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pressurized vs nonpressurized storage

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by ugenetoo, Apr 5, 2008.

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  1. ugenetoo

    ugenetoo New Member

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    im putting together (in my mind at the moment)a heat storage system for my older tarm solo boiler.
    what are the pros and cons of a pressurized versus ambient air pressure tank in the 1000 gal range?

    steve

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  2. guy01

    guy01 Member

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    I am learning myself so I could be wrong but I think the advantage of pressure is the ability to run hotter water. the disavantage would be the potential to go boom
  3. Gooserider

    Gooserider Mod Emeritus

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    Pressured tanks can be integrated into a pressured system without needing extra pumps, heat exchangers, etc, they can sort of be thought of as just another zone. Tank design is a bit fussier - a home-brew tank is harder, which is why a lot of the pressured tanks you see people talk about are made from used propane tanks. They also require extra plumbing for expansion tanks, pressure relief (so they DON'T go boom...) etc.

    Unpressured tanks are easier for a home built approach, and work well for those with systems that are otherwise unpressurized, however they will require additional heat exchangers, circulator pumps, etc. to interface with a pressured system. There are more concerns about potential corrosion issues on other system components.

    There are multiple trade-offs no matter which direction you go, so there really isn't a "right" answer for most setups. If I had an otherwise unpressured setup I'd use unpressured storage, otherwise it's more a case of pick which set of pros and cons you prefer, as a good case can be made for and against both.

    Gooserider
  4. Nofossil

    Nofossil Moderator Emeritus

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    To amplify Goose's comments, pressurized saves you an in-tank heat exchanger - hundreds of dollars at today's copper prices, or it saves you an external flat plate hx and pump combination. I suspect it also allows more perfect stratification and absolutely eliminates the temperature differential between the loop temperature and the tank temperature.

    If my open storage ever fails, I'll likely go pressurized next time - two 500 gallon propane tanks would be my choice based on what I know now.

    Unpressurized makes it much easier if you have multiple isolated systems, as in my current situation. My solar panels are isolated from my boiler, and both are isolated from my domestic hot water. All theree exchange heat via the storage. Harder to do with pressurized tanks, but I think I'll do some clever stuff with large thermosiphoning sidearm heat exchangers.
  5. ugenetoo

    ugenetoo New Member

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    my idea is to install a tarm boiler in a 42 foot storage trailer along with an insulated 1000 gal propane tank for storage. this setup would be moved from my shop 1/3 mile away to the home in the fall with a winters worth of wood loaded into it. at the home, i would have buried insulated pex that would be hooked up to the trailer boiler outside and to my existing 100k oil boiler inside.
    my thought was to use a plate exchanger to interface between the two boilers and run an open system outside since i will be draining the wood boiler and storage tank each spring before moving it back to the shop.
    i prolly eventually will build a detached boiler house but for now, i think this might work.
    any additional ideas or drawbacks that anyone can see?
  6. Bob Rohr

    Bob Rohr Minister of Fire

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    heat on wheels sounds like a fun project. Throw some solar panels on the roof and aim the rig it solar south for some additional, mostly free, energy gain.

    hr
  7. Nofossil

    Nofossil Moderator Emeritus

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    Cool idea! Maybe you could set up a traveling heat delivery business, sort of like the old iceman. Get a bunch of clients with the right hookups and onsite storage, and charge them up every couple of days....

    I don't know how much of a difference you'd see in corrosion running an open system in the trailer - I think it would be worse than a closed system that's drained once per year. I think I'd also worry about corrosion during the summer when it's drained but presumably pretty wet, with plenty of air. Gasification boiler is an expensive investment, Rust happens.
  8. free75degrees

    free75degrees New Member

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  9. Gooserider

    Gooserider Mod Emeritus

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    Good start, but there were some things I edited - IMHO current version isn't perfect, but it's improved...

    One point that I fixed was semantics - we generally refer to appliances that heat water for house heating as "boilers" - "furnace" is more commonly taken to mean a hot air based unit, i.e. HVAC systems.

    Gooserider
  10. free75degrees

    free75degrees New Member

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    cool, that's the whole idea of a wiki - anybody can edit to improve the content. I think the wiki can be a great tool for referring people to questions that get asked a million times.
  11. jebatty

    jebatty Minister of Fire

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    I'm just finishing conversion from 700 gal open to 1000 al (LP) pressurized storage. I already had my Tarm isolated from storage with a plate hx because of the open storage and because I use antifreeze in the Tarm (just to make sure no freeze up during a long winter vacation; -30F is not unusual where I live). So no disadvantage to the change-over by way of need for hx and pump.

    The only disadvantage so far is deciding to go with a bladder expansion tank. It is a bit costly.

    I'm questioning the BOOM issue. The Tarm itself is controlled to prevent pressure in excess of 30 psi; also shuts down to prevent overheat; max heat output to tank is 190; the LP tank is rated in excess of 200 psi; other plumbing should be able to handle at least 50 psi, probably much more. I will soon know, but with the expansion tank (and maybe even a little dead space in the tank if needed) and isolation by the hx from the boiler, I'm failing to see how excess pressure can build in the tank beyond the system capability.

    Two very large advantages so far. First, 1000 gal of storage is much better than 700. The Tarm handles 1000 gal effortlessly. Second, corrosion should now be about none. It's taken me awhile to balance the PH, water was a bit acidic at the start, should be in the 8-9 range now or soon. With the PH balanced and O2 scavenger, I already have noticed no visible rust/discoloration in the tank water, which was there initially. Any available O2 should be eliminated, and no new O2 should be entering the system. The open system had continual access to air, and I did not do the best job of balancing PH -- got lots of rust from the open steel tank.

    My vote is for pressurized.
  12. jebatty

    jebatty Minister of Fire

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    One more comment -- the LP tank inside, and water, stinks! Mine is located in a shop/storage building, so not an issue. Through the setup, and few water spills, the stink also in the building.

    How does one (in the future) eliminate the smell from a used LP tank? If I had to do this over again, I would give more attention to this issue.
  13. MarcM

    MarcM New Member

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    The stink comes from the mercaptan (actually ethyl mercaptan) add to the LP... I think the best way to neutralize the smell is to change it chemically... i.e. oxidize it with household dilute sodium hypochlorite (bleach) or something similar (hydrogen peroxide would probably work).
  14. Bob Rohr

    Bob Rohr Minister of Fire

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    It doesn't go away without a fight, that smell :0

    I tried Tide, Pinesol, bleach... I finally just put it away from the shop and let it air out for a few months before I brought it in. Never did get the smell out of some of my clothes.

    I wonder that a chemical engineer or guru would know how to counteract that odor. It's in ther for a good reason.

    If they could only build trucks with that staying power.

    hr
  15. MarcM

    MarcM New Member

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    The bleach might not have been concentrated enough, or you might not have used enough to react with all the mercaptan. Remember, it doesn't smell in the exhaust of propane after it's burned because it's oxidized and becomes a different compound. It would be impractical and dangerous to burn the inside of an empty pressure vessel and would probably comprimise it's structural integrity anyway.

    In any case, hydrogen peroxide is a stronger oxidizer than sodium hypochlorite but is also a fair bit more expensive.

    The bottom line is, that smell is not going away without changing it chemically, as it takes so little for humans to detect its presence (about 3 ppb).
  16. ugenetoo

    ugenetoo New Member

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    ive used a number of these tanks for different projects and about the only way to get rid of the smell is to air them out. it will go away eventually. i also have a couple of these available if any one is interested and is willing to travel to get one. im in patten which is 90 miles north of bangor. 1000GAL/$500
  17. Gooserider

    Gooserider Mod Emeritus

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    The "Boom" issue applies to pressured systems in general, whether it be water heaters or boilers, etc. If you don't provide proper pressure relief, the water in the system will expand, and SOMETHING in the system will let go, possibly / probably explosively. It might not be the storage tank, probably won't be, but SOMETHING will go boom - That is what is being referred to, not the idea that the tank would go. OTOH, with a non-pressure system the worst case failure modes can be messy when the tank boils over, but there is no explosion hazard. In a mixed system, the non-pressure part won't go boom, but the pressure part could if not designed and built properly.

    As long as proper pressure relief and expansion is provided for, it isn't a big deal, I would say the boom is a relatively low likelyhood problem. The issue that I would say is most likely to cause a boom is if the system builder fails to appreciate the amount that large volumes of water can expand when heated - I could see a person that wasn't used to this sort of hardware making an initial effort and doing something like not providing a big enough expansion tank, etc.

    Gooserider
  18. jebatty

    jebatty Minister of Fire

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    Goose - full agreement with your point. A system plump full of cold water with no expansion space for the heated water is a disaster waiting to happen. In my comment I assumed adequate expansion to handle up to the 190 or so max temp of the water in the tank. A pressure relief valve is essential.
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