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What is the “Mid Point” of a Refrigerant Blend?
As we have mentioned in several previous articles (such as this one HERE), many blended refrigerants have glide, which simply means they boil and condense over a range of temperatures instead of just one temperature.
As an example, consider refrigerant R407c. It is a zeotropic blend, which means it has enough glide to make a big difference if you fail to consider it.
For example, on an evaporator coil running R407c, the refrigerant leaving the TXV will begin boiling at the bubble point. Let's say that the pressure in the evaporator is 80 PSIG; the bubble temperature will be 40°.
As the refrigerant continues boiling, the temperature will increase towards the dew point, which is 50.8°. Any temperature gained ABOVE 50.8° on an R407c system at 80 PSIG is superheated, meaning the refrigerant is 100% vapor.
So, we calculate superheat as the temperature above the dew point and subcool as the temperature below the bubble point. The condensing temperatures and evaporator temperature aren't fixed, but they GLIDE between the bubble and dew and back again when the refrigerant is changing state.
But what does this mean for the evaporator and condensing temperatures when calculating target head pressure (condensing pressure) and suction pressure (evaporator pressure), also known as evaporator TD and condensing temperature over ambient?
The simplest way is to use the midpoint between the dew and bubble points to calculate CTOA and DTD.
In the case above, you would simply calculate 50.8° + 40° = 90.8 | 90.8 ÷ 2 = 45.5° average evaporator temperature or midpoint
Emerson points out that evaporators would be better calculated using 40% of bubble and 60% of dew, but the extra complexity generally doesn't make enough difference to mention.
I made this video to demonstrate further: