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Venting for metal melting furnace

egansenegansen Member Posts: 23
I have been working on installation plans for my Johnson Furnace metal melting furnace that I posted about awhile ago. The previous ower used garden hose to supply natural gas to it.

What I am struggling with is how to vent it. I'm not worried about the heat it creates I'm more concerned with the fumes generated during the melting process. The manufacturer's recommendation is to have 200 cfm of airflow at the base of the vent hood and that it should be 1' wider than the furnace on each side. What I am trying to figure out is what type of fan to use to vent the combustion gasses. I am planning on using Class A flue to go from the hood through the roof mostly because I am not sure how to determine the temperature of the gasses to vented. I belong to a couple of foundry groups on facebook and they didn't have many suggestions because they move their furnaces outside when they use them. Since this is a commercial model it really needs to be permanently installed and it's too heavy to move around.

I realize that this out of the ordinary but suggestions would be welcomed.



  • ratioratio Member Posts: 2,329
    There are flue draft inducers available, Tjernlund comes to mind. The one I installed modulated to control the static pressure in the flue, interlocked both the flue fan & a make up air damper to the unit both ways (started automatically, failure to fan/open shut down the unit), & were basically pretty neat to see operating. Probably a little more than you want to spend, but it's a starting place.
  • egansenegansen Member Posts: 23
    Thanks for the sugestion on Tjernlund. I will check that out. I was able to find some additional information on the attached image. Hopefully I will be able to find a fan that will be able to withstand the temps involved.
  • ratioratio Member Posts: 2,329
    According to that, you need an exhaust fan that'll move 1500 CFM at whatever static your duct ends up at. That 1500 CFM is composed of 115 CFM of flue gasses at ≈2000° and (1500-115=) 1385 CFM of ambient (guess at 70°) air. According to the mixed air formula here, your actual air temperature (assuming adequate mixing) will be (2000*(115/1500))+(70*(1385/1500)) ≈218°—a lot easier to manage, that!
  • egansenegansen Member Posts: 23

    That is exactly what I was looking for and gives me a better idea of what I am dealing with. Thermodynamics was a long time ago.

    Thanks again!!
  • egansenegansen Member Posts: 23
    Here is the rest of the installation details for what I am planning. Since I am using an exhaust fan am I correct in thinking that the not having the roof with in 10' horizontally of a chimney does not apply?
  • ratioratio Member Posts: 2,329
    I couldn't guess about the codes. Maybe @Jamie Hall could offer an opinion?
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