Homemade spiral heat exchanger?

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by jimnorth, Jan 28, 2008.

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  1. free75degrees

    free75degrees
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    Does anybody think that the amount of coil that STSS recommends is overkill? 540' of 3/4" copper gives about 15,000 square inches of copper surface area. In another post somebody said that about 4000 square inches was needed for an eko 40. I hope it is overkill because I am planning on using 5600 square inches (3 section of coil in parallel, each one is 100 feet of 1/2" copper).

    Assuming the STSS numbers are overkill then the cost comparison is closer. I am already committed to copper since I have already bought and coiled a bunch of it, but if I was starting over, I might go with a flat plate. I will have spent about $500 for all of my copper, including fittings, so I am about twice the cost of flat plate.
     
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  2. bbb123

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    I have the Tarm, STSS setup your saying 540'. It's 360' for the HX for boiler and 180' for domestic hot water. Mine handles any heat the boiler makes with no idle. You should check NoFossils site I think he says how much he put in and he gets some idling. I wouldn't short the heat exchange one reason for storage is to keep boiler from idling.
     
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  3. free75degrees

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    I see, so I should compare to the 360' feet from stss rather than the 540'. 360' gives about 10,000 sq inches.

    I checked nofossil's site, and it looks like there is roughly 100 feet total of 3/4" copper for the boiler, which comes out to about 2,800 sq inches.

    Nofossil - how much idling do you get? I looked at your example chart and I didn't see anything that looked like idling, in fact it looks like the opposite - i.e. you tank circulators shut of periodically because the boiler water is getting too cool, which implies that the HX is taking away more than enough heat. I am reading this correctly?
     
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  4. Adios Pantalones

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    I made a heat exchanger (smaller scale) to be used as a "wort chiller" for beer making. For a 22" spiral, coul around anything round and roughly 22". You can also get a spring-looking doohickey that slides over the pipe and prevents kinking.

    More efficiency, but way more back ressure, would be gained by introducing right angles in the pipe which would create turbulence inside the pipe and bring hotter (or colder, depending on which exchanger) water to the pipe surface.
     
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  5. chuck172

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    I'm correct in saying STSS 's coils are 3-180' coils, that is 540'
    bb123, How do the coils sit on the floor of the tank? Is there any special support STSS uses?
     
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  6. Adios Pantalones

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    I looked at their site recently, and they claim that the coils sit unsupported. They are 22" wide and 43" tall. I would guess that they are more than rigid enough that the connectors hold them in place.
     
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  7. bbb123

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    I don't remember anything on floor but it has been awhile. There are 3 coils, as far as HX for excess boiler heat there are 2 coils 180' x 3/4". The third coil in mine is for my domestic hot water. There is room for a 4th coil which I wish I put in for solar. I had a (hot) water maker on its own zone from my oil boiler but Tarm recommend removing that and getting domestic from the main tank (3rd coil). Back awhile ago we were doin efficiantcy tests and Nofossil obviously has precise monitoring equipment. His boiler was idling at a lower temp. than mine was. I have mine set up to heat tank up to 180 deg mine (depending on stage of burn) will not idle till it gets up around 170 deg. Course when I put mine in i think the coils were under $400 each. I understand the push for flatplate HX with current copper prices. I just looked at Nofo's tank he has 50' for his domestic, 70' for solar, and his BOILER HX is 50' x3/4 plus 40"x60" square section. The square section appears to be 3/4 or 1" with 1/2" paralel going between them. Plus remember if you do figure out how many square inches he has he only has a eko 25 not a 40.
     
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  8. bbb123

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    They are rigid they have 3 or 4 pieces of 1/2 copper up middle for support so the don't slinky on you.
     
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  9. free75degrees

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    Yes, mine (with a Tarm 40) needs to be about 65% larger than if I were using an eko 25 (based on relative btu output for eko 25 and tarm 40). So 2800 sq inches for an eko 85 would convert to about 4600 sq inches for a tarm 40 or eko 40.

    If nofossil gets some idling around 170* then I'll want to add some more oomph than 4600 sq in would give. I only have baseboard heat with 800 gallons of storage so 170* would only give me about 30* of range. I'd like to get that a bit higher if I can.

    Using 1/2 tube I would need 244 feet to get 4600 sq in. So my plan is to use about 300 feet, which is 23% more nofossil's system scaled to a Tarm 40. I know this is probably an over simplification, but to do it right I'd probably have to solve some unsolvable differential equations. If 300 feet is too small I guess I can add another coil later.
     
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  10. Nofossil

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    I wish I had more coil surface area - I'm actually contemplating forming up some fins from copper flashing and clipping them onto the coils. It works great until the tank top starts getting near 170. Then the coils can't transfer as much heat as the EKO generates, and I go into temporary idling.

    When withdrawing heat from the tank in the summer, I have to restrict the flow rate to get out water that's hot enough to be useful.

    Bottom line - more surface are is better.
     
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  11. jebatty

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    Although I use a plate hx to isolate my Tarm from the pressurized storage, with pressurized storage you can eliminate the coils for tank heating and heat the tank directly from the boiler. With the cost of coils compared to the cost of picking up a used LP tank, it seems that more attention to pressurized storage might be worthwhile. A hx would be needed for DHW, and a plate hx here would seem to be a very economical and efficient way to go due to the difficulty of putting any coil in a LP tank.

    Space issues come up with large LP storage compared to a STSS or home-made open storage tank.
     
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  12. Ron Lloyd

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    That’s the route I’m going only I intend to put a copper coil inside the propane tank for DHW. I ordered a 12” flange and gasket no more than two hours ago from here.

    http://www.hydro-eng.com/flanges_gaskets.htm

    I will have a short section of 12” pipe welded on top of the propane tank and the above flange will be welded to the pipe. I intend to make my own lid (blank flange) from ½” thick steel. I’m going to bore 2 holes in the lid (in addition to the bolt holes) slightly larger than the outside diameter of a ½”x 4” bronze nipple. Then I’ll have those two nipples brazed in those holes. The first fittings on the outside of the lid will be 1/2” unions (for assembly and disassembly purposes) and then I'll transition to PEX for both inlet and outlet. The idea is for the DHW coil to just “hang” from the lid but if it looks like it needs some additional support I will add it. The guy I’m buying the tank from will take care of all the welding and pressure testing.

    Ron
     
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  13. Adios Pantalones

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    Is a pressurized tank needed for direct tank heating because the outflow is forced by tank pressure? Could it be done with 2 pumps instead (I know- more power etc- just thinking is all)
     
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  14. Ron Lloyd

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    I’m not sure I understand your question either. From my understanding this type of system is pressurized because there is a fill valve that keeps the water pressure in the boiler and therefore in the tank relatively constant. There is only one circulator needed to move the heat from the boiler to the tank. Then you can either use one more circulator and a series of zone valves or a series of separate circulators for each zone to move the heat from the tank to the load.

    Ron
     
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  15. Adios Pantalones

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    Ah- got it- it's open circuit to the boiler (derrr- I should have realized that)
     
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  16. jebatty

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    I am confused by the question and the answers. A pressurized system does not force water by means of the pressure through the system. The whole system is of equal pressure. It is pressurized to remove all air and seal the water in the system. The pump is a circulator, meaning it is neither pushing nor pulling water through the system. It is not creating pressure, it simply moves water through the system by creating a pressure differential.

    An open system is open to the air and is constantly "boiling" or evaporating water out of the system, thus the need to add water, exposure to air, and thus also corrosion issues. These are avoided or greatly alleviated in a closed, pressurized system. There are other factors involved in a pressurized system.

    Suffice it to say that a single system cannot be both open (non-pressurized) and closed (pressurized) unless they are separated. Else pressure would alway flow to point of less pressure until pressures were equal, and a pressurized system would lose its entire pressure to the unpressurized, open system.
    .
    I don't know if this helps anything.
     
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