Propane: Forced air is more efficient is my experience

Well, I tried out more burner designs: Fronty T, Reil for my furnace. They get hot, but when I use forced air (using an air mattress inflater) with propane, I use less gas while it gets hotter which allows me to reach 1400 C (cast iron) in the crucible.
When melting copper, it costs more gas when I use Reil or Frosty T rather than using forced air for the same amount of metal.

What are your experiences ?

 

You are absolutely on the right track! I started with a T-Rex burner and modded it to forced air with an inline duct fan then a bounce house blower which made a HUGE difference. Bounce house blowers can be overkill but hold up to the back pressure well and are controllable by air blockage. Search around and you'll see all the different kinds of blowers people use. Its all good! Not sure the volume of air a mattress inflater puts out. But you want to be able to control both fuel and air to make adjustments as the furnace heats up, watching the size and color of the flame shooting out the top. Lots of gas burning outside the furnace is wasted. Different blowers like to be controlled different ways but then you can get to iron melting temps in a small propane furnace. Though its a hairy ride!

 

If you use forced air, you're burning the propane at much faster rate than a naturally aspirated burner while everything else stays constant like heat losses, so you'll get to temp faster. Melting the metal at a faster rate means shorter run times that reduce energy loss and fuel burnt. I find heating the furnace mass is my biggest bottleneck at the moment: it takes 75 minutes to melt three crucibles full of aluminium: the first crucible takes 38 minutes, the second is ready at 22 minutes and the third is ready at 15 minutes.

 

Ok I'm extremely confused by that statement, what do you mean by atmosphere and how are those psi's being measured,
is this statement for a particular furnace/burner arrangement, because my understanding of Burner psi is that it is related to orifices' size

The general concept i get: more air allows burning more fuel with a neutral flame, but the specific numbers evade me

Thanks in advance

V/r HT1

P.S. probably one of those college edjumication things i don't understand

 

Not sure this will answer your question but David is referring to the normal pressure the atmosphere pushes at which is 14.7 psi and will only let you burn gas so fast from a given volume of air. Usually you will go by the pressure on your tank valve to see where things are at gaswise. My setup runs at 18 - 25 PSI depending. Your air supply will be by whatever arbitrary method you have of controlling it. As your hotface, or refractory, heats up you can burn gas even faster at a neural flame as you keep adjusting. Then as HT1 said, each successive melt goes faster because the hotface is like a heat sink storing energy. Sort of the thermal version of a flywheel.

 

The psi pressure is IMHO the pressure which comes out of the regulator, shown on its pressure gauge. And that is not affected by the amount of air blown over it, more are makes a leaner flame and less a richer flame. Usually adding less air (but not too little !) makes the temperature inside the chamber higher. Yet it is still more air than the same gas flow / psi with natural aspiration, which makes an ultra rich, sometimes even sooting flame in a burner designed for forced air.

 

this is where the stuff gets confusing, alot of this stuff depends on your rig,
generally if someone says PSI they are talking about how much propane is going into the burner, BUT that is not a direct correlation to the amount of propane going into everyones burner, because PSI to volume depends on the size of the orafice the propane is going through,

in venturi burners, PSI CAN directly affect the amount of Air mixing with the propane , But most burners use some sort of air choke so you can adjust the amount of air going into the furnace.
old school reil burners did not have this so they often where not efficient if not built perfectly and kept clean .

I suspect as Cactusdreams pointed out ,DavidF was just basically saying in an open atmosphere Propane will burn rich, so you have to add air to get a neutral flame ... he and Al203 have forgotten more about propane burners then I have ever known, when I was building food trucks the boss had a book on Burners Propane and NG that he would not let out of his sight for a second that was just filled with more info then you could ever want . but it was toned different , it would tell you haw to get a certain amount of BTU's per hour, pipe size, just crazy stuff, that we needed to plan out the propane appliances in a food truck, it topped out well below hat we are doing, but boy was it interesting

V/r HT1

 

Very interesting conversation gentlemen.

Like a lot of other apparatus we use, we often go well beyond the item's original intended use. I don't know much, but I know that both the Reil and Frosty burners were developed by very experienced forging guys with specific goals in mind. We're all pretty aware of what too much oxygen can do to bare metal, furnace interiors, etc., so those burners as-designed work great for their intended purpose. If we want to use them for our purposes, then we have to tweek'em.

Pete

 

the stoichiometric ratio (the perfect balance of air to Gas) for propane is 15.6:1 (by mass) That makes it about 4.02% . Propane is picky is doesn't burn if it's lower that 2.1% or Higher that 10.1%
As long as you keep these ratios the same the amount of propane brunt at a given time can be limitless. The more burning propane at one time the more heat output you will get. Since we're not looking to have an explosion there are limits to this. Adding forced air allows more propane to be added. I think Mark summed this up pretty good in post #4 in this thread.
I have a simple burner (I don't remember the name it's been years since I built it) It' an 1.5" pipe with a 1/4" pipe welded in perpendicular with a 1/8" hole in the middle of the main bore. I blow air into the main bore with a shopvac (now a bouncy house blower). propane is injected into the 1/4"pipe. I have a valve on the propane line and a air damper on the main bore to adjust the mix. I found that this setup is enough to glass the inside of my 5 gallon furnace (ie. it's over kill!) Hopefully it will be better suited for the Keg Furnace I'm hoping to built over winter.

Hopefully some of my ramblings here were helpful.
Bruce

 

That's more or less what we have at the foundry: A 2" air pipe with 1/2" copper pipe opening in the center of the airflow from a blower with an adjustable restrictor. The propane flow is set by a larger regulator sold for pottery kilns. So the setup just dumps an adjustable fuel gas flow into an adjustable airflow, both of which can flow at higher rates than is actually needed.

 

I've always used forced air propane. Used to catch grief for it on the old AA, too much hassle to run power out for the fan. I use a small squirrel cage blower for Aluminum and Bronze, I was told it wasn't big enough but I have to choke the air down. I want to try it for iron sometime. Just a little 12-volt computer cooling fan.
I use it on my forge as well, just tip my furnace on it's side and fire it up.

 

Do you mean that more forced air sucks more propane out of its tank, regardless of its valve / pressure regulator setting ?? In don' t think so .... Problably I interpret this statement wrongly.

 

I'm noticing a zone of the furnace where the oil spray hits is noticeably cooler than the rest of the interior: At the highest temps of over 1300 deg C, with plenty of time preheating the refractory and tuned for hottest temps, the refractory is glowing bright orange-yellow with a black zone around the fuel spray. Oil takes energy to vaporize and convert to flammable gas and high rates of flow would tend to cool things too much. Propane is already a gas ready to combust so it's much more tolerant of furnace design.

Super high oil flow rates tended to cool things right down and make lots of noise.

 

Peak temps are only achieved when every bit of fuel is consumed over a period of time. Any deviation either high flow rates or low flow (rich or lean) will always result in a lower temperature. As with everything in life, it too operates on the bell curve.

 

Propane tanks are under pressure so there's no sucking gas out. The valve controls the amount of gas that is allowed to release at a given time. The Pressure regulator sets the max pressure that the gas can exit the tank at. Because the propane is liquid until it expands. This process of sublimation means that the pressure inside the tank isn't constant. That's way you need the regulator. This process also means you have a limit on how quickly you pull gas from the propane tank. Expanding gas causes it to cool if you try to pull gas too quickly you will freeze your propane tank. You are only trying to burn the vapor. Folks get around this by putting the tank in hot water or switching to a larger tank. As tobbo rightly pointed out it's a balance that you create the more air the more gas you need to allow to escape (opening the valve).
You can blow out a forced air burner if you feed to little gas. With metal melting you want a neutral burn so just enough gas and air to burn. too much gas it can get absorbed by the molten metal, too much air you oxidize the metal quickly.
It's a big game of how hot can I get the system in what time. If you can get the system to a higher heat in shorter time burning more fuel per minute a system can be more efficient and use less fuel than a system that burns less fuel per minute but takes longer to get to temperature. Efficiency factors in the mass of the furnace, the number of heats you plan to do in a set time frame, the burn rate of the burner. etc.

A furnace that takes a whole day to heat up but runs for 3 days straight pouring 1oooKg of Aluminum might very well be much more efficient than my little 5 gallon bucket that can pour an A4 (4KG) of aluminum in 25 mins. It just depends on the time frame and what you want to do.

 

I started with a Reil. I then added a blower to get hotter temps and that is all I run now. After warm up, I run the propane regulator as high as it will go (30 PSI on mine). I then adjust the blower gate to the correct stoichiometric ration using a highly accurate optical device. By that, I mean watching the flame out of the furnace and adjusting until there is no orange flame above the lid. Depending on the metal, I adjust the ratio after the melt is becoming liquid. For Aluminum I make it oxidizing (lean) by adding air to minimize H2 absorption. I have been told that does not work and I have no way to verify this but it's easy to do. If it is red metal, I make it reducing (rich) by restricting air to prevent slag formation and zinc loss. That makes a noticeable difference. You can definitely see the melt surface is cleaner after you make the furnace reducing (rich.)

 

I don't remember the source and I can't find that thread. I still follow the practice anyway. With red metals the difference in atmosphere is dramatic.
Robert