RE: Dew point / bubble point?

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Hi, I was a little confused when watching your videos, you say to use dew point for superheat, and bubble point for sub cooling. 

But, as far as I see it, dew point would occur in a condenser, and bubble point would occur in an evaporator? Would it not? 

Any info greatly appreciated

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6 Answers

Hey Macmaw, we use dew point as the reference in the evaporator because we want to measure how much the refrigerant is superheated going back to the compressor and conversely in the liquid line we use the bubble point in the condenser to measure how subcooled the liquid is. The bubble point is the exact point where the refrigerant is 100% liquid and the dew point is the exact point when the refrigerant is 100% vapor. To answer your first question, BOTH the dew point and bubble point happen in the evaporator AND condenser. Referencing the PH-diagram below and talking through what is happening in this basic refrigeration cycle starting with the liquid (subcooled) refrigerant as it enters the expansion device, the liquid refrigerant goes through an adiabatic (fancy word for constant enthalpy) pressure drop where it transitions (absorbs heat) from a 100% subcooled liquid, through the bubble point, and into the saturation dome where it is a liquid-vapor mix. As it evaporates it becomes less and less liquid and more and more vapor until it runs out of liquid and hits the dew point. At this time the refrigerant vapor continues to absorb sensible heat (superheat) until it gets back to the compressor as a superheated vapor (hopefully). Here it goes through isentropic (fancy word for constant entropy) compression. Coming out the compressor discharge it does the exact opposite that it did in the evaporator – it transitions (cools) from a 100% superheated vapor, through the dew point, and back into the saturation dome where it is once again and vapor-liquid mixture. As the refrigerant cools (condenses) it becomes less and less vapor and more and more liquid until it runs out of vapor and is 100% liquid at the bubble point. That’s bubble and dew point broken down to the basics and that applies to ALL refrigerants. The next question is why dew and bubble point matter now more than they use to? The answer is refrigerant blends, which I think it’s best to address that in a separate comment.

Image result for ph diagram dew and bubble

Rookie Answered on August 21, 2018.
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