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Heat Exchanger Project

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by Eric Johnson, Nov 2, 2007.

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  1. Eric Johnson

    Eric Johnson Mod Emeritus

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    I went out and bought a drill press this evening along with some silver solder, 200 feet of type M 1/2-inch copper pipe and a fancy hole driller for my header-style heat exchanger project. I got the 1" pex-al-pex to replace 100 feet of my 1" copper (the headers), so I'm about ready to get started. The drill press works well. Since I have about 128 holes to drill, I figured this was a good time to buy something that I always wanted anyway. I also got a new blade for my tube cutter and a stone for reaming the pipe.

    I agonize about this stuff, but do enjoy planning and doing it.

    One concern I've had is that I'll get the hx built and installed in the tank, then fill the tank up and find out that the damn thing doesn't work for one reason or another. What if it leaks? I can't really pressure test it before putting it in the tank, because it has to go in in sections and be assembled in the tank. One of those things. Or, what if my design sucks and only part of the hx heats up, essentially crippling it? What do I do then?

    Well, it occurred to me that I can build the thing, put it in the tank and get everything all hooked up, and then--ta da!--run it for a couple of days in the empty tank to check everything out. Any leaks will be immediately evident, and I can make sure that the heat is being distributed evenly. If not, you guys can help me figure out why not, and we can redesign it.

    So that's where I'm at with that. Pics to follow.

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  2. Nofossil

    Nofossil Moderator Emeritus

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    I'm eager to see the results.

    I built mine in the tank, then soldered a cap on the outlet and soldered a pressure testing rig (gauge, air line quick-connect, and shutoff valve) to the inlet. Pressurize to 120 psi. Leave for a few hours, come back and see if gauge has moved. Soapy water finds the smallest leak instantly. The valve is to vent the air when you're done.

    Word of caution: If you do this and you have the HX pressurized, DO NOT take a torch and unsolder the cap. I did this and launched one into the next county with a bang that sounded like the end of the world. HX doesn't leak, though.....
  3. Gooserider

    Gooserider Mod Emeritus

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    You could make a temporary pressure testing setup - 2 high pressure rubber hoses w/ plug in the end of one and a pressure setup in the end of the other, and PT each section of manifold as you get it built. If you do it that way the only joints you'd have to worry about in the tank are the ones where you join the sections together.

    Gooserider
  4. Eric Johnson

    Eric Johnson Mod Emeritus

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    I've posted a couple of pics in another, similarly-titled thread.

    My plan is to do the final assembly in the tank, then hook it up to the system and run hot water through it with the tank empty for a day or two to see what happens. I'm not worried about the no-fitting soldered connections. They are easily inspected upon assembly and I'm confident they won't leak (I'd put money on it, which is rare for me). I do have the usual concern about the integrity of the fitting joints, as they have been known to start leaking after a few heating and cooling cycles. So, if after a couple of days I don't have any water on the tank floor, I think we'll be good to go.

    Here's the pix again. I should have it finished this weekend.

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  5. Eric Johnson

    Eric Johnson Mod Emeritus

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    I'm a little more than curious. The original cistern was square and held about 3,000 gallons. It was made of reinforced, poured concrete. Three walls remain intact. I built the fourth wall from cinderblocks. That's the one I'm concerned about. I have two strategies for addressing that concern. First, I put a couple coats of that fiber-enhanced surface bonding cement on the block wall at the suggestion of slowzuki, who checked it out when he was in the area picking up his Jet Stream boiler. Secondly, after I get the hx into the tank and tested, I'm going to put the weight of the house onto the wall, by jacking it up slightly and building a wooden framework to transfer the weight to the top of the wall.

    If necessary, I can shore the wall up with wooden supports, since the foundation wall is only about 6 feet away. I think the first two approaches will do the trick. We're talking about 9,000 pounds of lateral force at the top of the tank when filled. Hopefully if the wall starts to move out, the surface bonding cement will crack, telling me it's time for the extra support.

    I don't have an air compressor, but I do have a rig for pressuring up cast iron radiators with a bicycle pump. How much pressure should I pump in there? I know compressed air can be dangerous; I don't want to blow it up.
  6. Eric Johnson

    Eric Johnson Mod Emeritus

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  7. Gooserider

    Gooserider Mod Emeritus

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    I would probably go for the lowest of "100% of the rated pressure" or at least 2-3X the "working pressure" A while back I was playing with the idea of making a liquid cooled PC, time and finances have put that project on hold, but I got most of the cooling system parts made. I pressure tested the rad (repurposed automotive heater core) at 25 PSI, as that is all those are supposed to be rated for. I tested my other cooler parts (machined from copper billet, and / or soldered) at 100 PSI because they "Should" be able to take that much, and that is all my compressor will do... Working pressure on the system would probably be on the order of about 3-5 PSI - the maximum head pressure the pump I was using could push is about 10, and obviously I'd be doing much less than that. :coolsmile:

    I'm not sure what your test value should be, but that is how I figured it on my setup.

    Another note, is that one way to GREATLY reduce the hazards of dealing with compressed air in pressure testing is to fill the device under test with water or other liquid, then just pressurize the tiny area at the pressure connector. This puts the exact same amount of pressure on the DUT, but if something lets go, your worst case is a flood followed by an air leak of whatever volume your supply hose can carry... This is the basic way they test high pressure vessels like SCUBA tanks - A catastrophic failure of an air filled tank will easily level a building, but a hydrostatic test setup's worst case failure (@ 5/3 working pressure, or 5,000 PSI for an aluminum tank) only results in blowing out a rubber diaphram and spewing ~20 gallons of water out the safety vent line. Messy but no big deal...

    Gooserider
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