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Need ideas for large buffer tank expansion vessel

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by termite, Oct 31, 2007.

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  1. termite

    termite New Member

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    New to the forum. I am installing a Biomax 60 and a 1200 gallon tank as a buffer. I have gained lots of knowledge by searching and lurking. I haven't been able to find a solution for expansion if I pressurize my 1200 gallon tank. I thought I might use a couple 80 gallon well pressure tanks but I don't think they can stand the temperature. To buy a large enough boiler expansion tank or combination of tanks would cost more than my budget allows. Can I leave the top of the tank empty and use that air space for expansion sort of like the older bladder-less air control expansion tanks? Total system volume would be 1100 gallons if I left 200 gallons out of the tank. If I could do this the obvious advantage would be not having to use a heat exchanger for the tank input and output. The top 1/3 of the buffer tank is above the rest of the system.

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  2. Eric Johnson

    Eric Johnson Mod Emeritus

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    Welcome to Hearth.net and our new Boiler Room, termite. You're our second BioMax owner that I'm aware of, and I'll be interested to see how it works for you. I gather that the BioMax and the EKO are similar (at least from the looks of them), except that the BioMax has that neat curved gasification chamber. Probably some other features that I'm unaware of.

    I'm no engineer, but I'd say offhand that trying to turn a 1,200-gallon tank into a pressure vessel is not something you can practically (or safely) do. Period.

    Take the Garn as a case in point: It has a 1,500-gallon tank surrounding the boiler and it's not pressurized. Why not? Well, if you look at the construction of even a modest, 50-gallon pressure vessel like the one surrounding my old boiler (photo attached), you can see that it's quite the piece of engineering and fabrication. And as I understand these things, containing the pressure gets a lot harder the bigger you go.

    I understand where you're coming from. Been there & done that.

    What you need, IMO, is a heat exchanger for a nonpressurized tank. What's your tank made of, by the way?

    EDIT: in rereading your post, it appears that you might actually have a 1,200-gallon pressure vessel. If that's the case, then I don't see why leaving an air cushion at the top wouldn't work. As you point out, that's how the old expansion tanks worked, and other than having to be drained periodically, they worked pretty well. But again, I'd defer to somebody who knows what they're talking about. Compressed air is a lot more dangerous than compressed water. How about posting a pic of your tank?

    I do know a guy on another website who uses a 500-gallon propane tank for a pressurized buffer. If you like, I can ask him what he uses for expansion.

    Attached Files:

  3. termite

    termite New Member

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    It's a 1200 gallon concrete lined boiler blowdown tank (uncle-in-law is in the scrap business). I don't know what the tank is rated but its thick and heavy ...very heavy... with convex ends. Has a 10" flange on one end and a multitude of places to connect nipples of various sizes. I'll attach a picture when I get home. If you could ask the gentleman with the propane tank how he addresses expansion I would appreciate it. Thanks for the reply.
  4. rreihart

    rreihart New Member

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    I see an expansion tank beside the 500 gallon propane tank in H-R's set up. It doesn't look that big, maybe 20 gallon or so. I don't know if there is another one some where else.
  5. Eric Johnson

    Eric Johnson Mod Emeritus

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    I emailed him with the details, so we'll see what (if) he says in reply. I'm guessing you could get by with an Extrol 60 or two. No big deal.
  6. Nofossil

    Nofossil Moderator Emeritus

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    Air in the top of the tank might cause a lot more corrosion in the system. All the oxygen will combine with iron somewhere to create rust.

    By my calculation, the water would expand by about 60 gallons between 4 and 100C (worst case range). The tank would only expand by about 6 gallons over the same temperature range, so you'd need to plan a good 60 additional gallons of expansion space.

    Can anyone else verify these numbers? Seems higher than my gut would have suggested, but I get 958 kg/cubic meter at 100C - works out to better than a 4% expansion.
  7. Eric Johnson

    Eric Johnson Mod Emeritus

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    My guy replied with the following:

    "I had my Bell & Gossett rep size mine. It takes into account volume, temperature
    delta T, fill pressure, and relief valve size.

    I'm not sure about the air space in the tank. I think you would need
    an airtrol fitting to make it work. And it may need to be above the
    boiler."

    He suggested posting it on this website where you might get a definitive answer from some really competent pros. And I second that suggestion:

    http://forums.invision.net/index.cfm?CFApp=2l.
  8. termite

    termite New Member

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    Here is the tank. I'll use an airtrol fitting on a separate tank mounted horizontally above the buffer tank. Now off to the scrap yard to find a 100 gallon tank. My wife is going to love that. I really appreciate all the feedback. At the risk of hi-jacking my own thread I attached a rough diagram (without the controls) showing what I have in mind for my system. Any questions or comments are welcome.

    Attached Files:

  9. Eric Johnson

    Eric Johnson Mod Emeritus

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    Nice boiler room, man. Wow, that's some tank.
  10. Nofossil

    Nofossil Moderator Emeritus

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    Pretty impressive, but too much is never enough....

    I won't hijack your thread any further than you already have, but I agree totally with your bidirectional strategy. Thermal stratification is your friend. I didn't have a four-way valve, so I accomplished the same thing with an extra circulator and a zone valve. My system uses a heat exchanger (actually three of them) and an unpressurized tank, but the principal is the same.
  11. termite

    termite New Member

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    Thanks nofossil. A brand new 96 gallon bladder expansion tank with an acceptance volume of 64 gallons appeared today on ebay solving my expansion tank problems. I'll put enough concrete in the tank to displace the volume required to get a 1+ safety factor (unless 180 degree chunks of concrete are a bad idea). Your site has been very inspirational as I install and think through controlling my system. I'll post a another diagram to a buffer plumbing thread when I figure out how to control this monster.
  12. Nofossil

    Nofossil Moderator Emeritus

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    I wouldn't be too paranoid. When I calculated the worst case volume, I was really paranoid. I used a 'fill' temperature of 4 degrees C, which is the absolute highest density that water can have. I then looked at the volume at 100 C, which is boiling. You will never (hopefully) see either temperature, so your volume change won't be as great.
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