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Freezer superheat target calculation

8120 Views 3 Replies 2 Participants Last post by  beenthere, 
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Can anyone tell me what the superheat target calculation would be for a freezer? I recently had to add some R134a to my freezer and I need to know what my target superheat should be. I know that the formula for a typical HVAC is [(3*return wet bulb temp)-outdoor dry bulb temp-80]/2. But will this work for freezers? If so, how exactly do you use it with a freezer? After running about 15 minutes, my freezer is measuring about 0 deg F inside, 1psi on the suction line near the input of the compressor and the line temp at this same point is about 88 deg F. According to my gauge, R134a should have a saturation of about -13 deg F at 1psi. So, the measured superheat is about 88-(-13)=101 deg F, right? The freezer is in my garage with an ambient of about 76 deg F. If I am able to use the HVAC formula, do I get the wet bulb temp by simply placing my temp probe inside a wet shoelace and put that in the freezer for 15 min? Doing this will probably not change the temp much. And will the dry bulb temp simply be the ambient temp that the freezer is in, 76 deg F? For sake of arguement, let's assume the wet bulb is -8 deg F, which should be too much of a drop at this temp. Anyway, the HVAC formula gives -90 deg F. Should I just use magnitude and drop the neg sign? Still not close to 101 deg F leading me to think I need to add R134a. But I can already see the line freezing up a little about a foot from the comp. Which means the saturation point is already outstide the coils indicating that I need to actually purge the R134a a little, not add anymore....argh!
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#2 ·
Can't use that formula for a freezer, the wetbulb is too low to be useful.

Is the cap tube is wrapped around the suction line to act as a desuperheater? If so, then you can't use the suction line temp after that point to determine charge level. Need to take it before that point.

You may be slightly low on charge yet. On a cap tube freezer, super heat should be about 10 degrees just after the freezer, or as soon as the suction line leaves the insulation of the box.
 
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Discussion Starter · #3 ·
Thx Beenthere, I have an update to post. The thermometer I used was out of calibration! I calibrated it a couple of weeks ago but I guess putting in and out of the freezer so many times caused it to un-calibrate. The freezer is a Figidaire FFC04 model with a 216787700 model controller. The control setting is on "3" and the control range is 1 to 7.
Here are my new numbers in a 74deg F ambient:
Cut-in=14PSI, Box Temp=7deg F
Cut-out=0PSI, Box Temp=-3deg F
After running 20 minutes:
Suction Valve P&T=2PSI, 66deg F (saturation temp=-10deg F for R134a)
Box Temp=3deg F
SH measured=66-(-10)=76deg F
SH calculated=[(3*3)-74-80]/2=-73deg F (magnitude=73deg F)

These numbers look much better. But I really don't know what they should be to begin with based on the controller setting? After running about 45 minutes, frost builds up on the suction line about 2-3 inches after it exits the box but it does not reach the heat exchanger loop. In the SH calculation, I used the dry bulb box temp instead of a wet bulb because at temps this low they should be basically the same. However, I will try your suggestion of getting measured temp value from where the suction line exits the box before it gets to the exchanger. It should be a few degrees lower there and hopefully the temp will not lower too much since the psi will also increase so that the measured SH will drop by only a few deg. For example, if the psi increases to 3 and the temp drops to 60, then the meas SH=67deg F which is too much. But if the temp doesn't change and the psi=3, then the calculated value will equal the measured value! :)
 
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