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  1. #1
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    Pilot operated, thermal expansion valve?

    Hey, wondering if someone can help explain how this valve works or point me in the right direction.

    So under my McQuay chiller is the main thermal expansion valve (TXV) which meters the flow of refrigerant between the condenser and the evaporator.
    There is also a refrigerant line running from the bottom of the condensers, through a small TXV and then into the base of the main TXV.

    The small TXV is a typical a/c type with a capillary tube and bulb measuring the superheat off the evaporator.
    But how does this small TXV control the flow in the main valve? I've added a photo from the manufacturer spares manual to help show this.
    Attached Images Attached Images

  2. #2
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    dunham bush used this setup for years , I cant give you much technical details now, but essentially they`re designed to work together , pilot txv does the the sensing opening and closing main txv , which in turn is main feed to chiller stan
    Keep it simple to keep it cool!

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  4. #3
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    Similar in action to a pilot operated on/off solenoid. TEV modulates the opening/operating pressure under the big ones diaphragm.

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  6. #4
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    Quote Originally Posted by stanbyyourword View Post
    dunham bush used this setup for years , I cant give you much technical details now, but essentially they`re designed to work together , pilot txv does the the sensing opening and closing main txv , which in turn is main feed to chiller stan
    Yes, very similar to a pilot operated pressure reducing valve in plumbing. Too hard to get a valve that size to operate correctly directly. The smaller valve does the sensing and adjusting. Only difference is that it's hooked to a larger valve to get more work done.

    Think of a car. In the Flintstone age, it was difficult to start and stop by foot. Modern gas cars use a gas engine. Easy to operate a gas pedal and easy to modulate speed with it. More power (more work) and better control.

    Stop, step back, relax and have another go at it.
    Last edited by BennyD; 03-20-2017 at 06:58 PM. Reason: Typo
    -----Stop, step back, relax and have another go at it.-----

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  8. #5
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    very similar also of pneumatic valves with pilot control
    Keep it simple to keep it cool!

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  10. #6
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    Quote Originally Posted by stanbyyourword View Post
    very similar also of pneumatic valves with pilot control
    Pneumatics... Now that's becoming a lost art! We only have a couple guys left that are good at it, and I am NOT one of them! Started too late in the game. Not to mention so many buildings have converted.

    Stop, step back, relax and have another go at it.
    -----Stop, step back, relax and have another go at it.-----

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  12. #7
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    Jun 2008
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    The pilot TX valve operates like any TX Valve except it runs at very low superheat. The main valve consists of a spring loaded valve that is operated by a piston with a bleed hole in it. If the pilot valve determines more refrigerant is needed in the evaporator it feeds refrigerant to the main valve piston which pushes the valve off its seat further, when the pilot valve reduces flow, refrigerant bleeds through the bleed hole in the main valve piston and the spring pushes the valve closed.

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  14. #8
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    Thread Starter
    Quote Originally Posted by nyrb View Post
    The pilot TX valve operates like any TX Valve except it runs at very low superheat. The main valve consists of a spring loaded valve that is operated by a piston with a bleed hole in it. If the pilot valve determines more refrigerant is needed in the evaporator it feeds refrigerant to the main valve piston which pushes the valve off its seat further, when the pilot valve reduces flow, refrigerant bleeds through the bleed hole in the main valve piston and the spring pushes the valve closed.
    Perfect, thank you Nyrb. I must have spent weeks searching through books, google and Youtube trying to find an explanation. Even the manufacturer ignored my e-mail. Then I came across this forum and within a few hours there's an answer.
    Thank you all for your insight also.

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