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  1. #1
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    A couple of questions on capillary tube design



    I am trying to get a refrigeration system working (Norcold 4407) that is a capillary tube design. It has a new compressor and a good evaporator that are matched (same model). What is missing is a section of the lines between the two. The original design had two 1/4" lines at the compressor end, but then the Delivery line merged inside the Suction line. I believe this is where the smaller diameter capillary tube starts - pls correct if I have this wrong. And it appears that the capillary tube ends at the evaporator, where the liquid turns to gas.

    I think I can figure out how to solder up new lines, to include the previously mentioned junction point, if I only knew the original length. The missing original line(s) was about 10' in length from compressor to evaporator, but the exact length from the start of the small diameter capillary tube to the evaporator is the mystery.

    How important is the capillary tube length? In other words, will it still work if my replacement capillary tube is off by 1", 3", 12" from the original design?

    Also, would it be better, worse or as bad to make the new cap tube longer rather than shorter? I'm trying to figure out if I have to take an educated guess, should I err in one direction over another.

    Thank you in advance for your expertise.



  2. #2
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    Re: A couple of questions on capillary tube design

    .

    The capillary tube works by resistance over its length.
    The longer the tube the greater the resistance, the shorter
    the tube the less the resistance will be.

    The diameter also plays a big part, the bigger the diameter
    (the larger the hole is) means there will be less resistance across
    its length but if the hole was smaller then the resistance would be more.

    The capillary is a combination of length and diameter and is a very precise
    measurement. A couple of inches each why might make a big difference to
    the overall performance. If you cut it too long it will have too great a pressure
    drop and that will affect (effect??) the system, but if you cut it too short it will have too
    little a pressure drop and that will affect (effect??) it also.

    You will find that most systems have about a meter (a yard to you ) of cap
    tube coiled up just after the dryer and before it goes into the suction pipe.
    Inside the suction pipe it travels all the way inside the suction (for protection
    and heat exchange) all the way into the evap.

    Your system sounds like the dryer and cap tube coil is missing? Without something
    to compare it to, you will struggle to size it correctly unless you are prepared to
    run it on test and keep changing it until it is correct?

    Regards
    Rob

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    Re: A couple of questions on capillary tube design

    Quote Originally Posted by Rob White View Post
    ....Your system sounds like the dryer and cap tube coil is missing? Without something to compare it to, you will struggle to size it correctly unless you are prepared to
    run it on test and keep changing it until it is correct?

    Regards
    Rob
    Thank you for the response, Rob.

    lol on " vs m. I'm a Brit (Fleet) but was raised in the US - either works for me

    The dryer is in place, but most of the cap tube is missing. To help envision, this unit is designed for converting a boat's ice box to a fridge - the evap is remote from the compressor. Given the 2.5m of distance between evap and comp, there was no need to coil the cap tube in this design. Instead the Delivery line comes out of the cond coil, thru the dryer, and out as 1/4" line, but then at some distance (about 1.5') it changes to the small diameter and enters the Suction line. Presumably this is where the cap tube starts. Not knowing the exact lengths of the old line(s) is a dilemma.

    Maybe I'll just try 10' and see what happens. If it floods, I'll reduce the charge.

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    Re: A couple of questions on capillary tube design

    Quote Originally Posted by mashenden View Post
    Thank you for the response, Rob.

    lol on " vs m. I'm a Brit (Fleet) but was raised in the US - either works for me

    The dryer is in place, but most of the cap tube is missing. To help envision, this unit is designed for converting a boat's ice box to a fridge - the evap is remote from the compressor. Given the 2.5m of distance between evap and comp, there was no need to coil the cap tube in this design. Instead the Delivery line comes out of the cond coil, thru the dryer, and out as 1/4" line, but then at some distance (about 1.5') it changes to the small diameter and enters the Suction line. Presumably this is where the cap tube starts. Not knowing the exact lengths of the old line(s) is a dilemma.

    Maybe I'll just try 10' and see what happens. If it floods, I'll reduce the charge.
    Right, OK then you have a converted system already?
    Most cap tubes I have worked on were about 2 to 3mtr long
    so a 10" tube will be way too short??

    Make it a couple of mtrs at least and then cut it back if too long.

    Rob

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    Re: A couple of questions on capillary tube design

    .

    Just re-read your post 10', ten foot, ten of your US (ex UK) feet

    Ten foot might just do it but be prepared to add or cut back.

    Start longer if you have the choice, it is easier to cut out rather
    than add in.

    Rob

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    Re: A couple of questions on capillary tube design

    Quote Originally Posted by Rob White View Post
    Right, OK then you have a converted system already?
    Most cap tubes I have worked on were about 2 to 3mtr long
    so a 10" tube will be way too short??

    Make it a couple of mtrs at least and then cut it back if too long.

    Rob

    .
    Nope, not converted. This is the design, but with damaged/missing lines to include the point where the Delivery line changes to the cap tub then goes inside the Suction line. Hence the dilemma.

    Yep 10' or about 3 m. Didn't the US adopt that unit of measure from the UK?? Not saying it is good, though. At least the UK had the sense to change to a more logical system of measure.

    Does my idea sound reasonable of reducing the charge a smidge (not sure what unit of measure that is) if it is close but a bit too short (flooding)?

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    Re: A couple of questions on capillary tube design

    .

    The problem is with a cap tube it is critically charged and it
    requires just the right amount of liquid inside the evap, at
    the correct pressure. The length of the tube will regulate the pressure,
    if it is too short the pressure will be high and therefore the evap temp
    will be higher and if the tube was too long then the evap pressure
    would be too long and therefore the evap temp too low.

    The amount of liquid inside will then have an effect on the pressure
    which affects the evap temperature. It's a very fine balance.

    Start long and charge it up until it just starts to freeze back down the suction pipe.
    If the pressure / temp is too low cut out an inch or two and try again
    until you get the correct pressure / temp for R134a

    Evaporating pressure for a fridge will be about 1 to 1.5 barg (15 to 20 psi) which = about -10 degC
    Evaporating pressure for a freezer will be about 0.1 to 0.4 barg (2 to 6 psi) which = about -25 degC

    Rob

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    Re: A couple of questions on capillary tube design

    Sounds like a plan. I knew the length was critical, but did not know if that was in terms of +/- centimeters vs +/- meters to get that sweet spot. Luckily I have some time over the holidays so I'll just have to mess with it.

    Thank you for your insight - it really helped me develop a plan.

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    Re: A couple of questions on capillary tube design

    hi
    compressor size-refrigerant-refrigerator temperature so u can have more&accurate help

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    Re: A couple of questions on capillary tube design

    Hi mashenden
    You have probably got the capillary size and length sorted by now but if not here is soem info that may be helpful.
    Paul
    Capillary Tube Selection 2.pdf
    Born to fish, forced to work

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  12. #12
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    Re: A couple of questions on capillary tube design

    Some good links there Jason I have saved them.
    Paul
    Born to fish, forced to work

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    Re: A couple of questions on capillary tube design

    Why not download the free captube selection programme from Danfoss?

    http://www.danfoss.com/United_Kingdo...ils/DanCap.htm
    I'm back on the Pale

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    Re: A couple of questions on capillary tube design

    Quote Originally Posted by frank View Post
    Why not download the free captube selection programme from Danfoss?
    If only I'd known about this before!! I made myself a little chiller a few years back to cool a single beer barrel. I spent days messing about with the capillary before I go it right. Oh well, nailed it eventually and it still keeps the home brew at tasty 7 deg c now.
    Last edited by frank; 09-12-2013 at 10:15 PM.

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