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  1. #1
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    cap tube as a bleed port for TXV?

    Hey guys. I'm an engineering student working on a small liquid chiller project. I'm trying to find a couple R-22 expansion valves in the 1/3 to 1/2 ton range and I need them to have a bleed port since my compressors are PSC type rotaries. I'm scrounging around ebay since I don't want to spend a fortune on retail priced valves and it has proved very difficult to find them with a bleed port (either that or nobody properly identifies the valve numbers). Is there any reason why I couldn't just braze in a length of small diameter cap tubing parallel with the valve to act as an external bleed port, choosing a small enough dia/long enough length so as to not significantly affect valve operation, but large enough to be able to recycle the compressor after a couple minutes? Seems to me like a simple way around the problem.

    Thanks in advance!

  2. #2
    Join Date
    Jun 2006
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    Why not just go with AXV's?

    Sent from my C771 using Tapatalk 2

  3. #3
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    Are you talking about a valve that would equalize pressure on off cycle ?

    That is usually done with a check valve that mounts at the TXV distributor and at the liquid inlet of the txv valve. Not on the valve itself.

    Some TXVs are externally equalized too.

  4. #4
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    Northern VA
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    I am trying to maximize flexibility for the system to accommodate as wide a range of loads as is reasonable (anywhere from liquid cooling gaming computer hardware, general space cooling, rapid temp pulldown of party drinks, to operating a small scale working ice rink model for a school project). Seems to me that a TXV would be the more flexible and safer solution for the compressors. I plan to use balanced port valves to further stabilize operation across as wide a range as possible.

    Just a little background on the project...the compressors are rated for 1300W and 1525W cooling at 7.2 deg C and they will run separate refrigerant circuits that share the same tube for the shell and tube evaporator and condenser. I have a programmable microcontroller to use as the control system to control the temp monitoring, cycling, staging, startup sequence, anti-recycle timers, etc., etc. The condenser is liquid cooled to provide flexibility in different cooling options, but it will normally implement a liquid to air radiator with a speed controlled DC fan using the liquid line temp as feedback to control head pressure over a wide range of ambient conditions.

    Quote Originally Posted by commtech77 View Post
    Are you talking about a valve that would equalize pressure on off cycle ?

    That is usually done with a check valve that mounts at the TXV distributor and at the liquid inlet of the txv valve. Not on the valve itself.

    Some TXVs are externally equalized too.
    Yes, exactly. From reading manufacturer's literature, the port seems to be non checked and (at least for parker/sporlan valves) is sized as a percentage of nominal valve capacity, all built into the valve body itself. It seems that I can replicate the same effect by mounting a length of cap tubing the same way as you explained for the check valve, since cap tube is cheap and easy to deal with.

    I'm not referring to internal versus external equalized valves, chosen depending on the pressure drop across the evaporator.

  5. #5
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    Quote Originally Posted by twophase View Post
    I am trying to maximize flexibility for the system to accommodate as wide a range of loads as is reasonable (anywhere from liquid cooling gaming computer hardware, general space cooling, rapid temp pulldown of party drinks, to operating a small scale working ice rink model for a school project). Seems to me that a TXV would be the more flexible and safer solution for the compressors. I plan to use balanced port valves to further stabilize operation across as wide a range as possible.

    Just a little background on the project...the compressors are rated for 1300W and 1525W cooling at 7.2 deg C and they will run separate refrigerant circuits that share the same tube for the shell and tube evaporator and condenser. I have a programmable microcontroller to use as the control system to control the temp monitoring, cycling, staging, startup sequence, anti-recycle timers, etc., etc. The condenser is liquid cooled to provide flexibility in different cooling options, but it will normally implement a liquid to air radiator with a speed controlled DC fan using the liquid line temp as feedback to control head pressure over a wide range of ambient conditions.



    Yes, exactly. From reading manufacturer's literature, the port seems to be non checked and (at least for parker/sporlan valves) is sized as a percentage of nominal valve capacity, all built into the valve body itself. It seems that I can replicate the same effect by mounting a length of cap tubing the same way as you explained for the check valve, since cap tube is cheap and easy to deal with.

    I'm not referring to internal versus external equalized valves, chosen depending on the pressure drop across the evaporator.

    Yes, a small check valve from the liquid line to the TXV distributor should do it.

    Be careful. putting it in backwards could be costly.

  6. #6
    Join Date
    Jun 2012
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    Northern NV
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    Bypass with cap tube not a good idea. Find a manufacturers chart and get one that bleeds internally. If you want to use a cap tube..... use a cap tube and not a TEV. Either that or TRY to figure out the combined capacity of the two. (you are an engineering student.) Supco has good literature on cap tube design and contrary to popular opinion cap tube is is a load sensitive device to some degree. Properly designed, it might be just what you need for the price of a bit of tubing and no TEV!

  7. #7
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    Chicago, IL
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    Just add a start kit to your compressor so it can start against pressure.

  8. #8
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    A rotary wouldn't require the high starting torque of a start capacitor/potential relay when used with a TXV...as I understand it...as they're similar to a scroll in that respect.

    If you go with a cap tube for a bleed, try Tecumseh's capillary tube selection program and select one for a percentage of the total capacity:

    http://boxload.tecumseh.com/SelectCapTube.aspx

  9. #9
    Join Date
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    Quote Originally Posted by icemeister View Post
    A rotary wouldn't require the high starting torque of a start capacitor/potential relay when used with a TXV...as I understand it...as they're similar to a scroll in that respect.

    If you go with a cap tube for a bleed, try Tecumseh's capillary tube selection program and select one for a percentage of the total capacity:

    http://boxload.tecumseh.com/SelectCapTube.aspx
    Thanks icemeister. I have ran some numbers in the Danfoss cap tube program and it looks like it actually won't be a very good solution due to the small cooling capacities. To get 10% capacity through the bypass requires at least several meters of the smallest listed size (0.026" IIRC), and I don't really want to go much higher than that. I was considering one of those hard start kits as an alternative solution, but if rotaries generally will start against backpressure, then that solves the problem easily. I'll probably get one of those Supco 3 in 1 kits for the 1500W compressor anyway, since I don't have the original overload clicker switch, but I may need to find one for it anyway since I don't know if the compressor has internal thermal overload protection. It says "thermally protected" on the label, but I don't know if that specifically refers to the external thermal/current overload it would normally have that is missing, or if there is additional internal protection. It's a Rechi precision compressor and the manufacturer's website is more or less useless.

  10. #10
    Join Date
    Feb 2008
    Location
    California
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    2,064
    why not pipe a Normally open solenoid valve around the TEV and wire it to cycle with the compressor?

    That won't mess with TEV operating capacity & ought to give ya some rapid pressure equalization on the off cycle.

  11. #11
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    Aug 2011
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    Northern VA
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    Quote Originally Posted by Phase Loss View Post
    why not pipe a Normally open solenoid valve around the TEV and wire it to cycle with the compressor?

    That won't mess with TEV operating capacity & ought to give ya some rapid pressure equalization on the off cycle.
    I thought about that, too. That's probably plan C.

  12. #12
    Join Date
    Mar 2009
    Posts
    49
    twophase, I have some thoughts on this. Tried to PM you but could not. PM me with contact info.

  13. #13
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    Trane uses a check valve that Ts off the liquid line and bypass's the TXV. Works just fine.

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