EXAMINATION CHAMBER

Description

[0001] The invention relates to a test chamber for conditioning air, comprising a test chamber which is closable from the environment and temperature-insulated for accommodating test material, and a temperature control device for temperature control of the test chamber, wherein by means of the temperature control device a temperature in a temperature range of -60 °C to +180 °C can be formed within the test chamber, wherein the temperature control device has a cooling device with a cooling circuit with a refrigerant, a heat exchanger in which the refrigerant undergoes a phase change, a compressor, a condenser and an expansion device.

[0002] Such refrigerants generally circulate within a closed cooling circuit of cooling devices and undergo various sequential changes in their physical state. Refrigerants should be designed such that they can be used in a cooling circuit within a specified temperature difference. So-called single-component refrigerants and refrigerant mixtures consisting of at least two substances are known from the state of the art. Refrigerants are designated according to DIN 8960, paragraph 6.

[0003] Due to legal regulations, a refrigerant must not contribute significantly to ozone depletion in the atmosphere or global warming. For example, fluorinated gases or chlorinated substances cannot be used as refrigerants, which is why natural refrigerants or gases are suitable. Furthermore, a refrigerant should be non-flammable to avoid, among other things, complicating the filling, shipping, and operation of a refrigeration circuit due to possible 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 property of a refrigerant to react with ambient oxygen, releasing heat.A refrigerant is particularly flammable if it falls into fire class C according to the European standard EN2 or DIN 378 classes A2, A2L and A3.

[0004] In addition, a refrigerant 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 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 20 years being specified here for 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.

[0005] A disadvantage of refrigerants with low GWP, for example, < 2500, is that these refrigerants sometimes exhibit significantly reduced cooling capacity in the temperature ranges relevant to a cooling circuit compared to refrigerants with a comparatively higher GWP. A low GWP can be achieved with refrigerant mixtures that have a comparatively high mass fraction of carbon dioxide. However, these refrigerant mixtures can exhibit zeotropic properties due to the different substances mixed together, which is undesirable in many cooling circuits.

[0006] 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. Carbon dioxide initially appears to be a suitable component for a refrigerant mixture because it is non-flammable and has a low GWP. When mixing carbon dioxide with another component, however, it is important that the mass fraction of carbon dioxide must be comparatively large if the other component is flammable. This is, however, disadvantageous because carbon dioxide has a freezing temperature orhas a freezing point of -56.6°C, which makes it difficult to achieve temperatures down to -60°C with a high carbon dioxide concentration.

[0007] Refrigerants should also be as easy to use as possible, i.e. they should 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 of the cooling circuit in question to the evaporation temperature of the refrigerant and to provide appropriate control. A further distinction must be made between refrigerants designed for static operation of a cooling system, i.e. a cooling system with an essentially constant temperature at the heat exchanger or evaporator over a longer period of time, and a dynamic cooling system with a comparatively rapid temperature change at the heat exchanger. Such dynamic cooling systems are installed in test chambers, among other places, so that the refrigerant used must be usable over a wide temperature range.

[0008] Test chambers are regularly used to test the physical and / or chemical properties of objects, particularly devices. Temperature test cabinets or climatic test cabinets are known, within which temperatures can be set within a range of -60°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. Such test chambers are usually, or sometimes, designed as a mobile device that is connected to a building only via the necessary supply lines and contains all the components required for temperature control and air conditioning. The temperature control of a test chamber containing the test object to be tested is usually achieved in a recirculation duct within the test chamber.The recirculation duct forms an air treatment chamber within 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 or ventilator 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, the temperature in the test chamber can then repeatedly fluctuate between a maximum and a minimum temperature. Such a test chamber is known, for example, from EP 0 344 397 A2 or US 2003 / 172751 A1.

[0009] The refrigerant circulating in a cooling circuit must be of such a nature that it can be used within the aforementioned temperature difference. In particular, the dew point temperature of the refrigerant cannot be higher than a minimum temperature within the temperature range to be achieved within the cooling circuit, since otherwise the minimum temperature would not be achievable if the refrigerant evaporates in the heat exchanger used to cool the test chamber. The dew point temperature of azeotropic refrigerants is reached immediately after the expansion element in the heat exchanger. Cooling circuits for test chambers, in particular, require very high spatial temperature constancy for precise temperature control of the test chamber, which is not achievable with zeotropic refrigerants, or can only be achieved with limitations.High temperature stability cannot be achieved here, as the dew point temperature or the dew point of the zeotropic refrigerant can shift locally due to temperature differences in the area of ​​the heat exchanger, depending on the temperature in the test chamber. The use of zeotropic refrigerants or refrigerants with temperature glide in cooling circuits of test chambers is therefore avoided.

[0010] Cooling systems are also known in which a zeotropic refrigerant mixture is successively evaporated. This means that components of the refrigerant are evaporated one after the other via an expansion device. Such cooling systems are also referred to as mixture cascade systems and are suitable for creating an essentially static low temperature.

[0011] DE 41 16 274 A1 describes a refrigerant for refrigeration machines and heat pumps that serves as a substitute for environmentally harmful refrigerants, in particular a refrigerant mixture containing 5 to 50 percent by weight of CO2 and 50 to 95 percent by weight of difluoromethane. This refrigerant mixture is intended to replace the refrigerant R13 B1.

[0012] US 2004 / 084652 A1 also relates to a refrigerant mixture of carbon dioxide and difluoromethane. According to one exemplary embodiment, 30 to 85 mass percent carbon dioxide and 15 to 70 mass percent difluoromethane are specified as the mixing partners.

[0013] The present invention is therefore based on the object of proposing a test chamber with which temperatures down to at least -60°C can be achieved in an environmentally friendly and safe manner.

[0014] This object is achieved by a test chamber having the features of claim 1.

[0015] The test chamber according to the invention for conditioning air, comprising a test space which can be closed off from the environment and is temperature-insulated for accommodating test material, and a temperature control device for temperature control of the test space, wherein by means of the temperature control device a temperature in a temperature range of -60 °C to +180 °C can be formed within the test space, wherein the temperature control device has a cooling device with a cooling circuit with a refrigerant, a heat exchanger in which the refrigerant undergoes a phase change, a compressor, a condenser and an expansion element, wherein the refrigerant is a refrigerant mixture comprising a mass fraction of carbon dioxide and a mass fraction of at least one further component, wherein the mass fraction of carbon dioxide in the refrigerant mixture is 44 to 48 mass percent, wherein the further component is difluoromethane, wherein a mass fraction of difluoromethane is 56 to 52 mass percent.

[0016] 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, trifluoroethylene under the designation R1123, 1,1-difluoroethene (C 2 H 2 F 2 ) under the designation R-1132a, fluoroethene (C 2 H 3 F) under the designation R1141 according to DIN 8960 in the last valid version before the priority date of the application.

[0017] According to the invention, a test chamber is provided with a refrigerant mixture of carbon dioxide and difluoromethane, which 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, since 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. For example, partially fluorinated refrigerants have a significantly higher GWP than carbon dioxide, although they also have an improved fire-retardant effect. Difluoromethane contains a considerable amount of fluorine atoms, which leads to an undesirably high GWP. However, it has surprisingly been found that a sufficiently low GWP, i.e., for example, < 150, can be achieved with a refrigerant mixture with a mass fraction of carbon dioxide of 44 to 48 mass percent with difluoromethane.Difluoromethane exhibits a lower freezing point with carbon dioxide than with pentafluoroethane. Consequently, a mixture of pentafluoroethane, difluoromethane, and carbon dioxide can achieve a lower freezing point than 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 final compression 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 it has a higher flame-retardant effect than carbon dioxide, which is advantageous.

[0018] In another embodiment of the refrigerant, the mass fraction of carbon dioxide can be 45.8 mass percent and the mass fraction of difluoromethane can be 54.2 mass percent. The refrigerant mixture can then consist solely of carbon dioxide and difluoromethane. With a mass fraction of carbon dioxide of at least 35 mass percent, the refrigerant mixture can be classified as non-flammable. The freezing point reduction is greater than that of R125, R1132A and R1141, which is why a reduction in the freezing point is possible. However, the GWP of difluoromethane is higher than that of R1132A, R1141, R1123 and carbon dioxide. Difluoromethane can be described as a small molecule, which means that when difluoromethane is compressed, the final compression temperature is higher than with larger and heavier molecules such as pentofluoroethane under the same technical conditions.The refrigerants R410A and R410B have lower final compression temperatures than difluoromethane, which is why they are particularly suitable as blend partners with carbon dioxide.

[0019] Consequently, the refrigerant mixture can be formed as a binary mixture.

[0020] The refrigerant mixture can contain pentafluoroethane, trifluoroethylene, 1,1-difluoroethene, or fluoroethene as an additional component. 1,1-Difluoroethene has a very low GWP, making it a suitable component for reducing GWP. It also has a very low freezing and normal boiling point, making it suitable for low-temperature applications. However, 1,1-Difluoroethene and fluoroethene are flammable and should not be added to the refrigerant mixture in excess of 10 wt%. Trifluoroethylene has a very low GWP, making it a suitable component for reducing GWP. Since it is thermodynamically similar to difluoromethane, refrigerant mixtures can be formed from carbon dioxide, difluoromethane, and trifluoroethylene. Trifluoroethylene, like difluoromethane, can lead to a significant reduction in the freezing point (more so than pentafluoroethane).

[0021] The following table shows an example of a refrigerant according to the embodiments described above. Table refrigerant R744 (wt%) 45,8 R32 (wt%) 54,2 Temperature glide absolute (K) 15,9 Temperature glide (°C) -78.7 to -62.8 Freezing point (°C) -87

[0022] Furthermore, the refrigerant may 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.

[0023] The refrigerant may have a relative CO2 equivalent, based on 20 years, of < 2500, preferably < 1500, particularly preferably < 500. Consequently, the refrigerant may have minimal environmental impact.

[0024] The refrigerant cannot be flammable. If the refrigerant is non-flammable, it becomes 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 transport method. With flammable refrigerants, filling may only be possible during commissioning at the installation site. Furthermore, the use of non-flammable refrigerants is possible if ignition sources are present.

[0025] By means of the temperature control device, a temperature in a temperature range of -60°C to +180°C, preferably -80°C to +180°C, particularly preferably -100°C to +180°C, can be achieved within the test chamber. In contrast to a mixture cascade system, the refrigerant, along with all of its components, can be evaporated simultaneously via the expansion device. Since the freezing point of carbon dioxide is -56.6°C, refrigerant mixtures containing a large mass fraction of carbon dioxide are, in principle, no longer suitable for achieving temperatures below -56.6°C. However, the use of the refrigerant according to the invention makes it possible to achieve a dew point temperature of the refrigerant of below -60°C.

[0026] The cooling circuit can have an internal heat exchanger, whereby the internal heat exchanger can be connected to a high-pressure side of the cooling circuit in a flow direction upstream of the expansion element and downstream of the condenser, and to a 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 be easily 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 device. Furthermore, it is only possible to use a zeotropic refrigerant with a temperature glide, as the location of the refrigerant's dew point temperature, or rather, the dew point itself, can then be shifted into the internal heat exchanger. Due to the temperature glide of the zeotropic refrigerant, the achieved dew point temperature of the refrigerant can be comparatively high, thus preventing further cooling of the heat exchanger.

[0027] Therefore, only a portion of the refrigerant can be evaporated in the heat exchanger, and the unusable portion of the wet vapor component of the refrigerant can be transferred to the internal heat exchanger. Overall, this makes it possible to use refrigerants with a mass fraction of carbon dioxide, which are both environmentally friendly and exhibit zeotropic properties, to achieve low temperatures in a test chamber. Furthermore, if a portion of the temperature glide, or a portion of the wet vapor of the refrigerant, is transferred from the heat exchanger in the test chamber to the internal heat exchanger, it becomes possible to achieve comparatively improved temperature stability with the zeotropic refrigerant.The cooling capacity delivered by the heat exchanger can then only be generated within a specific section of the temperature glide, so that a shift in the dew point of the refrigerant in the cooling circuit can hardly affect the temperature stability of the heat exchanger. Furthermore, it can be provided that only a single heat exchanger is used to cool one medium, in this case the air in the test chamber.

[0028] The heat exchanger can be dimensioned such that the refrigerant can only partially evaporate in the heat exchanger. This offers the advantage that the dew point, or the location of the dew point temperature, of the refrigerant can be shifted from the heat exchanger into the internal heat exchanger. Due to a temperature glide of the zeotropic refrigerant, a lower temperature is achieved in the heat exchanger during partial evaporation of the refrigerant in the heat exchanger than during the subsequent, remaining evaporation of the refrigerant in the internal heat exchanger.

[0029] In one embodiment of the test chamber, the heat exchanger can be arranged in the test space. The heat exchanger can then also be arranged in an air treatment chamber of the test space, so that air circulated by a fan can come into contact with a heat exchanger. This makes it possible to directly cool a volume of air circulated in the test space by means of the cooling device via the heat exchanger in the test space. The test chamber can then have the cooling circuit as a single, dedicated cooling circuit. The cooling circuit is then directly connected to the test space.

[0030] In a further embodiment of the test chamber, the condenser can be designed as a cascade heat exchanger of a further cooling circuit of the cooling device. Accordingly, the test chamber 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 cooling circuit. This embodiment of a test chamber makes it possible to achieve particularly low temperatures in the test space.

[0031] The temperature control device can comprise 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 means of 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 by means of the temperature control device. In this case, a temporal temperature constancy of ±1 K, preferably ±0.3 K to ±0.5 K or less than ±0.3 K, can be achieved in the test chamber during a test interval, regardless of the test material or the operating state of the test material.A test interval is defined as a period of time within a complete test period during which the test specimen is exposed to a substantially constant temperature or climatic condition. The heating heat exchanger can be combined with the heat exchanger of the cooling circuit in such a way that a common heat exchanger body is formed through which the refrigerant can flow and which has heating elements of an electrical resistance heater. The condenser can be designed with air cooling, water cooling, or another cooling liquid. In principle, the condenser can be cooled with any suitable fluid. It is essential that the heat load accumulating at the condenser is dissipated via the air cooling or water cooling in such a way that the refrigerant can condense until it is completely liquefied.

[0032] 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 a flow direction. The first bypass can be designed as a controllable internal supplementary cooling system. The first bypass can thus form a refrigerant injection device. Accordingly, refrigerant can be supplied from the controllable second expansion element in 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 a 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.

[0033] A second bypass with at least one third expansion element can be formed in the cooling circuit, wherein the second bypass bridges the expansion element downstream of the condenser and upstream of the internal heat exchanger in the direction of flow, wherein the refrigerant can be metered via the third expansion element such that a suction gas temperature and / or a suction gas pressure of the refrigerant on the low-pressure side of the cooling circuit upstream of the compressor can be regulated. This can, among other things, prevent the compressor, which can be a compressor, for example, from possibly overheating and then being damaged. Consequently, gaseous refrigerant located upstream of the compressor can be cooled via the second bypass by actuating the third expansion element by metering in still-liquid refrigerant.The third expansion element can be actuated by a control device which, in turn, is coupled to a pressure and / or temperature sensor in a cooling circuit upstream of the compressor. It is particularly advantageous if a suction gas temperature of ≤ 30 °C can be set via the second bypass. The refrigerant can also be metered in such a way that the operating time of the compressor can be regulated. In principle, it is disadvantageous if the compressor is switched on and off frequently. The service life of a compressor can be extended if it is in operation for longer periods of time. A refrigerant can be bypassed by the expansion element or the condenser via the second bypass, for example to delay automatic shutdown of the compressor and to extend the operating time of the compressor.

[0034] A further bypass with at least one further expansion element can be formed in the cooling circuit, wherein the further bypass bridges the compressor downstream of the compressor and upstream of the condenser in the direction of flow, such that a suction gas temperature and / or a suction gas pressure of the refrigerant on the low-pressure side of the cooling circuit upstream of the compressor can be regulated, and / or that a pressure difference between the high-pressure side and a low-pressure side of the cooling circuit can be compensated. The second bypass can additionally be equipped with an adjustable or controllable valve, for example a solenoid valve. By connecting the high-pressure side and low-pressure side via the further expansion element, it can be ensured that in the event of a system shutdown, the compressed and gaseous refrigerant gradually flows from the high-pressure side to the low-pressure side of the cooling circuit.This ensures that a gradual pressure equalization occurs between the high-pressure and low-pressure sides, even when the expansion device is closed. The cross-section of the additional expansion device can be dimensioned such that an overflow of refrigerant from the high-pressure side to the low-pressure side only marginally affects normal operation of the cooling system. However, it can be provided that gaseous refrigerant upstream of the compressor is cooled by adding liquid refrigerant via the additional bypass.

[0035] The internal heat exchanger can also 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.

[0036] The expansion device can have a throttle device and a solenoid valve, whereby refrigerant can be metered via the throttle device and the solenoid valve. The throttle device 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 a control device.

[0037] The temperature control device can also comprise a control device with at least one pressure sensor and / or at least one temperature sensor in the cooling circuit, wherein a solenoid valve can be actuated by means of the control device as a function of a measured temperature or pressure. The control device can comprise means for data processing 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 operational malfunction and, if necessary, initiate a shutdown of the test chamber in order to protect the test chamber and the test specimen from damage due to critical or undesirable operating conditions of the test chamber.

[0038] When using a refrigerant consisting of a refrigerant mixture with a mass fraction of carbon dioxide of 44 to 48 mass percent and a mass fraction of at least one further component, wherein the further component is difluoromethane, wherein a mass fraction of difluoromethane is 56 to 52 mass percent, the refrigerant is used to condition air in a test space of a test chamber for accommodating test material, which test space can be closed off from the environment and is temperature-insulated, wherein a temperature in a temperature range of -60 °C to +180 °C, preferably -70 °C to +180 °C, particularly preferably -80 °C to +180 °C, is formed within the test space by means of a cooling device of a temperature control device of the test chamber, with a cooling circuit with the refrigerant, a heat exchanger, a compressor, a condenser and an expansion device.

[0039] The refrigerant can be cooled by means of an internal heat exchanger of the cooling circuit, connected to a high-pressure side of the cooling circuit in a flow direction upstream of the expansion element and downstream of the condenser, and to a low-pressure side of the cooling circuit in a flow direction upstream of the compressor and downstream of the heat exchanger, the high-pressure side. The cooling of the refrigerant on the high-pressure side by means of the internal heat exchanger can be used to lower the evaporation temperature at the expansion element. While the evaporation temperature of the refrigerant on the high-pressure side is lowered, the suction pressure of the refrigerant on the low-pressure side can be kept constant. Major system engineering effort, such as additional control of the suction pressure or control of the expansion element depending on the suction pressure, is then not absolutely necessary.In particular, the compressor can then operate at a constant power level, regardless of the operating state of the cooling circuit. Especially when using piston pumps as compressors, it is essential that they operate over long periods and at a constant speed to achieve a long service life.

[0040] The high-pressure side refrigerant can be cooled by the low-pressure side refrigerant at a constant suction pressure on the low-pressure side by means of the internal heat exchanger. Consequently, the refrigerant can evaporate at a constant suction pressure along an evaporation section of the cooling circuit from the expansion element up to and including the internal heat exchanger. At the constant suction pressure or evaporation pressure of the refrigerant, the refrigerant can then evaporate from the expansion element with a low evaporation temperature to the internal heat exchanger with a high evaporation temperature, corresponding to the refrigerant's temperature glide. The dew point temperature resulting from the temperature glide can be higher than the temperature of the medium to be cooled or the air in the test chamber.As soon as the evaporation temperature of the refrigerant, at the same suction pressure, corresponds to the temperature of the air to be cooled in the test chamber, no further cooling of the air can be achieved. However, the dew point temperature reached in the subsequent heat exchanger is still below the liquid temperature of the refrigerant on the high-pressure side of the internal heat exchanger, so the liquid temperature of the refrigerant can be further reduced. Therefore, the evaporation temperature downstream of the expansion device can be lowered without changing the suction pressure, thus achieving further cooling of the air in the test chamber.

[0041] Thus, a first portion of the refrigerant passed through the expansion element can be evaporated in the heat exchanger, and a second portion of the refrigerant can be evaporated in the internal heat exchanger. An evaporation path of the cooling circuit, within which the refrigerant evaporates, can extend from the expansion element to the internal heat exchanger. The evaporation path can run through the internal heat exchanger, wherein a dew point of the refrigerant can preferably be located at an outlet of the internal heat exchanger upstream of the compressor in the direction of flow. A ratio of the first portion to the second portion can change during operation of the cooling circuit depending on a temperature in the test chamber or at the heat exchanger.For example, a comparatively large temperature difference between the temperature of the heat exchanger and a temperature in the test chamber can result in accelerated heating of the refrigerant in the heat exchanger, leading to a shift in the dew point of the refrigerant upstream of the compressor in the flow direction upstream of the internal heat exchanger or the outlet of the heat exchanger. Such a shift in the dew point can be tolerated as long as a comparatively low temperature or target temperature has not yet been reached in the test chamber. If the temperature of the heat exchanger approaches the temperature in the test chamber, a shift in the dew point occurs, and thus an increase in the second partial quantity of refrigerant relative to the first partial quantity.

[0042] The evaporation temperature of the refrigerant on the high-pressure side can be reduced automatically. Depending on the temperature at the heat exchanger, refrigerant that no longer evaporates can be discharged in the direction of flow, as in this case the temperature at the heat exchanger is no longer sufficient to cause a phase change in the refrigerant. Wet vapor or liquid refrigerant is subsequently evaporated in the internal heat exchanger, as the temperature difference between the high-pressure and low-pressure sides can always be greater than at the heat exchanger. If the temperature of the liquid refrigerant upstream of the expansion element is reduced by the internal heat exchanger through heat exchange, the energy density of the refrigerant upstream of the expansion element increases, and so does the achievable temperature difference at the heat exchanger.In principle, it is not necessary to regulate the interaction between the expansion device, heat exchanger and internal heat exchanger.

[0043] It is particularly advantageous if the cooling system is operated exclusively below the critical point of the refrigerant. If the cooling system is operated below the triple point of the refrigerant, the refrigerant cannot reach a supercritical state. This eliminates the need to design the cooling system for operation in the supercritical state, thus saving costs for the cooling system.

[0044] The constant suction pressure can be maintained, in particular, even during a reduction in the evaporation temperature of the high-pressure side refrigerant by the internal heat exchanger. Accordingly, the cooling of the high-pressure side refrigerant via the internal heat exchanger can also be used, at least partially or exclusively, to reduce the evaporation temperature of the refrigerant at the expansion device.

[0045] A dew point temperature of the refrigerant can be higher than a minimum temperature of the temperature range. In the test chambers known from the prior art, such a refrigerant can no longer achieve the minimum temperature of the temperature range in the test chamber, but rather a comparatively higher minimum temperature that essentially corresponds to the dew point temperature of the refrigerant. However, in the test chamber according to the invention, a refrigerant can be used whose dew point temperature is higher than an achievable minimum temperature of the temperature range, since the liquefied refrigerant can be cooled on the high-pressure side by means of the internal heat exchanger, so that an evaporation temperature of the refrigerant at the expansion element can be comparatively lower.

[0046] The refrigerant can be evaporated at a suction pressure or evaporation pressure in a pressure range of 0.3 to 5 bar absolute. Using the refrigerant within this pressure range enables a cost-effective cooling circuit design, as no special, pressure-resistant assemblies and components are required to form the low-pressure side of the cooling circuit.

[0047] The refrigerant can also be condensed at a condensation pressure in a pressure range of 5 to 35 bar absolute. Here, too, the high-pressure side can be designed with assemblies and components that do not need to be adapted to comparatively higher pressures.

[0048] Further embodiments of a use emerge from the descriptions of the features of the subclaims referring back to device claim 1.

[0049] Preferred embodiments of the invention are explained in more detail below with reference to the accompanying drawings.

[0050] 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.

[0051] The Fig. 2shows 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.

[0052] 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. Starting from position A, after a review of the Fig. 1 and 2the 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 wet vapor of the refrigerant then enters the internal heat exchanger 19 on the low-pressure side 18, where the refrigerant undergoes post-evaporation until it reaches 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. It is essential here that the suction pressure of the compressor 13 on the low-pressure side 18 is kept constant on the evaporation section 22, even when the evaporation temperature at the expansion element 15 changes.

[0053] The refrigerant is a refrigerant mixture consisting of a mass fraction of carbon dioxide of 44 to 48 mass percent and a mass fraction of at least one other component, the other component being difluoromethane. In principle, it is possible to use the refrigerants listed in the table above in the cooling circuit 11 and the cooling circuits described below.

[0054] The Fig. 3shows 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.

[0055] The Fig. 4 shows a cooling device 30 which, in contrast to the cooling device from Fig. 3with 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.

[0056] The Fig. 5 shows a cooling device 36 which, in contrast to the cooling device from Fig. 4has 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. The cooling circuit 39 also 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.

[0057] 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 test chamber for conditioning air, the test chamber comprising a test space which serves to receive test material and which is configured to be sealed against an environment and is temperature-insulated, and a temperature control device for controlling the temperature of the test space, a temperature in a temperature range of -60 °C to +180 °C being establishable within the test space by means of the temperature control device, the temperature control device having a cooling device (10, 23, 30, 36, 43) comprising a cooling circuit (11, 24, 39, 44) with a refrigerant, a heat exchanger (12, 25, 47), in which the refrigerant undergoes a phase change, a compressor (13, 26), a condenser (14, 27, 38) and an expansion element (15, 28), characterized in that the refrigerant is a refrigerant mixture composed of a mass fraction of carbon dioxide (CO2) and a mass fraction of at least one other component, the mass fraction of carbon dioxide being 44 to 48 mass percent, the other component being difluoromethane (CH2F2), a mass fraction of difluoromethane being 56 to 52 mass percent.

2. The test chamber according to claim 1, characterized in that the mass fraction of carbon dioxide is 45.8 mass percent.

3. The test chamber according to claim 2, characterized in that a mass fraction of difluoromethane is 54.2 mass percent.

4. The test chamber according to any one of the preceding claims, characterized in that the refrigerant mixture is a binary mixture.

5. The test chamber according to claim 1 or 2, characterized in that another component is pentafluoroethane (C2HF5).

6. The test chamber according to any one of the preceding claims, characterized in that the refrigerant is a zeotropic refrigerant and has a temperature glide of ≥ 10 K, preferably ≥ 15 K, particularly preferably ≥ 18 K.

7. The test chamber according to any one of the preceding claims, characterized in that the refrigerant has a relative CO2 equivalent of < 2500, preferably < 1500, particularly preferably < 500, over 20 years.

8. The test chamber according to any one of the preceding claims, characterized in that the refrigerant is nonflammable.

9. The test chamber according to any one of the preceding claims, characterized in that the cooling circuit (11, 24, 39, 44) has an internal heat exchanger (19, 29, 46), the internal heat exchanger being connected to a high-pressure side (17) of the cooling circuit upstream of the expansion element (15, 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 (12, 25, 47).

10. The test chamber according to any one of the preceding claims, characterized in that the heat exchanger (12, 25, 47) is of such a size that the refrigerant can only partially evaporate in the heat exchanger.

11. The test chamber according to any one of the preceding claims, 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.

12. The test chamber according to any one of the preceding claims, characterized in that the temperature control device has a heating device comprising a heater and a heating heat exchanger in the test space.

13. The test chamber according to any one of the preceding claims, 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), the first bypass being realized as an additional controllable internal cooling (34).