How dumb is an indoor LPG furnace..?

Discussion in 'Furnaces and their construction' started by J.Vibert, Jan 7, 2018.

  1. master53yoda

    master53yoda Silver

    This chart is a portion of a flue sizing chart from The Natural gas association or GAMA. Bear in mind that the BTUH rating is based on 500 F exhaust temps, for our furnaces we would need to divide the btuh rating by 4 to deal with the 2000+ flue temps our furnaces put out. they are also taking into consideration the chimney effect of hot air rising, the height in ft is the distance to the top of the chimney. so a 10' rise 6" vent with out a fan straight up can handle a 235 KBTUH normal furnace but only a 58K foundry furnace with a fan it could handle 104 kbtuh foundry furnace. Keep in mind that the flue temp will be about 500 over the metal temp, so the 2000 that i am working with is alum. if you are working with brass/bronze you are working with a 2500 temp and iron 3000. so you would need to divide the 500 the the chart is based on into the exhaust temp for the metal you are working with to get the conversion factor.
    The full chart is at this site.
    https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwj_iaubn87YAhVFymMKHU1LA3gQFggpMAA&url=https://www.questargas.com/ForEmployees/qgcOperationsTraining/Misc/gama%20venting.pdf&usg=AOvVaw1Avu4Jht_oZ2d1F-KvYDDa

    Art B



    flue chart.jpg
     
    Last edited: Jan 10, 2018
  2. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Thanks Art. That's a handy reference and good points about the exhaust temps for higher melt point alloys. Does the ceiling immediately above the holding furnace in your garage get (too?) hot?

    Since J said small aluminum melts was all he was considering I figured <100kbtu/hr and that jives with where I was coming from in post 26 when I said 6" duct might get him home. But like I said in post 40, a hood seems much more versatile than direct venting.

    Best,
    Kelly
     
  3. J.Vibert

    J.Vibert Silver

    Ya, if I go through with this at all it will be for only small (A10) aluminium melts.

    I don't do much of anything outside of that scope anyway. ...and truth be told, once I get a covered area somewhere out on my lot hopefully next summer, I'll keep the casting out there. The hood vented area will end up being for painting/welding.
     
  4. master53yoda

    master53yoda Silver

    the ceiling above my furnaces see about a 30 f rise over ambient temp some form of a power ventilated exhaust hood is probably the best method for our use. I am running a 2500 cfm exhaust with the shop garage door open anytime i am firing tilting furnace, the furnace is also setting right at the door.
     
  5. Peedee

    Peedee Silver

    Wargrade is still on the go, maybe he'll chime in on his extractor soon, I've pointed him this way.
     
  6. Robert

    Robert Silver

    My powered system uses a whole house blower from a heating system and a 10" vent so it obviously draws in a lot of ambient air in addition to combustion air. I was shocked to find that the exhaust pipe temperature right over the furnace is barely above ambient. I have a thermometer on it but it really doesn't move at all. Next time I run it I will turn off the fan and see how the temp rises just for fun.
    Robert
     
  7. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I'll bet that puppy can suck the doors shut. Excepting now, I suspect you enjoy a relatively moderate climate. I'm envious of that, particularly at this time of year. Here, when it's 0F outside and you invite 2000+cfm of air through your 4500ft3 shop, the inside temp quickly approaches the outside temp.

    Best,
    Kelly
     
  8. Robert

    Robert Silver

    It sure moves a lot of air! These blowers are easily obtained for free whenever someone replaces a furnace. I think I have 2 more! I just paid for a little sheet metal and duct work.
    Robert
     
  9. Petee716

    Petee716 Gold Banner Member

    I think I have 3 of them stashed in my barn. Some things you just can't let go by!

    Pete
     
  10. master53yoda

    master53yoda Silver

    must home heating furnaces are in the 75 to 120 kbtuh range so those fans will vent the heat off of our furnaces quit nicely

    just an idea about how much air they will put our
    HP , ducted cfm, open air cfm
    1/4 , 600 , 1100
    1/3 , 1000 , 1400
    1/2 , 1200 , 1600
    3/4 , 1600 , 2200
    1 , 2000 , 2500

    Art b
     
    Last edited: Jan 17, 2018
  11. Al Puddle

    Al Puddle Silver

    During yesterday's casting session I took some CO measurements. My shop is about 2400 cubic feet, has a 2000 cfm exhaust fan, and I'm burning propane.
    The CO levels went from 0 to >140 ppm (and climbing fast!) in seconds with no ventilation. I opened a 32" door and switched on the fan and the CO leveled out at 17 ppm. Then I put my ingot tray on top of the furnace to heat up. CO levels went up to 48 ppm. I opened up another door for more make up air and the levels stabilized at 12 ppm. When I turned off the burner, the levels went to 0 ppm in a few seconds.

    The video didn't work out, so no pictures. But, it did happen, honest.
     
  12. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Al P, What are you using for a CO sensor?

    Best,
    Kelly
     
  13. Al Puddle

    Al Puddle Silver

    I'm using a PYLE PCMM05. It's a hand-held meter that alarms at >30 ppm (most annoying). It didn't occur to me to use the max level feature, doah.
     
  14. master53yoda

    master53yoda Silver

    When checking CO, are you measuring the flame gases or the surrounding room. The surrounding room would be considered to the side and about 2 ft away above the top of the furnace.

    CO is formed anytime the flame touches something colder then 1100F or if there is insufficient air for complete combustion,

    It forms when a burner first fires and until the furnace/ flame temp gets above 1100f, it will also form when an item is placed into the flame that is colder then 1100 F.

    if the flame is leaving the furnace the furnace size is either to small for the amount of fuel burned or there is insufficient combustion air. if the flame leaves the furnace yellow it is insufficient air if the flame is blue or infrared it is a larger flame then furnace size, when melting iron or higher temp metals the furnace size must be larger to accommodate the higher fuel flows required for higher flame temps.

    Art B
     
  15. Al Puddle

    Al Puddle Silver

    The CO measurements were taken 4ft from the furnace at chest high level. There are no visible flames out of the furnace and the interior is basically blue flame.

    I was assuming the CO levels went up when placing the ingot mold atop the furnace due to a change in exhaust trajectory.
     
    Last edited: Jan 19, 2018
  16. Robert

    Robert Silver

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