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  1. #1
    Join Date
    Jun 2010
    Location
    Miami
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    258

    Questions about "critically charged" systems

    Okay, due to my big mouth and willingness to help I am now in over my head. I just hated to see a pricey unit thrown away over a start relay. Problem is the terminal welded to the compressor pin. I thought it was coming off until I pulled the pin out of the compressor! Chalk that up to stupidity and no good deed goes unpunished.

    So I need a little help understanding what symptoms I'd see if a critically charged system was under or over charged. There is no data on charge weight.

    I'm tending to think if one charged a small system during cooler temperatures one might find as the ambient air goes up you are no longer geting a column of liquid to the cap tube. At the other extreme I guess one could stack the condenser totally full of liquid once ambient air temps dropped.

    I really have no idea what sub cooling and superheat numbers I'd be looking for on a small lab refrigerator running 134a. I'm guessing like 10 degrees superheat and upwards of 20 degrees subcooling?

    BTW, if the system has a receiver and expansion valve need I give any more thought than getting a clear sight glass and maybe adding enough extra to get 10/20??? degrees of sub cooling?

  2. #2
    Join Date
    Jul 2012
    Location
    Western KY
    Posts
    1,221
    Assuming its a self contained RI cooler/freezer. Hard to guess on the charge. Charging by weight only way to hit target center mass. A call to the manufacturer may get you the correct data if u have M# & S#. When your talking total charge in ounces a little too much or not enough is alot so trying to guess it in is very difficult for even the most seasoned tech. As for the receiver TXV question a full glass does not mean a full charge. May work great when it's warm but fall on its face as ambient drops. Alot of guys use different methods to charge these systems. Full glass + 20% I have heard. 95% of receiver capacity is another. Fill the glass and come back in the fall and top off is another. I have done all 3 at one time or another. 95% receiver capacity has worked the best for me. Super heat and sub cooling have to be watched as they are the final determining factors.

  3. #3
    Join Date
    Jun 2003
    Location
    Chicago, IL
    Posts
    4,428
    Quote Originally Posted by Russ57 View Post
    BTW, if the system has a receiver and expansion valve need I give any more thought than getting a clear sight glass and maybe adding enough extra to get 10/20??? degrees of sub cooling?
    Do not charge to subcooling on a receiver. If the condenser is indoors, charge to a clear sight glass and then add a little more so you have some reserve in the receiver. once the site glass is clear, you can only increase subcooling by filling the receiver completely so that refrigerant starts stacking in the condenser.

    If the condenser is outdoors with a flooding control, then you have to calculate charge.

  4. #4
    Join Date
    Oct 2008
    Location
    Ohio, USA
    Posts
    177
    If the unit has a receiver, it is not a critically charged system.

    If it IS a critically charged system (without a receiver), then you charge to subcooling.

    It's safe to assume you are talking about a small system here, with probably not much more than a pound full charge. (maybe less like 8 oz, etc)
    Keep this in mind when charging, especially if you use long hoses. (don't dump the remainder of your high side liquid charge back into a properly charged system, consequently over-charging it)

    I would add charge and until you have 10-12* of subcooling. Certainly not more than 15* or you will start to use the bottom of your condenser coil as a receiver.

    At this point you are at the mercy of the cap tube. Your system is 134a/POE, which is notorious for plugging up capillaries.
    Ideally you want a superheat above 20* when the unit is at design temperature. If your cap tube is restricted you will not achieve even close to this.

  5. #5
    Join Date
    Oct 2003
    Location
    las vegas
    Posts
    1,505
    i agree with dolman that if it has a reciever it is not a critially charged unit.
    call the mfg. & get the correct specs on the unit & weigh in the required ammount.

  6. #6
    Join Date
    Jul 2000
    Location
    Guayaquil EC
    Posts
    10,359
    Quote Originally Posted by Russ57 View Post
    Okay, due to my big mouth and willingness to help I am now in over my head. I just hated to see a pricey unit thrown away over a start relay. Problem is the terminal welded to the compressor pin. I thought it was coming off until I pulled the pin out of the compressor! Chalk that up to stupidity and no good deed goes unpunished.

    So I need a little help understanding what symptoms I'd see if a critically charged system was under or over charged. There is no data on charge weight.

    I'm tending to think if one charged a small system during cooler temperatures one might find as the ambient air goes up you are no longer geting a column of liquid to the cap tube. At the other extreme I guess one could stack the condenser totally full of liquid once ambient air temps dropped.
    Once you study up on how cap tubes really work, you'll see that they actually rely on an ebb and flow between subcooled liquid and flash gas to do their job. Here's a very good explanation from Henry Ahrens of Supco:

    http://hvac-talk.com/vbb/showthread.php?t=163167

    Quote Originally Posted by Russ57 View Post
    I really have no idea what sub cooling and superheat numbers I'd be looking for on a small lab refrigerator running 134a. I'm guessing like 10 degrees superheat and upwards of 20 degrees subcooling?
    You really don't need to be concerned with the subcooling numbers...at least not as much as you need to be with the superheat...when charging these systems. The truly important thing is to have the box at or very near design temperature, at which point you should slowly add (or remove) refrigerant until you get between 20-40F superheat at the compressor.

    While doing this, it's always a good thing to keep an eye on the subcooling however. I'm saying this, not because subcooling is a good metric for charging cap tube systems, but only to get an early warning in case you may have a restricted cap tube. If the SC starts rising well above 10F or so and the SH is still way high, then you may have a problem.

    Quote Originally Posted by Russ57 View Post
    BTW, if the system has a receiver and expansion valve need I give any more thought than getting a clear sight glass and maybe adding enough extra to get 10/20??? degrees of sub cooling?
    As has been mentioned, receivers don't exist in critically charged system. But to answer your question, a receiver system typically will give you around 5F liquid subcooling and that's it. Adding more refrigerant won't change it because the added refrigerant isn't stacking up in the condenser...it's going to the receiver.

  7. #7
    Join Date
    Jun 2010
    Location
    Miami
    Posts
    258
    I should be more careful how I word things. Almost everything I work on is water cooled. The shell of the condenser is the reciever. My climate doesn't have large swings. I can set a water regulating valve and dial in a stable, year round, SCT regardless of my actual cooling tower water temperature. It makes matters very easy. I have a lot of leeway, or so it seems to me, compared to other systems.

    Now when I start thinking about air cooled condensers on units that will be charged in a shop with good airflow......and then be shoved up against a wall with other equipment....I start questioning just what happens. I agree that something with a reciever isn't critically charged. It was a different question.

    Still I had hoped for some discussion as to what one would see in critically charged systems if charge was a little off. I can make some educated guesses but that is about all and I could be very wrong.

    I have mentioned some of this before but suppose I should again. I am a building engineer, not a refrigeration tech. Most of my experience is from long ago helping my dad, primarily in auto air. I do have a universal license. I consider myself capable with the water cooled walk-ins I have and I'm not the type to overstate my abilities. Mostly I get pressed into this field when the normal guys can't handle a problem. My so-called refrigeration tech doesn't think superheat/sub cooling matters nor does he think leak detector/thermometer are required tools. This only underscores why my boss keeps getting me involved in stuff I'd rather not mess with. But hey, 28 years on the job only buys you so much privilage when the data center/MRI/CAT/PET/lab system is down. I could whine but I reckon it keeps life interesting.

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