rockstar.esq Posted August 25, 2010 Share Posted August 25, 2010 I'm looking for some sage wisdom to help a new guy out. I built a set of double bellows. They are 4' long by 2' wide tapering to the spout which is 1-1/2" diameter. The intake holes are 2-1/2" diameter with a flap valve. Both the bottom board and the stationary middle board have them. There's about 12" of movement at the back of the bellows. The fire pot is an automotive brake drum piped with 1-1/2" black pipe set up with the typical flange, tee, and trap. I've connected the bellows to the pipe with a section of flexible 1-1/2" exhaust pipe which is duct taped to seal on each end. I made a clinker breaker out of a towing ball that I cut down, drilled and tapped to allow me to mount it in the tuyere just below the flange connecting the pipe to the drum. The clinker breaker has about 1/4" on all sides to allow air flow around it. I'm using coke as fuel. The issue is that I can't get sufficient air flow to get the coke lit let alone keep it going. I tied a weight to the bottom board to get allow faster recovery. The bellows are covered with vinyl and there don't appear to be any major leaks. There is definitely air flow it's just not very fast. When I've put wood in to get the fire started, it pumps plenty of air to get it going quickly, it's just the coke never seems to get lit. Just to see what would happen I pre-heated some of the coke with a MAPP gas torch while pumping. Once lit, the best I could do was a dim orange, never yellow let alone white. Needless to say it went out quickly when I stopped pumping. I tried to mimic a set of bellows I saw working at a hammer in early this summer. They were only 3' long and it was easily handling the needs of the smith who was demonstrating forge welding with a coke fire. So that's it, I'd love your insights, thank you in advance! Quote Link to comment Share on other sites More sharing options...
Fosterob Posted August 25, 2010 Share Posted August 25, 2010 Coke needs a steady flow of air to stay lit. You can mix with coal if you use bellows or handcrank blower. If your bellows are not putting out enough air then some pics may help. Rob Quote Link to comment Share on other sites More sharing options...
John McPherson Posted August 25, 2010 Share Posted August 25, 2010 How many 2.5" intake holes in each chamber? A typical great bellows of that size should have at least two 6 to 8" intakes in the bottom board, and two 4 to 6" intakes in the middle board. This is a low pressure devise, so volume is important. Quote Link to comment Share on other sites More sharing options...
Nakedanvil - Grant Sarver Posted August 25, 2010 Share Posted August 25, 2010 "A weight tied to the bottom board" would also indicate too small of inlet valves. They can be HUGE to no ill effect. Quote Link to comment Share on other sites More sharing options...
ThomasPowers Posted August 25, 2010 Share Posted August 25, 2010 Bigger valves! My great bellows were slightly wider than that but I could blow coal out of the firepot if I pumped them hard. I used rectangular inlets with a light sheetmetal flapper on hinges and with felt glued to the wood around the inlet. Quote Link to comment Share on other sites More sharing options...
Wayne Posted August 25, 2010 Share Posted August 25, 2010 Definatell big intake holes, mineare 5-6" square and i have no trouble with coke Quote Link to comment Share on other sites More sharing options...
rockstar.esq Posted August 25, 2010 Author Share Posted August 25, 2010 Thank you all for the insights. I only have one 2-1/2" inlet on the bottom and one on the middle board. It sounds like I need to add another one to each board. I was concerned that if the hole was too large it wouldn't work properly but I can't think of a reason why. I'll add the inlets this weekend and post my results. Thank you all for your help! Quote Link to comment Share on other sites More sharing options...
Rob Browne Posted August 25, 2010 Share Posted August 25, 2010 While you have it apart consider making the holes MUCH bigger. A 2 1/2" diameter hole is nowhere near the size of a 5" hole. Quote Link to comment Share on other sites More sharing options...
Francis Trez Cole Posted August 25, 2010 Share Posted August 25, 2010 question your center board dose it have a flap valve? Sounds to me like you made a double chambered bellows if so you will need weight on the top to keep the air flow even. more weight the faster air will be driven out through your turee Quote Link to comment Share on other sites More sharing options...
Nakedanvil - Grant Sarver Posted August 26, 2010 Share Posted August 26, 2010 A 2-1/2 inch round hole has less than 5 square inches. A single 5 inch round is nearly 20 square inches! Many of the suggestions are for 50 - 100 square inches! Even two 2-1/2 inch holes will only be a little under 10 square inches. Double the size on the hole and you get four times the area! Quote Link to comment Share on other sites More sharing options...
rockstar.esq Posted August 26, 2010 Author Share Posted August 26, 2010 Good observations Rob Brown and Nakedanvil, I've got a 2-1/2" drill bit which is why I've used that size. Sounds like it's time to step up the size to get closer to what's being recommended. Francis, the centerboard does have a valve that's the same size as the bottom board. I made my bellows so that the centerboard is always stationary. The support's are connected to it. The top board may end up needing a weight on it to keep up with the bottom bellows once the new inlets are added. Thankfully that's easy to accommodate since I can just store hammers there until I've got the desired effect. I've very relieved to hear that the fix is so simple. Thanks again for all of your help. Quote Link to comment Share on other sites More sharing options...
ThomasPowers Posted August 26, 2010 Share Posted August 26, 2010 Can you get to the center board easily to increase the inlets (and flapper valves) on it? Some smiths have made the inlet/flapper valve pieces a removable piece that is screwed into place and so you can remove them to work on the inner set or take the inner set out for tweaking and then replace it. Most double lunged bellows work with the center board stationary and the top acts as an air store to provide for a constant output even with a punctuated input. Quote Link to comment Share on other sites More sharing options...
rockstar.esq Posted August 27, 2010 Author Share Posted August 27, 2010 I built the center and bottom board identically so the inlets line up with each other. Sadly, they are both simply a hole with a vinyl flap attached on the inside. I'm thinking that I'll make a square inlet valve that's mounted to a small frame. That way I can saw the square opening, slip the valve through at an angle and still have the frame to fasten to the board. Now that I've made this mistake I'd definitely make the valves removable the first time since it's much easier to fix mistakes when things are adjustable. Quote Link to comment Share on other sites More sharing options...
pkrankow Posted August 27, 2010 Share Posted August 27, 2010 Gonna make some mess but: Mark a square or rectangle to remove and plunge cut with your circular saw. Finish the corners with a hand saw. Vacuum out the bellows, make new valves to fit the large access ports and fasten them in. Make the hole big enough to pass your entire saw so you can make a new valve body for the stationary panel. Phil Quote Link to comment Share on other sites More sharing options...
rockstar.esq Posted September 4, 2010 Author Share Posted September 4, 2010 Well folks, I cut new inlets that are 8" square. The valves are flaps of vinyl that are 10" x 12" with the long side the being the side fastened down. My new problem is that the valves aren't sealing at all. I'm thinking of gluing a piece of sheet metal to the vinyl flap to stiffen and hopefully seal it better. Before I tear into it again I'd like some input. Thanks in advance. Quote Link to comment Share on other sites More sharing options...
Nakedanvil - Grant Sarver Posted September 4, 2010 Share Posted September 4, 2010 Or even plywood. Quote Link to comment Share on other sites More sharing options...
ThomasPowers Posted September 7, 2010 Share Posted September 7, 2010 my flaps were sheetmetal---Al sign material; of course I used light hinges and contact cemented felt on the contact side of the flap. Quote Link to comment Share on other sites More sharing options...
Ramsberg Posted September 9, 2010 Share Posted September 9, 2010 Here is a simple test, your new valve is 8" square so that is 64 sq in. When one is really pumping one pound per sq in of pressure developed inside the bellows isn't out of the question, so that would be 64 lbs. If you were to take place 64 lbs on top of the valve would it hold its shape and stay sealed? Wood is by far the best material to use for a valve(to me at least), it readily seals against a felt strip and even if just 3/8" thick is very stiff compared to a sheet of steel of the same weight. Ironically you may have made the valve to big. There are a few reasons that multiple valves are used, one is that they spead the flow of air across the carcas of the bellows, another is that they allow comparatively smaller valves so the valves can be smaller and lighter. For two holes to have a total of 64 sq in they would need to be roughly 5 5/8" square, for three holes 4 5/8" square. I would use the vinal as a hinge for a light plywood valve. A fun fact, for an open exit one pound of air pressure will produce a velocity of 236 mph. V = 66 * sqrt(H) H = head in inchs of water : divide by 27.6 to get lb V = feet per second : divide by 1.46 to get mph I have always liked small bags of sand or rocks as weights for the top of the bellows, that way you can hand them at hand and they won't disapear on the other side of the shop. Also they will give just the right flow for the work and fuel at hand. As a side note you shouldn't need any weight on the bottom plank of the bellows, it should drop rapidly from its own weight. What is happening is that the small inlet holes are producing a significant vacuum inside the bottom part of the bellows and this tends to give them an artificial lift, you probably noticed that when you put a bunch of weight on the bottom board the sides of the bellows sucked in really good, that is why, the vacuum. Hope that this helps some. Caleb Ramsby Quote Link to comment Share on other sites More sharing options...
Nakedanvil - Grant Sarver Posted September 10, 2010 Share Posted September 10, 2010 It ain't the area of the valves that creates the pressure, it's the area of the bellows. Quote Link to comment Share on other sites More sharing options...
John B Posted September 10, 2010 Share Posted September 10, 2010 When does volume kick in then? Quote Link to comment Share on other sites More sharing options...
Nakedanvil - Grant Sarver Posted September 10, 2010 Share Posted September 10, 2010 Here is a simple test, your new valve is 8" square so that is 64 sq in. When one is really pumping one pound per sq in of pressure developed inside the bellows isn't out of the question, so that would be 64 lbs. If you were to take place 64 lbs on top of the valve would it hold its shape and stay sealed? Wood is by far the best material to use for a valve(to me at least), it readily seals against a felt strip and even if just 3/8" thick is very stiff compared to a sheet of steel of the same weight. Ironically you may have made the valve to big. There are a few reasons that multiple valves are used, one is that they spead the flow of air across the carcas of the bellows, another is that they allow comparatively smaller valves so the valves can be smaller and lighter. For two holes to have a total of 64 sq in they would need to be roughly 5 5/8" square, for three holes 4 5/8" square. I would use the vinal as a hinge for a light plywood valve. A fun fact, for an open exit one pound of air pressure will produce a velocity of 236 mph. V = 66 * sqrt(H) H = head in inchs of water : divide by 27.6 to get lb V = feet per second : divide by 1.46 to get mph I have always liked small bags of sand or rocks as weights for the top of the bellows, that way you can hand them at hand and they won't disapear on the other side of the shop. Also they will give just the right flow for the work and fuel at hand. As a side note you shouldn't need any weight on the bottom plank of the bellows, it should drop rapidly from its own weight. What is happening is that the small inlet holes are producing a significant vacuum inside the bottom part of the bellows and this tends to give them an artificial lift, you probably noticed that when you put a bunch of weight on the bottom board the sides of the bellows sucked in really good, that is why, the vacuum. Hope that this helps some. Caleb Ramsby It ain't the area of the valves that creates the pressure, it's the area of the bellows. "A fun fact, for an open exit one pound of air pressure will produce a velocity of 236 mph". Gotta be something wrong there, so just a few pounds would produce super-sonic velocity? Quote Link to comment Share on other sites More sharing options...
Ramsberg Posted September 10, 2010 Share Posted September 10, 2010 It is the volume of flow and the restriction to said flow that produces a pressure differential, either a negative pressure trying to fill a container or a positive pressure trying to empty it. I made two references to the valve and pressure, one being its restrictive nature producing a negative pressure inside the first chamber, the second being the delivery pressure of the air acting on the valve face producing a load on said valve. The area of the bellows doesn't effect the pressure per square inch, but it does effect the total weight required produce the delivery pressure through the nozzle. Lets say that you are burning 50 lbs of coal an hour with the forge and it is being fired 1/2 of the time, that would be a firing rate of 100 lbs per hour requiring at least 210 CFM of air. Delivered through a 2 sq in nozzle that would be 252 fps air velocity, the head pressure for that velocity would be 14.5" of water head, adding in the loses it would be at least 17" and closer to 20" or more. That is the pressure per square inch that would be required inside the delivery chamber of the bellows. The bottom feeding chamber would have to be a bit greater to force the air past the middle plate valve and against the pressure in the top chamber. The quantity of coal consumed in this example is rather high and the nozzle size is a bit small too. So if you were burning half of the coal in the prior example with the same nozzle area then you would require at a minimum 105 CFM of air which would give a velocity of 126 fps, which would require a head pressure of 3.63" of water, the flow loses would also be a bit less so an estimate would be around 5" - 7" of head pressure in the bellows. For the 8" by 8" valve described above that would be 11 3/5 lbs to 16 1/4 lbs on the valve to seal it. This is probably what one would normally be running at. But if you built a really deep fire to heat a really big chunk of steel the head pressure must be really great to supply enough air for the big fire. Really neat stuff, I love it! Caleb Ramsby Quote Link to comment Share on other sites More sharing options...
pkrankow Posted September 10, 2010 Share Posted September 10, 2010 How big of a forge burns 50# of coal an hour? How big are typical production fire pots like from Centaur? Regardless resisting 20#+ of force with an edge supported 8x8 square without buckling will require a very rigid valve. Maybe a grill that has 2 inch slots or perforations so the valve panel is well supported would be a good idea. Phil Quote Link to comment Share on other sites More sharing options...
Ramsberg Posted September 10, 2010 Share Posted September 10, 2010 Grant, It gets a lot more complicated when the speed of sound in approached, the density of the medium also changes the speed of sound, so the more the air is compressed the higher the speed of sound is. When it exits and expands into the atmosphere a bunch of wild stuff happens. The speed of sound in the atmosphere(sea level) is roughly 1,085 fps, in theory it would require just over 9 3/4 psi to produce a flow approaching Mach 1, in reality things are a bit different. That formula is for lower velocities. A blow off tool using compressed air is well above supersonic, that is one of the reasons that it is so loud, that is also why whips crack. One psi for 236 mph doesn't sound like much, but if you put your head into an air at that speed(mine is roughly 70 sq in) if the coefficient of drag was 1 then that would be 70 lbs trying to rip my head off! The force on a 24" by 24" window pane would be 576 lbs! Caleb Ramsby Quote Link to comment Share on other sites More sharing options...
Tim McCoy Posted September 10, 2010 Share Posted September 10, 2010 IMHO - A convergent duct (area of inlet/passage is decreasing) the pressure on the side of input is increased causing a slowing of velocity and conversely on a divergent duct (area of egress is increasing) the pressure is decreased causing an increase in velocity ... the size of the bellows and the rate at which they are pumped will determine the volume and mass of air pressing against the valve (inlet/passage) to supply air to the fire. As the air passes through the inlet/passage to the fire the pressure drops allowing the air to travel at a faster rate. Bellows volume/size, the rate at which it is refilled and emptied, the size of the hole allowing the air to reach the pipe going to the fire will all determine the efficiency of the "bellows". It is how a jet engine works and why air coming out of a small nozzle goes "faster" ... works with fluids also. More air moving across the burning medium makes it burn faster and hotter so volume and speed both are important up to the point that the coal blows out of the hole - es verdad, no?? :lol: Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.