REFRIGERANT BLENDS AND GLIDE
Many of the HFC refrigerants are mixtures or blends of two or more individual chemicals. Mixtures can be azeotropes, near azeotropes or zeotropes.
Azeotropes exhibit a single boiling point, strictly speaking at one particular pressure, but nevertheless they may be treated as a single substance. The first azeotropic refrigerant was a CFC, R502, so the use of refrigerant blends is not new. Where the boiling point varies throughout the constant pressure boiling process, varying evaporating and condensing temperatures exist in the phase change process.
R407C is the most widely used zeotrope. Referring to Figure 3.5, the shape of the well-known refrigerant vapour compression cycle is unchanged, as shown. Constant P1, evaporating, and P2 condensing pressure processes are represented by horizontal lines, but the lines of constant temperature are now sloping. The temperature at which condensation starts is called the dew point, denoted here as T2(Dew). As condensation progresses, the temperature falls to T2(Bubble) so that the dew point temperature T2 of 40°C corresponds to a bubble point temperature of approximately 34°C. The temperature during the evaporation process changes from T1 (Evaporator Inlet) to T1 (Dew), as the lighter components of the mixture, R32 and R125, evaporate preferentially to the R134a, and so the remaining liquid becomes R134a rich, its boiling point gradually increasing until all the liquid is evaporated. Note that this does not mean that the lighter components boil away leaving liquid R134a at the very end of the process. The composition shift during the process is limited and quite small. Further superheat then occurs after evaporation is complete, raising the temperature to Ts, the suction temperature at the compressor inlet.
The temperature glide can be used to advantage in improving plant performance by correct design of heat exchangers. A problem associated with blends is that refrigerant leakage can result in change in the proportion of components
4Q 30 Q 4n fill SO I no 130
J — lJ» 130 1 40 I <50 1.30
120 1(50 200 2-B 280 320 3ЖI 4M 443 4SJ 320
Figure 3.5 Vapour compression cycle with R407C
In the blend. However, the changes are small and have negligible effect on performance. The following recommendations apply to the use of blends:
• Equipment must always be charged from the liquid phase, or the component concentrations will be incorrect.
• Ingress of air must be avoided.
• Blends which have a large temperature glide, greater than 5 K, should not be used with flooded type evaporators.
Some mixtures exhibit a glide of less than 2 K, and these are called ‘near azeotropes’. For practical purposes they may be treated as single substances. Examples are R404A and R410A.
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