Get Tech Tips
Subscribe to free tech tips.
Glide at Static Pressure
We've been pretty spoiled in residential and light commercial in the USA because we haven't needed to deal with glide much. R22 has no glide, and R410a is a near-azeotropic blend, which means it has almost no glide.
The days of being able to ignore glide are coming to an end.
Carrier has announced their replacement for R-410a will be R-454b, which they will call “Puron Advanced.” It still has very little glide (only 0.2°F), but many of the other options (like R-407c shown above) have a rather severe glide.
Glide comes down to the fact that some blended refrigerants boil and condense over a range of temperatures rather than at a single pressure/temperature point.
The point at which a refrigerant is fully liquid before subcooling (or the point of the very first bubble in the liquid) is called bubble point, and we use the bubble point to calculate subcooling.
The point when the mixture becomes fully vapor before superheating (or the first drop of liquid dew in a vapor) is called the dew point, and we use it for calculating superheat.
Zeotropic blends (blends with glide) have several impacts on the system, but the one we notice most is in the evaporator. When a blend with glide enters the evaporator coil, it will start by boiling at a lower temperature. As it moves through the coil, the refrigerant temperature will increase until it hits the dew point before it starts to superheat. This means that neither the dew nor the bubble temperature is REALLY the evaporator temperature; the true effective evaporator temperature is somewhere in the middle. We call this the mid-point.
Because some of the refrigerant flashes off right at the start of the evaporator, the effective midpoint isn't really the middle between the dew and bubble; it tilts more towards the dew. Emerson recommends a more accurate estimate that would account for that “inlet quality.” So, merely multiply bubble by 0.40, dew by 0.60, and then add the two together to get a more accurate evaporator midpoint.
But let's say you connect to a system that is off or connect gauges to a tank. You think you know which refrigerant you're dealing with, but you can't be sure. How can you know for sure that the refrigerant in the tank or system is what you think it is?
Do you use bubble, dew, or midpoint for static pressure?
The answer is you use the bubble point. Now, I've not had anyone fully explain why to me, but it stands to reason in my head that, in the static state, the majority of the refrigerant mass in the system (or tank) is in the liquid state. Since it is neither in the process of boiling nor condensing, it would be at the bubble point. That's probably a very unscientific way of thinking about it, but it's what I've got for now.