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  1. #1
    Join Date
    Jun 2012
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    Northeast
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    305

    Confused Checking system charge on a cold room box

    The system is a cold roombox that holds 39*F. It has 404-A refrigerant. The condensing unit sits about 22'-28' above the evaporator. There is a reciever, accumulator and I think there was a EPR valve at the compressor. Can't remember. The condensing unit sits in a rack with two identical units below it. They are water cooled with head pressure regulating valves. There is a hot gas line that runs down to the box. I could not locate the solenoid valves, I think they got sheet rocked in above the box. So I don't know how they got piped.

    The sight glass was flashing. My first question is how do you charge a system like this? I have read that you cannot charge a system with a reciever by sub-cooling. Is that correct? I was told that the vertical drop of the liqiud line will cause the sight glass to flash. Is this correct? Also being a blend the sight glass may flash a little to begin with.

    One thing I did notice between this unit and the other two was that the sight glass was not flashing on the other two. I expect it was because of the way the liquid line was piped. On the unit with the flashing SG the LL comes out of the reviever and travels horizontal until it makes a drop. The SG is piped in the horizontal line. On the other two that do not flash the LL comes out of the reciever and is pipied up about 1' to 1.5' before it runs horizontal and then goes down. The SG is located in the vertical prtion of the LL. My second question is can piping the SG this way in a system like this were the condensing unit is above the evap have the advantage of indicating when the system has a correct charge?

    What do you guys think? Thanks for the help.

  2. #2
    Join Date
    Mar 2009
    Location
    Mid-Mo
    Posts
    3,596
    Are you having a problem with the one that is flashing? Not pulling temp? What type of setup do you have for your water medium, is it city water then goes down the drain? Hook up any gauges?

  3. #3
    Join Date
    Jun 2004
    Location
    St Paul, minnesota
    Posts
    1,256
    clear the glass by adding charge.This is always done in refrigeration when you have a receiver. sight glass location is irrelevant. solenoid valves if not inside condensing unit are probably inside evaporator coil

  4. #4
    Join Date
    Jun 2012
    Location
    Northeast
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    Quote Originally Posted by superfittertech View Post
    clear the glass by adding charge.This is always done in refrigeration when you have a receiver. sight glass location is irrelevant. solenoid valves if not inside condensing unit are probably inside evaporator coil
    With these boxes the soleniods are usualy on top of the box. At this site at least. Never seen them any where else.

  5. #5
    Join Date
    Jun 2012
    Location
    Northeast
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    Quote Originally Posted by ryan1088 View Post
    Are you having a problem with the one that is flashing? Not pulling temp? What type of setup do you have for your water medium, is it city water then goes down the drain? Hook up any gauges?
    The call was for the box that had the flashing SG. But when I got to the site it was almost at temp. By the time I got gauges and stuff on it pulled temp on it's own. There are 3 evap fans and 1 was out. So I am guessing the door was left open which happens alot with these boxes. With one fan being down it just took some time to reach set point.

    I did try to add gas which is why I asked about how the sight glass was flashing. I was unable to clear the glass before the head pressure got to high. I remember a guy telling me years ago that a SG will flash if the condenser is above the evap. That was with a split A/ac unit though. I tried to adjust the head pressure regulating valve but it was wide open. All three units get the same water. A condenser water loop. The building probaly has 30 of these boxes in it. It was a night time call. I told the customer we should come back to replace the fan motor and check on the head press valve for a restriction or replace if needed. But they have there own maintenance gy=uys so we never got called back. My guess is they replaced the fan motor and left the SG flashing.

    I wish they would let us finish these jobs. If there guys are too lazzy to come in at midnight they should not get to do the follow up.

  6. #6
    Join Date
    Mar 2009
    Location
    Mid-Mo
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    3,596
    Quote Originally Posted by Push It View Post
    I remember a guy telling me years ago that a SG will flash if the condenser is above the evap. That was with a split A/ac unit though.
    Ok now I see why you were wondering that. I have no idea where he came up with that, but I don't find that to be true.

    Sounds like you have a water flow issue to that unit that needs to be addressed, but sounds like you won't be able to. Atleast until a compressor blows up...

  7. #7
    Join Date
    Aug 2004
    Posts
    20
    Quote Originally Posted by Push It View Post
    The sight glass was flashing. My first question is how do you charge a system like this? I have read that you cannot charge a system with a reciever by sub-cooling. Is that correct? I was told that the vertical drop of the liqiud line will cause the sight glass to flash. Is this correct? Also being a blend the sight glass may flash a little to begin with.
    Snipped
    My second question is can piping the SG this way in a system like this were the condensing unit is above the evap have the advantage of indicating when the system has a correct charge?
    Superheat and subcooling. For your first question. It's not either/or it's both on all systems every time.

    No. Measure the liquid line temperature 6" out of a receiver. Compare it to your condensing temperature. That'll show you what the deal is. Assuming a properly charged system of course.

    You lose going up, you gain going down.
    http://www.hvacsalesandsupply.com/Li...0practices.pdf

    Second question, again no.
    You need the condensing, evaporating, suction line (at the bulb and the compressor), liquid line, discharge line, air in/out, water in/out, temperatures. Compressor model number and amp draw.
    Looking at the results would paint a picture of the complete system. Any system or charging problems would be easy to "see".
    Without measurements, it's just guessing.

    Digging into piping, PE charts, etc and the attendant math will give you better understanding of what you're looking at.
    There are huge amounts of Youtube video's and some books and resources available to flesh out your knowledge.
    From Sporlan's Cold War (http://www.sporlanonline.com/coldwar-newsletter/) to RSES's SAM's Manuals (Being a RSES member gives you access online, http://www.rses.org) to books by Dossat and King. I mention these two because they specifically address your questions. Either is good.

    I'd buy a book or two written by experts who are actually knowledgeable in the areas you want to know more about.
    Principals of Refrigeration by Dossat or Basic Refrigeration by King and TECH Method by Lloyd for charging and troubleshooting.
    The first two are older books (still valid) and easy to find that cover the theoretical guts. The other you'll have to buy new because nobody gives up their TM book.
    http://prostores1.carrierzone.com/se...com/StoreFront

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