Building a Boiler similar to a garn

[allan]

I'm in the midst of designing my garn like boiler. I have a 2800 gal tank that is 7 foot in diameter and 10.5 feet long. I figure that I can store heat in the water to last at least 24 hours or about 1.4Mbtu's. The total heat storage is 2800 gal * 8.33 lbs/gal *(180-120 degrees F). What I'm trying to figure out is how to calculate my firebox so that my burn time to heat 1.4Mbtu's will only take 3 hours (about 500Kbtus). I can calc the wood required (lbs) but I'm not sure of the right air (cfm) to get the 500kbtu/h burn rate. Once I know this, I can calc my exhaust hx size. The air required to burn completely (Stoichiometric) plus the excess air 40-100%??? I may have to dig deep in the old thermo book. If anyone has suggestions, I'm all ears.

 

[TCaldwell]

eup, my model 1900 runs about 350cfm at 2in water column, it has a 24inch long secondary reaction chamber, the largest garn has a 36 inch src for longer gas residence, i think they both run the same draft induce fan and motor, i think the garn specs section might tell the fire box size.one note, be careful not to incorporate too much exhaust resistence, as with the sealed induced draft there is no way to manipulate the primary/secondary air ratio,,,, unless you add a true secondary air inlet, i wonder if garnification could shed some info.

 

[brad068]

Welcome to the faith you-per. Glad to see that your going the contained way. As far as the specs for the firebox and cfm, I would just build the box out of a 40x96" sheet of .250" sheet rolled. A full 48" is pretty deep. Round is simple, equal expansion and one seem. The fan, I would go with a wheel size @12" with a 1hp motor. Its easier to slowdown/restrict the fan. Its hard to size exactly what cfm of combustion air is needed because unless you are burning "crib wood" the mc can vary the combustion rate greatly. One time I can throw in the driest sticks of oak and the boiler can't get enough air and starts puffing and the next time I throw in green oak and the air flow is almost to great. The best I got was @ 500Kbtu/Hr, High E got 800K and the new kid on the block got @ 600K so far. The next unit I build is going to have A bigger fan and better preheated incoming fresh air. That's what I can see for improvements so far.

 

[allan]

Thanks garnification and tc. I will make the fire box 40" long and 30" diameter. Do you think that I should go with a bit bigger diameter secondary reaction chamber? I will go with a 30 or 36" long secondary chamber. Currently it is 12"steel pipe and 1-1.5 inches of some kind of refractory or insulation. Also I was going to use 6" pipe vs 5" to reduce the WC pressure in the pipe due to friction, esentially reducing the feet per second flow of the air in the pipe from 25.5 ft/s to 18 ft/s assuming that the fan gives a constant cfm which is not the case, but I don't have the fan curves. I have a 9 inch wheel the same diameter as the 1500 garn so I will try this wheel to begin with and I have found a source for bigger wheels if needed. I would like to make the air turbulent in the exhaust pipe if possible as it is always better to mix the air so that the hx is more efficient. I appreciate all input. There is a lot of experimenters here and I like it. Thanks

 

[TCaldwell]

eupi, the od. of the shrouded radial blade for the garn 1900 is 9,15/16", the open id. facing the hx pipe is 5,1/2", the width of the fan is 3,1/8". if you have 6' hx you might need a larger open id. for the fan, to accomodate this. also since you are in the design stage, might consider a 2 hp 56c frame 3 phase motor, controllable with a variable speed drive to modulate fan speed.insulation spectialties of america will vacuum form the secondary reaction chamber sleeves to your dimensions, 219-733-2502. also you might consider a recirculation loop to homogenize the water temp in the boiler. you will find that your actual supply water temp is about 10 deg lower than the boiler temp gauge indicates, cause the supply water is tapped at the bottom of the boiler. i think switzer has a recirc. loop at 40gpm to combat stratification. the 2 hp motor came from grainger, the hitachi vsd from drives warehouse.com. also i moved the combustion air inlet inside. tom

 

[brad068]

Eup, you need to get a vacuumed formed or the moist pac blanket for the secondary chamber. Regular ceramic fiber blanket looses is firmness after it get heated @ 2000*F and acts more like cotton so it needs to be rigidized. You can buy the saturation chemical and moisten the blanket yourself also. I used 2",4" and 6" pipe for the hx...don't know where you are going to sub. the 6" for the 5". I'm curious as to where you have a line on fan wheels? I will build my own but if the price is right I may purchase one too.

I don't know if you noticed that the secondary chamber is placed lower out the back of the firebox. The original unit I had had it centered out the back and from what I found the lower chamber seems to burn hotter longer. As far as size, mine is 10"x26" piece of 10" SCH 40 pipe. I think 12" will work fine, and if you think it is to big put another sleeve inside that one to size it down. I think a foot or two longer wouldn't hurt. Of my whole unit, the door latch and fan wheel were the two most time consuming things to build. About the door, I used two tank ends one 20"(inner) and one 22"(outer) and turned one inside the other which leaves a perfect 1" gap for 1" rope gasket, works great. Good luck and hopefully we can see some pics when you get it done.