Zeotropic Refrigerant Mixtures in Vapour Compression Refrigeration Systems-Issues and Implications

Use of zeotropic refrigerant mixtures introduce a number of novel issues related to vapour compression systems used in refrigeration and air conditioning industry. Certain attributes associated with evaporation and condensation behaviours of zeotropic mixtures are responsible for most of these. This paper discusses how the mixture attributes influence the established practices, design and operation of conventional vapour compression systems.


Introduction
Before 1980s, chlorofluorcarbons (CFC) and hydrochlorofluorocarbons (HCFC) took the leading stand as the most suited category of working fluids for refrigeration and air conditioning (RAC) applications.However, with the advent of environmental implications associated with the CFCs and HFCFs, presently a mechanism for gradual global cut down on the use of these substances is in place.Various fluids; for example hydrocarbons (HCs), organic and inorganic fluids, hydrofluorocarbons (HFCs), and blends of certain HCFCs and HFCs are considered as replacements.To obtain working fluids that a) do not damage the environment and b) match the thermal-physical characteristics of CFCs or HCFCs being replaced, mixing of suitable refrigerants (HFCs, HCFCs and HCs) became a preferred concept.
Unlike with pure refrigerants, with many mixtures the phase changing process is nonisothermal, and the composition of mixture does not remain constant during condensation and evaporation.This situation has left the RAC industry with numerous issues to deal with.Design norms and operational guidelines, component and lubricant selection procedures etc., are a few to mention.Further, non-isothermal phase change behaviour of mixtures creates ambiguity in selecting heat exchangers or compressors from manufacturers' catalogue based on a single temperature [1].Changes in mixture composition causes variations in capacity, flow temperatures and coefficient of performance (COP) etc., compared to the original design [2].
Invariably, the change over from pure to mixture refrigerants demand reassessing and revising many established practices associated with vapour compression systems.Thermodynamic and heat transfer behaviour is one of the important aspects in this context.Further, influences of two special attributes of mixtures; temperature glide (TG) and composition shift (CS), too require detailed attention.These two attributes exert significant influence on heat transfer behaviour [3] to the extent that errors of omitting the effects in heat exchanger design can be substantial.On the other hand, with appropriate system design, the two attributes contribute towards improving, and optimizing system performance.A conceptual discussion on issues and implications of using refrigerant mixtures in vapour compression systems forms the theme for this paper.

Background -refrigerant mixtures
Evaporation or condensation of a pure fluid is isothermal.However, a mixture with two or more components exhibits non-isothermal phase change behaviour.For example, start and finish of evaporation of a mixture occur at bubble point and dew point temperature respectively.Fig. 1 shows this behaviour for a binary mixture.During phase changing process, the vapour phase is enriched with more volatile components, while the liquid phase is depleted of the same component(s) of the mixture.

Dr. Leelananda Rajapaksha, BSc.Eng. (First class Hons. Peradeniya). M.Eng. (AIT, Bangkok). PhD (University of London. UCL) is a Senior Lecturer attached to the Department of Mechanical Engineering, Faculty of Engineering, University of Peradeniya.
At a given pressure and composition, the difference between the dew point and the bubble point temperatures is defined as the temperature glide (TG).
When different refrigerant mixtures are considered based on their phase changing process, two main groups; azeotropes and zeotropes (also termed non-azeotropes), can be identified.The magnitude of temperature glide of azeotropic refrigerants generally depends on the boiling points of the components and the composition, and decreases with increasing pressure.The magnitude of composition shift during zeotropic phase change can be estimated using the corresponding composition values (Z -P and Z -Q, Fig. 1) along the bubble and the dew lines respectively for a binary rnixture.When the HTF flow rate for obtaining TG match is not available, under or over glide condition occurs.A horizontal line (case a, Fig. 3) represents perfect TG match.For cases b and c, the inclinations (b and (c correspond respectively to a certain degree of mismatch due to lower (i.e.dT is decreasing along the length, relatively to the cold end of the HTF) and higher flow rates (i.e.dT is increasing along the length) than that of case a.A large gradient implies a higher mismatch that generally leads to poor performance.

At
However, evaporator will not usually have the full glide corresponding to the operating pressure, due to the fact that the refrigerant is most likely to be in the two-phase state when entering the evaporator.
One implication of temperature glide is that at equal degree of condenser subcool (Fig. 4 Further, non-linearity could influence or limit the potential for decreasing the temperature differences at inlet/outlet, and may lead to increased heat transfer area requirement to achieve a desired capacity.Errors in the order of 50% in the heat exchanger area estimate could occur due to use of conventional LMTD method with non-linear mixtures [14].

Shift in circulating composition
When a vapour compression system uses a single component refrigerant, the concentration of the refrigerant through out the system remains the same.However, when zeotropes are used, researches have shown the existence of a running mixture composition different to the original mixture [15,16].A different running composition exists due internal fractionation of zeotrpes [16] and hold up of liquid and vapour phases in twophase sections of heat exchangers and system components that hold refrigerant in equilibrium [17].

The fractionation is a result of preferential boiling (or condensation) of mixture components that result in different compositions in vapour and liquid phases during phase-change heat transfer.
In boiling the vapour phase is usually enriched with more volatile components of the mixture while the liquid phase is depleted of the same.This situation results in holding up of less volatile components in the liquid phase during phase change while the circulating mixture composition within the system becomes different to that of original composition.In addition, shift in the circulating composition occurs due to refrigerant leakages (mostly in vapour phase) and differential solubility of mixture components in the lubricants [18].

ENGINEER
Shift in circulating composition leads to modified system pressures, temperatures, capacity and efficiency [19], and the magnitude of which depends on the design and operational parameters of the system concern [16].For a given refrigerant mixture and a system, the magnitude of the concentration shift varies with the amount of system charge, operating conditions, volumes of heat exchangers and the availability of liquid receiver and suction line accumulator.To estimate the circulation composition, refrigerant hold ups in different parts of the system (where the liquid and the vapour phases coexist) need to be estimated [17].Refrigerant hold up estimate does not involve the single-phase sections such as suction and liquid lines.

• Capacity control
As indicated above, in normal system operation, the circulating mixture composition within the system is different to the original composition.
In other words, if the running composition can be varied, the thermodynamic properties of the mixture can be altered providing an additional means of controlling the capacity.
Different methods can be employed to achieve the necessary changes in running composition.However, all these can be classified under two generic categories.Active systems, which use rectification, and passive systems that use accumulators [20].

Systems using passive techniques require a large amount of refrigerant charge and the potential of composition change is theoretically limited between dew and bubble line compositions [21].
Usually, the variation of composition resulting from refrigerant staying in various system components such as a liquid receiver or an accumulator is manipulated to change the system capacity with this method [20].On the other hand, the achievable limit of capacity control in active systems depends on the effectiveness of rectifications and distillation process implemented within the system.

Phase change heat transfer
In vapour-liquid flow, the two phases exhibit various geometric configurations or flow regimes.During flow boiling or condensation, the HTC depends on the flow regime.Usually mixtures exhibit relatively lower HTCs compared to pure fluids

• Condensation
Condensation heat transfer behaviour of mixtures is quite complex.The effect of the mixture composition is complicated so that the HTC have not been found to vary in any familiar manner with the composition [22].The mechanism of condensation is very sensitive to operating conditions and involve theoretically unexplained phenomenon [3].

Fig. 1 :
Fig. 1: Phase changing process of a binary mixture When in equilibrium, azeotropes exhibit the same liquid and vapour phase compositions and can be considered single component refrigerants for practical purposes [4].In these mixtures, a unique phenomenon occurs along specific locus of pressure, temperature and composition such that the vapour and liquid compositions during phase change remain relatively unchanged.Examples of this kind of mixtures are well known R500 (i.e.R12+R15ia), R502 (i.e.R22+R115) used in freezing and transport refrigeration applications.Temperature glides of these mixtures are less than 0.25 oC in the range of pressures found in common vapour compression systems.On the other hand, zeotropic mixtures show different vapour and liquid compositions when in equilibrium.This is a direct result of nonsimilar boiling points of the individual refrigerants that make up a zeotrope [5].During phase change, different boiling points of the constituents promote preferential boiling (or condensation) of certain components relative to the others.This leads to a change (or shift) in the composition of the phase changing mixture.Associated with the composition shift are the changes of the dew (Tdew) and the bubble (Tbub) point temperatures of the remaining mixture.This results in a non-isothermal phase change occurring within dew and bubble point temperatures.

3. Implications and influences of mixture behaviour 3 . 1 Fig. 2 .Fig. 2 :Fig. 3 :
Fig. 2 : Heat exchanger temperature profiles during phase change of (a) pure refrigerant, (b) zeotrpic refrigerant mixture with glide matchingIn addition, indirect advantages of glide matching are that (1) compressor will be working across a relatively reduced pressure range improving the COP, (2) relatively higher flow temperatures can be achieved using appropriate mixtures at relatively smaller compression work compared to a CFC refrigerant based system delivering similar capacity and flow temperature.However, the use of adequately sized heat exchangers is an important prerequisite for glide-matched systems, as smaller dTs invariably necessitate larger heat transfer areas to deliver required capacity.Water, water/glycol mixture, atmospheric air can be considered as suitable HTFs for glide matching in the evaporator and the condenser of vapour compression systems[6].In general, liquid-toliquid systems are better suited for implementing glide matching than air-to-air systems[7].The actual performance gain through glide matching, however, is largely a matter of achieving the right combination of heat exchanger sizing, selections of HTF and refrigerant[8].When using a linear refrigerant and a HTF, perfect glide matching can only be obtained at one specific HTF flow rate[2].This is usually estimated based on the heat transfer within the phase changing section considering the magnitude of TG at selected operating pressure (i.e.design conditions).Any subsequent change in the pressure or the HTF flow rate influence the degree of glide matching achievable.The influence of the HTF flow rate on glide matching can be shown in Fig.3, which presents 3 cases of different flow rates, plotting dT versus vapour quality.

and the temperature in the vapour phase reduces the HTC [ 3 ]Fig. 7 :changes to boiling curve for a binary mixture [ 24 ] 3 . 4
Fig. 7: Principal changes to boiling curve for a binary mixture [24] 3.4 Heat exchanger design for refrigerant mixtures LMTD and NTU-effectiveness (e) methods are the two conventional heat exchanger design methods.LMTD method requires the knowledge of all four temperatures (inlet and outlet)

Table 1 presents composition details and temperature glides of few selected mixtures presently used to replace common CFCs and HCFCs indicated. Table 1: Refrigerant mixtures replacing
Note: TG = T dew -T^ presented in table 1 at 100 kPa