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  1. #1
    Join Date
    Mar 2007
    Location
    NW AR
    Posts
    2,478

    Walk- in cooler kicking my arse...

    Russell condenser with dual evaps. First call was for bad time clock. The unit was keeping the box at a cool 35 degrees, just freezing up occasionally. Sight glass full. While diagnosing the clock I hear it pumping down. It never shuts off so i put my gauges on it and its pulling into a vacuum. Bad low pressure cut out, tap on it and it shuts down. Stick a new pressure control on it but have to order a clock. Return this morning with a clock and its kicking on and off, on and off. Box is getting warm. Adjust press control so it will run. 125 on the head and about 10 suction. r22. I start searching for a restriction. Start at the filter drier and work my way through the pump down solenoid all the way to the txvs. No temp drop anywhere. I think maybe screens. Nothing in them. New drier, evac, and start it up. Same thing. Pretty cool when I started, about 60-65 degrees outside. So im scratching my head, several hours have passed, and compressor opens on thermal. Great. Warmed up to about 80 outside by then. I hose it down and sit and wait, and wait. Food in the box is getting hot and so is my temper. Finally the compressor starts up. Check my gauges and pressures are great...55 suction 225 head??? WTF?? Im starving so have the owner cook me some grub and let it run about an hour. Everything still looks great, box pulling down quick. Its got one of those head pressure control valves on it which I know basically nothing about. Could really use some insight here.

  2. #2
    Join Date
    Jun 2009
    Location
    DFW, TX
    Posts
    675
    Sounds like low on charge. When it got cold, the liquid stacked up in the condenser and receiver went empty. Try reading up on "low ambient charging". Most here advocate the proper mathematical way using Sporlan's 90-30-1 bulletin to calculate the low ambient charge required. I was taught different ways, which are not as perfect but get the job done for most low ambient conditions in TX where I live: On a simple unit like this, I typically front seat receiver then start charging into low side. The head pressure will stabilize at a certain pressure (depending on the ambient temp at that time). Once it rises about 25#-50# about that pressure, you know the receiver is almost full. Then you are done. Another way I have been taught is to soak down the condenser with ice water and charge to clear sight glass. You just have to be careful, because if the condensing unit pumps down at the receiver during defrost, you must make sure all of the charge will fit into the receiver, because overcharge will cause it to go off on high head after you leave during the first defrost on a hot afternoon. By charging directly into a closed receiver and watching head pressure and waiting for the rise, you are charging the unit to a certain receiver level. If that amount of refrigerant is not enough to maintain a clear sightglass on a cold day then the unit may have an undersized receiver.

  3. #3
    Join Date
    Mar 2007
    Location
    NW AR
    Posts
    2,478
    Did a search and have been reading about how the valve works. You say front seat the receiver? If I do that it will immediately start pumping down. Are you saying if im adding refrigerant, with that valve closed, it wont pump down?

  4. #4
    Join Date
    Mar 2007
    Location
    NW AR
    Posts
    2,478
    And can you clarify what this means:

    The head pressure will stabilize at a certain pressure (depending on the ambient temp at that time). Once it rises about 25#-50# about that pressure, you know the receiver is almost full.

  5. #5
    Join Date
    Nov 2009
    Location
    Richmond, working under tarps
    Posts
    526
    when you close, front seat the king valve, you can then charge the unit after the king valve in liquid state.

    it will go through the evap, back to compressor then to the condenser and receiver.

    once the SCT is approx 25-50psig above ambient saturated pressure the receiver is nearly full, you have a little room because the condenser can hold liquid.


    it does sound low on charge for lower ambients, vapor bypassing the condenser.

  6. #6
    Join Date
    Sep 2007
    Location
    NH
    Posts
    1,191
    Yep needs some gas.
    http://www.sporlanonline.com/90-30-1.pdf
    Here the way to charge it.

  7. #7
    Join Date
    Mar 2007
    Location
    NW AR
    Posts
    2,478
    Thanks all. Yea i read up on the 90-30-1. Ill do the math, recover, and weigh it in later when its slower. Right now I just need to get it close so I can finish the multiple jobs im already in the middle of.

  8. #8
    Join Date
    Sep 2007
    Location
    NH
    Posts
    1,191
    Quote Originally Posted by ar_hvac_man View Post
    Thanks all. Yea i read up on the 90-30-1. Ill do the math, recover, and weigh it in later when its slower. Right now I just need to get it close so I can finish the multiple jobs im already in the middle of.
    The water hose method has worked well for me in a pinch... Or with microchannel coils.

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