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Plate Heat Exchanger Setup

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by jebatty, Jan 2, 2008.

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  1. jebatty

    jebatty Minister of Fire

    Joined:
    Jan 1, 2008
    Messages:
    4,432
    Loc:
    Northern MN
    I placed a post seeking assistance on high flue temp on a Tarm in which I indicated that I was using a plate HX, and I have received several replies asking for more info on the HX and water tank heat storage setup. Here goes.

    Issue – Tank storage: cost is a big issue here, especially for an inexperienced home experimenter like me, and also because I did not know how well it actually would work with my new Tarm Solo Plus 40. I wanted about 1000 gallons and searched (and continue to search) for a tank of that size at a price I am willing to pay. I have settled for now on three 275 gallon used fuel oil tanks, which I cleaned out as best I could. I got all three of these for $125 so the price was right for a starter on tank storage. For the future I am leaning towards a buried, insulated, concrete septic tank of about 1200 gallon size. If I go this route, I would include some sort of a mixing setup, like is used in a milk tank, or a recirculating pump. This would largely eliminate the stratification issue and simplify the heat draw down plumbing.

    The venting and plumbing turned out to be a bit perplexing, but what I have now works, and works exceptionally well. This is a vented tank setup, non-pressure. In essence the tanks are plumbed in series, but I won’t go into the details on this because most of the details relate to using three tanks and would not be relevant to use of a single tank.

    Issue – Stratification: this clearly is an issue but is not significant based on my application. Hot water enters at the top and cold water is drawn off the bottom. I use my steel storage tanks as large radiators, as the whole installation including the boiler is in my shop. I just keep the water hot enough to provide a comfortable temperature, hotter when it is colder outside (a couple of weeks ago it was -26F), and not so hot when it is warmer. If the shop get too warm, I just open a window. I also partially insulated the tanks to reduce the heat radiation. I have a thermometer mounted in one of the storage tanks, and so far I have brought the tanks up to a maximum of 155F. In fact, depending on outside temperature, I only need to fire the boiler not more than 8 hours in a day and as little as once every three days. Heat radiation from the heat storage is better than I anticipated (and doesn’t use any electricity). As mentioned above, stratification is not an issue with my application. It is obvious that interesting stratification is taking place in my storage tanks, and is not simply hot on top and cold on bottom.

    Issue – Dip tubes: if the stratification issued is resolved, I think this could be dealt with reasonably by drawing near the top and returning to the bottom. With a recirculation pump, I would use that pump to feed the input side of a separate plate heat exchanger, and then run the heating circuit off the output side of the plate exchanger.

    Issue – Pump placement on non-pressure tank side: this caused some initial difficulties caused by air in the plumbing. This was recurring because I have a high temperature filter installed on the supply side to trap any crud before it reaches the plate HX. When I replace the filter, some air always gets into the plumbing. I now have the pump mounted below the water line on the tank and mounted vertically so that it pumps water up into the output side of the plate HX. Any air in the plumbing moves through the pump quickly, and I have a vent installed at the top of this run to expel any air that gets into the plumbing. With this setup I don’t think cavitation remains an issue and air locks no longer occur.

    Issue – Cast iron or bronze pump. I didn’t give this much thought, mostly because I got new Taco 007 pumps for $20 each on Ebay.

    Issue – Plate heat exchanger. The Tarm Solo Plus 40 is rated at 140,000 BTU. I first tried a plate HX which was too small, and without much calculation have now replaced that with a 30 plate, 5" x 12" HX with 1" in/out ports. This probably is a little large for the boiler, as it will draw all the heat the boiler can deliver. In other words, the burn will continue non-stop until it burns out delivering maximum heat output to the input side of the HX. I think this setup is the most efficient for burns, as restarting burns is not efficient, and heat storage makes this possible. I paid just over $200 for the HX.

    I largely followed Tarm’s schematics on this installation, including the Thermovar valve. The boiler is filled with a 50% antifreeze mix to prevent any possible freeze-up of the boiler. The boiler loop, powered by its own circulating pump, simply feeds the input side of the HX as the heat load. I have a surface mounted thermostat on the boiler return line which turns on both the circulating pump for the boiler and the separate circulating pump for the HX output side when the boiler return reaches 150F (shuts off at 140F). Thus, the boiler is fired, by gravity water flow the boiler return pipe gradually heats to 150F, the pumps turn on, and heating continues until the fire burns down, the pumps then shut off. The input and output sides of the HX are plumbed counter-flow, as recommended.

    I am very impressed by the HX setup. I installed meat thermometers on the in and out sides of both sides of the HX to get a feel for performance. Real world example: boiler hot water in to HX at 165F, water return to boiler at 120F; cold water in on output side of HX at 90F, hot water out on output side of HX at 160-163F. Input pump is Taco 009 on 1" copper lines; output pump is Taco 007 on 3/4" copper/high temperature hose lines. Output volume (through filter) is about 6 gpm.

    More questions? Feel free to ask.

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  2. tuolumne

    tuolumne New Member

    Joined:
    Mar 6, 2007
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    Now living in VT, and building new home in VT
    What are the specs of the heat exchanger? I plan to use a small one to heat my domestic water off the storage, and if I am unable to track down an old propane tank for pressurized storage, I'll need a larger HX to heat my storage as well.
  3. jebatty

    jebatty Minister of Fire

    Joined:
    Jan 1, 2008
    Messages:
    4,432
    Loc:
    Northern MN
    Take a look at http://www.aicheatexchangers.com/l-line_brazed_plate.html for info which will get you close. The HX I now have is LB31-30. The small one I started with is not AIC brand but is close to the LA14-20 in specs, which appears to have optimal rating of 70,000 btu. In fact, I will be listing my small one on Ebay soon. I had it in use less than 3 months. It's performance with the same setup as the LB31-30 was approx as follows: input hot water at 165F; output cold water in 100F, output hot water out 140F.
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