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  1. #1
    Join Date
    Jun 2003
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    GA
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    395

    Confused

    I've read in many commercial refrigeration contexts that "a TXV sys requires a receiver", yet most every residential comfort cooling sys with a TXV has none. My fist thought is that obviously the bottom sub-cooling part of the condenser in these cases is something of a receiver.

    Which drags me to the next question, why use a separate receiver if you can do the same thing with a little more condenser? I've read enough that suggest that using the condenser to SC comes at a cost to head pressure. I understand the effect of head press on VE, COP, M dot, and EER, but I don't understand what a "separate" receiver does for the sys beyond providing a reservoir for liquid.

    Commercial evap loads ranges wildly from pull-down to end-of-cycle, are my answers hiding here? What the hells going on in side the condenser and "sep" rec as the evap load flys all over the place? What SC would I see?

    Residential evap loads are very constant by comparison, the biggest change on these systems is the rejection CST which follows the ambient DB which can range a lot too. All the SC measurements I see are point-in-time, so the questions are killing me here. I know CST follows ambient plus {split / approach / ATD}, but what will SC be doing across the same range?

    Is the hidden justification for a "separate" receiver to avoid paying the extra head? Also, how do you charge a receiver / TXV system?? You can't charge for 10-20 SC leaving the condenser. If you did charge for condenser exit SC, would you not flood the receiver to the top?

    BTW, the saturated (but not SCed, yes?) liquid dumps into the top of the receiver(no?) and the law states there can be no SC at the surface of the liquid within.

    Can somebody tie all this crap together before my IPR pops ??

    Thanks,

    md

  2. #2
    I wouldn't get hung up on receivers, they are just resovoirs to store excess subcooled liquid when the system doesn't need it,

    [Edited by top mechanic on 05-06-2005 at 05:37 PM]

  3. #3
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    Jun 2003
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    GA
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    395

    Talking

    That's not the end to this thread.

  4. #4
    refrigeration condensers have no subcooler rows in the condenser,air conditioners do.

  5. #5
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    GA
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    Talking

    A Spin-off from my original post would be...How does the refrigerant know the difference?

    The questions may be for a hard core RSES CMS type or ME.

    Thanks,

    md

  6. #6
    Join Date
    Oct 2004
    Posts
    3,400
    Air conditioners have a relatively steady heat load.

    Commercial refrigeration will have hot pull down, defrost, low ambient, and several other factors to work around.
    A receiver is pretty cheap, an ensures a liquid head to the valve under varying load conditions.

    At least that's the way I heard it.

  7. #7
    A receiver gives thew system a lot of benefits, the expansion valve needs to see a column of liquid, I'll try to answer your questions about it.

    The residential systems not having it ?

    It would be a better system if they did, but the cost and the space are prohibitive,the systems do not have much capacity control.So they in a asense do use the last few rows of the condender to store refrigerant.

    Why use a receiver in place of a condenser ?

    because it will give you unecessary subcooling. And could bring your head prssure down into a range that the TXV does not want to see.Also, in low ambients the last thing you want is a big condenser, some head pressure controls actually try to fill the condenser with liquid to maintain a pressure by making the condensing area smaller, this type system must have a receiver.

    What SC will you see when loads change ?

    Measurements should be taken at constant loads, subcooling is affected by all the things you have already mentioned, just know that the receiver is not expected to do any subcooling,that is one of the condensers job. If you have a column of liquid and the subcooling changes it will not be detrimental to the total operation of the system.

    What will subcooling be doing across all those ranges ? depends what type of headpressure you maintain,If you maintain the head you will advertantly have a lot of control over the subcooling.

    How do you charge a system with a receiver ?

    By subcooling or by weight. Having ten to twenty degrees will not flood the receiver, the piping config of a receiver will usually not permit it to flood, but if you do your subcooling and your headpressure will start to climb quickly.

    The liquid in the receiver may or not be subcooled if you were to measure the pressure inside the receiver, but it doesn't matter. The job of the condenser was to condense and subcool the liquid below the pressure in the condenser, not the pressure in the receiver. So when the refrigerant leaves the receiver it may not be subcooled as much as it was when it left the condenser, but it will be about the same temperature,and that liquid at that temperature with the right pressure differential across the TXV is what the low side wants to see.

    Can someone tie all this together?

    I just think your looking at it like it has a mechanical function when it really is just a storage tank.




  8. #8
    Join Date
    Apr 2002
    Location
    Dallas, TX
    Posts
    2,987
    Originally posted by mdman
    but I don't understand what a "separate" receiver does for the sys beyond providing a reservoir for liquid.

    You’ve got it. Keep in mind that it would also allow the condenser to be more efficient by not having liquid backed up into it.

    Originally posted by top mechanic
    The residential systems not having it ?

    It would be a better system if they did, but the cost and the space are prohibitive
    Residential systems are designed to be very cost competitive…

  9. #9
    Join Date
    Oct 2004
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    3,400
    Originally posted by Andy Schoen

    Residential systems are designed to be very cost competitive…
    Yes.

    That too.

  10. #10
    Join Date
    Apr 2002
    Location
    Omaha, NE
    Posts
    1,561
    Andy,
    Airworx mentioned that refrigeration condensers don't have
    subcooler rows like A/C. I've heard this somewhere else, too. Is this really true, and if so why? Is it because the subcooling would just be lost in the receiver?

    Also, I 've always wondered why you can use a cap tube with both refrigeration and A/C, but a piston type metering device is only used in A/C. What capability does the cap tube have on refrigeration that a piston doesn't have?

  11. #11
    Join Date
    Jun 2004
    Location
    Olean, NY
    Posts
    1,449
    Originally posted by bwal2
    Commercial refrigeration will have hot pull down, defrost,
    hot pull down?
    Guinness for strength

  12. #12
    Join Date
    Jun 2003
    Location
    GA
    Posts
    395

    Cool

    The evap load on a box after you "upset the load" by stocking say, an empty WIF with 1,000 lbs of product at room temp. Or when you turn it on for the first time in a while.

    There may be a more academic definition.

    md

  13. #13
    Join Date
    Jun 2003
    Location
    GA
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    395

    Talking

    unless somebody corrects me here, there is...

    pull down load
    standby load - at cut in
    end-of-cycle load - at cut out

    somebody have better nomenclature for these?

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