Thread: Superheat question

1. Superheat question

I’m having a brain fart right now.

R-448a WIF
Suction temp at evap 7deg
Suction pressure at evap 11 psi = -18 deg dew
7 minus -18 = 25. I have 25 deg SH at my evap. Does that sound right?

2. With 404 I come up with 34*f

3. Originally Posted by VanMan812
I’m having a brain fart right now.

R-448a WIF
Suction temp at evap 7deg
Suction pressure at evap 11 psi = -18 deg dew
7 minus -18 = 25. I have 25 deg SH at my evap. Does that sound right?
Your math is correct (I didn't get my PT chart out to verify that part).

4. I try to imagine a number line when dealing with subtracting a negative number. Start at -18 and couynt how many numers there are to get to +7...ie, 25

-18 -17 -16 -15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 - 4 -3 -2 -1 0 +1 +2 +3 +4 +5 +6 +7

5. I was thinking I need to compensate for the glide. Yes?

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25degree superheat is way too much on freezer. Is it new install?

7. Originally Posted by VanMan812
I was thinking I need to compensate for the glide. Yes?

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No, the dewpoint is the saturation temperature of the refrigerant gas leaving the evaporator, so by definition, the superheat is the measured temperature minus the dewpoint temperature.

Some guys have been using the midpoint or average evap temperature to determine superheat, but that isn't the way to do it properly.

8. Originally Posted by icemeister
No, the dewpoint is the saturation temperature of the refrigerant gas leaving the evaporator, so by definition, the superheat is the measured temperature minus the dewpoint temperature.

Some guys have been using the midpoint or average evap temperature to determine superheat, but that isn't the way to do it properly.
That makes sense and that’s what I usually do. The p/t chart I was using gave the average temp (bubble and dew) and it made me think for a minute that I should be using that instead of the dew.

Why are some guys using the midpoint? Wouldn’t that give a false SH that’s higher than it actually is? That’s a slippery slope.

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9. Originally Posted by VanMan812
I was thinking I need to compensate for the glide. Yes?

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https://www.honeywell-refrigerants.c...012&download=1

10. Originally Posted by VanMan812
That makes sense and that’s what I usually do. The p/t chart I was using gave the average temp (bubble and dew) and it made me think for a minute that I should be using that instead of the dew.

Why are some guys using the midpoint? Wouldn’t that give a false SH that’s higher than it actually is? That’s a slippery slope.

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It's been mentioned a couple of times in recent threads by some market techs that the chain they do work for requires they use the midpoint for superheats with blends because the cases run better and are more able to meet the design temps. I don't agree with that method as it's not technically correct. It may work for them but not for the reason they think. The techs say they're just doing as they're told...and you can't knock that too much.

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