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  1. #1
    Join Date
    Mar 2005
    Location
    N.J.
    Posts
    149

    expansion tank pressure

    We have a school with a chilled water system. The system is 300 tons of d.x. and quite large in reference to the piping layout and length of run.We are gong to replace the verticle expansion tank due to a ruptured bladder. My question is ,what is the formula for figuring out what to charge the new tank to . I'm told it will come pre charged to 12 psig,but I dont think that will be enough pressure to satisfy our verticle lift . Many thanks in advance.

  2. #2
    Join Date
    Mar 2007
    Posts
    78
    Adjust the air charge to the system fill pressure.

  3. #3
    Join Date
    May 2004
    Location
    Illinois
    Posts
    2,843
    Quote Originally Posted by richper
    I dont think that will be enough pressure to satisfy our verticle lift.
    Ok, how tall is your system? You need to know that before you can figure your fill or tank pressure.

    Quote Originally Posted by richper
    quite large in reference to the piping layout and length of run.
    Short-loop syndrome? This is a completely different problem, but you probably already knew that..
    "If I had my life to live over again, I'd be a KA." - Albert Einstein

    It's later than you think.

  4. #4
    Join Date
    Jun 2005
    Location
    The Gray Northwest
    Posts
    661
    How can you have a chill water system with 300 tons DX?
    You need to know your system volume, delta T of your loop, how many feet of pump head you have and so on.
    Check this out: http://www.taco-hvac.com/en/library.html

  5. #5
    Join Date
    Mar 2007
    Posts
    22
    Quote Originally Posted by maggio View Post
    Adjust the air charge to the system fill pressure.
    This is the correct answer. Make sure this is done with no pressure on the water side.

    Take empty tank.
    Fill air side equal to you system fill pressure.
    Install tank.

  6. #6
    Join Date
    Jun 2005
    Location
    The Gray Northwest
    Posts
    661
    Quote Originally Posted by RLJ79 View Post
    This is the correct answer. Make sure this is done with no pressure on the water side.

    Take empty tank.
    Fill air side equal to you system fill pressure.
    Install tank.
    System static fill pressure, plus system head, plus 3 psig.

  7. #7
    Join Date
    Mar 2007
    Posts
    22
    Where are you getting this info? I am looking at an install manual right this minute.

  8. #8
    Join Date
    Sep 2002
    Location
    Hampton Roads, Virginia
    Posts
    1,600
    As long as the expansion tank is piped very close to the system fill pressure regulating valve, then precharge to system fill pressure before connecting the water side. If expansion remote from fill, then other factors need to taken into consideration.
    "Profit is not the legitimate purpose of business. The legitimate purpose of business is to provide a product or service that people need and do it so well that it's profitable."

    James Rouse

  9. #9
    Join Date
    Mar 2007
    Posts
    22

  10. #10
    Join Date
    Jun 2005
    Location
    The Gray Northwest
    Posts
    661
    Quote Originally Posted by RLJ79 View Post
    Where are you getting this info? I am looking at an install manual right this minute.
    Just finished a new tank installation. You have to know system volume so you can determine the tanks acceptance volume if it's a new tank. All tanks come with a 12 psi pre-charge which works for most single story applications. If you're pumping to a rooftop or in a multi strory building you also have to take into account the head pressure on the system. What is the highest point of your system? If it's 30 ft the formula would be: 30 x 0.5 + 3 psi residual = 18 psi air charge. The residual is added to compensate for the head pressure so you can eliminate the air in the system. If you undercharge the tank, you'll reduce the acceptance capacity.

  11. #11
    Join Date
    Sep 2002
    Location
    Hampton Roads, Virginia
    Posts
    1,600
    Yes thats how to calculate the required fill pressure, the system volume is indeed needed to correctly size the expansion tank.




    Quote Originally Posted by big johnson View Post
    Just finished a new tank installation. You have to know system volume so you can determine the tanks acceptance volume if it's a new tank. All tanks come with a 12 psi pre-charge which works for most single story applications. If you're pumping to a rooftop or in a multi strory building you also have to take into account the head pressure on the system. What is the highest point of your system? If it's 30 ft the formula would be: 30 x 0.5 + 3 psi residual = 18 psi air charge. The residual is added to compensate for the head pressure so you can eliminate the air in the system. If you undercharge the tank, you'll reduce the acceptance capacity.
    "Profit is not the legitimate purpose of business. The legitimate purpose of business is to provide a product or service that people need and do it so well that it's profitable."

    James Rouse

  12. #12
    Join Date
    Mar 2007
    Posts
    22
    System volume is one of the factors needed to size an expansion tank. Also, the system fill pressure should be already determined by you building height...correct? Otherwise the water pressure would be insufficient to make it to the top of the building.

  13. #13
    Join Date
    May 2004
    Location
    Illinois
    Posts
    2,843
    Quote Originally Posted by RLJ79
    Take empty tank.
    Fill air side equal to you system fill pressure.
    Install tank.
    So how do you know the fill pressure is right?
    "If I had my life to live over again, I'd be a KA." - Albert Einstein

    It's later than you think.

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