# Thread: Question on calculating box temp from suction pressure

1. LOL- more than funny boys- I just submitted by application prior to my last two posts.

2. Originally Posted by WestcoastApprentice
Ok guys- my brain won't shut off and Im gonna have no fingernails left if I have to wait until Tuesday morning to figure this stuff out. Feel free to tell me to take a hike the moment I become annoying by the way.

I'm going to try and go back a step to make sure that I understand this kind of a medium temp-charging scenario correctly:

Okay so the REASON that you can directly correlate a refrigerant temperature (the temp of the gas in your suction line) from your suction line pressure (and not have to factor in any degrees of subcooling) is because you charge to a full site glass and then stop thus allowing for full saturation, but no more.........am I correct so far?

Okay and then the calculations regarding box temp and temp diff. are no problem for me, but I think what my j-man was referring to was the fact that the txv adds 10*F superheat, so when measuring the pressure at the outlet of the evap coil (suction line) it's neccesary to factor that in. Full sight glass means saturation temperature at whatever pressure we read, PLUS the addition of 10*F from the txv.............

So if what you guys are saying is true, and I'm not saying it's not, then why do you not have to factor that 10*F in when making your calculations based on the temp/pressure relationship at the outlet of the evap?
By what your journeyman said, if you read the equivalent to 30 Deg F saturated on your gauge, you would then add 10 Deg for the TD and another 10 Deg for the superheat...meaning you're estimated box temp would be 50 Deg F.

This method contradicts the basic definition of the evaporator TD...ie, TD = (Tbox - Tevap).

What I say is the mathematics of the TD rule will hold true, even in reverse. Rearrange the formula, solving for the box temp...Tbox = Tevap + TD

3. Icemeister, I completely agree that the definition of evaporator as well as TD is basic and clear- air on and air off, as the kiwis like to put it. The 10*F isn't part of the calculation for deriving TD, it's part of the calculation in going from suction line pressure (or equivalent temperature) to box temp- temp of gas in the suction line, plus the TD of the evap, plus the 10* added by the txv. Im failing to see how the 10* superheat added by the txv is unnecesary in calculating the box temp, when the suction line pressure/temperature AFTER the evaporator has had the TXV's superheat added to it........

4. Another way of explaining his theory of it would be to say that the pressure temperature relationship at the outlet of the evaporator is NOT a fully saturated vapor condition- it's 10*F superheated because of the txv and therefore when working backwards to determine the box temp that superheat must be factored in.

5. Originally Posted by WestcoastApprentice
Icemeister, I completely agree that the definition of evaporator as well as TD is basic and clear- air on and air off, as the kiwis like to put it. The 10*F isn't part of the calculation for deriving TD, it's part of the calculation in going from suction line pressure (or equivalent temperature) to box temp- temp of gas in the suction line, plus the TD of the evap, plus the 10* added by the txv. Im failing to see how the 10* superheat added by the txv is unnecesary in calculating the box temp, when the suction line pressure/temperature AFTER the evaporator has had the TXV's superheat added to it........
By stating TD = air on - air off tells me you've missed the meaning of the evap TD...which is TD = Tbox - Tevap. The Tbox is the air temp entering the evap coil and the Tevap is the temperature of the refrigerant inside the coil.

When you read a pressure on your suction gauge and correlate that to a saturated temperature, does that saturated temperature from your gauge not represent the saturated temperature of the refrigerant in the evaporator?

6. Originally Posted by WestcoastApprentice
Another way of explaining his theory of it would be to say that the pressure temperature relationship at the outlet of the evaporator is NOT a fully saturated vapor condition- it's 10*F superheated because of the txv and therefore when working backwards to determine the box temp that superheat must be factored in.
Study that Heatcraft Engineering Manual I linked to earlier (starting on p.21).

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