Low-temp freezer will not reach setpoint
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  1. #1
    Join Date
    Apr 2012
    Location
    Central Pennsylvania
    Posts
    6

    Low-temp freezer will not reach setpoint

    I accidentally posted this in another forum earlier, but it was in the wrong area. So...

    Good Evening,

    I am a licensed HVAC&R Tech with 11 years experience, and I mostly work on A/Cs, small fridges/freezers, and general appliances. I do not have a lot of experience with walk-in freezers. Also, I am new to this forum.

    I have a small 6X6 that will not reach a setpoint of -15 degrees. To the best of my knowledge the unit was maintaining temp before a compressor failure 3 days ago. I was able to get an exact replacement for the compressor. After intallation, it pulled down very slowly, but there was a good deal of moisture in the box (floors and walls) and it has been very humid (around 70 percent with temps in the high 80s to low 90s). With all of the moisture, I was not too concerned that it was pulling down a bit slow, and I was at the end of the day.

    I checked on the unit this morning, after 14 hours of run time, and it was hovering around 9 degrees. By this afternoon, a full 24 hours running, it never went lower than 2 degrees. I was replacing a compressor on another unit - so I did not have much time to check this one out again. I did get the following information:

    -15 Ice Cream Freezer - R-404A

    Box Temp: 2 degrees

    Suction Pressure: 20 psig (Sat. of -15 degrees)

    Ambient Temp around condensor: 95 degrees

    Discharge Pressure: 330 psig (Sat. of 124 degrees)

    Suction Line Temp at Compressor: 32 degrees (it is not very easy to get a temp at TXV bulb while it is running as you have to drop the fan housing to get to it)

    Compressor amp draw: 9.5 (Rated RLA of 11.4)

    I also did try to do a quick pump down (but it is not a pump down system - just to clarify) to check the compressor even though it is new. It took a few minutes to even approach zero, and I could not wait long enough for it to get all the way down (which it did not).

    Any suggestions? I am just stuck right now, and I find it hard to believe that a new compressor is bad.

    Thanks.

  2. #2
    Join Date
    Nov 2009
    Location
    wasilla, alaska
    Posts
    29
    Superheat and subcooling? also in the old days we subtracted 15 degrees from the box temp and converted that to pressure to see what we needed.like 9.5 suction i think. you ever see this thing at minus 15 yourself? You need sh and sc to help troubleshoot what you can't see.

  3. #3
    Join Date
    Aug 2002
    Location
    Sacramento area
    Posts
    69
    Quote Originally Posted by minghex View Post
    ...

    After intallation, it pulled down very slowly, but there was a good deal of moisture in the box (floors and walls) and it has been very humid (around 70 percent with temps in the high 80s to low 90s). With all of the moisture, I was not too concerned that it was pulling down a bit slow, and I was at the end of the day.

    ... I did get the following information:...

    Suction Pressure: 20 psig (Sat. of -15 degrees) ...

    Suction Line Temp at Compressor: 32 degrees ...

    I also did try to do a quick pump down (but it is not a pump down system - just to clarify) to check the compressor even though it is new. It took a few minutes to even approach zero, and I could not wait long enough for it to get all the way down (which it did not).

    Any suggestions? I am just stuck right now, and I find it hard to believe that a new compressor is bad.

    Thanks.
    So you have 47 degrees of superheat at the compressor. That seems pretty high. I'd open the TXV to bring the SH down, which will also increase the suction pressure and provide more capacity out of the compressor.

    You mentioned you tried a quick pump down. How was this done, closing the suction service valve, or closing the receiver king valve? If you closed the valve at the compressor suction, it should have pumped down very quickly.

    Also, if the walls and ceiling have wet insulation, this could be the problem. Maybe you can drill a screw into the wall, and see if water comes out of the screw hole. First try it down low just in case only the lower portion is wet.
    I'm still learning this trade.

  4. #4
    Join Date
    Apr 2012
    Location
    Central Pennsylvania
    Posts
    6
    Well, this morning I find this box at -10 degrees. I am not sure what the change was. I am still going to check it further. Particularly, I want to see it recover if I open it for a bit (which they do here every day). I have seen this box in the past, and it does get below zero. I cannot say if I ever noticed it at -15 though.

    Superheat and subcooling? also in the old days we subtracted 15 degrees from the box temp and converted that to pressure to see what we needed.like 9.5 suction i think. you ever see this thing at minus 15 yourself? You need sh and sc to help troubleshoot what you can't see.
    I knew superheat would be one of the first questions. As I said I just wasn't able to rig something up for that yesterday. As for subcooling, I didn't realize that I didn't include that. I did take a temp off of the liquid line, but I didn't write it down - 13 days on in a row has left me a bit tired. I definitely thought pressure-wise that everything looked reasonable considering the box temp and the ambient in the condensor space.

    So you have 47 degrees of superheat at the compressor. That seems pretty high. I'd open the TXV to bring the SH down, which will also increase the suction pressure and provide more capacity out of the compressor.

    You mentioned you tried a quick pump down. How was this done, closing the suction service valve, or closing the receiver king valve? If you closed the valve at the compressor suction, it should have pumped down very quickly.

    Also, if the walls and ceiling have wet insulation, this could be the problem. Maybe you can drill a screw into the wall, and see if water comes out of the screw hole. First try it down low just in case only the lower portion is wet.
    I think the SH at the compressor is high due to the line running through a very warm space (regardless of the line insulation), but it did seem high to me. I have always tried to avoid adjusting TXVs until a last resort (especially if this worked last week), and I would try to rig something for a true SH first.

    I did front seat the king valve at the receiver - there is no suction valve on this unit (just a process tube for low-side readings).

    The moisture was all surface moisture. They had some things melt in there, and they didn't fully dry everything up before I started the new compressor. I was working on multiple units at once.

    Thank you both.

  5. #5
    Join Date
    Feb 2010
    Location
    NE Alabama
    Posts
    301
    Is TXV bulb mounted horizontal or vertical. On horiz. should be at 4 or 8 o'clock for proper operation. I check and adjust superheat after any changes in a walkin

  6. #6
    Join Date
    Nov 2009
    Location
    wasilla, alaska
    Posts
    29
    I always heard 20 degrees SH at the compressor was max you want. But since we don't live in a perfect world I try to shoot for as close as i can get. We need SH at TX bulb in a perfect world. In Alaska we see some real strange stuff. But suction pressures don't tell us flow-put some heat on that TX bulb aand see if anything changes. I had this same thing on a open scoop ice cream freezer. Try running that TX all the way open and then closed. It took me 3 days to fix ice cream freezer and client lost 2 thousand in ice cream sales.

  7. #7
    Join Date
    Jan 2010
    Posts
    5,939
    You need to check that superheat and zero it in at 4-6*. Forget about the sub cooling on refrigeration. It is what it is. Is there a sight glass on the liquid line ? Is this a remote condensing unit ? Does it have a receiver ?

  8. #8
    Join Date
    Apr 2012
    Location
    Central Pennsylvania
    Posts
    6
    Thanks for all of the advice - I appreciate it. Unfortunately, I could not get back to this unit today. As I said in an earlier post, the box was a -10 this morning. I believe the main reason for that is a drastic drop in ambient temperature. That said, a -10 box made it a lower priority for now. I have more work than I can handle right now!

    I plan to check this unit thoroughly on Tuesday when I go back there. There is a lot of good information on this site in general.

  9. #9
    Join Date
    Apr 2012
    Location
    Central Pennsylvania
    Posts
    6
    Just to answer a few of the questions that were asked…

    The TXV is mounted horizontally around 4 o’clock.

    The condenser is remote with only about 10 feet of line set.

    The unit does have a receiver and a sight glass (the sight glass has been clear during operation).

    I just really need to get back in there, turn the unit off, and rig something up to get a true superheat. I wish they would make it a little easier on us to access some of these units for proper testing!

    Have a good night and a good weekend.

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