WO1996002605A1

WO1996002605A1 - Refrigerant compositions

Info

Publication number: WO1996002605A1
Application number: PCT/GB1995/001595
Authority: WO
Inventors: Richard Llewellyn Powell; Stuart Corr; Frederick Thomas Murphy; James David Morrison
Original assignee: Imperial Chemical Industries Plc
Priority date: 1994-07-13
Filing date: 1995-07-06
Publication date: 1996-02-01
Also published as: GB9414134D0; AU2892595A

Abstract

A non-azeotropic refrigerant composition is described comprising (A) a first component comprising fluoromethane and (B) a second component comprising at least one hydrofluorocarbon selected from the group consisting of difluoromethane, 1,1,1-trifluoroethane and pentafluoroethane.

Description

REFRIGERANT COMPOSITIONS

The present invention relates to non-azeotropic refrigerant compositions which boil over a temperature range and thus provide temperature glides in the heat exchangers of the heat transfer devices in which they are used.

Heat transfer devices of the mechanical compression type such as refrigerators, freezers, heat pumps and air conditioning systems are well known. In such devices a refrigerant liquid of a suitable boiling point evaporates at low pressure taking heat from a surrounding heat transfer fluid. The resulting vapour is then compressed and passes to a condenser where it condenses and gives off heat to another heat transfer fluid. The condensate is then returned through an expansion valve to the evaporator so completing the cycle. The mechanical energy required for compressing the vapour and pumping the liquid may be provided by an electric motor or an internal combustion engine. In addition to having a suitable boiling point and a high latent heat of vaporisation, the properties preferred of a refrigerant include low toxicity, non-flammability, non-corrosivity, high stability and freedom from objectionable odour.

Hitherto, heat transfer devices have tended to use fully and partially halogenated chlorofluorocarbon refrigerants such as trichlorofluoromethane (Refrigerant R-l 1), dichlorodifluoromethane (Refrigerant R-12), chlorodifluoromethane (Refrigerant R-22) and the azeotropic mixture of chlorodifluoromethane and chloropentafluoroethane (Refrigerant R-l 15); the azeotrope being Refrigerant R-502. Refrigerant R-22, for example, is widely used in air conditioning systems.

However, the fully and partially halogenated chlorofluorocarbons have been implicated in the destruction of the earth's protective ozone layer and as a result the use and production thereof has been limited by international agreement.

Whilst heat transfer devices of the type to which the present invention relates are essentially closed systems, loss of refrigerant to the atmosphere can occur due to leakage during operation of the equipment or during maintenance procedures. It is important, therefore, to replace fully and partially halogenated chlorofluorocarbon refrigerants by materials having low or zero ozone depletion potentials. In addition to the possibility of ozone depletion, it has been suggested that significant concentrations of chlorofluorocarbon refrigerants in the atmosphere might contribute to global warming (the so-called greenhouse effect). It is desirable, therefore, to use refrigerants which have relatively short atmospheric lifetimes as a result of their ability to react with other atmospheric constituents such as hydroxyl radicals.

Replacements for some of the chlorofluorocarbon refrigerants presently in use have already been developed. These replacement refrigerants tend to comprise selected hydrofluoroalkanes, i.e. compounds which contain only carbon, hydrogen and fluorine atoms in their structure. Thus, refrigerant R-12 is generally being replaced by 1,1,1 ,2-tetrafluoroethane (R- 134a).

Although suitable replacement refrigerants are available, there is always a need for new replacement refrigerants that exhibit the required low or zero ozone depletion potential. For example, very real benefits could be realised by a new replacement refrigerant having a higher refrigeration capacity and/or a higher energy efficiency (coefficient of performance) than the replacement refrigerants known in the art. The provision of more energy efficient refrigerants is particularly desirable since it can lead to a reduction in indirect global warming. One possible route to achieving an increase in the energy efficiency of a heat transfer device is to employ a non-azeotropic refrigerant blend which boils over a reasonably wide temperature range. The present invention provides a non-azeotropic refrigerant composition comprising a mixture of compounds having low or zero ozone depletion potentials which can boil over a temperature range making it possible to increase the energy efficiency of the equipment in which the composition is used. The temperature range over which the refrigerant composition of the invention boils, i.e. the so-called temperature glide, can be adjusted by appropriate selection of the components and the amounts thereof forming the composition and in this way it is possible to tailor the composition to its intended application. In addition, the refrigerant composition of the invention can exhibit an advantageously high refrigeration capacity.

According to the present invention there is provided a non-azeotropic (zeotropic) refrigerant composition comprising: (A) a first component comprising fluoromethane (R-41) optionally together with carbon dioxide (COj); and

(B) a second component comprising at least one hydrofluorocarbon selected from the group consisting of difluoromethane (R-32), 1,1,1-trifluoroethane (R-l 43 a) and pentafluoroethane (R-125).

The zeotropic refrigerant composition of the invention comprises at least the first and second components defined above.

The first component (component (A)) comprises fluoromethane (R-41) or a mixture of R-41 and carbon dioxide (CO.). Both of these compounds have a low temperature refrigeration action, with CO₂ subliming at around -78.5°C and R-41 having a boiling point of around -78.4 °C.

The second component (component (B)) comprises at least one hydrofluorocarbon selected from the group consisting of difluoromethane (R-32), 1,1,1-trifluoroethane (R-143a) and pentafluoroethane (R-l 25) which have boiling points of around -51.6°C, -47.6°C and -48.5°C respectively. It is apparent that the boiling points of the three hydrofluorocarbons specified for the second component are appreciably higher than the boiling point of the R-41 and the sublimation temperature of the CO₂ (if included) making up the first component which means that the refrigerant composition of the invention is capable of boiling and condensing over a temperature range, i.e. it can exhibit a temperature glide in both the evaporator and condenser. The second component may contain just one of the three specified hydrofluorocarbons or it may comprise a mixture, for example an azeotropic or azeotrope-like mixture, of any two or all three of these compounds. In a preferred embodiment, the second component is R-l 25 or a mixture comprising R-l 25 and R-32. The amounts of the first and second components in the refrigerant composition may be varied within wide limits, but typically the refrigerant composition will comprise from 2 to 35 % by weight of the first component and from 65 to 98 % by weight of the second component. Preferably, the refrigerant composition will comprise from 2 to 25 % by weight of the first component and from 75 to 98 % by weight of the second component. In a particularly preferred embodiment, the refrigerant composition comprises from 2 to 15 % by weight of the first component and from 85 to 98 % by weight of the second component.

The refrigerant composition of the invention may also be combined with one or more hydrocarbon compounds in an amount which is sufficient to allow the composition to transport a mineral oil or alkyl benzene type lubricant around a refrigeration circuit and return it to the compressor. In this way, inexpensive lubricants based on mineral oils or alkyl benzenes may be used to lubricate the compressor.

Suitable hydrocarbons for use with the refrigerant composition of the invention are those containing from 2 to 6 carbon atoms, with hydrocarbons containing from 3 to 5 carbon atoms being preferred. Propane and pentane are particularly preferred hydrocarbons, with pentane being especially preferred.

Where a hydrocarbon is combined with the refrigerant composition of the invention, it will preferably be present in an amount of from 1 to 10 % by weight on the total weight of the refrigerant composition. The refrigerant composition of the invention may also be used in combination with the types of lubricants which have been specially developed for use with hydrofluorocarbon based refrigerants. Such lubricants include those comprising a polyoxyalkylene glycol base oil. Suitable polyoxyalkylene glycols include hydroxyl group initiated polyoxyalkylene glycols, e.g. ethylene and/or propylene oxide oligomers/polymers initiated on mono- or polyhydric alcohols such as methanol, butanol, pentaerythritol and glycerol. Such polyoxyalkylene glycols may also be end-capped with suitable terminal groups such as alkyl, e.g. methyl groups. Another class of lubricants which have been developed for use with hydrofluorocarbon based refrigerants and which may be used in combination with the present refrigerant compositions are those comprising a neopentyl polyol ester base oil derived from the reaction of at least one neopentyl polyol and at least one aliphatic carboxylic acid or an esterifiable derivative thereof. Suitable neopentyl polyols for the formation of the ester base oil include pentaerythritol, polypentaerythritols such as di- and tripentaerythritol, trimethylol alkanes such as trimethylol ethane and trimethylol propane, and neopentyl glycol. The esters may be formed with linear and/or branched aliphatic carboxylic acids, such as linear and/or branched alkanoic acids. Preferred acids are selected from the C,._g, particularly the C₅.₇, linear alkanoic acids and the C₅.₁₀, particularly the C_j.,, branched alkanoic acids. A minor proportion of an aliphatic polycarboxylic acid, e.g. an aliphatic dicarboxylic acid, may also be used in the synthesis of the ester in order to increase the viscosity thereof. Usually, the amount of the carboxylic acid(s) which is used in the synthesis will be sufficient to esterify all of the hydroxyl groups contained in the polyol, although residual hydroxyl functionality may be acceptable.

The single fluid refrigerants and azeotropic refrigerant blends which are used in conventional heat transfer devices boil at a constant temperature in the evaporator under constant pressure conditions, and so produce an essentially constant temperature profile across the evaporator. The temperature of the heat transfer fluid being cooled, which may be air or water for example, drops fairly rapidly on first contacting the cold surfaces provided by the refrigerant evaporating in the evaporator owing to the large difference in temperature between that fluid and the evaporating refrigerant. However, since the temperature of the heat transfer fluid is progressively reduced as it passes along the length of the evaporator, there is a progressive reduction in the temperature differential between the fluid and the evaporating refrigerant and a consequent reduction in the heat transfer or cooling rate.

In contrast, the refrigerant composition of the invention is a non-azeotropic (zeotropic) composition which boils over a temperature range under constant pressure conditions so as to create a temperature glide in the evaporator which can be exploited to reduce the energy required to operate the heat transfer device, e.g. by making use of the Lorentz cycle. One technique for exploiting the temperature glide involves the use of a heat transfer device equipped with a counter current flow evaporator and/or condenser in which the refrigerant and the heat transfer fluid are caused to flow counter currently to each other. With such an arrangement, it is possible to minimise the temperature difference between the evaporating and condensing refrigerant whilst maintaining a sufficiently high temperature difference between the refrigerant and the external fluid(s) to cause the required heat transfer to take place.

The consequence of minimising the temperature difference between the evaporating and condensing refrigerant in the same system is that the pressure difference is also minimised. As a result, the overall energy efficiency of the system is improved as less energy is consumed to bring about the refrigerant pressure rise from evaporator to condenser conditions.

The zeotropic refrigerant composition of the present invention may be used to provide the desired cooling in heat transfer devices such as air conditioning and low temperature refrigeration systems by a method which involves condensing the refrigerant composition and thereafter evaporating it in a heat exchange relationship with a heat transfer fluid to be cooled. In particular, the refrigerant composition of the invention may be employed as a replacement for refrigerant R-22.

The present invention is now illustrated but not limited with reference to the following example.

Example 1

The performance of three refrigerant compositions of the invention in a refrigeration cycle of the type prevailing in an air conditioning system was evaluated using standard refrigeration cycle analysis techniques in order to assess the suitability thereof as a replacement for R-22. The operating conditions which were used for the analysis were chosen as being typical of those conditions which are found in an air conditioning system, and counter current flow at the heat exchangers was assumed. In order to illustrate the benefit of the zeotropic refrigerant compositions of the invention in terms of their improved energy efficiency, it was first necessary to define the inlet and outlet temperatures of the heat transfer fluids at each heat exchanger (evaporator and condenser). The temperatures in the evaporator and condenser, assuming that a single fluid refrigerant was used in the cycle, were then chosen and these temperatures together with the inlet and outlet temperatures of the heat transfer fluids referred to above were used to determine a target log mean temperature difference for each heat exchanger. In the cycle analysis itselζ the refrigerant inlet and outlet temperatures at both the evaporator and condenser were adjusted until the target log mean temperature difference was achieved for each heat exchanger. When the target log mean temperature difference for each heat exchanger was achieved, the various properties of the refrigerant composition in the cycle were recorded. The following refrigerant compositions were subjected to the cycle analysis:

(1) A composition comprising 5 % by weight R-41 and 95 % by weight R-l 25. (2) A composition comprising 10 % by weight R-41 and 90 % by weight R-l 25.

(3) A composition comprising 10 % by weight R-41, 45 % by weight R-l 25 and 45 % by weight R-32.

The following operating conditions were used in the cycle analysis.

EVAPORATOR.

Evaporator Temperature: 10°C

Inlet Temperature of Heat Transfer Fluid 25°C Outlet Temperature of Heat Transfer Fluid 15°C

Log Mean Temperature Difference for Evaporator 9.1 °C

CONDENSER:

Condenser Temperature: 42°C

Inlet Temperature of Heat Transfer Fluid 30°C

Outlet Temperature of Heat Transfer Fluid 40°C

Log Mean Temperature Difference for Condenser 5.58°C

Amount of Superheat: 15°C

Amount of Subcooling: 5°C

Isentropic Compressor Efficiency: 75 %

Cooling Duty: 1 kW

The results of analysing the performance of the three refrigerant compositions in an air conditioning cycle using these operating conditions are given in Table 1. The performance parameters of the refrigerant compositions which are presented in Table 1, i.e. condenser pressure, evaporator pressure, discharge temperature, refrigeration capacity (by which is meant the cooling duty achieved per unit swept volume of the compressor), coefficient of performance (COP) (by which is meant the ratio of cooling duty achieved to mechanical energy supplied to the compressor), and the glides in the evaporator and condenser (the temperature range over which the refrigerant composition boils in the evaporator and condenses in the condenser), are all art recognised parameters.

The performance of refrigerant R-22 and R-l 25 under the same operating conditions is also shown in Table 1 by way of comparison.

It is apparent from Table 1 that the refrigerant compositions of the invention boiled over a temperature range in the evaporator and condensed over a temperature range in the condenser, i.e. they exhibited a temperature glide in both heat exchangers, and that this property appeared to enhance the energy efficiency of the air conditioning cycle as is evident from the higher values recorded for the coefficient of performance for the refrigerant compositions of the invention in comparison with pure R-l 25. It is also apparent from Table 1 that the refrigerant composition of the invention exhibited high refrigeration capacities in comparison with both R-l 25 and R-22.

TABLE 1

Claims

Claims:

1. A non-azeotropic refrigerant composition comprising: (A) a first component comprising fluoromethane; and (B) a second component comprising at least one hydrofluorocarbon selected from the group consisting of difluoromethane, 1,1,1-trifluoroethane and pentafluoroethane.

2. A non-azeotropic refrigerant composition as claimed in claim 1 wherein the first component (A) additionally comprises carbon dioxide.

3. A non-azeotropic refrigerant composition as claimed in claim 1 or claim 2 wherein the second component is pentafluoroethane or a mixture comprising pentafluoroethane and difluoromethane.

4. A non-azeotropic refrigerant composition as claimed in any one of claims 1 to 3 which comprises from 2 to 35 % by weight of the first component (A) and from 65 to 98 % by weight of the second component (B).

5. A non-azeotropic refrigerant composition as claimed in claim 4 which comprises from 2 to 25 % by weight of the first component (A) and from 75 to 98 % by weight of the second component (B).

6. A non-azeotropic refrigerant composition as claimed in claim 5 which comprises from 2 to 15 % by weight of the first component (A) and from 85 to 98 % by weight of the second component (B).

7. A refrigeration or air conditioning system containing a non-azeotropic refrigerant composition as claimed in any one of claims 1 to 6.

8. The use in a refrigeration or air conditioning system of a non-azeotropic refrigerant composition as claimed in any one of claims 1 to 6.

9. A method for providing cooling which comprises condensing a non-azeotropic refrigerant composition as claimed in any one of claims 1 to 6 and thereafter evaporating it in a heat exchange relationship with a fluid to be cooled.