A refrigerant may be either a pure compound or a mixture (blend) of two or more refrigerants. Examples of pure refrigerants are R12, R22 and R134a. Examples of mixtures are R502, R404A and R407C. A mixture can behave either as a pure refrigerant (azeotropic mixtures), or differently (non-azeotropic, or zeotropic, mixtures).
Although it contains two or more refrigerants, at a certain pressure an azeotropic mixture evaporates and condenses at a constant temperature. Because of this, azeotropic mixtures behave like pure refrigerants in all practical aspects. Figure 5.8 shows that the temperature is constant in the liquid-vapor mixture region for a given pressure.
Zeotropic mixtures have a gliding evaporation and condensing temperature (see Figures 5.9 and 5.10). When evaporating, the most volatile component will boil off first and the least volatile component will boil off last. The opposite happens when gas condenses into liquid. Figure 5.8 shows that for a given pressure, the temperature will change in the liquid-vapor mixture region. This results in a gliding evaporation and condensing temperature along the heat transfer surface. In practice, the saturation temperature at the inlet of the evaporator will be lower than at the outlet. In the condenser, the saturation temperature at the inlet will be higher than at the outlet.
Heat exchangers and refrigerants with glide
To exploit the temperature glide optimally, it is necessary for the heat exchanger to operate with a counter-current flow. This gives an advantage for BPHE heat exchangers compared with Shell & Tube (S&T) heat exchangers. S&Ts do not operate with a truly counter-current flow, and test results have shown a decrease in capacity for this type of heat exchanger that is not experienced with BPHEs.
For refrigerants with glide, e.g. R407C, it has been indicated that a substantial sub-cooling is required to receive pure liquid from the condenser. Thus, if the condenser is operating with co-current flow, an increased condensing temperature could be the result.
During the evaporation and condensation of zeotropics, there is a negative effect on the heat transfer coefficient due to mass transport phenomena in the refrigerant. High turbulence and good mixing in the heat exchanger neutralizes this negative effect, which also suggests that BPHEs have an advantage over S&Ts.
Another problem with zeotropics can occur if refrigerant liquid is allowed to collect somewhere in the circuit, e.g. in suction line accumulators, flash tanks, receivers or pool boiling/flooded evaporators (often S&T). A change in the composition of the refrigerant circulating through the system could follow from this, resulting in unpredictable performance. To avoid this, all components should have a continuous flow of refrigerant without the opportunity to collect any liquid. Hence, flooded evaporators will probably disappear from systems containing mixtures with glide.
A considerable leak of refrigerant in the liquid-vapor region can lead to the same problems, i.e. changed composition of the remaining refrigerant charge, giving unpredictable system performance.