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  1. #1
    Join Date
    May 2005
    Location
    Arizona
    Posts
    145

    Question

    I have mostly been working on residential air conditioning. Due to a shortage of techs here in my town I have found myself making the transition into walk-in refrigerators and freezers. I am not really too concerned about the electrical as I am the refrigerant charging.

    I know superheat and subcooling. Do most of these walk-ins come with TXV's? If so, will I use 10 - 15 degree subcooling on walk-ins as I have in the past with comfort cooling?

    Will I be using superheat? What are the biggest differences from walk-ins and comfort cooling with regard to refrigerant charging?


  2. #2
    Join Date
    Jul 2000
    Location
    Guayaquil EC
    Posts
    10,478
    All walk-in system use TEVs for refrigerant metering with the exception of some proprietary systems like Heatcraft's Beacon system that use electric expansion valves. They also generally have receivers except for Russell's High Sierra system.

    The basic difference between walk-ins and typical split A/C systems then is that A/C systems are critically charged whereas refrigeration systems are not. For that reason they are not charged in the same manner.

    You already know that on an A/C system with a cap tube you're essentially charging by suction superheat and on a system with a TEV by liquid subcooling. This is done because the amount of refrigerant in the system will cause these values to change. When you hit your mark, you're done.

    On a refrigeration system you basically charge to a full column of liquid by clearing the sightglass. If the system includes some form of condenser flooding control like a Headmaster, you must then add more refrigerant to account for the amount of refrigerant that gets stacked up in the condenser in cold weather.

    It's always best to follow the manufacturers' recommendations when installing any system. Heatcraft has what is probably the best manual out there for commercial walk-ins. Read it, study it and understand it. You'll be far ahead of those who don't.

    Heatcraft Installation and Operating Manual


  3. #3
    Join Date
    Sep 2001
    Location
    prov.r.i.
    Posts
    1,441

    Hmm

    dear, ice meister,not too break your bubble but i think you are miss informed,refrigeration systems are also critically charged as well,as for a/c systems, have have yet to see one with a cap tube.could be wrong mind you, butttttttttttttttt.
    if at first you dont succeed,then skydiving is not for you

  4. #4
    Join Date
    Jan 2006
    Posts
    1,120
    Just changed a 2 horse cast iron body on a walk in cooler,3 phase,changed out filter dryer,contactor,fixed freon leaks,new condensor fan moter,added 1 oz.of acid away just incase,system didnt seem to be cooling that well,pressures was 18 and 175,so i came back about 4 hrs later and checked walkin temp it was at 40,so i played with charge somewhat,took superheat after adding a little 409 and suction stayed about 18 and line temp was around 34,16 degree superheat then i checked subcooling,pressure at 185 and line temp at 110.Had about 22 degree of subcooling,so letting it sit over night to see how it looks in the morning,ya i think the charge is somewhat pretty critical in refrigeration but hard to go from one to another.
    There are three signs of old age.
    The first is your loss of memory,
    the other two I forget.


  5. #5
    Join Date
    Jun 2001
    Location
    San Diego, CA
    Posts
    1,149
    critical - a minor difference in charge volume has a major impact on performance.

    I haven't seen a TXV/RCVR/Sg unit that required a critical charge yet. I have seen small appliance size restaurant equipment that did require a critical charge, but it had a cap tube.

    You can overcharge a TXV and it'll be just fine as long as the head pressure remains nearly constant. Thus it is not a critical charge.

    The basic concepts of charge apply in the same manner and to the same degree regardless of application. Gas is gas man.

  6. #6
    Join Date
    Jul 2000
    Location
    Guayaquil EC
    Posts
    10,478
    Originally posted by len
    dear, ice meister,not too break your bubble but i think you are miss informed,refrigeration systems are also critically charged as well,as for a/c systems, have have yet to see one with a cap tube.could be wrong mind you, butttttttttttttttt.
    Len.....The subject concerned the difference between A/C and walk-in system charging. I didn't want to confuse the issue with self-contained wannabe refrigeration systems like those in True and Beverage Air reach-ins.

    As for A/C system and cap tubes, I should have said fixed metering type systems instead of cap tubes.....it would have been clearer and a little more up-to-date. Many split A/C systems had capillary tube feeds before most manufacturers switched over to flow check pistons type feed.

  7. #7
    Join Date
    Apr 2005
    Location
    where the beer flows like wine
    Posts
    2,871
    Originally posted by hillbilly tech
    Just changed a 2 horse cast iron body on a walk in cooler,3 phase,changed out filter dryer,contactor,fixed freon leaks,new condensor fan moter,added 1 oz.of acid away just incase,system didnt seem to be cooling that well,pressures was 18 and 175,so i came back about 4 hrs later and checked walkin temp it was at 40,so i played with charge somewhat,took superheat after adding a little 409 and suction stayed about 18 and line temp was around 34,16 degree superheat then i checked subcooling,pressure at 185 and line temp at 110.Had about 22 degree of subcooling,so letting it sit over night to see how it looks in the morning,ya i think the charge is somewhat pretty critical in refrigeration but hard to go from one to another.
    Hill, did you liquid charged it? at 18psi suction the evaporator should be near 20 degrees.

  8. #8
    Join Date
    Sep 2001
    Location
    prov.r.i.
    Posts
    1,441

    Talking

    i stand to be corrected
    if at first you dont succeed,then skydiving is not for you

  9. #9
    Join Date
    Jan 2006
    Posts
    1,120
    Hvacpope,ya i liquid charged it on the high side,vacumm sucked most of the freon in,added a little later on the low side,i know its a no no but what else do ya do,anyway box is running good and cold now,but i will be keepen a eye on it,pass by there everyday.
    There are three signs of old age.
    The first is your loss of memory,
    the other two I forget.


  10. #10
    Join Date
    Jun 2001
    Location
    Michigan
    Posts
    12,077
    well.

    let me see here.

    40 degree coil with a 15 degree TD is 55 degree discharge air, at design load. sounds like refrigeration to me.

  11. #11
    Join Date
    May 2005
    Location
    Arizona
    Posts
    145
    Ice, you said it is charged to a full column of liquid by clearing the sightglass. I have never charged to a sightglass and am sure it is not difficult. I just need a little guidance. Do I just add refrigerant till I start to see the liquid in the sightglass? Where do I stop charging, when liquid is present at the bottom, center, or all the way at the top of the sightglass?

    Also, if I wanted to charge by superheat would that be acceptable also? If so, is between 20 - 30 degrees a correct superheat?

    Would I completely leave out subcooling?

  12. #12
    Join Date
    Jan 2004
    Location
    Earth
    Posts
    4,879
    The biggest difference with a txv system I see would be the presence of a receiver to store excess liquid. Superheat is set at the evaporator outlet, not the compressor. The goal is a clear glass, plus the needs of any low ambient device.

    Superheat in boxes, cooler 8-12 degrees freezer 4-6 degrees.

    I feel installing any walk-in without a in line heat exchanger is foolish.

    This will flash any returning liquid in the suction line and sub cool and insure a full Colum of liquid at the txv.
    A Diamond is just a piece of coal, that made good under pressure!

  13. #13
    Join Date
    Jun 2006
    Posts
    107
    Do not charge by sub-cooling. Add refrigerant until you can see it flowing through the sight glass. Keep adding refrigerant until the sight glass clears. Let the system run for a few minutes to make sure the sight glass stays clear. Add more if necessary. At this point add flooding charge if you have a flooding control valve.


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