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7.4 Glide refrigerants

Non-azeotropic, or glide, refrigerants are mixtures of two or more refrigerants where the components have different saturation temperatures at the same pressure level. When a glide refrigerant enters a condenser, the least volatile component condenses first. As the concentration of this lowvolatile refrigerant decreases, the temperature of the remaining refrigerant mixture will also decrease, approaching the saturation temperature of the second least volatile refrigerant, and so on. Hence, the condensing temperature is higher in the beginning of the condenser than in the end, even if the condensing pressure remains constant. The temperature glide in a condenser is shown in Figure 7.16. This process is the opposite of a glide refrigerant in an evaporator, where the most volatile refrigerant evaporates first, at the lowest temperature.

In a condenser operating with glide refrigerants, there are three temperatures of special interest: the dew point (the highest condensing temperature), the mean condensing temperature and the bubble point (lowest condensing temperature), which is achieved just before all refrigerant has been transformed to liquid. The three temperatures are shown in Figure 7.17.

The available desuperheating should be considered as the difference between the inlet discharge gas temperature and the dew point. The subcooling achieved is calculated as the difference between the leaving temperature and the bubble point. The middle point is a purely theoretical temperature and is calculated as:

Counter-current vs. co-current Flow for refrigerants with glide

The decreasing condensing temperature of a glide refrigerant improves the already favorable temperature program over a plate heat exchanger operating in counter-current mode. The initial condensing temperature of the least volatile refrigerant is higher than the final condensing temperature. The increasing temperature of the secondary fluid will therefore not approach the temperature pinch as described above (see Figure 7.18). The mean temperature difference will increase, resulting in a more efficient heat transfer process. This additional heat transfer efficiency may be utilized by (1) increasing the outlet temperature of the secondary fluid, (2) decreasing the heat transfer area, i.e. reducing the number of plates, or (3) lowering the condensation pressure.

A co-current condenser with a glide refrigerant will instead have an impaired temperature profile, as described in Figure 7.18. The decreasing refrigerant and increasing water temperatures will converge at the condenser outlet, resulting in very poor heat transfer. To avoid the need for an "infinite" heat exchanger with a very high number of plates, the temperature difference between the refrigerant and secondary fluid sides must be increased.

The sub-cooling must also be kept low for a co-current condenser, because the converging temperatures greatly reduce the already low heat transfer coefficient. More sub-cooling requires a very large heat transfer area, leaving a smaller surface available for the condensation of the gas. Thus, the condensation pressure and temperature increase, and the system performance (COP) decreases.

Potential problems with glide refrigerants

Non-azeotropic (glide) refrigerants are reported to have lower heat transfer coefficients than azeotropic refrigerants, because of the two-way mass transfer of refrigerant molecules from the heat transfer area to the bulk gas. The refrigerant gas condenses at the heat transfer area, which creates a constant underpressure that attracts more refrigerant. However, the more volatile refrigerants do not condense if the temperature is too high. There will therefore be an increased concentration in the gas of non-condensing refrigerant. This must diffuse from the heat surface into the bulk gas to allow more condensable refrigerant to access the heat transfer area, as shown in Figure 7.19. The mass transfer resistance results in a lower heat transfer coefficient. This can be overcome by a higher temperature difference between the refrigerant and the secondary side, similarly to the problem of inert gases discussed above.

The favorable temperature program of a true counter-current condenser counteracts this negative effect in a way that is impossible for cross-flow heat exchangers such as Shell and Tubes (S&T). In an S&T heat exchanger with the refrigerant on the outside of the tubes, the most volatile refrigerant may even accumulate and cause problems similar to inert gas, with low general heat transfer and high system pressure. This is generally not a problem in plate heat exchangers, because all refrigerant is forced through by the channel geometry.

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