CFM parameters for electric forge blower?

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I'm building a gas forge right now, but one thing I'm not sure about is what the CFM parameters for a forge blower should be. The forge I'm making is a tube forge based on a modification of Tim Zowada's design. 12" dia. steel pipe 12" long, 1 1/4" insulation lining, refractory blanket wrapped, 3" x 4" doors at both ends. The gas and air inlet is side mounted toward the top of the cylinder. I need something I can consistently holding welding temperatures with.

The problem I'm having here is that I've never dealt with installing an electrical blower before. I've worked with them, but that doesn't mean knowing anything about them. I normally use a hand-crank blower on my coal forge.

So, what are the CFM requirements for a forge blower? I've got a couple of places to buy them, but I just don't know what to buy. Since it's going to be on a rheostat I can adjust it down, but what should the max be?

I'm thinking an 80 CFM would be sufficient, but that's really just a guess. I would really appreciate any assistance anyone can provide.

Marshal


B.J. Severtson's picture

Check out the site

tzknives.com is Tim Zowada's web site, in the manifold description page he says that the manifold requires 110 cfm @ 7" H2O ... interesting site. Brad


visitor's picture

It does?

Well, I feel dumb. I couldn't find it and I've looked at the blasted site about 10 times. Must have looked right at it and passed over it.

Thanks for pointing out my lack of attention to detail. Do you think that's a trait that will negatively impact my metalworking? =)


B.J. Severtson's picture

hmm

Not for the first few years. As you get more creative in your problem making, problem solving will get more difficult. Little story: I live in Florida, we have two things here, lots and lots of sun and a bunch of tourists. I feel some moral obligation to tell red getting tourists that they should put on sun screen. I feel no obligation what so ever, to ever tell any of them twice. instructions are a lot like that. Just an observation. You'll be fine.
Hammer on something. Brad


Rich Waugh's picture

The best forge blower I know

The best forge blower I know of is the one sold by Kayne and Sons' Blacksmith's Depot.  It is a 160 cfm blower withy an outlet pressure of around 2-1/2 psig or so.  I just installed on yesterday on a sideblown masonry forge, in fact.  Great blower.  Jymm Hoffman recommends this same blower for the blown gas forges he builds, as well.

One thing about electric blowers you may not know:  the easiest way to damp their output is to restrict their input.  Rather than use a rheostat or SCR to limit their electrical power, simply restrict the input air.  This is actually easier on the motor, in many cases, and is certainly easier on the motor than restricting the output air. 

With the input restricted, there are fewer air m olecules for the impeller to move, so the effort is actually lessened - on the other hand, with the output restricted, the impeller is actually trying to compress the air against increased resistance.  Similarly, when you cut back the juice with a rheostat, you're starving the motor of what it needs to move the available air, straining it unnecessarily.  SCR's, since they clip the waveform, aren't as bad as rheostats, but they still starve the motor for power to some degree.


visitor's picture

Best Forge blower

Hey Rich,

Thanks for the link, this is the type of thing I was looking for, a bit more expensive than I was looking for, but if I'm building a forge for the long run, then the blower will be the one thing that's going to stay together as the forge gets rebuilt over the years.

Your explanation of restricting intake airflow vs. output airflow is great and really helps me to overcome some of the questions I've had in my mind about how much work the impeller is doing trying to pressurize a manifold that is restricted by gate valves.

I think 160 cfm would allow me to weld... well just about anything. Or push my truck out of a ditch, either way, not a bad thing.


Julia S-S's picture

I'm not sure, because my

I'm not sure, because my forge is not finished yet. I was told to use a 65 or 70 CFM "squirrel cage" blower motor.
(available from Grainger as a "pole blower" or something like that.

My forge will be about the same size as yours (made from a 10 gal propane tank).

~~~Julia


Rich Waugh's picture

65 cfm may or may not be

65 cfm may or may not be sufficient, depending on the type of burner and type of forge you are building. Generally, squirrel cage blowers are very low-pressure devices, only putting out mere ounces of pressure against static pressure. If your forge is designed to operate with minimal exhaust openings, then you may find that a squirrel cage blower doesn't work well for you. Also, if you are building burners with restricted outlets, you'll need more pressure than that little squirrel cage can deliver. In those situations, you need a positive displacement blower like the one I mentioned in my prevous post.

With a forge that sufficient exhaust opening and a simple tube burner with no restriction at the flame end and no fancy vortex mixing devices in the tube, a small squirrel cage might be just fine. You can probably scrounge a used one at the dump, or an appliance store. Or try a computer fan, a microwave oven fan or other low output blower to see if the concept is viable. If that works, then buy a decent squirrel cage. If not, you'll have learned something and can try other options. Electric clothes dryers have large blowers designed to provide some pressure to overcome lint traps, etc. An old blow dryer will work for many forges and can be bought new for under ten bucks lots of places.

Things to consider.


B.J. Severtson's picture

Let me see?

Richard,
Let me see if I'm getting a grasp on this. Would your second example result in a bigger fire but not as hot? So them If I'm understanding this cfm's are only part of the equation.. A desired relationship between cfm and pressure is the goal? Learning Brad


Rich Waugh's picture

You're pretty much correct,

You're pretty much correct, Brad. No one single factor controls the final result. You can get a fire goinjg pretty well waving a sheet of cardboard at it, or you can do it with a jet of compressed air. When you're workin gin an enclosure, things get muddled pretty quickly with back pressure, radiant combustion, altered atmosphere, etc. To say nothing of the mixing efficiency of the burner itself. Propane doesn't mix with air as easily as some gases, so a poorly designed venturi burner may be terribly inefficient at mixing the fuel and air and therefore run poorly, while a well built burner will get the most out of the fuel under the circumstances. This is one reason that blown forges generally run hotter than venturi types. With a blower, you can force the fuel and air to mix in a shorter distance due to the increased turbulence.

The better the fuel and iar are mixed, the greater the liklihood that the flame will burn cleanly. The burner and enclosure still need to be designed to foster a good burn, though. With too much back pressure or too little velocity in the burner tube, the fuel-air mixture may be moving slower than the flame front propagation rate, resulting in burnback, the situation when the flame travels back up the burner tube. At the opposite end of the issue, too high a velocity can result in the flame literally being blown off the burner tip and combusting in the forge chamber. The ideal is a flame that stays at the burner opening and completely combusts all the fuel with no unburned excess and no excess air. It is a tricky balancing act.

Lots of variables and lots of misunderstandings about what is happening. Heck, I may be misunderstanding a good bit of it myself, though my engineer friend tells me I have it right. I keep learning and trying new things in the never-ending search for the perfect forge. If you discover the secret before I do, please share it with me!


B.J. Severtson's picture

Thank you, Rich

Beautiful answer. The likely I'll discover the perfect forge before you do is about the same as the likelihood that my wife will ever let me use her hairdryer to power a foundry. But hey..I'll let you know. Thanks Brad


Julia S-S's picture

I'll have to think on that.

I'll have to think on that. If I am understanding correctly, a hair dryer or squirrel cage would be more appropirate for a tube-shaped forge(open on both ends). Since mine will be mostly closed, I might need more air pressure, to bring in the same amount of air?

~~~Julia


visitor's picture

Grainger pole blower

I was looking at the blower and was asking one of my friends who has a better understanding of wiring and electrical than I do. He was concerned with the idea of putting a rheostat on a motor like that because many of them are only set to one speed, so reducing the juice to it can damage the motor. I can't remember if it was this particular blower or not that we were discussing. He was also concerned with the positive pressure in the manifold causing the impeller to have to work too hard and possibly damaging it.

Does anyone know roughly how much time you get out of a tank of propane 10, 30, 60 gallon? I know it will depend radically from forge to forge, but I don't know if I should expect a 10 gallon tank to last a day or a year.

Marshal


Julia S-S's picture

If your forge is running

If your forge is running pretty efficiently, and you are working 8-10 hours at a time, I think a 10 gal tank should last a day or two. With really proper air-fuel ratio and insulation, I have been told that you can keep your forge in the 3000 degree range with very little propane.

Perhaps someone with more experience can tell you more.

As for the air supply, i'm really hoping to switch to venturi system in the near future. I don't like the idea of fire, propane, and electricity in the same system, and all this talk of rheostats is making me nervous!

I have a great story about burning off my eyebrows and eyelashes.

~~~Julia


Rich Waugh's picture

Marshal, See my above post

Marshal,

See my above post on restricting intake air versus output air. Rheostats vary the voltage/current and only work on some kinds of motors (universal) and will destroy induction motors. Low voltage is a killer for all motors, as it creates stalling and heating. A silicon-controlled rectifier (SCR) controller will work better because it changes the wave form to control the speed by clipping part of each cycle of the AC but keeping the peak voltage up where the motor needs it to be. At least this is how I understand it anyway. Easier and safer to restrict the intake air.

On your fuel consumption question, it is way to broad to have any accurate answer. How may burners, what size, what type, how hard are you planning to run the forge, at what altitude, etc. I have a well-insulated small forge that uses one 1/2" pipe aspirated burner and when doing forging demos where I don't do any welding, it will run for three or four days on a 20 pound gas grille type tank. On the other hand, I have a large forge with four 3/4" pipe burners that will gobble up a 100 pound cylinder in a day when run at welding heat. In fact, I have to manifold two 100# cylilnders in order not to freeze the valve after about three hours of running the forge flat out. So the answer to your question lies somewhere between the two extremes. In routine shop use, my two-burner forge uses a 100# (23 gallon) cylinder of propane in about a week. Less if I'm doing much welding or workin greally big stock, more when I'm doing light work and the forge is idling more than half the time.

You will not be able to run a forge at 3000 degrees for very long, if at all. The refractories will break down below that temp and it would require a nearly perfect stoichometric mixture burning in an incredibly well insulated enclosure to even reach that heat. 2300 degrees is a more reasonable expectation.


visitor's picture

cylinder gas forge

Here's what I have done and experianced.
My homemade gas forge was 10"dia.x12"long
I used castable 3000 deg. ref. 2" thick wich
gave me 6" dia. inside dia. I walled off the openings with firebrick. my air/gas feed tube was 2" id I tried the grill propane tank but after a couple hours of use it started to freeze up so I went to a !00 gal tank prob solved my blower was the dayton 4" 110 cfm squirrel cage and I did the air restriction method it worked great for many years. Try not to sweat the design so much experiment just pretend your running a oxy act torch adjust the gas adjust the air and try for the best burn ..not hot enough...increase the air..increase the gas....burning to rich back off on the gas....etc. heck I made a coffee can forge the same way same blower same gas setup but smaller pipes still worked great ...I sent this little unit to Bill Moran...he said nobody ever sent him anything like it .....neat!! Hope all goes well....A Bladesmith.