Positive or negative draft for my homemade gasifier?

[deerefanatic]

Hi all... I'm building a tarm/eko clone stove for myself..... It's going to be designed to be an outdoor unit (insulated with a weather resistant covering) and as such, won't have much of a chimney. (Think OWB...... Scary, I know! )

I was originally going to do a positive draft like Tarm and EKO do currently with blowers for secondary and primary air separately; both being speed variable.......

Now I'm reading where these companies are building their new crop of stove with negative draft blowers in the exhaust stack and air control sliders to control primary and secondary air... This seems much better to me as I could have a single speed fan and then have air "sliders" to adjust my air ratios.. Much less hassle and more reliable I would think.. Plus, I could get away with a short (4 or 6 foot tall) stovepipe (which I already have) vs needing a tall chimney (that I don't have) for draft......

If folks think that this would work well, where is a good source for a draft inducer for a stove like this.. I'm having a tough time finding draft inducers that can cope with wood fuel. (Lots of ones for propane/fuel oil)

I'm thinking that my output when finished will be around 300,000 to 325,000 btus....... Just as an FYI

 

[Redox]

I think there may be some European manufacturers of draft inducers designed for wood, but I don't think you are going to like the price. The Tjernlund inline inducers aren't made for wood, but if you kept it clean, it would probably work OK, assuming you achieve gasification. Using a combustion air fan (positive pressure) is probably a lot cheaper and simpler and seems to be working for a lot of the manufacturers.

Let us know how it goes with the prototype; I'm guessing it's going to be JD green?

Chris

 

[2.beans]

would this one work? http://www.draftinducers.com/draftinducerinstall.htm

 

[TCaldwell]

the garn is draft induced with a 56c frame single phase jet pump motor, grainger sells them for about $100.00 , they are able cause the motor mounts outside the boiler and the mounting plate has a insulated interior cover isolating the boiler flue temps 400/500degf at that point from the motor mounting plate, only the motor shaft that the fan attaches to perforates the omsolation, i think wood gun has a similar set-up. check out garnifications build pics to see a garn style

 

[steam man]

would this one work? http://www.draftinducers.com/draftinducerinstall.htm

I've never seen one of these operate but my first guess is that that are meant to "assist" the draft thus probably can't do what a gassifier really needs, especially a 300,000btu+ unit.

Mike

 

[deerefanatic]

Yah, I wondered about the Tjernlund AD-1 if it would flow enough air........

My other thought is a pair of these: http://surpluscenter.com/item.asp?UID=2008060808421424&item=16-1258&catname=electric (One for Primary, one for secondary) with a transformer and variable DC supply..... For positive draft....

 

[brad068]

http://us.exhausto.com/files/pdf/ProductInformation/3932001.pdf. I think this would be a better option than the AD-1.

Doesn't that Garn setup just keep looking better and better?

 

[steam man]

The problem choosing a fan is that it needs to be rated for the flow and the pressure drop through the system. I've seen different HVAC systems that used twice the horspower for the same flow rate required. Maybe someone else here could tell you how much air is needed to burn wood as in lbs air/lbs wood and base your airflow on that. Next, without being able to measure the actual pressure drop with the required airflow presents a problem. You could end up oversizing or undersizing the horsepower required. Maybe someone here with a boiler of the rating you have can give you the fan specifics you need to get you into the ballpark. This isn't insurmountable it sounds like with your skills, I just wouldn't want to waste alot of time and money getting it right.

 

[brad068]

In the HVAC industry all the parameters are controlled right down to the precise amount of fuel needed. In a chunk wood combustion that is almost impossible. For one, unless you can dry your wood down to that exact same moisture content, cut it to the same dimention, and stack it perfectly every time, the rest of the combustion controlling process is going to be difficult. When you burn chunk wood, there is so much surface area that can either want to combust rapidly or slowly. The fuel is not metered. When you can start to breakdown the fuel into chips, pellets, and then sawdust you can control the combustion process better.

Thats why the Garn process works so well. One thing that I noticed with mine is that when it goes into the puffing stage (3:10 to Yuma), That is when I see the biggest temp rise in the water. My best is 500kbtu, High E is 850kbtu, and the Newkid is 720kbtu. I don,t think that is anything to sneeze at. When that "puffing" is going on, there are excess combustible gases in the firebox that were driven off from the heat just waiting to be mixed with the right amount of o2 and when that happens there is a explosion in the firebox and secondary chamber which disrupts the exh. flow creating pressure in all directions. I know it don't sound right but it sure works well.

 

[deerefanatic]

Garnification: No, a garn isn't looking better........ LOL!

Yah, I'm thinking I'm going to put the nix to the negative draft deal.. I can use DC motors with PWM for speed control and get a really adjustable air ratio system......

What do yall think of the blowers I found earlier for positive draft? I've been told that for my size stove, 100-150 cfm's of primary air is probably enough and about half that for secondary.... So Those should easily be big enough... Then I can slow them down......

Oh, FYI, I know that those blowers have their own speed control circuitry, but I'm going to bypass it.....

 

[brad068]

df, Do you got any of the stove built? I think you better stop thinking and start building. Snow will be here before you know it!

 

[deerefanatic]

Oh yah....... I had an OWB that I'm using as my base to build off of (lots of steel in those, plus the loading door is already built, which saves me alot of work..) I have a little more cutting to do, then I can start reassembling... I'm using the OWB firebox and lining it with firebrick... I'm going to cut out the firegrate (almost done with that) and put a refractory floor in place.. Then enlarge the ashpan and use it as a secondary combustion chamber. Then it's just a matter of building the heat exchanger and various shrouds, and it will be done........

 

[mikeyny]

DF,
Yes, you are really on to something. This has been my own scheme of things for some time now. It will be very necessary to know the cfm req. at given btu outputs. As you know wood varies from monemt to moment. You need to have something to monitor the output and regulate the blowers (negative draft). A plc would realy simplify things. Do it.
Mike

 

[deerefanatic]

Oh, where did you get the blower wheel for your motor for your unit Garnification? If I could get the wheel, maybe I could fab up something myself with a motor.... The biggest deal I see is getting the housing built for it.......

 

[deerefanatic]

DF,
Yes, you are really on to something. This has been my own scheme of things for some time now. It will be very necessary to know the cfm req. at given btu outputs. As you know wood varies from monemt to moment. You need to have something to monitor the output and regulate the blowers (negative draft). A plc would realy simplify things. Do it.
Mike

Yah, that's why I'm going to select a blower much in excess of what I need.. You can always slow them down... But you can't speed them up past what they are able to do.....

I'm picking DC motors so that I can use a simple PWM circuit to speed control them. (manual speed control...... Not automatic, but still better than no control at all.)

 

[brad068]

Oh, where did you get the blower wheel for your motor for your unit Garnification? If I could get the wheel, maybe I could fab up something myself with a motor.... The biggest deal I see is getting the housing built for it.......

I made it and the housing it goes into. Is the owb unit you are using sound? It may be a whole lot of work for nothing if it rusts through in a year or two. Another great thing about the garn units is that every weld joint is double weld except the pipe weld fittings which are beveled good.

 

[heaterman]

would this one work? http://www.draftinducers.com/draftinducerinstall.htm

I've never seen one of these operate but my first guess is that that are meant to "assist" the draft thus probably can't do what a gassifier really needs, especially a 300,000btu+ unit.

Mike

That would be absolutely correct. Not what you want.

 

[deerefanatic]

Oh, where did you get the blower wheel for your motor for your unit Garnification? If I could get the wheel, maybe I could fab up something myself with a motor.... The biggest deal I see is getting the housing built for it.......

I made it and the housing it goes into. Is the owb unit you are using sound? It may be a whole lot of work for nothing if it rusts through in a year or two. Another great thing about the garn units is that every weld joint is double weld except the pipe weld fittings which are beveled good.

Ahh.... Well, designing a blower wheel AND a housing is a bit beyond my capabilities.... Looks like it's gonna be a positive draft system......

Yes, the OWB seems sound.... It's giving me a bugger of a time trying to cut it apart with the Oxy torch.... And I've already trashed several recip saw blades when I was using the saw.......

I plan to clean off the surface rust and stuff and give the outsides of the panels a shot of that hi-temperature manifold paint...... You know, the stuff they use on exhaust manifolds and stuff.

 

[TMonter]

Ahh.... Well, designing a blower wheel AND a housing is a bit beyond my capabilities.... Looks like it’s gonna be a positive draft system......

And likely a waste of your time as you can almost always purchase a fan for far less than you could engineer and build one.

Definitely go positive draft on the forced air side. You can always add an ID fan but you need the static and flow that a FD fan on the combustion side generates

 

[heaterman]

Oh, where did you get the blower wheel for your motor for your unit Garnification? If I could get the wheel, maybe I could fab up something myself with a motor.... The biggest deal I see is getting the housing built for it.......

I made it and the housing it goes into. Is the owb unit you are using sound? It may be a whole lot of work for nothing if it rusts through in a year or two. Another great thing about the garn units is that every weld joint is double weld except the pipe weld fittings which are beveled good.

Ahh.... Well, designing a blower wheel AND a housing is a bit beyond my capabilities.... Looks like it's gonna be a positive draft system......

Yes, the OWB seems sound.... It's giving me a bugger of a time trying to cut it apart with the Oxy torch.... And I've already trashed several recip saw blades when I was using the saw.......

I plan to clean off the surface rust and stuff and give the outsides of the panels a shot of that hi-temperature manifold paint...... You know, the stuff they use on exhaust manifolds and stuff.

A major thing to consider if you're going with negative draft is the flue temp where the blower wheel is located. On a Garn the wheel is rarely exposed to more than 400*, well within the tolerance range of even mild steel. In a typical OWB stack however, I commonly measure temps above 900* during a burn. This gets into the area of metal fatigue rather quickly and will be an issue for you if the boiler design lacks good heat transfer capability like a Garn.

 

[TMonter]

That's a good point heaterman. Stack temperatures need to be kept below 450 on most fans to prevent metal creep when operating. Otherwise you are into exotic fan materials for the blade and housing not to mention special cooled outboard bearings.

 

[Redox]

That's a good point heaterman. Stack temperatures need to be kept below 450 on most fans to prevent metal creep when operating. Otherwise you are into exotic fan materials for the blade and housing not to mention special cooled outboard bearings.

Not to mention that all the fan curves go out the window at that temperature...

Chris

 

[TMonter]

Not to mention that all the fan curves go out the window at that temperature…

Not necessarily, you just need to get revised curves for the new temperature which most manufacturers should be able to provide if the temperature is within the design range of the fan.

Higher temperatures just mean more horsepower is required to get the same flow in lb/hr for the lower density.

 

[Redox]

Not to mention that all the fan curves go out the window at that temperature…

Not necessarily, you just need to get revised curves for the new temperature which most manufacturers should be able to provide if the temperature is within the design range of the fan.

Higher temperatures just mean more horsepower is required to get the same flow in lb/hr for the lower density.

True, but there was discussion about homemade fan wheels. My point was: what works on the bench probably won't on the stack.

I worked on a VSD for a draft fan on a Chiptec gasifier once. Gross overload when cool, but once it was up to operating temperature, everything was fine.

Chris

 

[deerefanatic]

Ok then. Positive draft it is! (I've always been a positive person anyway! )

I'm seriously thinking I'm going to use two of these: http://surpluscenter.com/item.asp?UID=2008060808421424&item=16-1258&catname=electric

Cheap, Hi flow, all metal.... What more could one want?