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[paperman]

In laying out the gas path for my unit I was thinking of a reason why I could not have the gas port in the floor of the primary in the front (toward the door) of the chamber opposed to the back that I see most of the time.  This would allow the secondary chamber to be unidirectional.  All the flue gas would flow to the back and not have to make a loop.  With 5' of secondary chamber I would hope all the combustion has happend by then.  Should primary air come in high and away or close into the wood?  The upper rear of the primary would give all the wood a chance to burn opposed to only the wood closest to the nozzle using all the air with an in the bed manifold.  Secondary air would route low almost through the fire bed to the front of the secodary chamber and blow back to the rear of the unit toward the first pass heat exchanger. 

Is 48" to long of a firebox?  Figured I would just fold up a sheet and make my dimentions fit that but if its to deep I can shorten it before its broke.  Would a radial blower work better for air supply?  The high pressure of a radial would help secondary mixing and burn I would think.

   

[peacmar]

The biggest reason for a rear inlet is to keep things simple. Most of us and most manufactures like to have the fan enclosed in the back of the burner with all the other mechanical parts of the burner so everything is packaged nicely and access is easy. If the inlet is in the back then there is less routing and less chance of future failure. As far as a low inlet, this could complicate the process. A high inlet provides two major import aspects.1 Even air distribution around the charcoal bed, or more properly described as the "hearth zone." 2 preheating of the air before it reaches the hearth zone as it passes through the reduction zone. If your air is too cool when it hits the hearth zone it will kill efficiency drastically. Likewise, an uneven distribution of air will cause bridging in The hearth zone and inhibit proper gassification. Bridging is when the coals burn faster then the wood above reduces to charcoal and gassification stops completely. You are on the right track when it comes to secondary mixing, this can be very critical to achieve proper and complete burn, but be wary of too much primary chamber pressure and too much nozzle velocity as this too can cause bridging. Size your nozzle to your heat needs, and make an air flow adjustment for the primary chamber. The ideal nozzle velocity for wood is 33 feet per second. This will determined e how many nozzles and their size. A 48" chamber is not to much, but keep in mind you want all the wood to get to the nozzles on its own so design accordingly. Use gravity here. If you need some help with designing the nozzle or calculations just ask, time is a little tight for me lately but ill be happy to help as much as I can.

[paperman]

Thank you for the answers.  Can I assume that if I can fashion a swirling nozzle that it would help mixing in the secondary and should promote better burn?  I am taking delivery of a small radial blower today to play with and see how well it performs.  After I get a feel for the curve of this fan I can find out how many inchs of water it can develop and start looking at nozzle sizing. 33 FPS seems pretty quick. 

[peacmar]

A nozzle that developes a swirl in the secondary is ideal and is what I'm after. The secondary chamber does not need to be round as I have tested this theory. Square actually makes more turbulence which seems ideal. You need to think in terms of CFM with your blower instead if in/h2o to work out the calculations. So that information would be vital to know. Also, might I make the suggestion that you keep all your questions for your build in one thread? If you start a new topic with every question it can make things get very cluttered. I watch for the NEW marker at each thread and check accordingly. Just a helpful thought to pass along to you and others.

[paperman]

As I am still in the drawing stages I have to make a few assumtions and Im sure my math is off abit. My question is can I achive complete combustion in the secondary firing 150,000 btu in a 50" tunnel?  I see some commercial units make the path longer while others have a fairly compact chanmber.  That should burn 22# of wood/hr and need 37CFM of air to make happen.  I have a 120 CFM radial blower to use.  Would 2" ball valves be usefull for air metering?  The actuator and fan arrived by Big Brown yesterday.  Also picked up the 10 ga. for the tank walls and the flue material. 

[paperman]

Being forced draft are we limited by the flue out let size?  I was planning on ( 2" flues out of the secondary but that is only 24.8" and a 6" flue is 28.26 or about 13% smaller.  Does the FD make that a none issue?

[willieG]

Being forced draft are we limited by the flue out let size?  I was planning on ( 2" flues out of the secondary but that is only 24.8" and a 6" flue is 28.26 or about 13% smaller.  Does the FD make that a none issue?

im gonna guess at this one so take it as just that....the forced draft may be restricted slightly due to friction but for sure the tighter pipes will be cause for increase in speed of air movement through the smaller diameter

think of a stream..a large wide area in teh stream looks like it is hardly moving but choke that stream down around the bend and it appears to be wildly roaring

[peacmar]

Forced draft can be as forgiving as it can be finicky I have found. you can push, but only so hard before gasses push back. Gasses act like a spring. You can cram in more than can escape and you get a back draft situation. I've managed to make my boiler that weighs 1200 lbs, plus 400 gallons of water, dance around like and Irish jig. There are 2 bad situations that can come of this, flammable gasses can push back into the primary chamber and ignite, key word kaboom! Or the sides will flex like a drum, and eventually you will have fatigue failure of the metal. Could you please be a little more specific of what your numbers mean? Are you talking surface area of tubes? Cross sectional area of flue? It would be easier to help if I understood what those numbers where.

[paperman]

Cross sectional area.  My ( 2" ID flues will have a total of 25.12", these will feed a chimney outlet of (6") 28.26.  My main question is would say is will ( 2" flues support the Btu output of a 6" stack assuming it is in a forced draft situation?  Is there a way to figure mass air flow of a gasification furnace at a given Btu or #/hr of wood burned.  I assume it would be fairly long equation as you have heating and cooling at different stages of the furnace.  I dont want to put the work into this unit and have it be a pig, but I also dont want to dig so far into every detail that I research for 2 more years.  I started forming the transition boxs yesterday.  I would like to start cutting the flue sheets tonight, assuming the 2" flues will work. 

[peacmar]

By flue I'm going to assume that you are talking about the heat exchanger tubes (Hx from now on). The flue is actually just the chimney portion. As long as your cross sectional area of the combined Hx tubes are reasonably close to the flue area you won't have a problem. Now I have to ask, are you forming the heat Exchange tubes? Why not use pipe? Also, keep in kind that the heat exchanger is the area of major wear on any boiler. The tubes erode on the inside from the hit gasses and comrade on the water side from the changes in temperature. Industrial boilers use special tubing but for us pipe is fine. The idea is to make replacement as easy as possible. Round shapes also take heat expansion better, and pipe is anealed so there are no points of stress within the structure. Also keep in mind that you will have to clean them regularly to get the best efficiency.

As far as calculations, they are all quite simple and short.

Btu/hr requirements translate into lbs/wood burned

Lbs/hr wood burned translated into cfm air needed

Cfm air translates into cross-sectional area of hx tubes and flue diameter

Hx tubes should be somewhat more area than flue diameter, as the gasses are heated, Charles law explained that volume will decrease as they cool. Therefore will require less area after the hx. Theoretically, if your hx cools all the way back to ambient temp, the flue must only be as large as the fan inlet.

[paperman]

Don’t know how my (eight) turned into a dude with shades but oh well.  The Hx path is made from 2" Id sch 10 pipe not formed.  I had thought the calcs would be that easy but I do not think they are.  We are taking the wood as a solid fuel and turning it into a gaseous fuel.  I was hoping for a constant to use such as X#'s of wood = X# or CFM of exhaust by product.  I am a powerhouse maintenance supervisor so I am fairly comfortable with building the unit. There is a big reason our ID fans are MUCH larger than the FD.   I am no engineer; I want to make this unit the best I can so I am asking the questions that come to mind.  Wish I had all the answers but I am far from educated in fluid/thermal dynamics.     

[peacmar]

83.177 cubic feet of fresh air to 1 pound of wood for complete combustion.

Primary air to secondary air is 1:5 respectively

All wood grows at different densities, but the accepted btu value of wood her pound is 8600.
6880 btu per pound at 20% moisture content, which is the ideal moisture content for a gasifier.