burner air requirements

[Dodge]

My apologies if we have covered this B4, but I can

[MattBower]

I've never actually asked, but I've always thought that people tend to use bigger pipe with blown burners because it reduces back pressure. Many blowers (especially common squirrel cage blowers) don't handle back pressure so well, and the ones that do tend to be more expensive.

Probably the reason you don't see a lot of specific plans for these burners is that they're usually so simple. Look at one and you pretty much know how to build it.

[Dodge]

Hmmmm There's one item checked of my compressed R&D list. Longevity of air compressor vs. blower I haven't bought any parts yet but I'm brainstorming a design that starts with a 1 1/2" Tee for the air/fuel "mixifier" and then reduce to a 1" pipe to "nozzelate" it into the forge. (Also R&Ding some new nomenclature) LOL

[ThomasPowers]

Big long to-do about using compressed air to blow a forge here just recently. In general unless you have a source of compressed air that must be there anyway and has excess capacity it's not such a good idea.

For a forge you need low pressure high volume; a compressor makes high pressure low volume. Why put more wear on a piece of equipment that is expensive to buy and maintain (and NOISY!) rather than spending a couple of bucks on a small fan?

Workshops that have constant air with excess capacity did sometimes use compressed air but they would use a system to entrain outside air with the blast---sort of like how an aspirated propane burner works to get more air at a lower pressure. This has been done for over 100 years so it's not new. You can tell how popular it is compared to using a small 150 cfm blower though by how much info is easily available on it...

[Dodge]

Big long to-do about using compressed air to blow a forge here just recently. In general unless you have a source of compressed air that must be there anyway and has excess capacity it's not such a good idea.

For a forge you need low pressure high volume; a compressor makes high pressure low volume. Why put more wear on a piece of equipment that is expensive to buy and maintain (and NOISY!) rather than spending a couple of bucks on a small fan?

Workshops that have constant air with excess capacity did sometimes use compressed air but they would use a system to entrain outside air with the blast---sort of like how an aspirated propane burner works to get more air at a lower pressure. This has been done for over 100 years so it's not new. You can tell how popular it is compared to using a small 150 cfm blower though by how much info is easily available on it...

Thanks for the info and history, Thomas. IIRC, the recent thread about compressed air to blow a forge was about a coal forge wasn't it? Same reasoning for air requirments granted. Thats why I would for sure incorporate a valve to reduce the amount of air. Like I say, I'm still brainstorming. I do, however, as a matter of practice, tend to turn on my compressor as the first thing I do when I work in my shop as I use a lot of air tools. I'm used to that beast cycling frequently, so that wouldn't be a bother. Just looking for alternatives and darn it, while trying hard not to re-invent the wheel, I just gotta be unconventional LOL

[courtiron]

A couple of thoughts for your blown burner project, air propane ratio is about 17:1 and about 10:1 for natural gas.
The larger diameter blower wheels generally develop higher the static pressure.
The higher the static pressure the less effect the opening and closing of the furnace door has on fuel mixture. A blown burner is influenced less by opening
and closing of the door than an atmospheric burner is but it is still affected.
To approximate the burner BTU input you can use the CFM capacity of the blower. If your blower pushes 30 CFM that's 1800 CFH which requires 180 CFH of natural gas. For propane that would be about 72 CFH. That is about 180000 BTU. Remember that gas pressure is pretty independent of office size down stream of the orifice, i.e. less pressure and large hole = higher pressure and smaller hole. You just have to have more gas pressure than air pressure to prevent the combustion air pressure from affecting the gas pressure regulator. Not to likely but possible, and more likely with a natural gas system than propane cause the propane pressure is generally higher anyway.
One problem that often occurs with a blown system is mixture stratification in systems that inject the gas downstream of the blower. I had 'dark' spots inside the furnace and gas burning outside of the furnace when it hit the atmosphere. I inserted two small 'windmills', one ahead of the gas inlet and one downstream of the gas inlet along the pipe that fed the air and gas into the forge body. The first windmill (it is a 1/2" fender washer welded to the 1/4" gas inlet pipe slit radially and the resulting 'petals' were then twisted slightly to make sort of a propeller). The other one is twisted the opposite way. The gas inlet pipe is cross drilled thru the 1/4" pipe between the windmills and the end is plugged. This stirs the gas mixture and results in more complete combustion inside the forge. This entire assembly was inserted in the run of a 2" tee that has the air injected into the branch and a 2" pipe out of the other end leading to the forge. You can avoid this altogether by injecting the gas into the blower it self, but I have never been as comfortable with that setup although a some manufactures use that method. Sure gives great mixing though...
Keep it hot.
Paul

[HWooldridge]

I use a 60cfm squirrel cage with a two burner - each tube is 3/4" black pipe. Box size is 15"Lx12"Wx4"T and it will just barely reach welding temp and it is really more useful for forging because welding is a little hit and miss. I feed this with about 5lbs of propane at the regulator and set the gas flow with a needle valve. The fuel dump is just downstream from the blower so the mix goes about 18 linear inches before entering the forge. I have hot spots right under the burners but because it will not consistently reach welding temps, I think the air flow and fuel mix is just a bit too small for the box size so it could stand to have a third burner and maybe 90 cfm.

Forgot to add that this forge consumes a 100lb (25 gal) tank of propane in about 50 hrs of runtime.

Edited March 18, 2009 by HWooldridge
Added content

[ChrisB]

Dodge,

Take a look at Jymm Hoffmans burner. It's about the simplest out there and doesnt require an expensive regulator or needle valve.

[Dodge]

Courtiron, bet ya cant say that three times real fast

HW, That leads me back to my question about bigger tubes for blown ?!?

Chris, Jymm PM'd me and hes gonna send me some more info In the mean time, where can I see his burner?

Edited March 18, 2009 by Dodge

[HWooldridge]

HW, That leads me back to my question about bigger tubes for blown ?!?

Dodge,

You may be correct as I was guessing when I built it. I do not have a cold spot in the forge under each burner but I just don't know enough about it to make conclusions without a bunch of experimentation. I converted an old naturally aspirated Sandia style forge and used the same size pipes that were there from the first iteration. I assume that for a given pressure and CFM, a larger pipe would make for less pressure and a shorter flame length.

[ChrisB]

http://www.iforgeiron.com/forum/f7/need-help-you-7371/index2.html#post69840

[ChrisB]

Dodge my above post will how the Hoffman burner system. That title carried over from the older thread.

[CRAFTBENDER]

Dodge,

Take a look at Jymm Hoffmans burner. It's about the simplest out there and doesnt require an expensive regulator or needle valve.

I saw his setup last Monday while visiting his shop. Developed by trial and error. If I remember right it only uses five psi and it do get hot quick.

[Dodge]

HW, I have read so much material over that last few days that my brain hurts with information overload. LOL But I seem to remember reading somewhere that a shorter flame isn't necessarily a bad thing, depending on the size of your forge. It means that all of the gas is being burned inside the forge. If its too long, you may actually not be burning all the fuel inside the forge. Can any body confirm or deny this and maybe expand on it a little??:)

[HWooldridge]

HW, I have read so much material over that last few days that my brain hurts with information overload. LOL But I seem to remember reading somewhere that a shorter flame isn't necessarily a bad thing, depending on the size of your forge. It means that all of the gas is being burned inside the forge. If its too long, you may actually not be burning all the fuel inside the forge. Can any body confirm or deny this and maybe expand on it a little??

Well, I can tell you that I can closely control the flame and minimize "dragon's breath" out of the front door with the needle valve - or I can dump enough gas to have a 10" long tongue of flame. However, I do believe my blower is too small because I can run it wide open with no choke on the air intake and easily overwhelm the air supply with fuel. In other words, very rich to very lean is less than 1/4 turn of the needle valve. I think I should be forced to restrict the air and not run the blower at full blast - i.e. have a little extra oomph on the throttle.

[Jymm Hoffman]

Barbecue regulator set at house hold pressure is 11 inches of water column, that is about 1/2 pound pressure.

Pretty much what I was going to send you is in the link that Chris posted.

One of the photos shows a needle valve, it was used for fine adjustment. It has been removed because it would change on me and I had to constantly adjust it. Went back to just using the ball valve for adjustments in the gas.

Edited March 19, 2009 by Jymm Hoffman

[TASMITH]

Scott;

Just another small conundrum for you on the blower for the forge. If you can, get a blower with 'paddles' as apposed to the squirrel cage type of fan. The paddle type will produce more air pressure per volume than the squirrel cage type will and will withstand more back pressure. A good gate type valve can be installed in the air output pipe to control the amount of air delivered to the burners.

Terry

[HWooldridge]

Barbecue regulator set at house hold pressure is 11 inches of water column, that is about 1/2 pound pressure.

Pretty much what I was going to send you is in the link that Chris posted.

One of the photos shows a needle valve, it was used for fine adjustment. It has been removed because it would change on me and I had to constantly adjust it. Went back to just using the ball valve for adjustments in the gas.

My forge works better if I run a bit more pressure at the gauge and fine tune with the needle valve. I get an inconsistent flame if the pressure drops below 3 lbs so I usually run around 6-7 lbs.

Burner design eventually boils down to sufficient fuel volume and proper mixing but I personally believe that good refractive insulation is one of the most important characteristics of a hot gas forge - so you don't lose the heat once it's created.

[Jymm Hoffman]

HW, I don't think you have enough pressure from your blower, maybe the burner tip configuration being different as well. To get enough pressure, most blowers need to run at 3,450 rpms or more. Volume alone is not enough as well as pressure alone, you need both, but can easliy over do it with some blowers on the market.

Tasmith, You are absolutely correct. The problem I have is finding such a new blower at a reasonably low price. I was actually reluctant to try the ones I now recommend for the reasons you mention. However, these blowers are not your average squirrel cage, and since I wanted a readily available source at what I call a more reasonable price, I tried the ones from Kayne & Son/Blacksmiths Depot. I was getting prices of over $500.00 for new paddle type blowers and the manufacturers wanted to sell large volumes. Now if you can get a used one from a Johnson Furnace, you will get a great blower. I just checked the prices of the blower I would recommend from Johnson is $785.00 compared to Blacksmith's Depot for about $110.00. And as Paul Harvey would say, and now you know the rest of the story, at least why I use these blowers.

[HWooldridge]

HW, I don't think you have enough pressure from your blower, maybe the burner tip configuration being different as well. To get enough pressure, most blowers need to run at 3,450 rpms or more. Volume alone is not enough as well as pressure alone, you need both, but can easliy over do it with some blowers on the market.

Jymm, I agree with your diagnosis and plan to upsize the blower but it's like a lot of things - it works OK so I just keep using it. If I ever get a break in my work schedule, I plan to build several more gas forges; all with quick disconnects so I can move between propane tanks with minimal setup. These will consist of a smaller "pocket rocket" with a 6"x6" box for doing tips and short forge welds, a long thin forge (probably 4 burner and 30" long) for pickets, and a large flat enclosed box with a slit on one side so I can get scrolls in and out without fighting a door. Thanks for your comments...Hollis

[Valentin]

Dodge: my blower always blows off my flame so i quited trying this blowers... but hardley wait to see yours work so maybe i will copie it if it worth ''reinventing the weel'' ??? Why do u whant a blown forge if u have normal burner already ? The best improvement i can think of now to a gas forge is a stainles steel pipe for burner

[Dodge]

Val, Just want to try something different as I am building a new forge from scratch anyway. The flares are toast and would need to be replaced to re-use the burners, and although they worked fine from the beginning, I always wanted to try another design "next time." :)