Blends and Midpoint w/ Jim Bergmann

Jim talks about dew point, bubble point, and midpoint in refrigerant blends. He also covers the purpose of each and why MeasureQuick displays midpoint on the gauges.

We've formerly used mostly pure refrigerants. However, as new refrigerant blends come on the scene, we have to deal with glide, which indicates that we have a range of boiling temperatures instead of a fixed boiling point. We have bubble point and dew point, which are when the refrigerant starts to boil and finishes boiling, respectively; you generally use dew point to determine the superheat and bubble point to determine the subcooling. Zeotropic refrigerants have larger glides than near-azeotropic refrigerant blends; azeotropes have no glide at all.

The midpoint is the halfway point between the bubble point and dew point in refrigerant blends. Coil temperature typically corresponds with the midpoint.

To find the midpoint of refrigerant in the condenser coil, add the dew and bubble points and divide the sum by two. The process is a bit trickier on evaporator coils. In the evaporator, you run refrigerant through the metering device and get some flash gas; when the refrigerant undergoes that change, the bubble and dew points change. As a result, the midpoint becomes a bit more weighted towards the dew point (60%).

In MeasureQuick, the temperature-pressure charts go a step above and beyond to give you the superheat, subcooling, and midpoint. The midpoint is the effective temperature of the evaporator coil, which is a critical piece of information in refrigeration systems where food products are at stake. You can also use the midpoint for coil DTD and TD.

Jim and Bryan also discuss:

• R-410A and near-azeotropic refrigerants
• Metering devices as reactive components
• Coil temperature misconceptions and uncertainty
• Pressure differentials and drops in the system
• Maintaining food quality in refrigeration
• MeasureQuick mathematical models and formulas

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