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  1. #1
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    Super heat and sub cooling of walk in coolers/ freezer and reach in cooler/freezer

    So Im a residential and commercial air conditioning tech whos starting to work on commercial and residential refrigeration systems and Im really only familiar with how to charge a system based on the super heat and sub cooling methods. I know on a residential AC condenser it will list the desired sub cooling that the manufacturer recommends so I usually charge to that when theres a TXV but when it comes to a fixed orifice on a residential system or a capillary tube on a commercial walk in/reach in or even a txv on a commercial walk in reach in I cant even seem to find a specification on what the superheat or sub cooling should be. How exactly do you guys who are refrigeration seasoned pros know how to check the charge on a refrigeration system or how to charge it properly if there is no rates super heat or sub cooling on the specs?


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  2. #2
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    There's a couple of variables depending on what's being stored,

    In a commercial walk in freezer, 2-3* SH at evap is pretty good, but you still want to maintain 20-40 *SH at compressor which is vital.

    In a commercial walk in refer, anywhere between 5-12*SH could be fine depending on what's in the box and if you want it more humid than others, increasing runtime can decrease humidity and by raising or lowering SH you are changing the "effeciency" of the evap coil as you will be using more or less of it. In rough terms.

    And for the never ending discussion of subcool, most walk in's have liquid receivers, which can make it difficult to get a SC reading, I've tried measuring it between the outlet of the condenser and the receiver but it won't tell you much, there's extra refrigerant when there's a lighter load but when there's a heavy load, there might not be so it's difficult to base on that.

    If your TXV is set right, you clear the sightglass and you dial in the SH to match whatever you are doing, you'll be in pretty good shape. IMO the comp SH is more important than the evaporator, if you have to run lower SH in the box to keep a lower SH at the comp, do it. But remember that conditions can change, I try to get it at around 30*SH at the comp, that's a happy medium, but if you set it at 20*SH when it's warm out, when it gets cold it could in theory go below 0 and you would be slugging liquid.

    Things to consider. But in general and this goes for any system, SH at the comp should ALWAYS be between 20-40* if it's higher than that, your cooking the compressor, lower than that you could risk slugging liquid and wreck the compressor.

    When I check a system, I normally go for the evaps first, if they got ice on them that's a bad indicator most of the time, it could mean it's running forever and the door doesen't close, or it could mean it's low on charge, it could mean a txv is starving the evap and so forth.

    Hopefully that answers some of it.

  3. #3
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    6-8 SH low temp and 8-10 SH medium temp
    Officially, Down for the count

    YOU HAVE TO GET OFF YOUR ASS TO GET ON YOUR FEET

    I know enough to know, I don't know enough
    Liberalism-Ideas so good they mandate them

  4. #4
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    Aug 2002
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    Find the book
    Refrigeration for the A/C Tech by Dick Wirtz.

    A must read!

  5. #5
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    Aug 2002
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    Systems with a x-valve clear the SG adjust SH for 20 - 40*f at the pump

  6. #6
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    Thread Starter
    What about a reach in cooler or refrigerator? SH on those is the same as a walk in?


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  7. #7
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    Quote Originally Posted by HyperFusionHVAC View Post
    What about a reach in cooler or refrigerator? SH on those is the same as a walk in?


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    Comp manufacturers want 20- 40* at the pump.

    Small self contained 20 or less!

  8. #8
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    Thread Starter
    Thanks for the advice. Just bought book, gonna be looking into it when it arrivdz

  9. #9
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    Generally speaking, refrigeration systems with a receiver, the subcooling doesn't vary with the addition or removal of refrigerant because the receiver will store the added refrigerant rather than the condenser coil. Normally one can only expect around 5 Deg F of liquid subcooling, so clearing the liquid line sightglass is used to establish a baseline or benchmark for charging. If the system has a condenser flooding type of head pressure control such as a Headmaster, then additional refrigerant will be required. Often the condensing unit manufacturer's installation manual will include information on the amount of additional refrigerant needed, but short of that Sporlan has a tried and true method for calculating this in their Bulletin 90-30-1:

    https://www.parker.com/literature/Sp...90/90-30-1.pdf

    For systems with a capillary tube it's advisable to charge by weight using the manufacturer's required refrigerant charge as stated on the unit's nameplate. If there is no such charging information available, it's best to slowly charge until about 10 Deg F superheat at the evaporator outlet is obtained or 20-40 Deg F SH at the compressor.

    An old-school method of charging a cap tube system is to charge slowly until you get a frost line on the suction leaving the box. Another is to shut off the evaporator fan and charge until you get frost on the suction line at the compressor.

    A third method is from a vacuum, charge the system to the high side with liquid until the high and low side pressures are equal. This is often useful to get you started and then fine tune using evaporator superheat.

  10. #10
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    Quote Originally Posted by icemeister View Post

    A third method is from a vacuum, charge the system to the high side with liquid until the high and low side pressures are equal. This is often useful to get you started and then fine tune using evaporator superheat.
    That's how I do it most of the time when shut down after a repair. I kind of enjoy fine tuning evaps and getting a system dialed in really well, It's like the final touch after a day or 2 of working on a repair.

    I did wonder though, the receiver is fairly large in most cases, so if you have a sight glass that's half full or so, I would think you are still getting a pretty good column of liquid to the txv considering it's backed up to a certain extent and so is showing a level whereas I feel if you just dump refrigerant into a receiver and the system has a leak, it just takes so much longer before the system acts up due to the leak, but if a full sight glass was required for the system to work properly, how come it doesn't affect it until there's virtually nothing visible int he sightless anymore indicating you no longer have solid liquid to the txv, and that $500 worth of R22 is now out in the air.

    Not to hijack the thread, but it's something i've wondered about a couple of times.

  11. #11
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    There is a situation where the SG has what we called "Rivering". A solid column of liquid going thru the SG yet vapor is still present. It does happen, maybe 1 in 100. Resident expert Andy Schoen touched on it a while back. I'll let Ice get into the details.

  12. #12
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    One quick additional note on charging to a clear sight glass.

    There are times that you will over charge the system when clearing the sight glass. It is important to know when a sight glass is showing a low charge, or rivering. When a sight glass is 'rivering', it will look like a river, and only be maybe half full. This typically occurs during a low or very low load condition.

    The difference is that you will not see foam or bubbles, which IS a symptom of a low charge.
    If you were a real tech, you'd solder a relay on that board and call it good to go.

    I do a triple evac with nitro to remove non condensables.

    I use 56% silver on everything except steel.

  13. #13
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    Ah heck, you just beat me to it.


    Quote Originally Posted by pecmsg View Post
    There is a situation where the SG has what we called "Rivering". A solid column of liquid going thru the SG yet vapor is still present. It does happen, maybe 1 in 100. Resident expert Andy Schoen touched on it a while back. I'll let Ice get into the details.
    If you were a real tech, you'd solder a relay on that board and call it good to go.

    I do a triple evac with nitro to remove non condensables.

    I use 56% silver on everything except steel.

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