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  1. #1
    Join Date
    Nov 2003
    Posts
    501
    Had a call today for a warm walk in. Found unit low on charge. Located and repaired leak service valve packing nut. Also added a cap. Now...on to the problem.

    Charged 408a to full sight glass.
    Suction pressure - 30 (0*)
    Head Pressure - 320 (128*)
    Superheat - 56* at condensing unit (couldn't easily get to suction at TXV bulb)
    Subcooling - 15*
    Ambient - 93*
    Box - 50* (at time of these readings)

    It was cooling very well, but I couldn't wait to see the final temp because we were so backed up today. I left it this way assuming that since the box was warm, the TXV was punched wide open and alowing the high superheat. I figured that when the temp came down, the TXV would close, and also allow higher subcooling. I saw the box pull down to about 30* before I left from the 70* that it was when I got there. The product is already ruined, so if it doesn't cool completely over the weekend it isn't a big deal, but it would be nice if I knew that it would be fine.

    I should know this, but I'm really out of it today, so I bring it to you all.

  2. #2
    Join Date
    May 2004
    Posts
    12,030
    Thoughts...

    Charging to full sight glass on a warm box will cause an overcharge situation. Charge just enough to have a liquid seal on the glass outlet and give the TXV time to pinch. This will back up the liquid. Let the box temp come down before you clear bubbles.

    If you can't get to the suction line at the evap coil then use a temp drop across the coil. Good TD will be around 10*. If you can remove the side panel and take a temp reading of the suction line just at the outlet of the suction header. We do this if there is a rack under the coil that is not easy to move out.

    Your pressures are a little high for your ambient conditions and 408. Look for 20# on suction as the box settles. If the box got to 30* then your suction should have dropped from 30# to near 20# or lower if your TXV is working properly and assuming all other components are in working order.

    All of this takes time that you didn't have. I think when you go back to check the unit you will find a little overcharge and possible flood back.

  3. #3
    Join Date
    Sep 2002
    Posts
    186
    Sound like your overcharged or a very dirty cond 320 head is too high. 30 suction is a little high. Is the amp draw high or low. I don't care much for 408 always runs too hot.
    Because chicks dig it.

  4. #4
    Join Date
    Nov 2003
    Posts
    501
    Condenser is very clean. All it ever collects is dust, which I blew out well.

    It sounded overcharged to me too, but the high superheat had me question even that. I just figured that with the ambient, and high load made up for it being slightly high.

  5. #5
    That high superheat back at the compressor you noticed will certainly drop as the box cools off and the TX settles down.
    However, that amount of subcooling on the liquid line is unusually high for a hot box on a hot day.

    You want ten degrees of subcooling when the load is down to temp and the t-stat is about to be satisfied.

    And that is also the moment you want to be observing your other temperatures and pressures.


  6. #6
    Join Date
    Sep 2001
    Location
    East Stroudsburg, PA
    Posts
    13,215
    Originally posted by dorrmann
    I left it this way assuming that since the box was warm, the TXV was punched wide open and alowing the high superheat.
    If the TXV were "wide open", you should have LOW superheat, not high.

    408A doesn't normally run a very high head, when applied right. If you were seeing 320 with a 93 degree ambient, go looking for air or other non-condensibles in the system.

    If that was a low side leak, I hope that you replaced the filter-drier, and evacuated.

  7. #7
    Join Date
    Apr 2001
    Location
    Camel City, NC
    Posts
    6,233

    superheat

    Probably has a ZP charge head on the txv limiting suction pressure and increasing superheat during pull down. Keeps compressor from over amping like a cpr does.
    Be safe not fast. body parts don't grow back

  8. #8
    Join Date
    Jan 2004
    Location
    Earth
    Posts
    4,879
    How did you determine your low charge condition? Was a flashing glass your indicator? Is your liquid filter before your sight glass?

    You may have overcharged a unit with a plugged filter.
    A Diamond is just a piece of coal, that made good under pressure!

  9. #9
    Join Date
    Jul 2000
    Location
    Guayaquil EC
    Posts
    10,377
    I agree with selfemployed that the thermostatic element on the TXV must be (or at least should be) an MOP type like the ZP which will throttle back to hold a maximum evap pressure of around 30# during initial pulldown or post defrost operation.

    The superheat and subcooling readings are totally meaningless until the system is at or at least close to design temperature, so forget about the overcharged system and all that.

    I've seen a lot of low temp units run high heads like this on pulldown. They typically have relatively small condensers and at 30# suction you're right at the top end of the envelope for a low temp CU, so I wouldn't get all excited about 320# discharge with R408A at those ambients.

  10. #10
    Join Date
    Feb 2004
    Location
    northern colorado
    Posts
    90

    Unhappy Never enough time

    We all have been in your shoes, 4pm and two more calls to do and each at the opposite side of town through rush hour. It is these times that try men's souls.

    In my 25 years I have found out if I leave a job with doughts, I will be called back. Agian I known it's hard to stay with more call ahead and the boss yelling, "gota go there're piling up." Take your time, relax and you will have a more harmonious outcome. Besides think of the OT you'll be racking up$$

    The guys are right you could be overcharged. I do have a question, is you condenser a remote and does it have a headmaster? and, Did you happen to have your amp meter on the compressor? With that kinda load you could be overloading the condenser.

  11. #11
    Join Date
    Nov 2003
    Posts
    501
    Thanks again for all the input guys. Let's see if I remember the questions that you all asked so I can answer them.

    I determined the low charge because the the sight glass was almost totally empty, and running at -20* evap temp, and high superheat (I can't remember the exact line temp). The fans wouldn't even come on. I did think about the plugged filter, but I didn't read any temp drop across it.

    It is a remote condenser, but it's located in a mech room, not outside. No headmaster or fan cycle control.


    I was under the assumption that if the load was high enough (on a normal TXV), and it was wide open, it would increase the superheat because of the high load. Under normal conditions, I can understand how it would be low superheat, but I was thinking with a really high load, it just couldn't feed enough even wide open.

    The leak was on the packing nut of the outlet of the receiver, so I wasn't too concerned with it pulling in air, moisture, etc.

    Stormrider - I totally agree with what you said, especially the OT part. Unfortunately, with other refrigeration equipment down at the time, I couldn't follow it. Milk stores get a little pissy when their milk warms up.

    Thanks again everybody!

  12. #12
    Join Date
    Apr 2001
    Location
    Camel City, NC
    Posts
    6,233

    Milk

    Yea sticking it in the corner doesn't help it either and soda pop pays to be front and center.
    Be safe not fast. body parts don't grow back

  13. #13
    Join Date
    Jun 2002
    Location
    Bemidji, Mn
    Posts
    3,573
    Any updates on this?
    You picked a fine time to leave me loose wheel...

    http://rapalaguy.spaces.live.com/

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