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How measureQuick calculates superheat & subcooling

Once in awhile, we get a question on superheat and subcooling calculations and why they don’t appear to agree.  You will notice this difference on blended refrigerants only. So we have a blended refrigerant here, r458a, and you’ll see what looks like a saturation temperature of 45 degrees and a suction line of about 65 degrees. Let’s just keep the math simple, it looks like a superheat of about 20. Then on the high side see 90.5 & 102. It looks like a sub cooling of approximately 12 degrees yet the superheat and the subcooling are 14.7 and 7.3. 

And if we were to look at the Fieldpiece app you’ll see we have the same suction line temperature but the vapor saturation temperature is 50 and liquid saturation is 98.  You’ll see that on our app, we are at 45 & 102.  They’re definitely different.  But if you’re looking at superheat and subcooling at 14.7 and 7.3 we’ll see that those are almost identical ratings. So why, in this case, does measureQuick do some things different than most other apps?

measureQuick does some things different than most other apps do because we’re doing them the right way, which is to show you the average coil temperature instead of the dew point or the bubble point. 

If you tap on the target for a gauge (as seen in the image above) you’ll see that this refrigerant is a high glide refrigerant. So the center of the glide is 45º and it’s got a dew point temperature of 49.6º (Remember: Dew for heat and Bub for cool). We use dew point for calculating the superheat and bubble point for calculating the subcooling.  If we go back to the mQ app (1st image) and say okay, 49 (dew point temp) minus 64 (suction line temp), that’s where we get our 14.7 degrees of superheat.

 A lot of people have not seen this before but this actually goes back to some work that Emerson’s done on refrigerants.

This is an Emerson presentation about average coil temperature, and to get the average evaporator coil temperature of a blended refrigerant we have to take the pressure in the bubble point column and the pressure in the dew point column, find the corresponding temperatures and use the following equation.

Ave Coil Temp = 0.40(bubble point temp) + 0.60(dew point temp)

If we go to the condenser, you’ll see that to find the average condensing temp, we take the bubble point plus the dew point divided by two and that gives us an average.

In measureQuick, we’re calculating average coil temperature because we’re calculating where those targets should be for the high and the low side temperature and we use dew point and bubble point in the background to calculate superheat and subcooling. So you’ll notice that our app always agrees with other apps with the superheat and subcooling because they’re like we are, using dew point and bubble point, but the difference is we’re displaying average coil temp under your gauge, which is much more representative of the temperature you should be looking for (especially if you’re doing refrigeration) than displaying the dew point or the bubble point of the refrigerant on the gauge set.

Click on the video below to listen to Jim explain.

Updated on June 11, 2021

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