 25.This ensures that no refrigerant evaporates upstream of the expansion device. The first bypass 31 therefore makes it possible to respond to changing load conditions of the cooling device 30. The second bypass 32 has a third expansion device 35 and is connected to the cooling circuit 24 downstream of the condenser 27 and upstream of the internal heat exchanger 29, as well as downstream of the internal heat exchanger 29 and upstream of the compressor 26. This makes it possible to reduce the suction gas mass flow upstream of the compressor 26 via the second bypass 32 to such an extent that excessively high discharge temperatures are avoided.

[0043] The Fig. 5 shows a cooling device 36 which, in contrast to the cooling device from Fig. 4 has a further cooling circuit 37. The further cooling circuit 37 serves to cool a condenser 38 of a cooling circuit 39. The condenser 38 is designed here as a cascade heat exchanger 40. Furthermore, the cooling circuit 39 has a further bypass 41 with a further expansion element 42. The further bypass 41 is connected to the cooling circuit 39 in the flow direction of the cooling circuit 39 downstream of the compressor 26 and upstream of the condenser 38, as well as downstream of the internal heat exchanger 29 and upstream of the compressor 26. Thus, compressed but not yet liquefied refrigerant can flow back upstream of the compressor 26 via the further bypass 41, whereby a suction gas temperature and / or a suction gas pressure of the refrigerant can be regulated.

[0044] The Fig. 6shows a cooling device 30 with a cooling circuit 44 and a further cooling circuit 45 and in particular an internal heat exchanger 46 in the cooling circuit 44. A heat exchanger 47 is arranged here in a temperature-insulated test space of a test chamber (not shown).

Claims

1. A method for operating a cooling device (10, 23, 30, 36, 43), a temperature of at least -80 °C or lower being established at the heat exchanger by means of the cooling device having a cooling circuit (11, 24, 39, 44) comprising a refrigerant, a heat exchanger (12, 25, 47), an internal heat exchanger (19, 29, 46), a compressor (13, 26), a condenser (14, 27, 38) and a controllable expansion element (15, 25, 28) of the cooling device, the cooling device being realized as a compression refrigerating system, the refrigerant undergoing a phase transition in the heat exchanger, the refrigerant of a high-pressure side (17) of the cooling circuit being cooled by means of the internal heat exchanger, the cooling of the refrigerant of the high-pressure side by means of the internal heat exchanger being used to reduce an evaporation temperature at the expansion element, a zeotropic refrigerant being used as refrigerant, the refrigerant being nonflammable and having a relative CO2 equivalent of < 2500 over 100 years, characterized in that the cooling device is operated either having the refrigerant in a liquid and vaporous phase in a first operating state, in which the carbon dioxide is, also during the expansion, only in the liquid phase, or in a second operating state as a sublimation refrigerating system having the refrigerant in a partially solid phase, in the second operating state, the expansion element being controlled by means of a control device of the cooling device in such a manner that the refrigerant partially freezes during an expansion at the expansion element.

2. The method according to claim 1, characterized in that in the cooling circuit (11, 24, 39, 44), downstream of the expansion element (15, 25, 28) and upstream of the heat exchanger (12, 25, 47), the refrigerant is formed with a homogeneous mixture of a liquid and / or gaseous phase with particles of a solid phase.

3. The method according to claim 1 or 2, characterized in that the expansion element (15, 25, 28) is controlled by means of the control device in such a manner that a pressure of 1 bar or lower is produced on a low-pressure side (18) of the cooling circuit (11, 24, 39, 44).

4. The method according to any one of the preceding claims, characterized in that a refrigerant mixture composed of a mass fraction of carbon dioxide (CO2) and a mass fraction of at least one other component is used as refrigerant.

5. The method according to claim 4, characterized in that a ratio of the components of the refrigerant remains constant in the cooling circuit (11, 24, 39, 44) at all times.

6. The method according to claim 4 or 5, characterized in that the other component has a freezing point which is below a freezing point of the carbon dioxide (CO2) at a pressure produced in the cooling circuit (11, 24, 39, 44).

7. The method according to any one of claims 4 to 6, characterized in that the mass fraction of carbon dioxide (CO2) is 10 to 50 mass percent, preferably 30 to 50 mass percent.

8. The method according to any one of claims 4 to 7, characterized in that the other component is pentafluoroethane (C2HF5) and / or difluoromethane (CH2F2).

9. The method according to any one of the preceding claims, characterized in that the refrigerant partially freezes at a temperature below -75 °C and is liquid and / or gaseous at a temperature above -75 °C.

10. The method according to any one of the preceding claims, characterized in that a temperature of at least -90 °C, preferably -100 °C or lower, is established at the heat exchanger.

11. The method according to any one of the preceding claims, characterized in that the refrigerant has a relative CO2 equivalent of < 1500, preferably < 500, over 100 years.

12. A cooling device (10, 23, 30, 36, 43) for conditioning air, the cooling device having a cooling circuit (11, 24, 39, 44) comprising a refrigerant, a heat exchanger (12, 25, 47), an internal heat exchanger (19, 29, 46), a compressor (13, 26), a condenser (14, 27, 38) and a controllable expansion element (15, 25, 28), the cooling device being realized as a compression refrigerating system, a temperature of at least -80 °C or lower being establishable at the heat exchanger by means of the cooling device, the refrigerant being able to undergo a phase transition in the heat exchanger, the internal heat exchanger being usable to cool the refrigerant of a high-pressure side (17) of the cooling circuit and to reduce an evaporation temperature at the expansion element, the refrigerant being a zeotropic refrigerant, the refrigerant being nonflammable and having a relative CO2 equivalent of < 2500 over 100 years, the cooling device having a control device by means of which the expansion element is controllable, characterized in that the cooling device is operable either having the refrigerant in a liquid and vaporous phase in a first operating state, in which the carbon dioxide is, also during the expansion, only in the liquid phase, or in a second operating state as a sublimation refrigerating system having the refrigerant in a partially solid phase, the expansion element being controllable by means of the control device in such a manner that the refrigerant can partially freeze during an expansion at the expansion element.

13. The cooling device according to claim 12, characterized in that the internal heat exchanger (12, 25, 47) is connected to the high-pressure side (17) of the cooling circuit (11, 24, 39, 44) upstream of the expansion element (15, 25, 28) and downstream of the condenser (14, 27, 38) and to a low-pressure side (18) of the cooling circuit upstream of the compressor (13, 26) and downstream of the heat exchanger.

14. The cooling device according to claim 12 or 13, characterized in that the control device has at least one pressure sensor and / or at least one temperature sensor in the cooling circuit (11, 24, 39, 44), the expansion element (15, 25, 28) being actuable by means of the control device as a function of a measured temperature and / or a pressure.

15. The cooling device according to any one of claims 12 to 14, characterized in that the condenser (14, 27, 38) is realized as a cascade heat exchanger (40) of another cooling circuit (36, 43) of the cooling device (10, 23, 30, 36, 43).

16. The cooling device according to any one of claims 12 to 15, characterized in that a first bypass (31) having at least one controllable second expansion element (33) is realized in the cooling circuit (24, 39, 44), the first bypass being connected to the cooling circuit upstream of the internal heat exchanger (29, 46) and downstream of the condenser (27, 38) and upstream of the internal heat exchanger and downstream of the heat exchanger (25, 47), the first bypass being realized as a controllable internal additional cooling system (34) and a re-injection device for refrigerant.

17. A test chamber having a cooling device (10, 23, 30, 36, 43) according to any one of claims 12 to 16, the test chamber comprising for conditioning air a test space which can be sealed against an environment and is temperature-insulated and which serves to receive test material, and a temperature control device which comprises the cooling device and serves to control the temperature of the test space, a temperature in a temperature range of -80 °C to +180 °C, preferably -90 °C to +180 °C, especially preferably -100 °C to +180 °C, being establishable within the test space by means of the temperature control device.

18. The test chamber according to claim 17, characterized in that the temperature control device comprises a heating device having a heater and a heating heat exchanger in the test space.