apples Posted May 30, 2011 Share Posted May 30, 2011 I am working on making a gas (propane) fire pit. I have read that you gotta buy or make an air mixer valve / venturi. Otherwise the flame will be yellow and very sooty. I assume you guys have make your own venturi things, how hard is it to do? I don't need a hot flame to melt stuff, just don't want the sooty black stuff coming off. Also how long does a 9kg / 20lb gas bottle last in a forge? I'm just trying to work out how long my fire pit will burn for. I might need two bottles for the night. I guess it is all about pressure and how big a hose/ nozzle/jet you are using. I'm after flames about 3-4ft high, like flames in a good sized wood campfire. Looks like the 0.8mm mig tips are good for the forges from about 0.1psi to 15psi. Will be using an LPG welding regulator, not a pissy little BBQ regulator. My BBQ reg is about 2.8kPa which is only 0.4psi. That isn't much hey. Any way I'd appreciate some help. And you can buy the air mixer valve things but I'm in Australaia and I think our gas fittings have a different thread so it may be harder for me to just buy a air mixer thing? Our LPG bottles have a 5/8 UNF Left hand thread. So I guess the other fittings are also UNF? Peter Link to comment Share on other sites More sharing options...
John B Posted May 30, 2011 Share Posted May 30, 2011 You don't say what you want your fire pit to do, I can only assume you want it to be decorative. Some years ago I was asked to make a Beacon Brazier and silly me suggested gas fired, this was the outcome, whether it is similar to what you are requiring, the technology to make it will be similar. Flame flare size is regulated by gas pressure and orifice size, a deflector plate over the orifice will help prevent sooting, trial and error will give you the sizes required, the amount of gas you use will depend on variables also. Here are some pictures to give you an idea of what it turned out like. First the brazier and its mounting post to give some idea of scale. Prototype burners that fit inside the brazier Brazier with burner assembly fitted inside and being tried And finally the brazier in situ and alight Because I only required the flame effect, no air addition was required to the propane. The orifice sizes were 1/16" diameter with a deflector plate welded onto the gas supply tube which was made of 1/2" dia stainless tube with the tops blanked off with a central orifice and deflector plate fitted over , These deflector plates reduced sooting which was negligable anyway and helped disperse the gas to give a better flame spread. All the burner tubes went to a made up manifold that had a single in feed from a variable adjustable regulator. The flames you see were produced at a very low pressure, with the regulator only just cracked open, I did not have gauges on the feed line so cannot give an accurate pressure reading. Ignition was by piezzo or flare, It was made this way to help prevent seagulls nesting in the brazier, Have fun, if it's any consolation, all the approved gas engineers I contacted could not help or advise on how to make this item, they did not know where to start, but were well impressed with the finished item and certified it as fit to use. Link to comment Share on other sites More sharing options...
apples Posted May 31, 2011 Author Share Posted May 31, 2011 Well here is my fire pit. It's had about 3 wood fires in it so far, so she ain't that clean anymore. I am going to get a lpg welding regulator so that is good for about 60psi probably. Then I will make up a pipe setup and drill holes for the gas to escape. The venturi part I guess can go under it, I will also buy about 10meters of hose (33ft) and a flashback to be safe. Was thinking to fill the pan up with either water and have it bubble up, or vermiculite. So the flames have gotta be at least 3ft tall, it's coming on to winter down here in OZ and we are going to need to keep warm around that fire. Looking forward to the gas, not having the smoke smell and stinking clothes, and I won't have to go looking for wood. Rolled over a log just two days ago and there was a brown snake there, crikey. Link to comment Share on other sites More sharing options...
apples Posted June 2, 2011 Author Share Posted June 2, 2011 How long does a bottle of gas last? Can you calculate it? I know my pressure will be anything from 0-60psi, my bottle of gas is 9kg. What else do you need to know to calculate how long the gas will flow out of it at a set pressure? Nozzle/jet size and volume of gas tank/cylinder? What about just roughly. What psi do you guys run at and how long will that make a 9kg (20lb) bottle last for? Peter Link to comment Share on other sites More sharing options...
Steve Sells Posted June 2, 2011 Share Posted June 2, 2011 9kg is going to be a problem if you even think about running it at 60. All forges operate differently all operators use them differently, How long does a loaf of bread last? Read the forum you may find answers before you post. Being new its hard to know who or what to ask. To try to answer your question, A 9kg bottle should last from a few hours to longer in normal usage. Welcome to the forum. Link to comment Share on other sites More sharing options...
don't tread on me Posted June 2, 2011 Share Posted June 2, 2011 Running at 20psi my 20 lb. tanks last 2-4 hours. Link to comment Share on other sites More sharing options...
ThomasPowers Posted June 2, 2011 Share Posted June 2, 2011 Don't forget ambient temperature too! Last week I took my forge camping and using the same forge, bottle, pressure I ran several hours longer in "summer" temperatures than I would get in winter time! 60 psi seems extremely high as most small forges I know will melt steel at lower pressures than that! The calculation can be done but it will be far easier to just hook it up and run the experiment! Link to comment Share on other sites More sharing options...
pkrankow Posted June 2, 2011 Share Posted June 2, 2011 The calculation can be done but it will be far easier to just hook it up and run the experiment! Yes, run the experiment! My one burner gets 6 hours or so off a grill bottle at 10-15 psi. Freezing of the bottle is more of a concern than anything else,at least from an operation standpoint, since this is a small bottle. A strong fan (like a household box fan) moving fresh air over the bottle seems the safest recommendation. The idea is to improve the heat transfer into the propane bottle without increasing the temperature over ambient. "Warm" water baths have also been frequently done, I use cold tap water as my "warm" bath. Phil Link to comment Share on other sites More sharing options...
MattBower Posted June 2, 2011 Share Posted June 2, 2011 Running 60 PSI through a 0.5" orifice is very different than running 60 PSI through a 0.001" orifice. Make sense? Then there's the fact that the size and design of the forge and how well insulated it is will determine how much gas you need to deliver to get to the temperature you want. And the temperature you want will vary depending on what you're trying to do. Etc., etc. Point is, as others have suggested, it's much easier to determine the answer to your question through experimentation with your own burner and forge than it is to try to calculate it from theory. Link to comment Share on other sites More sharing options...
DennisG Posted June 2, 2011 Share Posted June 2, 2011 Go to this link, HERE there is a BTU calculator that has a "usage" section based on psi, bottle size, and orifice size. Link to comment Share on other sites More sharing options...
MattBower Posted June 2, 2011 Share Posted June 2, 2011 Go to this link, HERE there is a BTU calculator that has a "usage" section based on psi, bottle size, and orifice size. Thanks for that, Dennis. I haven't seen that one before! Link to comment Share on other sites More sharing options...
Nakedanvil - Grant Sarver Posted June 2, 2011 Share Posted June 2, 2011 By the time you have all the data you need to calculate it, you'll already know how long a bottle lasts! How long does a loaf of bread last? Is that a trick question? :lol: Link to comment Share on other sites More sharing options...
CurlyGeorge Posted June 2, 2011 Share Posted June 2, 2011 Peter, go to this link and go to post #19, in it. http://www.iforgeiron.com/topic/18817-start-of-a-new-gasser/ I kept track of burn times until my bottle emptied out. But like the guys said, there are a lot of variables. Good luck. :D Link to comment Share on other sites More sharing options...
apples Posted June 2, 2011 Author Share Posted June 2, 2011 Thanks for all the comments guys. Yeah I guess I will have to fire her up and see. I will post some pictures later on. I get my regulator, hose and flashback today. Link to comment Share on other sites More sharing options...
apples Posted June 3, 2011 Author Share Posted June 3, 2011 Well I fired her up tonight. Filled up the fire pit with water and had it bubbling through. The gas burned like gas I guess. It was very bubbly, too bubbly actually. This was with only the one gas jet/outlet. The flame went to about 3-4ft. Tomorrow when I have time I will try to add a heap more holes to the setup. Most fire rings have a heap of holes that are spaced about 1" apart. Where as I only had the one hole. They also mention using a air/fuel mixer adapter thing, otherwise you get black soot from the flame not burning with not enough air. But my flame seemed okay, no soot at all that I could see. Link to comment Share on other sites More sharing options...
pkrankow Posted June 3, 2011 Share Posted June 3, 2011 Now we need pictures! Phil Link to comment Share on other sites More sharing options...
CurlyGeorge Posted June 3, 2011 Share Posted June 3, 2011 Well I fired her up tonight. Filled up the fire pit with water and had it bubbling through. The gas burned like gas I guess. It was very bubbly, too bubbly actually. You got me confused for sure, now, Which isn't hard to do anyway. What do you mean that you filled the fire pit with water and had it bubbling through? :blink: Link to comment Share on other sites More sharing options...
apples Posted June 3, 2011 Author Share Posted June 3, 2011 I wanted to try the fire on water effect. I have attached a pic of my fire pit I made which should make it easy to see what i mean, lol. I think I need more gas holes. Link to comment Share on other sites More sharing options...
pkrankow Posted June 3, 2011 Share Posted June 3, 2011 After you get it lit again, take some pictures, hopefully with the desired effect. Phil Link to comment Share on other sites More sharing options...
MattBower Posted June 3, 2011 Share Posted June 3, 2011 Whoa. That's not what I thought this discussion was about! lol That's pretty cool, though; I hope you get it working. Link to comment Share on other sites More sharing options...
Guest Posted June 3, 2011 Share Posted June 3, 2011 This person should be aware that he is posting on a black smithing site, so the answers given will reflect that usage. But since its entertaining, it will remain but be relocated to a place more appropriate. NOTE posters 2 threads from 2 places have been merged into this one. Link to comment Share on other sites More sharing options...
alexhhisc Posted June 4, 2011 Share Posted June 4, 2011 My forced air forge uses about 1.3 lb per hour. So 20lb tank (filled 15lb) will work for 10-12 hours. Link to comment Share on other sites More sharing options...
Fe-Wood Posted June 4, 2011 Share Posted June 4, 2011 I played with that idea a bit in College. Running propane under still water and igniting. It snaps like caps and splashes water all about... Sorta fun for a simple mind like mine...... As far as how long my propane tank lasts.... Until its empty. I know that because my forge stops working :P Link to comment Share on other sites More sharing options...
clinton Posted June 5, 2011 Share Posted June 5, 2011 This is a very simple equation to solve- every blacksmith knows this. You take the volume and divide by PI X R Sq then you follow the rotation of a top on a bottle of scotch and cube this number when the sum equals the pounds of butter in a chicken soup and the moon is in line with Venus the answer will be right in front of you Link to comment Share on other sites More sharing options...
Fe-Wood Posted June 5, 2011 Share Posted June 5, 2011 Thanks Clinton!!! For the life of me I couldn't remember that formula..... Into the archives it goes :lol: :lol: Link to comment Share on other sites More sharing options...
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