The relation between a pure refrigerant and its saturation temperature was discussed above. If a mixture of two or more refrigerants with different evaporation temperature is used instead, the phenomenon of glide may occur. This is a non-azeotropic, or zeotropic, mixture.
When a non-azeotropic mixture boils, the vapor formed first will be rich in the refrigerant with the lowest boiling temperature. The composition of the liquid will therefore change during the evaporation process, with an increasing concentration of the refrigerants with higher evaporation temperatures. Consequently, the saturation temperature will increase, reaching a maximum at the outlet saturation temperature, just before the superheating. This is the opposite situation to a pure refrigerant, where the outlet saturation temperature is the minimum refrigerant temperature.
Because the evaporation temperature of a refrigerant with glide is not constant at constant pressure, but will change with the composition of the refrigerant mixture, there will be a start (low temperature) and stop (high temperature) evaporation temperature. To distinguish these, the temperature at which boiling starts is called the bubble point and the point where all liquid has evaporated is called the dew point. The dew point temperature corresponds to the outlet evaporation temperature of a pure refrigerant.
Some refrigerant mixtures have a very small temperature glide of less than 1K, for example R404A, which has a temperature glide at 0°C of 0.5K. Such refrigerants are called near-azeotropic and the glide can normally be neglected for all practical purposes.
The thermodynamic properties of interest in heat transfer applications are less favorable for refrigerant mixtures with glide than for pure refrigerants. There are three possible reasons for this:
- The least volatile refrigerant in the mixture at any time accumulates at the heated surface because the more volatile refrigerants boil off. This increases the bubble point locally near the heated surface and thus also creates a temperature difference between the wall and the bulk liquid.
- The viscosities and conductivities of the refrigerants are usually less favorable than the weighted average of the components themselves.
- The start of nucleation in the boiling process may become weaker, leading to fewer nucleation sites. Furthermore, there is a risk of partial distillation of the most volatile components if there is an open vessel to which vapor is led, i.e. a low-pressure receiver or a pool boiling evaporator. The refrigeration composition will then vary throughout the system, causing a lower evaporation pressure and a higher condensation pressure, with reduced system performance as a result.
If there is a leak, it is possible that a disproportionate amount of the most volatile component will escape. Refilling the system with new refrigerant without controlling the actual composition of the refrigerant mixture inside the system will create a refrigeration mixture with a new composition. Its thermodynamic properties are unknown, and may not be as good as those stated for the original composition.
Because of the low heat transfer and the risk of refrigerant separation, non-azeotropic refrigerants are not recommended for use with flooded or pool boiling type evaporators.
Glide in direct expansion (DX) evaporators
For dry expansion evaporators operating with high turbulence and true counter-current flow, e.g. brazed plate heat exchanger evaporators, non-azeotropic refrigerants are much easier to handle. They may even have some advantages over pure refrigerants.
The high degree of turbulence induced by the winding brazed plate heat exchanger channels minimizes the effect of local accumulation and temperature increase close to the heat exchanger wall.
Because of the glide, the evaporation temperature will increase through the evaporator with increasing vapor quality. If the heat exchanger is arranged in counter-current flow, the increase in boiling temperature will be compensated by the decrease in temperature of the secondary fluid (see Figure 6.11). Consequently, the mean temperature difference over the brazed plate heat exchanger evaporator is higher for zeotropic refrigerants compared with refrigerants with a constant evaporation temperature (see Figure 6.11).
The higher mean temperature difference for glide refrigerants can be exploited to obtain a higher heat flux without the penalty of a lower system efficiency (COP). In reality, this means that a smaller brazed plate heat exchanger evaporator can be used. Alternatively, the heat transfer surface is kept constant and higher system efficiency is obtained by an increased evaporation pressure and, consequently, higher bubble and dew points.
A potential problem that should be considered is the heat surface required for superheating. Because the temperature difference at the end of the evaporation process is lower for zeotropic mixtures than for refrigerants without glide, the superheating will require more heat transfer area. For a normal level of superheating, 5-7K, the difference is negligible. However, if the evaporator is designed with a very small temperature approach or if a high level of superheating is necessary, the difference increases.
Effects of pressure drop
The effects of pressure drop on non-azeotropic (glide) refrigerants are identical to the effects on azeotropic (no glide) refrigerants, except that the glide of the refrigerant and the temperature decrease due to the pressure drop counteract each other. As discussed above, the saturation temperature increases gradually during the boiling process for non-azeotropic refrigerants, whereas the induced pressure drop will act to reduce the evaporation temperature by decreasing the saturation temperature (see Figure 6.12).
To a certain extent, pressure drop acts positively on the superheating, because the temperature difference in the end of the evaporator will increase due to the decrease in saturation pressure. As shown in Figure 6.12, it is possible to remove the effect of the temperature glide totally only if the pressure drop is large enough. However, it is difficult to obtain such a large pressure drop inside a brazed plate heat exchanger. To reach this condition for a common non-azeotropic refrigerant, R407C, would require a channel pressure drop of 6.1 bar at 0°C, excluding the pressure drop over the distribution system.