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  1. #1
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    when putting a condensing unit on a cap tube with a reciever must you remove the reciever for it to work right. or doesnt it matter. thanks for the replies

  2. #2
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    Yes, the receiver must be removed for the capillary tube to work properly.

    A cap tube works in a system with a fixed charge and amount of subcooling entering it and the pressure difference across it varies the amount of refrigerant flow through it to balance the load on the system.

    You get the change in subcooling by the amount of refrigerant that gets stacked up in the condenser (due to load or ambient variations, so if there's a receiver in the line the refrigerant will stack in the receiver and as such, the cap will alway see low subcooling so I would then expect to see a starved evaporator most of the time.

  3. #3
    Join Date
    Oct 2005
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    refrigerant will stack in the receiver and as such, the cap will alway see low subcooling so I would then expect to see a starved evaporator most of the time.

    Ice,
    Why low subcooling and also cap will always have liquid available from the bottom of the receiver so why srarved evaporator ?

  4. #4
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    Oct 2005
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    58
    sorry starved

  5. #5
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    As I said, you can only get subcooling when there is liquid stacked up in the condenser. If the refrigerant at the outlet of the receiver is at saturation, you have zero subcooling, right? As liquid starts to back up into the last passes of the condenser coil you gain subcooling because that liquid is being cooled below saturation.

    If there's a receiver in the liquid line. The condenser will continually drain to the receiver and not stack up any liquid, so no subcooling. A cap tube won't do its job without it.

    The cap tube is a pressure reducing device. It depends on two things....pressure difference and subcooling. You'll get more restriction from a cap tube with low subcooling because the liquid flashes to a vapor sooner inside the tube leaving more of its length with two-part flow. If you consider that you can move more lbs/min through a tube with liquid than you can with a gas you'll begin to grasp the concept here.

    With more restriction, the evaporator pressure reduces increasing the pressure difference which in turn will increase the flow.....maintaining a balance to the load. The refrigerant that was boiled out of the evaporator then ends up where?....in the condenser of course. That creates more subcooling, more flow and then what?.....the evaporator pressure increases and so the pressure difference decreases and the flow slows down.....again following the load.

    It's an intricate, but simple dance that's going on in there between delta P and LSC that allows the cap tube to do its thing. If you toss a receiver in the mix, nobody's dancing.

  6. #6
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    Mar 2006
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    OKC, Oklahoma
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    also, during an off cycle, refrigerant from the receiver will tend to migrate to the evaporator causing floodback on start-up.

  7. #7
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    "If there's a receiver in the liquid line. The condenser will continually drain to the receiver and not stack up any liquid, so no subcooling. A cap tube won't do its job without it."



    DOES THAT MEAN IF YOU FILL THE SYSTEM FULL OF REFRIGERANT IT WOULD WORK JUST THE SAME.

  8. #8
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    Originally posted by jat2020
    "If there's a receiver in the liquid line. The condenser will continually drain to the receiver and not stack up any liquid, so no subcooling. A cap tube won't do its job without it."

    DOES THAT MEAN IF YOU FILL THE SYSTEM FULL OF REFRIGERANT IT WOULD WORK JUST THE SAME.
    If you mean charging the system until the receiver is 100% full thus allowing liquid to stack up in the condenser the short answer is yes, I believe so.

    But I would also bet it wouldn't work the same very long considering the off cycle migration.*

    *(See the earlier post by freezerfixer. )

  9. #9
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    new york
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    ya thats what i meant, your probably right the liquid would migrate back to the compressor on the off cycle. thanks for the replies

  10. #10
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    Oct 2005
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    Thanks Ice

  11. #11
    Join Date
    Jan 2007
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    Northern NY
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    Thumbs up

    I posted this on a similar thread...

    A cap tube system will work fine with a receiver. The amount of refrigerant charged may have to be slightly higher to compensate for the volume required to fill the bottom of the condenser to cover the outlet tube. I'm only talking about an ounce or two. The risk, however, is that it will be easy to overcharge the system. There will be no precipitous rise in head pressure, bacause the liquid will simply fill the receiver. If the compressor is off for very long, the excess liquid will flow through the cap tube into the evaporator and cause problems on start-up. Liquid may flood back to the compressor, eventually causing bearing failure or the compressor will repeatedly cycle on the motor overload. The overload will then ultimately fail. These systems are usually built without receivers for the cost savings.

    As long as the compressor is running, and the condenser and receiver are operating in normal ambient conditions, there will be subcooled liquid in the receiver.

  12. #12
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    I agree tgood, most times there will be a lift to the receiver. In order to accomplish this lift, there must be a full colum of liquid from the condenser outlet, all the way up to the top of the receiver.

    Unless the reciever is below the condenser outlet, liquid must back up before a transfer to the receiver can begin.
    A Diamond is just a piece of coal, that made good under pressure!

  13. #13
    Join Date
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    Quote Originally Posted by TGoodrich View Post
    I posted this on a similar thread...

    A cap tube system will work fine with a receiver. The amount of refrigerant charged may have to be slightly higher to compensate for the volume required to fill the bottom of the condenser to cover the outlet tube. I'm only talking about an ounce or two. The risk, however, is that it will be easy to overcharge the system. There will be no precipitous rise in head pressure, bacause the liquid will simply fill the receiver. If the compressor is off for very long, the excess liquid will flow through the cap tube into the evaporator and cause problems on start-up. Liquid may flood back to the compressor, eventually causing bearing failure or the compressor will repeatedly cycle on the motor overload. The overload will then ultimately fail. These systems are usually built without receivers for the cost savings.

    As long as the compressor is running, and the condenser and receiver are operating in normal ambient conditions, there will be subcooled liquid in the receiver.
    It may work, it will not work "fine". The receiver will remove the pressure differential that allows the cap tube to provide the correct metering. A cap tube system relies on high head pressure at high evap temperature and lower head at lower evap temps. That and the variation in subcooling is what makes it work. While the unit may appear to be working alright, you will likely have superheat problems at one end of the temperature spectrum.

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