Furnace gas temperature


I built a new furnace out of ceramic fiber and a reflective ceramic coating material to make the 11 inch diameter furnace more efficient. The fuel is propane from a 20 gal. RV tank entering into the furnace through a 1.5 inch diameter pipe. I started out using a hairdryer to push oxygen into the furnace. The furnace is unable to provide a good melt to a 6" crucible filled with bronze. The color of the crucible is red/yellow, not yellow/white like a high temp burn should be. I switched to a leaf blower motor and had the same problem. Supposedly propane and air burns at 4,000 F+ which should be more than adequate. What's going wrong? I'm thinking of adding O from a welding cylinder.

johndach's picture

new furnace - Low temp


What size hole do you have in the gas input nozzle? If not large enough, there will not be enough gas to get the temps needed to melt the bronze no matter the pressure (I use about a 1/16 inch hole for my big furnace, you may get along well with 1/32 inch). Also what is the psi of the fuel entering the nozzle (I use a variable pressure regulator and have it cranked up to about 30 psi for my firing)? Also, there is a GREAT likely hood that the tank is going to freeze up and thus the pressure is going to go way down when you get things going, so you are going to need a larger tank or a number of smaller ones. Also, how are you getting the gas/fuel into the burner tube (where is the entry point for the fuel) and how are you getting the fuel and air mixed (is there enough pipe from fuel entry to the end of the burner)? Is the burner tube long enough to get a bit of mixing (see last line)? You should be able to easily melt steel with propane/air so most likely you are not getting enough fuel as a hair drier will give enough air to melt the furnace if you have enough fuel to go along with it...... Send me an e mail and I will send you a picture of the burner I use to give you an idea as to a way to get fuel and air mix into the furnace. Also is your burner tube entering the furnace at a 90 deg (straight in) or entering at an angle? Is the mix entering at a clockwise or counter clockwise direction (should be counter clockwise in the Northern hemisphere). You are on the right track and I bet that there is just not enough gas/fuel getting into the furnace - it takes q good deal!!!! We would use over 100 gallons to melt 3 100 lb melts and heat the shells to go along with the metal melt.

I am happy to help more if needed. You are welcomed to call if you like (360.681.4240 PST).

John Dach
web site: http://www.MLCE.net

ed dravo's picture

new furnace

1. The pipe entering the burning tube which enters the furnace broaches at 30 degrees.. It has about a 2 inch mixing depth before entering the furnace at a counterclockwise direction. The fuel pipe length I can vary somewhat but I find changing the length doesn't vary the flame much

2. The tank valve (removed regulator) did freeze up fairly regularly at first but I was running the tank upside down, flooding the shutoff valve. Turning it over corrected the problem but I think you're correct in assuming there's just not enough pressure.

 3. The gas nozzle hole is 1/4 inch. I will check the pressure tomorrow-I think the pressure varies greatly from the time it starts to the pressure a few minutes in. Will check tomorrow.

Rich Waugh's picture

Ed,I think you have more


I think you have more than just one problem with your setup.

First, in order for propane to burn efficiently, it must be mixed with air in the proper 14:1 ratio. This generally takes a mixing tube of 9" long or longer to achieve. Most of the guys I know have the propane coming in to the system at the intake side of the blower so as to get better turbulence to mix the gas and air. (I wouldn't do that with a blow dryer however, due to the possibility of igniting the propane in the dryer's motor.) After the blower is the mixing tube and then the burn area, which is larger than the mixing tube to slow the gas/air velocity to maintain a stable flame front.

Your 1/4" gas feed should be adequate at a feed pressure of 1/2psi to 2 psi. If you're running straight tank pressure you could use an orifice more like .035". A blow dryer should definitely put out more than enough air, though a proper vane-type centrifugal pressure bower would be better. I suspect your fuel problem is starvation from freezing.

For a furnace of that size, you may need to get either a bigger cylinder or manifold two or three of the smaller ones together. You are most likely drawing off the propane gas faster than it can evaporate from the liquid, resulting in starvation of the burner. I use a 100# cylinder in a similar application with good success.

If you feel your gas cylinder getting cold, that indicates you're drawing off a lot of gas. Draw off enough of it fast enough and you can freeze up the valve.

You could learn a lot about burner and furnace design from a book by Michael Porter entitled "Gas Burners for Forges, Furnaces, and Kilns.".

ed dravo's picture

fuenace heat

1. I extended the gas inlet tube to 9 inches.

2. I added a larger outlet  "combuster.'

3.  I stuck with the hairdryer for air flow.

 The furnace ran a lot smoother and did not freeze up. I did not reach bronze melting temperature though. I will add a nipple for a 5 gallon tank.

johndach's picture

furnace problems

I think you still HAVE to increase the pressure of the fuel to get enough into the burner to get high enough BTU's to melt the bronze. I think your problem is you are trying to heat the oxidizing air, the furnace, the crucible, the furnace and makeup for heat loss and there is just not enough fuel input to get the job done. It should sound like a jet engine, noisy,,,, not quiet and muffled. Can you take a picture of the exhaust flame? The flame should be visible, even in good daylight, it should be showing/burning well above the top of the kiln, 1 - 2 feet above on my kilns when firing. Bronze is just not that difficult to melt.

John Dach
web site: http://www.MLCE.net

ed dravo's picture

Furnace temp


1. I added a manifold to mix gases from two bottles of propane before entering inlet pipe.

2. I replaced the old cement refractory lid with a ceramic fiber lid.

 When I lit the furnace the first thing I noticed is the furnace didn't chug until hot like it did without the manifold. The furnace heated up to yellow hot with just the 20 gallon tank, melting the bronze.

 I substituted a leaf blower air supply from the hair blower and the furnace turned white hot, steel melting temp.

 Thanks everyone for the help.

You can never rely on pictures alone to get a good furnace working.

Rich Waugh's picture

I'm glad you got it to work

I'm glad you got it to work right Ed, and I'm pleased to hear that our advice was helpful to you.

I look forward to seeing a picture or two of your furnace and what you're casting, too.