Burner heat output - 1/2" versus 3/4" - I know what, but not why...

[jcornell]

I've been playing with burner and forge designs.  Forge #1 was a brick pile with a 3/4" Z-burner from Larry Zoeller.   Subsequently I started playing with small "bean can" forges, using a 3# coffee can for the forge body.  Attached picture is experimental forge #4.  It's got 1 inch of superwool HT, treated with colloidal silica as a rigidizer.  After the rigidizer was dry and cooked, I put on an experimental refractory coating, a mixture of 4 parts zircopax and one part kaolin.  Instead of using water to make a slurry, I used the colloidal silica rigidizer. It worked out fairly well on the rigidized superwool.  It's tough, it's very reflective.

 

Experimental forges 2, 3 and 4 (all bean can designs) all used a 1/2" scaled down version of the Z-burner, which I call the Wye Burner.  I like the Wye burner, but the 1/2" burner wasn't putting out as much heat as I'd like.  So I built a 3/4" burner loosely based on the micro-mongo design.  Attached picture shows the 1/2" Wye Burner and the 3/4" micro-mongo burner.  Both burners are using a Tweco 14T-23 (.023) mig tip.  Using the same regulator setting, same burner tip as the 1/2" burner,  the 3/4" burner puts out a LOT more heat than the slightly smaller burner.  

 

I don't understand why I'm getting so much more heat.  The regulator is set at the same pressure, the mig tip is the same diameter, but the 3/4" burner beats the 1/2" burner in heat output, hands down.  With the regulator and gas input being the same, why is there such a difference in heat output?

 

Can you wise blacksmiths explain this one to me?  Please?

[timgunn1962]

One of the difficulties when burners, forges, etc are under discussion is terminology. "Heat" and "Temperature" seem to be used interchangeably by many people and they are not the same thing at all.

 

There are really two things that affect the temperature achieved by a given gas forge/burner combination.

 

The first, and the one that most people "get", is the amount of gas being fed.

 

The second, and the one that some people seem to get but others don't, is the amount of air being fed along with the gas.

 

The most important thing is really the ratio of gas to air. This ratio is what governs the flame temperature.

 

Whatever burner is in use, the maximum flame temperature occurs at the "stoichiometric" mixture. This is where all of the Oxygen in the air, burns with all of the fuel gas, resulting in no remaining unburnt gas and no unburnt Oxygen. For Propane burning with air, the temperature of a stoichiometric flame is 1980 degC (3596 degF), assuming the gas and air are at 20 degC (68 degF) before they are burnt. This temperature is also called the "adiabatic" flame temperature.

Air contains about 79% Nitrogen. Although the Nitrogen doesn't play any appreciable part in the combustion, it is along for the ride. Because it is mixed in with the other stuff, it ends up at the same temperature as the Carbon Dioxide and Water vapour produced by the combustion process. The heat absorbed in raising the Nitrogen to 1980 degC (3596 degF) is quite significant. If there was no Nitrogen, that energy would be available to raise the temperature of the Carbon Dioxide and Water vapour instead. The adiabatic flame temperature for Propane burning with pure Oxygen (i.e no Nitrogen present) is considerably higher at 2820 degC (5108 degF).

Going back to burning with air, if we adjust the fuel/air mixture so that it is either side of the stoichiometric ratio, the flame temperature will reduce.

If we add extra air (a lean burn), there's no extra gas for the extra Oxygen to burn with, but the extra Oxygen and Nitrogen will absorb some of the energy produced, reducing the flame temperature

If we add extra gas (a rich burn) instead, there's no extra Oxygen to burn it with. The unburnt gas will absorb some of the energy produced and this will reduce the flame temperature.

If reaching a specific temperature is the only concern, it's best to go lean; gas is expensive and air is free.

Unfortunately, "we" usually need to use a rich flame as the free Oxygen in a lean flame causes scaling and Carbon reduction problems in "our" application.

The big difficulty for many is likely to be judging the mixture. If you can measure the temperature, increasing the amount of air slightly will show which side of stoichiometric you are on; the temperature will rise if the mixture is rich, fall if it's lean. Most folk don't have the kit to measure temperatures in the range needed for forging. Very few indeed will have the kit to measure welding temperatures. Almost none will have the kit to measure stoichiometric flame temperature.

 

In my, admittedly limited, experience, many (perhaps even most) Naturally-Aspirated gas forges will actually get hotter with a smaller gas jet, against the expectations of their builders.

 

I suspect you'll struggle to find a smaller jet that will directly swap with the .023 MIG tip, which is as small as MIG wire usually gets, but to get the 1/2" burner working well, it's what you need to do.
 

[knots]

Here is a link to useful information for orifice size selection.

 

http://www.joppaglass.com/burner/highp_chart.html

 

Burner size ( BTU capacity) needs to be matched to the forge chamber volume.  As it turns out I have a 1/2" burner that that I built in the early 90's that has an orifice drilled using a No 72 drill for a small chambered gas forge.  The burner works well as it turns out my drilled orifice is also very close to a .023 Mig tip .

 

My recollection is that the Rule Of Thumb for sizing the BTU requirements for a forge furnace is somewhere around 100,000 BTUH/ Cu Ft .   Perhaps some one here can confirm that ROT or provide a means of calculating the BTUH requirements for a well insulated forge with proper end closures.

[Frosty]

Fuel air ratio is indeed the main determining factor for burner output, all other things being equal. The thing that makes it so I can't offer an explanation or even WAG as to what's causing the differences between your burners is they're two different designs. If we were talking about two zoeler burners or two Porters or two "T"s I could give a decent account. As it is now you're comparing a Chevy and a dune buggy.

 

Generally speaking a burner's output is determined by the tube dia. a 3/4" tube has 2x the area of a 1/2" tube so puts out twice the BTUs.

 

Frosty The Lucky.

[knots]

Having had a closer look at your 1/2" burner, it appears that you have used a 3/4" wye fitting and have  used a pipe reducer to make the 1/2" pipe fit into the 3/4" wye.  My experience is that such a connection would create a very rough interior transition and therefore a lot of turbulence in the area of the burner where you most need a clean transition to promote induction of the combustion air.  Any turbulence in that area is going to reduce the velocity of the propane stream and therefore reduce the induction of combustion air.  

 

When using pipe fitting in burners I always chuck them up in my lathe and turn the interior of the assembly to clean up the transition.  Bottom line is that it doesn't have to be pretty on the outside but on the inside it matters.

 

Also you might play around with the orifice extension to position it further down the burner tube.  Seems to me that if you can see the orifice through the air port that you may be pushing the combustion air into the burner tube rather than inducing it to enter.

[basher]

Tim Gunn knows what he is talking about.

 my guess would be that one of your burners is behaving more optimally at that gas pressure for that orifice size .

 Ii the poor burning burner is burning lean or rich , you could tweak it by either choking the air to make it richer or by putting a smaller tip in to increase the gas speed and therefore the ventury action.

 Back pressure also plays a big part in how well a burner works for a given forge