FIELD OF THE INVENTION The invention relates to a heat transfer fluid, in particular; a fluid for use in heat transfer applications, such as cooling; use of said fluid in heat transfer applications; and heat transfer devices comprising said fluid.

BACKGROUND OF THE INVENTION

An air conditioner (or air conditioning unit or "air con" or "AC", as it is often referred to) is an example of a heat transfer device used to modify the conditions of an atmosphere, typically where the air in an environment is cooled and optionally dehumidified. The apparatus can be integral to a given environment (e.g. forming part of the ventilation system of a building or the climate controls of a vehicle) or may be a self-contained unit which can be deployed to a particular environment. Heat transfer devices of this kind process the air of an environment, typically cooling the air (though in some applications can provide heating) by bringing incoming air into thermal communication with a heat transfer fluid system. The heat transfer fluid, once it has absorbed the heat from the incoming air, is then moved to a heat sink where the heat is released .

A number of different heat transfer fluids have been used over the years in air conditioning and refrigeration systems. Of particular note are chlorofluorocarbons sometimes referred to as CFCs. As is well-known, CFCs are very damaging to the earth's ozone layer and contribute to global warming. Therefore, CFCs have been gradually phased out of use in countries around the world. Accordingly, various substitute fluids have been developed, without the negative environmental side-effects, as replacement for CFCs. A common fluid mixture that has been used to replace traditional CFCs is a mixture of difluoromethane (CH2F2, often in the industry referred to as "R32") and pentafluoroethane (CHF2CF3, often in the industry referred to as "R125"). This particular gas mixture (often referred to in the industry as "R410A") performs well as a heat transfer fluid and is less damaging to the environment. However, in order to achieve comparable properties to their CFC counterparts, it is often the case that heat transfer devices using CFC replacement fluids, must be operated at higher pressures. For instance, R410A is conventionally used in systems at pressures approximately 35% higher than those of an equivalent system using CFCs. This is disadvantageous for a number of reasons. Firstly, operating systems at higher pressure requires more energy and therefore reduces the overall efficiency of a heat transfer device. Higher pressures also place greater demands on the structural integrity of a heat transfer device and increase the rate of "wear and tear". Furthermore, operating systems at high pressures often leads to an increase in the amount of leakage from a system, which wastes heat transfer fluid. In addition, retrofitting older heat transfer devices with new (more environmentally friendly) heat transfer fluids often requires the incorporation of a compressor which increases the cost of recycling older equipment.

A typical example of a gas used to replace CFCs is 1,1,1,2-tetrafluoroethane (CF3CFH2 - often referred to in the industry as "R134A"). CF3CFH2 is primarily used as a "high- temperature" refrigerant for domestic refrigeration and previously in automobile air conditioners. CF3CFH2 is a replacement for dichlorodifluoromethane (often referred to in the industry as "R12" or "Freon 12"). CF3CFH2 works at low vapour pressures which limits the refrigeration capacity of these compounds alone. Another gas which has been proposed as a replacement for CFCs is the hydrofluoroolefin 1,3,3,3 tetrafluoropropene (CHF=CHCF3, often referred to in the industry as "R1234ze(E)"). This colourless gas has been proposed as a replacement for CF3CFH2 as a refrigerant in automobile air conditioners as it has been shown to have minimal environmental impact, having a "global warming potential" (GWP) rating one 335th that of CF3CFH2 and an atmospheric lifetime of about 400 times shorter. That said, such gases still must be pressurized in order to provide optimal heat transfer properties. Accordingly, what is desired is an effective heat transfer fluid which can be used in heat transfer devices at low pressures and without the negative environmental properties of CFCs.

The invention is intended to overcome or at least ameliorate the above problems.

SUMMARY OF THE INVENTION

There is provided in the first aspect of the invention, a heat transfer fluid comprising : a first component, present in an amount greater than 85% wt., comprising; (a) a C1-C6 tetrafluoroalkane, and (b) a C1-C6 tetrafluoroalkene; and a second component, present in an amount less than 15% wt., comprising; (c) a C1-C6 difluoroalkane, and (d) a C2-C6 pentafluoroalkane. The inventors have surprisingly found that making use of a two-component system containing the above claimed components leads to a surprising improvement in the efficiency of a heat transfer system. In particular, when a small amount of the second component is introduced into the first component, excellent heat transfer properties are achieved at pressures similar to those of CFC alternatives. The net result of this is that more efficient heating results. The magnitude of improvement is particularly surprising as the observed performance results in energy cost savings of up to 62% with the addition of only a relatively small amount of second component to first component when compared with existing industry standards.

The term "heat transfer fluid" is intended to refer to a liquid or gas which is used to absorb or dissipate heat. Such fluids are typically used to absorb heat from a first region of a system and dissipate heat from a second region of the system. A classic example of this would be, for instance, a refrigerator in which the heat transfer fluid passes through the fluid circuit, in thermal communication with a heat sink and chilled internal cabinet, in such a way as to extract heat from within the cabinet (where food is kept) and release heat to a heat sink. Whether or not the heat transfer fluid is a liquid or gas depends largely on the temperatures involved and the pressures at which the system is operated. It is conceivable that the fluid could be a combination of both liquid and gas depending on how a particular heat transfer device is operated. However, it is typically the case that the heat transfer fluid is a gas.

It is typically the case that component (a) is tetrafluoroethane, in particular 1,1,1,2- tetrafluoroethane (CF3CFH2, often referred to as "R134a"). Furthermore, it is also the case that element (b) is tetrafluoropropene, in particular Trans-1, 3,3,3- tetrafluoropropene (CHF=CHCF3, often referred to as "R1234ze(E)"). The inventors have found that these two components function well as ingredients of the first component of the heat transfer fluid. Typically, the ratio of (a) to (b) is in the range of 1 : 2 to 2: 1 by weight. It is often the case that the ratio of (a) to (b) is in the range of 2: 3 to 3 : 2 by weight. Most typically the ratio of (a) to (b) is about 1 : 1.05.

It is typically the case that (a) is present in an amount in the range of 40 wt.% to 50 wt.%., more typically 42 wt.% to 48 wt.%, more typically still 45 wt.% to 47 wt.% and most typically about 46 wt. % with respect to the total composition. Further, it is often the case that (b) is present in an amount in the range of 45 wt.% to 55 wt.% , more typically 46 wt.% to 52 wt.%, more typically still 47 wt.% to 51 wt.%, even more typically 48 wt.% to 50 wt.% and most typically about 49 wt.% with respect to the total composition. Unless otherwise stated herein, references to the percentage of the component of the composition is intended to refer to the weight percentage of the total composition.

Turning to the second component of the invention, (c) is typically difluoromethane (CF2H2, often referred to as "R32"). In addition, it is often the case that (d) is pentafluoroethane (CF3CF2H, often referred to as "R125"). The inventors have found that making use of these compounds as elements (c) and (d) results in a surprising improvement in the properties of the heat transfer fluid as compared to those currently prevalent in industry.

Any mixture of elements (c) and (d) may be possible in the second component of the composition. However, the ratio of (c) to (d) is typically in the range of 2 : 3 to 3 : 2 by weight. However, it is often the case that this ratio is in the range 4: 5 to 5 :4 by weight and typically, the ratio of (c) to (d) is often 1 : 1 by weight. In one preferred embodiment, the second component comprises a mixture of 40 wt.% to 60 wt.% difluoromethane and from 60 wt.% to 40 wt.% pentafluoroethane. More typically, the mixture comprises from 45 wt.% to 55 wt.% difluoromethane and from 55 wt.% to 45 wt.% pentafluoroethane.

In the heat transfer fluid of the invention, it is typically the case that (c) is present in an amount in the range of 2.0 wt.% to 10 wt.%, more typically 2.0 wt.% to 8 wt.%, and even more typically, 2.0 wt.% to 3.0 wt.%. Most often, (c) is present in an amount of about 2.5 wt.% with respect to the total composition.

In the heat transfer fluid of the invention, it is typically the case that (d) is present in an amount in the range of 2.0 wt.% to 10 wt.%, more typically, 2.0 wt.% to 8 wt.%, and even more typically, 2.0 wt.% to 3.0 wt.%. Most often, d) is present in an amount of about 2.5 wt.% with respect to the total composition. The inventors have found that these particular ratios and absolute quantities of the described compounds of the second component result in significant improvements in the heat transfer properties of the fluid as compared to those currently prevalent in the industry.

Moreover, employing compositions having the described components in the claims amounts has been found to reduce the flammability of the composition.

It is typically the case that the first component of the heat transfer fluid is present in an amount greater than 90 wt.% by weight of the total composition of the gas. Further, it is also the case that the second component is typically present in an amount of less than 10 wt.% by weight with respect to the total mass of the heat transfer fluid.

The heat transfer fluid of the invention may include further additives. For instance, inert odorous fluids may be incorporated into the heat transfer fluid of the invention in order to aid in the detection of leaks. The fluid of the invention may further comprise one or more dyes. There is no particular restriction on the choice of additive, provided that the additive does not (or at least negligibly) adversely interfere with the action of the heat transfer fluid. Catalysts and other additives that promote the heat transfer properties of the fluid of the invention may also be used. Antimicrobial or cleaning agents may also be introduced into the fluid. Moreover, a leak stopping reagent may be incorporated into the composition of the invention so as to promote the sealing of any cracks or punctures in containers in which the composition is contained. Typically, the total amount of additives introduced into the composition of the invention will be less than 15%, more typically less than 10%, more typically still less than 5%, even more typically less than 3% and most typically less than 1% (by weight of the total composition).

There is also provided in a second aspect of the invention, a heat transfer device comprising the fluid according to the first aspect of the invention.

The term "heat transfer device" is intended to refer to any device adapted to heating or cooling a particular environment, using a cyclable fluid circuit. There is no particular restriction on the types of heat transfer device with which the fluid of the first aspect of the invention may be used. However, typical examples include refrigerators, air conditioning units, heat pumps, centrifugal chillers and combinations thereof. Most typically, the heat transfer device is an air conditioning system .

The heat transfer device may include a compressor. There is no particular restriction on the type of compressor with which the heat transfer fluid of the invention is compatible. However, typically the compressor is an inverter compressor, on/off compressor or screw / screw-inverter type compressors. Most often the compressor will be an inverter compressor as theses have been found to provide optimal efficiency with the heat transfer fluid of the invention. Also provided in a further aspect of the invention is a use of the fluid according to the first aspect of the invention, as heat transfer fluid. It is often the case that the heat transfer fluid is used as a coolant. Also provided is a process for treating an atmosphere or environment using a heat transfer device comprising the heat transfer fluid according to the first aspect of the invention.

Although the composition of the invention may comprise any one of the above described features, it is also envisaged that said composition may consist, or consist essentially, of those features described herein.

DETAILED DESCRIPTION OF THE INVENTION A comparative test was conducted between a 50 : 50 mixture of difluoromethane and pentafluoroethane (often referred to in the industry as "R410A") and the gas composition according to the invention (referred to herein as "Composition 1") which has a composition as outlined below:

Amount

Gas

/ wt. %

difluoromethane 2.5 pentafluoroethane 2.5

Trans-l,3,3,3-tetrafluoropropene 49.0

1,1,1,2-tetrafluoroethane 46.0

Table 1. Composition 1, a preferred embodiment of the invention.

The skilled person will appreciate that these values are not exact and typically, these values are accurate to ± 1 wt.%. The properties of composition 1 are shown in table 2 below:

Property Blend

(a) Formulation Rl-32/Ί 25/134a/ 1234ze(E)

(2.5/2.5/46.0/49.0) {mass %)

(b) Molecular Mass as Formulated 105.3 g/mo

(c) Molecular Mass of the Vapor at 60°C ( 140"F) 102.8 g/mol

(d) Temperature at 14.7 psia. (101 kPa)

Saturated liquid (Boiling Point) -29.20 °C (-20.56 °F) Saturated vapor (Dew point) -26.02 °C (-14.84 °F)

(e) Maximum temperature glide

At the normal boiling point 3.18 °C (5.72 °F)

At 20°C(68°F} 2.7 °C ( 4.86 °F)

(f) Latent Heat of Vaporization

At 60°C{140°F) 131.2 kJ/kg (56.4 Btu/lb)

(g) Specific Heat Ratio of Vapor

At 60°C(140°F} 1.45

(h) Temperature at the Critical Point 99.5 °C (211.1 °F)

(i) Specific Volume at the Critical Point 2.05 Ukg {0.0328 f /lb)

shows the properties of the Composition 1

Table 2. continued

Measurements were performed on an air conditioning apparatus from Gree Electronic 2400BTU (British Thermal Unit) and Toshiba 12000BTU. Below average data are registered during a 15 minutes run. The measurements were made on two different days, and thus, there are some differences on some of the parameters such as the outdoor temperature.

Measurement Area R410A Composition 1

Consumed electrical power / A 3.8 1.6

Incoming air temperature to evaporator /

29.3 28.2 °C

TABLE 2. Power measurements of the tested gas composition versus R410A.

In addition a simple test to examine the heat effect of the fluid of the invention was performed . It was heated to about 44°C outside, from about 23°C inside. Outside temperature was about the same as cooling. Energy consumption on 410a was 3.8 A at a pressure of 8.7 bars. The Composition consumed 1.6A at a pressure of 3.4 bars.