Problem with burner.

[scrap metal hoarder]

I built this forge about 3 months ago. I am having a problem with the flame "snuffing out" when the chamber gets hot. The forge is built out of a 12" pipe, lined with kao wool, and three fire bricks internally, one on the bottom, and one on each side. It runs good for about 10 to 15 minutes, but as soon as it gets a little hot, the flame snuffs out, and restarts itself from the heat in the chamber. The hotter it gets the worse the problem gets. I notice the internal burner is glowing red hot when the problem starts to happen. The internal burner is about 5" from the bottom fire brick. The burner is a 1-1/4" tee reduced to 1", and is inserted into a stationary 1-1/4" pipe that goes about 3" into the chamber. But like I said it burns great until it gets hot. Any help to this newbie is appreciated. Thanks

[Rich Hale]

Did you use proven plans for that design,? if so you need to read the plans again,,,if you did not yoiu  may wish to rethink and use a plan that someone else has had luck with.

[knots]

First - No part of the metal burner assembly should be extending into the furnace chamber.  The fiber surrounding the burner opening should be stabilized with a liner wash of your choice or other refractory material.

 

One test that you can perform to determine if to much air is being induced is to use a hand held putty knife to partially close the open Tee port as a means of temporary adjustment of combustion air volume.  I believe that you need a swing away flap to fine tune the air volume being induced into the burner.

 

Another thing to be aware of with burners in general is the potential for the combustion air volume to be to low which can cause the flame to burn back into the burner tube.  This must be avoided.  This is usually accompanied by a high kind of hollow sound that some times kind of whistles .

 

Getting the fuel air mixture right is very important.

[swedefiddle]

Knots is correct. Either you don't have enough air flow or your fuel is too low of pressure. The burner tip CAN NOT be inside  the hot part of the forge, the flame HAS TO BE only at the very tip of the burner. If the burner pipe is getting hot you don't have enough flow to push the mix to the entrance of the forge. Try aiming a hair dryer toward the air opening of the burner and see what happens. If you are not using an adjustable fuel regulator, get one.

 

The fuel nozzle tip MUST BE in the center of the burner tube. Slide the fuel tip in and out of the burner until you hear the correct location (you will hear the burner roar)

 

Neil

[Frosty]

It'll really help if you post pics of your burner in operation. First to see the flame itself, then when it starts giving problems.

 

However, just looking at the forge my guess is it's getting exhaust fumes from the forge and choking out. Once the forge gets hot the exhaust characteristics change though I don't know what the changes are, they're different.

 

If possible try mounting the burner so it's farther out of the exhaust plume, even if it needs a deflector or you have to induce a little draft to blow the exhaust away from the burner intake. Only a LITTLE draft, naturally aspirated burners are susceptible to breezes so make it a light one.

 

Okay, that's all guess work from non running pics, I could be so far off base I couldn't see the ball park.

 

Frosty The Lucky.

[scrap metal hoarder]

Thanks to all for the help. I will move the internal burner out of the chamber, and see what happens. also the combustion air may be to low, because I do have "burn back" once in a while as Mr. Knots suggested. I will see if I can somehow adjust the air flow and adjust the fuel nozzle tip. Thanks again to all.

[knots]

If air induction volume is a problem, perhaps is a smaller orfice run at a higher pressure might do the trick. Upping the velocity of the fuel gas might induce more combustion air flow .

However before we get to far ahead of ourselves - What is the chamber size of your forge, and we know your burner tube is 1", but we do not know the orfice size, or the pressure used at which the is fuel supplied ? The thing is that a gas of a certain BTU content per Cubic Foot will deliver a known number of BTU's through an orfice of a certain size, when delivered at a certain pressure. The BTU quantity can and should be matched to the forge chamber volume, and will require that a fairly precise volume of combustion air be provided. Not rocket science just a little math.

So far all we can do is guess at what the problem may be .

[Frosty]

You're right as far as it goes Knots but the burner is working till the forge gets hot, then it goes out. We're not talking about proper fuel air ratio producing X BTUs at X psi. for X temp per X volume, it's arithmetic and the burner won't perform well unless the math's right. You have that on the nose.

 

This is a bug causing it to shutdown, not a tuning issue. I could be wrong though and if it is a tuning issue I'll be sponging this for everything I can learn.

 

Stay with us on this Knots, we'll lick it.

 

Frosty The Lucky.

[knots]

OK here is another WAG.   What if the extended burner, after heating to red heat, preignites ther fuel air mixture back into the the burner tube repeatedly afer the forge reaches a certain temperature.  For lack of a better analogy call it a puff cannon.   The confined igmition could result in a pattern of  strong puffs  resulting in flame out.   After which reignition results from the glowing interior liner surfaces .    Puff - puff - puff- puff  -----.

 

If that is the case SCM's plan to remove the extension will resolve this.    

[timgunn1962]

When you get the problem, can you increase the gas pressure at all, or are you at the limit of your regulator?

The thing I'd suspect is that you are seeing the flame run back down the tube until it runs out of mixture. There will then be a brief pause as fresh mixture reaches the hot forge and relights. It will then happen again.

In your bottom pic, gas enters on the left, draws in air through the tee and mixes the gas with the air from there to the end of the tube.

To get a stable flame, the speed at which the flame-front moves left-to-right through the gas/air mixture needs to match the speed that the gas/air mixture moves right-to-left.

The flame speed varies with air:fuel ratio and with temperature: it moves faster if it is hotter. As a fairly general rule, you need to start most gas forges on low pressure and build up the pressure as the forge heats up. Starting too high will cause the flame to detach from the burner. Running too low when hot will cause the flame to burn back down the tube.

When the flame burns back, it radiates heat in front of it and generates a pressure wave, both of which increase the rate at which the flame moves. It will therefore accelerate down the tube until it runs out of mixture somewhere near the tee and goes out. Once unburnt mixture reaches the hot chamber, the cycle can start again.

Often, the frequency of the cycle starts quite low and increases, as the temperature of the burner tube increases due to having the flame run down it.

One of the things a flare does is give a variable area so the mixture slows as the area increases. This gives a fairly wide range of gas feed pressures over which the flamefront and mixture speeds can be equal somewhere within the flare and maintain a stable flame.

The flare does other "stuff" as well, but it gets a bit more difficult to explain.

 

 

 

 

 

 

[knots]

When you get the problem, can you increase the gas pressure at all, or are you at the limit of your regulator?

The thing I'd suspect is that you are seeing the flame run back down the tube until it runs out of mixture. There will then be a brief pause as fresh mixture reaches the hot forge and relights. It will then happen again.

In your bottom pic, gas enters on the left, draws in air through the tee and mixes the gas with the air from there to the end of the tube.

To get a stable flame, the speed at which the flame-front moves left-to-right through the gas/air mixture needs to match the speed that the gas/air mixture moves right-to-left.

The flame speed varies with air:fuel ratio and with temperature: it moves faster if it is hotter. As a fairly general rule, you need to start most gas forges on low pressure and build up the pressure as the forge heats up. Starting too high will cause the flame to detach from the burner. Running too low when hot will cause the flame to burn back down the tube.

When the flame burns back, it radiates heat in front of it and generates a pressure wave, both of which increase the rate at which the flame moves. It will therefore accelerate down the tube until it runs out of mixture somewhere near the tee and goes out. Once unburnt mixture reaches the hot chamber, the cycle can start again.

Often, the frequency of the cycle starts quite low and increases, as the temperature of the burner tube increases due to having the flame run down it.

One of the things a flare does is give a variable area so the mixture slows as the area increases. This gives a fairly wide range of gas feed pressures over which the flamefront and mixture speeds can be equal somewhere within the flare and maintain a stable flame.

The flare does other "stuff" as well, but it gets a bit more difficult to explain.

 

 

Thanks Tim for that explanation .    In this case the wild card seems to be the extension of the burner tube into the forge furnace chamber.   How would the introduction of this direct exposure of the burner end to high temperatures affect the burner function ?

[Frosty]

I missed or got distracted from the over heating burner tube. This is a problem and burners should not be inserted into the chamber at all. Even if it doesn't back fire, what Tim is describing the forge heat will erode the burner tube and as soon as it gets rough the flow will slow and efficiency will drop.

 

Properly tuned you can turn the psi up and down and the flame will remain stable. There is a limit though, as the curve isn't straight. The more primary psi the greater the entrainment so the flame will become leaner the higher the propane (primary) psi. Regardless, for the most part the range of this type burner is wide enough for our purposes.

 

If everything is well tuned and balanced just heating the output end of the tube won't have a lot of affect on performance. At first but as soon as you turn the burner down and the rate of propogation (flame front speed) approaches the velocity of the air fuel mix in the tube it'll start puffing as Tim describes. As soon as that happens what Tim describes is what happens and as the tube heats farther up it gets worse till it's stops working.

 

I prefer to extend my burner tubes only JUST into the refractory liner so it's not exposed to much heat and the fuel air flow cools it. I also dip my thread protector flare analogue in kaolin slip to shield them from the heat and oxy exposure.

 

Flares, or in commercial induction devices the entire tube, is tapered at a max of a 1:12 ratio which works out to 12% full length which increases the cross sectional area. This increases the volume filled by the same volume of mix. First the mix has to slow down so it acts like a flame holder, it's NOT but it acts like one. Second and more important for efficiency it increases the vaccum generated so it induces more air and becomes much more robust. It isn't as susceptible to outside breezes nor back pressure.

 

Frosty The Lucky.

[knots]

I guess my point was that since the portion of the air tube has direct exposure to the forge interior,  and therefore  is heated to a red heat. This means  that there is an ignition source INSIDE the the burner tube.  This ignition is contained by the walls of the burner tube and therfore the fuel air mix would  expand in two directions only.  This confined ignition would rush out of the end of the burner tube, and manifest itself as a PUFF that could blowout the fire, rather than expanding three dimensionally inside the forge compustion chambed.  Then the process would recycle.