Galvanized hood for gas forge exhaust?

Hi. I am going to be installing my gas forge in my shop (walk out, unfinished basement), and I am planning on having an exhaust fan in a
hood to suck out the exhaust/gases. I was wondering about using a galvanized hood for such a setup? (Actually I was thinking that a trash-can cut in 1/2 would make a pretty good and cheap one). My forge is covered w/ BBQ high-temp paint, and this hasn't burned off (except a little around the opening), so I'm wondering if the relatively low exhaust temps will burn off the Zn? Should I paint the hood with the BBQ paint?
Also, while we are here - I did a quick calculation and figured that I need to exhaust for about 275 CFM, does this seem reasonable for a forge burning about a BBQ tank/6hours? (I.e. I'm groping for a sanity check). My calc was thus, and it's been a while:
My forge burns a 20lb BBQ tank/6 hours (conservative). C3H8 == 0.044kg/mol
20lb * 2.2kg/lb * 1mol/0.044kg = 1000mol C3H8/6 hours
At STP 1mol occupies 22.4L
22.4L/mol * 1000mol * 0.265gal/L * 0.1337ft^3/gal = 793ft^3 at STP
That is the volume of C3H8 at STP in the tank, but we to burn it we need 5 02/1 C3H8, which is
5 * 793ft^3 == 3968ft^3 of O2,
but air is 20% O2, so we need
3968ft^3 / 0.20 = 19841ft^3 Air at STP to burn a 20lb BBQ tank. But the exhaust is about 5x less dense, so that would be 99205ft^3 exhaust gas.
It takes 6 hours to burn the tank, so to remove that much gas we need to exhaust at 16534 ft^3/hour == 275 CFM.
Wow, that seems like a lot (previously I had forgotten to consider the incease in volume of hot air). It's 1/16th the volume of my shop. But I know that bathroom fans can run 150-200CFM, so I guess it's not completely out of order.
Anyone see a problem with this calc/reasoning (I corrected it twice while writing it out - see it helps to talk about such things ;) )? Of course I'll err on the high side and run a bigger fan and have ample fresh air intakes and have the CO/GAS detectors, but I was just trying to get a feel for the volume of air needed, and I (naturally) want to make sure that we are operating in a safe manner.
Thanks for your input,
don
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Like some other things in our society, the concern about galvanized coatings has been greatly exaggerated. You are perfectly safe to use a galvanized hood, and to use galvanized burner tubes, even if an inch of the galvanizing will be burned off. You can easily enough look up the effects of Zink on the body, so I will not go into it here....have a glass of milk of you are concerned. Feel free to proceed with whatever hood design you want to make.
When your hood is done you can check its effectiveness by opening a window in your shop, with everything else closed to seal it, get your forge and your exhaust hood going, and see if the flow is into the shop through the window. You can put some smoke into the shop for a good visual indicator of air flow. If you detect a flow out the window, or no flow either direction, you have a problem. As to the fan, where are you going to put it. There are several ways to go about the exhaust problem. One is to pressurize the shop with a fan blowing air into the shop, which forces air up the stack and outside. This works, but has its down sides. You can place an inducer fan in the stack itself....a high maintenance arrangement, or you can use a draft inducer which I will not describe here, but you can see images of by clicking the following;
http://www.reil1.net/shop4.jpg (Taken when I was building my shop)
http://www.reil1.net/draft.jpg (Close-up of the hardware)
The rubber hose connects to a 2" diameter section of 90 degree bend exhaust pipe that points up in the center of the chimney pipe. This system can induce a draft strong enough to suck a balled up piece of paper up the stack and out the top of the 12" diameter chimney. There is no maintenance with this system, and it works extremely well. Induced draft is the only way to go if it is going to be used long term.
Ron
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I love it. How big is that blower that you have up there (in whatever blower units are (e.g. CFM))?
don
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I am embarrassed to say I don't know how big the inducer fan is, as far as cfm, but because I have the ladder in the shop right now to install a centrifugal exhaust fan in the shop wall, I moved it over and checked, and the little blower is 1/45 hp. You can use any kind of blower. This one was brand new and just happened to be laying on a shelf at my local junk shop one day so I bought it, even though I had no use for it at the time. It is made by Dayton. Anything that will establish an upward air flow will work, but if you want it to generate a good suction, like this one does, you will need something of reasonable size like this little blower, not a little muffin fan.
You can use a 1/4" or 3/8" air lance too, from your compressor, but a compressor is a pain to listen to, and compressed air is expensive. There is one other alternative I should mention. I once had a 1972 Ford 4-WD pick-up. Because I used it for geology work off-road all the time, I removed the emission control system, and part of the system was an axial blade air compressor. It looks just about like the air conditioner compressor on a car. With a 1/3 hp motor hooked up, the axial blade compressor puts out a very good stream of air, and I used it for a time to run an air lance for my big coal forge hood. So if you have an old car or truck laying around you may have one of those compressors, and they last just about forever. They are a little noisier than a small blower though, so the blower is preferable.
Some further info that may be of value concerns the injection piping. As I mentioned in my last post, get a 2" diameter automobile exhaust pipe elbow for the injector. If you want to increase its efficiency you could neck it down a little, but I don't recommend it. When you get the exhaust pipe elbow, also get a set of the heavy exhaust pipe, three bolt, flanges that slide over the exhaust pipe, and the donut (gasket) that goes between them to provide the seal. My chimney is heavy gage steel (20 gage), and very rigid, but light gage will work as well. Heat the three corner flange fitting and bend it to match the radius of your chimney pipe. Then braze it to the elbow in the location needed to center the injector end in the chimney pipe pointing straight up. If you have light gage chimney pipe, use the second three corner flange fitting on the inside of the chimney pipe to bolt it all together, with the donut on the outside to provide the closure. If you have heavy gage pipe, you can just bolt to the pipe itself, and forget about the inner flange fitting. If you look at the second image I posted a link to, you can see the flange fitting against the chimney pipe.
I don't know where you live, but also visible in my two images is a little lever handle just below the draft inducer pipe. It is the damper handle. I made a damper that is a 12" diameter 20 gage disk and comes as close as I can make it to totally closing and sealing the chimney when in the closed position. It can get pretty cold here in the winter, and a 12" diameter chimney pipe can pass a lot of heat up the stack, so in the winter, when the forge isn't in use, I keep it closed to keep the shop heat in the shop. I also use it to control the amount of draft up the stack in the winter to reduce the amount of cold air that comes into the shop to replace it. I have to open a window or door when the draft inducer blower is running.
I think you will find this kind of draft inducer FAR superior to any kind of inline blower in the stack itself, especially if you use coal on occasion. I have two coal forges, a small and a big one, that I designed the exhaust system around, but I don't use coal any more, so it is a non issue with me now, but coal puts a lot of crud in the chimney, and the simple 90 degree elbow is much superior to having fan blades running in the exhaust stream. It is zero maintenance, and if you use coal, you could even clean the stack with it in place periodically. You don't have to worry about electrical overload problems with the blower motor either, in case the fan blades got sooted up to the point they can't turn. Also, if you open or close the damper, it makes no difference to the blower, so long as the blower is above the damper, as you see in the images.
BTW, the centrifugal exhaust fan I am installing in the peak of the shop wall is not intended to get forge smoke out of the shop, I never get forge smoke or fumes in the shop. It is to exhaust excess heat in the summer when I have the big door open, but even more, to get rid of welding fumes and smoke. I do my welding on an Acorn platen table that is not under the hood, so the welding smoke fills the shop. I can't afford to breath the smoke due to my lungs not being in the best of shape, so I have to keep the shop air clean. I just got the Acorn table set up in the shop, so prior to now I did my welding out on the shop patio. I had planned to have the Acorn table since when I first built the shop, so I put in all the wiring and the switch for the fan when I built the shop. Getting the Acorn table now makes the fan necessary. It is arriving today, so I have a new task to do this weekend, to cut the hole in the wall and get it installed and wired up. I do know it's rating, 1656 cfm, so it should keep the shop air pretty clean. So long as the door is open I should be able to run the centrifugal fan as well as the hood exhaust system, and have both function fine at the same time. With the big door closed, I am not sure...will have to try it. The induced draft in the hood is pretty strong so perhaps it will be strong enough to work with just a window or walk door open when the centrifugal fan is running...I will have to wait and see. If not, no problem. The centrifugal fan is for excess heat in the summer when the big door is open, or when I am welding, when the forge hood would not be running anyway.
One last tip, and it has already been mentioned in another post, get a CO detector. I have a section on my Web site about the "Nighthawk Gas/CO Detector," http://www.reil1.net/design1.shtml#Nighthawk that you may want to look at. It is a combo CO and flammable gas detector, and is very reasonably priced. I have two of them installed in my shop. Don't take chances with CO or explosive gas....can ruin your whole day!
Well, sorry for the digression...just my way of thinking online. If you have any questions as you are putting together your exhaust system, feel free to write me offline.
Yours,
Ron
Golden Age Forge http://www.reil1.net/gallery.shtml E-Mail: snipped-for-privacy@reil1.net Boise, Idaho

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Hi Ron,
Thanks very much for all that information. Sounds like a pretty good system, so I'll see if I can't fool around with it and get something to works. Also seems like it can be done pretty cost-effectively.
I really appreciate all the information/detail in your message.
Good luck with the fan this weekend.
don

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The fan arrived a little after noon. It is quite a piece of technology. It looks like a the front end of a jet engine and is about 3' in diameter. The hole I have to cut in the shop wall is only 12" in diameter too, which is pretty amazing. I guess these centrifugal fans are very efficient, and based on the blast that it put out when I connected it up to test it, I think it will be able to change out all the air in the shop in only a few minutes. It was very quiet too, which was more than I expected.
When you get your exhaust system working drop me a line and let me know how it came out.
Yours,
Ron
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don schad wrote:

Actually, it's kg/2.2lb, which puts you off by a factor of almost 5. Looks fine otherwise.
- ken
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Argh! Damn those unit conversions! Thanks for getting that, it feels a lot better to be running in the mid-50's again.
I after I posted that message I calculated the velocity in the burner pipes to be around 70 mph, and I was having a hard time believing that...14 is more reasonable (but still seems pretty high).
Thanks,
don
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Even tho you only burn the oxygen out of the air, don't you need to exhaust the nitrogen that came into the burner with that air, too? You will also be heating a certain amount of air around the forge due to the inefficiency of the insulation. Maybe you want to be able to get rid of that, too.
Pete Stanaitis ---------------
don schad wrote:

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