FREEZER

Abstract

The present invention relates to a freezer, comprising a cooled interior and comprising a wall that surrounds the cooled interior at least in regions, and comprising a refrigeration circuit, wherein the wall is formed at least in part by a vacuum insulation body and/or by an insulation body which comprises or is made of polyurethane, in particular comprises or is made of polyurethane foam, or only by a vacuum insulation body or only by an insulation body which comprises or is made of polyurethane, in particular comprises or is made of polyurethane foam, and in that the refrigeration circuit comprises a refrigerant mixture.

Description

The present invention relates to a freezer, comprising a cooled interior and comprising a wall that surrounds the cooled interior at least in regions, and comprising a refrigeration circuit.

Ultra-low temperature (ULT) fridges or freezers or ULT devices from the prior art reach a use temperature level of approximately −86° C. So-called cryogenic ultra-low temperature freezers, in which a storage temperature level between −150° C. and −120° C. can be set, are also known.

Usually two-stage cascade refrigeration systems or individual auto cascade circuits and Stirling engines are usually used as the refrigeration technology.

The flow diagrams of the two cooling-technology cascade variants are shown in FIGS. 4 and 5.

FIG. 4 shows a two-stage cascade refrigeration system comprising a condenser 24, two precoolers 22, two compressors 23, two expansion members 25, a cascade heat exchanger 26, and an evaporator 21, wherein these elements are connected, by lines configured for conducting a refrigerant, in such a way that a two-stage cascade refrigeration system comprising two refrigeration circuits which are connected, in terms of thermal technology, by the cascade heat exchanger 26 is formed.

FIG. 5 shows an auto cascade system comprising double intermediate expansion, comprising a condenser 24, a compressor 23, two precoolers 22, four expansion members 25, two phase separators 27, a collector 28, and an evaporator 21, wherein these elements are connected to one another, by lines configured for conducting a refrigerant, in such a way that an auto cascade system comprising double intermediate expansion is formed.

In the case of the cascade systems for the storage temperature region of approximately −86° C., primarily natural hydrocarbon refrigerants such as isobutane, propylene, or ethylene, having filling quantities of less than 150 g, serve as working fluids in the refrigeration circuit.

In the case of cryogenic ultra-low temperature freezers, in the high-temperature stage approximately 500 g filling amount of the refrigerant mixture R-407D is used, which mixture consists of 26% R32, 11% R125 and 63% R134a, and has a global warming potential (GWP) value of 1627, and in the low-temperature stage approximately 800 g filling amount of the refrigerant mixture MU-N721 having a global warming potential (GWP) value of 6952. These refrigerants are therefore considered to be refrigerants that are harmful to the environment.

In the case of the Stirling engine (not set out here in greater detail), helium in a filling amount of approximately 10 g is used as the working gas. In the case of a known system, the −86° C. storage compartment is connected to the cold head of the Stirling engine via a thermosiphon comprising less than 100 g ethylene.

The plurality of required chilling-technology components, such as compressors, heat exchangers, expansion members, phase separators and collectors requires energy-intensive material and production outlay in the manufacturing process for the known ultra-low temperature freezers, in particular in the case of the cascade circuits. On account of the low manufacturing numbers, Stirling engines are sometimes extremely expensive items of equipment.

The ultra-low temperature freezers comprising cascade systems and comprising Stirling engines achieve low efficiencies, for example a Coefficient of Performance (COP) of usually less than 0.3. At approximately 1 to 1.4 kWh per day, and 100 l usable volume, the ultra-low temperature freezers currently available have an annual energy requirement of approximately 2000 to 3500 kWh, at usable volumes of between 500 and 700 litres, and therefore high operating costs.

Refrigeration circuits that are configured as a mixed-refrigerant-cycle (MRC) or as mixed refrigeration circuits, comprising a refrigerant mixture as the circulating working fluid, are also known from the prior art.

U.S. Pat. No. 1,986,959 A discloses a mixture of R30 and R160 in a dual-temperature freezer.

The Lorenz-Meutzner cycle, disclosed by Lorenz and Meutzner in “LORENZ, A.; MEUTZNER, K.: On Application of Non-Azeotropic Two-Component Refrigerants in Domestic Freezers and Home Freezers. Proc. XVIth Int. Congr. Refrig., Moscow (1975)” uses a special system circuit for freezers and/or freezers comprising binary refrigerant mixtures of R22 and R11 which, however, were not able to win out, in terms of energy, compared with the circuit variants of the pure substance systems.

A particular property of MRC or mixed refrigerant circuits is the temperature drift that occurs in the heat transfer in the two-phase region, i.e. in evaporation and condensation. This is shown in FIG. 3.

The heat exchanger surface is plotted on the x-axis of the graph shown in FIG. 3, and the temperature is plotted on the y-axis. The evaporation is shown to the left of the dashed vertical line, and the condensation is shown to the right of the dashed vertical line.

Air and refrigerant now flow in countercurrent over the heat exchanger surface, and transfer thermal energy between one another.

During the condensation, the refrigerant temperature in a refrigerant (KM) mixture continuously decreases compared with a KM pure substance, in the case of which the refrigerant temperature remains approximately constant over a large range, and in the case of evaporation said temperature continuously increases, compared with a KM pure substance, in the case of which the refrigerant temperature remains approximately constant over a large range. The temperature of the air increases continuously during the condensation, and decreases continuously during the evaporation. The temperature change over the heat exchanger surface is shown by the corresponding curves on the graph.

By mixing fluids of different normal boiling points, a high pre-cooling capacity upstream of the expansion member and a use-specific boiling point drift at the evaporator can be achieved, in the sense of a “molecular engineering” development process.

The temperature drift furthermore opens up energy-related potential improvements for all heat transfer processes, which potential improvements can have a positive effect on the overall efficiency of a system. In order to achieve small temperature differences between the air and the refrigerant mixture, in a manner that increases energy efficiency, sufficient air volume flows or large heat exchanger surfaces should be provided.

In the field of cryogenic applications, MRC systems are used in LNG condensation processes or during cooling of high-temperature superconducting current feeds according to the prior art. The technological suitability of the MRC cooling technology for low-temperature applications can already be concluded from these MRC applications in the temperature range from around −160° C. to −200° C.

The thermal insulation of the housing for ultra-low temperature freezers from the prior art is composed of a main body made of polyurethane (PUR), and a plurality of vacuum insulation panels, which are applied in multiple layers to all the housing sides and in the door region.

Thermal insulation by means of vacuum technology or by means of vacuum insulation panels is described in DE 10 2013 005 585 A1, DE 10 2013 002 313 A1, DE 10 2015 008 131 A1 and DE 10 2015 008 157 A1.

Against this background, the object of the present invention is that of advantageously developing a freezer from the prior art, in particular in view of the refrigeration circuit and the thermal insulation.

This object is achieved by the subject matter having the features of independent claim 1. The dependent claims relate to advantageous developments of the invention.

According thereto, it is provided according to the invention that the wall is formed at least in part by a vacuum insulation body and/or by an insulation body which comprises or is made of polyurethane, in particular comprises or is made of polyurethane foam, or only by a vacuum insulation body or only by an insulation body which comprises or is made of polyurethane, in particular comprises or is made of polyurethane foam, and in that the refrigeration circuit comprises a refrigerant mixture.

In other words, it is advantageously provided that the wall comprises thermal insulation, wherein the thermal insulation is formed by a vacuum insulation body and/or by an insulation body which comprises or is made of polyurethane, in particular comprises or is made of polyurethane foam, or only by a vacuum insulation body or only by an insulation body which comprises or is made of polyurethane, in particular comprises or is made of polyurethane foam.

In this case, an insulation body preferably comprises an insulation region and can also comprise the support structures surrounding the insulation region, such as walls or foils.

The insulation region in an insulation body which comprises or is made of polyurethane, in particular comprises or is made of polyurethane foam, is preferably formed by polyurethane, in particular by polyurethane foam. The polyurethane foam can for example be arranged between an inner container and an outer container of a freezer. Thus, for example the inner and outer container and the polyurethane foam form the insulation body.

The insulation region in a vacuum insulation body is preferably formed by an evacuated or vacuumed region. For example, a foil filled with support material can be evacuated, and thus form a vacuum insulation body.

A mixed refrigeration circuit or a mixed-refrigerant cycle (MRC) system is preferably integrated into a thermally insulated housing.

This advantageously results in energy-efficient generation or reaching and maintaining of a storage temperature level in the cooled interior of between −150° C. and −36° C., in particular between −150° C. and −90° C. or between −90° C. and −36° C.

In this case, the housing can preferably comprise the following types of insulation:

• • housing in vacuum technology, which consists entirely or in part of evacuated elements;
  • housing in which one or more inner and/or outer vacuum elements are connected to a housing insulated by polyurethane, in particular by polyurethane foam;
  • housing that is insulated only by polyurethane, in particular by polyurethane foam.

There are two approaches for reducing the energy requirement of a freezer, in particular a ULT device:

• • a) improving the efficiency or the effectiveness of the chiller unit in order to reduce the power consumption, at a consistent effective cooling capacity, or
  • b) reducing the thermal conductivity of the insulation materials of the housing in order to reduce the heat flow entering the storage region from the surroundings.

The two energy-related optimization possibilities are preferably linked by a mixed refrigerant circuit or an MRC for providing cold according to (a), and the vacuum technology in the housing structure according to (b).

Preferably on account of the low thermal conductivity, the vacuum technology makes it possible, in particular compared with housings which are thermally insulated only by means of polyurethane, to bring about a smaller heat input through the housing at the same insulation thicknesses, or to achieve an increase in the usable volume at a comparable heat input, on account of a thinner required insulation layer thickness.

The freezer is preferably equipped with an MRC chiller unit.

It is conceivable that the cooled interior can be closed by a closure element, and that the wall is part of the closure element.

The closure element can be a door, a lid or a flap, or another element by means of which the cooled interior can be closed.

It is preferably provided that the wall forms part of a housing or carcass of the freezer. The housing can be a full-vacuum housing.

It is conceivable that an inner and/or outer vacuum insulation panel is arranged on the wall.

It is preferably provided that the refrigeration circuit is configured to cool the cooled interior.

In an advantageous embodiment it is provided that the refrigeration circuit comprises a or exactly one condenser, compressor, precooler and evaporator, and one or exactly one expansion member.

It is preferably provided that the compressor is configured to compress at a pressure ratio of less than or equal to 20, preferably of less than or equal to 15, more preferably of less than or equal to 10, and particularly preferably of less than or equal to 9.

It is preferably furthermore provided that the precooler is a microstructure, plate, tube-in-tube, or multi tube-in-tube heat exchanger.

In an advantageous embodiment, it is provided that the refrigeration circuit is a single-stage refrigeration circuit.

It is preferably provided that the refrigerant mixture comprises two or more than two natural hydrocarbons or consists thereof, and preferably comprises or is a binary, tertiary, quaternary, etc refrigerant mixture of natural hydrocarbons, wherein it is preferably provided that the refrigerant mixture comprises or is a quaternary or higher-order (>4 component) refrigerant mixture composed of or comprising the natural hydrocarbons isobutane, propylene, ethylene and methane.

In particular for the temperature level between −90° C. and −150° C. nitrogen can be mixed in as the fluid component.

It is conceivable that the refrigerant mixture, at storage temperatures of between −36° C. and −90° C., is made up of two or more fluids, and in this case natural hydrocarbons are used, and for cryogenic ultra-low temperature freezers having storage temperature levels of between −90° C. and −150° C. nitrogen is furthermore mixed in as a supplementary fluid component.

In principle, more than four components are also possible and are covered by the invention.

The successful conceptual design of the composition of the refrigerant mixture in an MRC is an important factor in order, on account of the infinitely large number of mixing variants, to identify precisely the mixture which, as a working fluid in the MRC system, together with a precooler matched to the refrigerant mixture, leads to the best possible energy efficiency of the refrigeration circuit.

Preferably a quaternary refrigerant mixture composed of the natural hydrocarbons isobutane, propylene, ethylene, and methane is possible for a storage temperature region of −86° C. Other refrigerant mixtures composed of two or more natural hydrocarbons are also possible and are covered by the invention.

It is preferably provided that the freezer is configured for storing vaccines, plasma and/or biological material.

It is conceivable that a temperature in the cooled interior is between −150° C. and −36° C., in particular between −150° C. and −90° C. or between −90° C. and −36° C.

It is preferably provided that the freezer is a ULT device or is integrated in a fridge and/or refrigerator.

The energy consumption of the freezer according to the invention, and the resource usage of materials of the components in the production process can advantageously be significantly reduced compared with the prior art.

At this point, it is noted that the terms “a” and “an” do not necessarily refer to precisely one of the elements, even if this represents a possible embodiment, but rather can also denote a plurality of the elements. Likewise, the use of the plural also includes the presence of the element in question in the singular and, vice versa, the singular also includes a plurality of the elements in question. Furthermore, all the features of the invention described herein can be combined with one another as desired, or claimed in isolation from one another.

Further advantages, features and effects of the present invention emerge from the following description of preferred embodiments with reference to the figures in which identical or similar components are denoted by the same reference signs. In the drawings:

FIG. 1: is a diagram of an embodiment of a freezer according to the invention.

FIG. 2: is a graph of the temperature profiles of a refrigerant mixture in the pre-cooling process.

FIG. 3: is a graph of the temperature profiles during condensation and evaporation in a pure substance system

FIG. 4: is a diagram of a two-stage cascade refrigeration system.

FIG. 5: is a diagram of an auto cascade system comprising double intermediate expansion.

FIG. 1 shows a cooled interior 10 of a freezer, which is surrounded, at least in part, by a housing configured as a full vacuum insulation body 11.

An evaporator 21 of a refrigeration circuit 20, in the form of an MRC comprising a refrigerant mixture, is located in or on the cooled interior 10.

The refrigeration circuit 20 further comprises a precooler 22, a compressor 23, a condenser 24, and an expansion member 25, wherein these elements are connected, by lines configured for conducting a refrigerant mixture, in such a way that a refrigeration circuit 20 is formed.

From the flow diagram of the component arrangement of an MRC system as in FIG. 1, it is clear that the component structure of the refrigeration circuit 20 is based on known component sets of freezers from the prior art.

A positive outcome is the reduced number of refrigeration circuit components compared with the cascade circuits. This results in a reduced resource usage for the production process for MRC systems. The refrigeration circuit components required for an MRC system are furthermore standard components in refrigeration technology, and preferably do not require any further development outlay beyond the process-adapted component design.

In order to achieve, in a single-stage MRC refrigeration process, the required temperature range between the ambient temperature level of approximately 25° C., the use temperature level of approximately −86° C. and/or the evaporation temperature level of approximately −120° C., pre-cooling of the refrigerant mixture is preferably required.

For this reason, the precooler 22 in an MRC system, which can be for example a microstructure, plate, tube-in-tube or multi tube-in-tube heat exchanger, assumes a decisive functional role.

The process sequence in the precooler 22 is shown in the graph of FIG. 2.

FIG. 2 shows, by way of example, the pre-cooling process of an MRC chilling process according to the invention for an ultra-low temperature freezer in the storage temperature range of −90° C. to −36° C.

For an ultra-low temperature freezer having a storage temperature level between −150° C. to −90° C., the mentioned process parameters are not relevant.

The standardized heat exchanger length is plotted on the x-axis of the graph in FIG. 2. The temperature in ° C. is plotted on the y-axis of the graph in FIG. 2.

The pre-cooling of the for example quaternary refrigerant mixture comprising a highest boiler, high boiler, low boiler and lowest boiler now takes place, as shown in FIG. 1 and in the graph in FIG. 2, wherein refrigerant mixture flows from the condenser 24 into the precooler 22, from point 1 to point 2, and, in counterflow, refrigerant mixture flows out of the evaporator 21 from point 3 to point 4.

In the process, the refrigerant mixture originating from the evaporator 21 cools the refrigerant mixture originating from the condenser 24, in the precooler 22, from approximately 30° C. to approximately −90° C. The pre-cooling to −90° C. is merely an example and is not limiting.

The pre-cooling of the refrigerant mixture can be achieved when the substance properties of the fluid mixture are matched to the heat exchanger characteristics of the precooler.

In the capillary/suction tube heat exchanger of an R600a pure substance or −18° C. freezing system, such a vast pre-cooling effect could not be achieved, since the different R600a capacity currents on the warm side or the fluid side, and cold side or the vapor side, limit the achievable pre-cooling capacity.

In addition to the huge pre-cooling effect, the low compressor pressure ratio in MRC systems furthermore has a positive effect on the energy efficiency.

The following section describes, by way of example, an MRC system according to the invention in the temperature range between −90° C. and −36° C. The process parameters are different for MRC systems in the storage temperature range of −150° C. to −90° C.

For example, in the MRC system the required temperature lift of approximately 110 K between ambient temperature level of approximately 25° C. and usage temperature level of approximately −86° C. can be achieved at pressure ratios of approximately p_c/p_o=9. In the case of a pure substance system, it would not be possible to overcome such a large temperature range at a comparable pressure ratio, and therefore, in the case of pure substances as refrigerants primarily two-stage cascade circuits are used to achieve the low temperature level. This applies correspondingly for the further temperature ranges and/or pressure ratios cited within the context of the invention. These are preferably a temperature in the cooled interior between −150° C. and −36° C., in particular between −150° C. and −90° C. or between −90° C. and −36° C.

The pressure ratio is preferably less than or equal to 20, preferably less than or equal to 15, more preferably less than or equal to 10, and particularly preferably less than or equal to 9.

Compared with cascade circuits, the significantly reduced number of components is furthermore decisive for the increased energy efficiency of the MRC system. Thus, on account of the heat and substance transport processes and the resulting temperature and pressure gradients within each refrigeration circuit component, processes associated with dissipation occur, which can be understood as energy loss characteristic variables. On account of the plurality of the components in the cascade systems, significantly more efficiency-reducing processes occur than in an MRC system.

The following considerations should preferably be made with regard to the structure of the refrigeration circuit 20: Checking the pressure resistance of the R600a circuit components used hitherto, since the MRC system operates at higher system pressures compared with the R600a refrigeration circuit.

Claims

1. A freezer comprising a cooled interior and comprising a wall that surrounds the cooled interior at least in regions, and comprising a refrigeration circuit, wherein the wall is formed at least in part by a vacuum insulation body and/or by an insulation body which comprises or is made of polyurethane, or only by a vacuum insulation body or only by an insulation body which comprises or is made of polyurethane, and in that the refrigeration circuit comprises a refrigerant mixture.

2. The freezer according to claim 1, wherein that the wall forms part of a housing or carcass of the freezer.

3. The freezer according to claim 1, wherein an inner and/or outer vacuum insulation panel is arranged on the wall.

4. The freezer according to claim 1, wherein the the refrigeration circuit is configured to cool the cooled interior.

5. The freezer according to claim 1, wherein the refrigeration circuit comprises a or exactly one condenser, compressor, precooler and evaporator, and one or exactly one expansion member.

6. The freezer according to claim 5, wherein the compressor is configured to compress at a pressure ratio of less than or equal to 20.

7. The freezer according to claim 5, wherein the precooler is a microstructure, plate, tube-in-tube, or multi tube-in-tube heat exchanger.

8. The freezer according to claim 1, wherein the refrigeration circuit is a single-stage refrigeration circuit.

9. The freezer according to claim 1, wherein the refrigerant mixture comprises two or more than two natural hydrocarbons or comprises a binary, tertiary, quaternary, refrigerant mixture of natural hydrocarbons.

10. The freezer according to claim 1, wherein the refrigerant mixture further comprises nitrogen.

11. The freezer according to claim 1, wherein the freezer is configured for storing vaccines, plasma and/or biological material.

12. The freezer according to claim 1, wherein a temperature in the cooled interior is between −150° C. and −36° C.

13. The freezer according to claim 1, wherein the freezer is a ULT device or is integrated in a fridge and/or refrigerator.

14. The freezer according to claim 5, wherein the compressor is configured to compress at a pressure ratio of less than or equal to 15.

15. The freezer according to claim 5, wherein the compressor is configured to compress at a pressure ratio of less than or equal to 10.

16. The freezer according to claim 5, wherein the compressor is configured to compress at a pressure ratio of less than or equal to 9.

17. The freezer according to claim 1, wherein the refrigerant mixture comprises a quaternary or higher-order (>4 component) refrigerant mixture comprising the natural hydrocarbons isobutane, propylene, ethylene and methane.

18. The freezer according to claim 1, wherein a temperature in the cooled interior is between −150° C. and −90° C.

19. The freezer according to claim 1, wherein a temperature in the cooled interior is between −90° C. and −36° C.