draft on top of fire?

draft on top of fire?

PostBy: sparky On: Fri Mar 02, 2007 8:07 pm

Is it really true that you don't want any draft over top of the fire? Even just a little bit?

Needless to say, the top two drafts on my SF 2500 have alway been opened just a crack and do draw a draft.

Is the subject up for debate or is it a no doubter to have none?

I'm willing to crank 'em down and see what happens if most think I should.
sparky
 
Stove/Furnace Make: Harman
Stove/Furnace Model: SF2500 Handfired furnace

PostBy: paving2007 On: Fri Mar 02, 2007 8:47 pm

Hi Sparky,

I am no coal burning expert, as I am new to this(burning coal) but everybody I talked to about draft and the different threads on this site that people talk about draft, it is better to have the door vents closed completely and have the ash pan door vent open

You want the air flow to come up from the bottom of the coal bed instead of over the top of the coal

my 2 door vents are always shut all the way and I regulate my air flow with the ash pan vent and that seems to work well for me

Tony
paving2007
 
Stove/Furnace Make: Fire Boss
Stove/Furnace Model: Wood/Coal Fireplace Insert

PostBy: LsFarm On: Fri Mar 02, 2007 9:19 pm

Hi Sparky, Any oxygen that flows over the top of the coal is wasted.. The coal seems to burn on the sides of the coal pieces as the air flows up through the coal bed.

The only time any above fire air is possibly desireable is when a load of fresh coal is added, there are some burnable gasses given off by the fresh coal that may get burnt by some above fire air. This is debatable in my opinion.

When the coal fire is going well, having the upper air vents open is wasting draft that should be pulling air up through the coal bed.

If you are burning bituminous coal, there is a lot of burnable gasses and soot above the coal bed, but the added air has to be HOT to ignite the fumes. Air through the door is not hot enough usually.

Hope this makes sense. Greg L

.
LsFarm
 
Stoker Coal Boiler: Axeman Anderson 260
Hand Fed Coal Boiler: Self-built 'Big Bertha' SS Boiler
Baseburners & Antiques: Keystone 11, Art Garland


PostBy: sparky On: Fri Mar 02, 2007 10:31 pm

Thanks Tony.
I don't consider myself new to this but I am humble enough to know I don't know it all and am willing to improve.

Greg,
You do make sense. I guess I always figured there was some sort of secondary burn near the top surface but the fire does burn completely vertically, bottom to top.
Sometime this weekend I'm going to put some penetrating fluid on the threads and try to spin those two door drafts close.

Thanks again,
Mark
sparky
 
Stove/Furnace Make: Harman
Stove/Furnace Model: SF2500 Handfired furnace

PostBy: ktm rider On: Sat Mar 03, 2007 1:12 pm

I have a natural draft on the door of my boiler for burning wood and a combustion blower on the ash door for burning coal. When I burn coal in my boiler the natural draft is supposed to be completely closed, or so I thought.
I noticed one day that the door for the natural draft does not completely close. SO, I called the manufacturer and asked them if it was supposed to be all the way closed. Jeff at AHS told me no, it shoud be open just a tad to allow the gases to burn off that will accumulate when the combustion blower is not running and the fire is dormant.
This is most likely depending on the style of firebox and boiler I would guess, so what is right for mine might not be right for yours.

I also noticed that when burning Bituminous coal , if I set the natural draft ( over the fire draft ) to work with my combustion blower ( under the grates for coal) I don't see the dreaded bridging problem as bad as when I have the natural draft all the way closed.
I only burn Bituminous coal so I have no idea how this would affect Anthracite coal .
ktm rider
 
Stove/Furnace Make: AHS Multifuel
Stove/Furnace Model: CO 55 with oil backup

PostBy: dirvine96 On: Sat Mar 03, 2007 1:52 pm

My Hitzer has a spin damper on the loading door. The manual say that it should be open a some to eliminate the possibilty of puffing and blowing out smoke.

Don
dirvine96
 
Stove/Furnace Make: Hitzer 82FA

PostBy: sparky On: Thu Mar 08, 2007 6:37 pm

After closer look, it doesn't look like the spin drafts on the loading door will close completely.
Half of the surface is flush and there is about a 1/16 to 1/8 inch gap around the rest. Maybe it's designed that way?
I'm not having any sort of problem, I just thought I'd try the 100% bottom draft.
Maybe just as well.
sparky
 
Stove/Furnace Make: Harman
Stove/Furnace Model: SF2500 Handfired furnace

hitzer 503

PostBy: hgmd1 On: Thu Mar 08, 2007 11:38 pm

I leave the inlets open on the door now since I once had an explosion in the insert. By leaving them open there is secondary ignition of the gases. I would rather have a little less efficiency and avoid the explosion. The explosion was forceful enough to blow the top off.
hgmd1
 

PostBy: Berlin On: Thu Mar 08, 2007 11:58 pm

"I also noticed that when burning Bituminous coal , if I set the natural draft ( over the fire draft ) to work with my combustion blower ( under the grates for coal) I don't see the dreaded bridging problem as bad as when I have the natural draft all the way closed. "

this is because with bituminous, especially those coals prone to swelling, heat must be added above the coal bed to burn away the tars and bitumin that causes midphase "bridging". cracks form in the solidified coal bed from the intense heat burning away the tars, if the heat is only from the bottom, the tars will only burn away and cracks will only began to form in the bottom of the coal, the top will stay solidified, leaving a "crust" that prevents air from going all the way through the coal bed and creats low heat output, and in some cases can put the fire out or be a general pain in the ass. the results are even better and no poking is needed if either A. preheated air is entering above the coal bed, and directed somewhat downwards toward the bed of coal, thus limiting the timespan of "bridging" to only a few minuites, or B. there are insulating, reflective refractory panels or brick in an arch, which effectively direct heat back towards the burning coal bed creating the high temps neccecery to prevent "bridging". both of these methods either alone or in combination reduce soot and prevent fuel waste. burning a high volitile bituminous, the smoke should turn quickly from yellowish white to a lite black or light grey for the remainder of the volitile flaring cycle, not remain thick white/yellowish which is what will happen if burning high vol coal w/out one of these methods in place.
Berlin
 
Hot Air Coal Stoker Furnace: Will-Burt Combustioneer 77B
Coal Size/Type: Ohio BITUMINOUS pea stoker coal

PostBy: LsFarm On: Fri Mar 09, 2007 1:01 am

I stand by my opinion that any above the fire oxygen is wasted when burning anthracite. The exceptions are burning bituminous during the time the volitiles are burning off, and burning wood. With both of these exceptions, if the secondary air is preheated it will work much better.

I took a quick look at the hand loaded Harman and Hitzer product lineup, and most of the stoves do not have any air vents in the door providing above the coal bed oxygen.

The exceptions are stoves meant to burn wood as well as anthracite coal.

The secondary air from a vent in a loading door is cool air. This cool air will not burn gasses unless the gasses are already at combustion temperature. Secondary air needs to be HOT, preheated by the fire bed, to be effective at burning gasses above the firebed and in the flue.

Take a look at Berlin's stove plans in the 'how to' forum, his design preheats the air in order to effectively burn bituminous coal

The most effective way to prevent explosive burning of gasses is to leave a small amount of the underlying red coal bed exposed when adding a layer of fresh coal. This acts as a pilot flame to burn off the gasses generated by heating the fresh coal. Just adding fresh cool air above the fresh layer of coal will not burn off the gasses, there is not enough heat. But once enough heat is generated, then the gasses will ignite, sometimes explosively. I've blown the chimney pipe off the back of my boiler by not loading it correctly.

As long as you have excess draft, then letting some air over the top of the coal bed does no harm. If when the weather warms up and your draft gets weaker, you may want to eliminate the air over the fire.

Greg L

.
LsFarm
 
Stoker Coal Boiler: Axeman Anderson 260
Hand Fed Coal Boiler: Self-built 'Big Bertha' SS Boiler
Baseburners & Antiques: Keystone 11, Art Garland

PostBy: Berlin On: Fri Mar 09, 2007 1:24 am

"I stand by my opinion that any above the fire oxygen is wasted when burning anthracite"

exactly. most of the combustion with anthricite takes place very very close to the bed of coal; unless you like turning your stove into a blast furnace, for this reason, it is generally unecessary to have much if any overfire air. additionally, any slight increase in combustion efficiency from trying to completely burn the gasses coming off the coal bed by introducing overfire air, may actually reduce overall efficiency and heat output of the appliance by introducing cool air above the fire, and lowering the overall temp in the firebox. this is why baro's and stack dampers are effective, they reduce or eliminate excess air in the firebox by lowering the overfire draft thus raising the temp and overall efficiency of the appliance.

btw, i would not reccomend, but rather caution agians the use of a baro or stack damper in a bituminous appliance for many reasons; trust me, you don't want either when burning bituminous coal.

"I would rather have a little less efficiency and avoid the explosion. The explosion was forceful enough to blow the top off."

I am not familer with your particular insert, however, i would send someone back to welding school if a coal gas explosion blew anything off from a stove/insert. if everythign is built correctly the explosions are not anything to be worried about. although they do occur more frequently with less draft and tend to be worse if using a stack damper (which is why i strongly advise against using one with bituminous, the explosions are much more violent than w/ anthricite and they must have somewhere to go, a stack damper prevents escape of the gasses, and a baro will cause soot and smoke to enter the home.)
Berlin
 
Hot Air Coal Stoker Furnace: Will-Burt Combustioneer 77B
Coal Size/Type: Ohio BITUMINOUS pea stoker coal

PostBy: rouxzy On: Sun Mar 11, 2007 2:33 am

On my Harman Mark III there is a constant draft going in around the top and bottom of glass on door. From what I am told this has a two fold purpose. It washes the glass of ash and keeps just enough draft on top of the coal so you don't get that "flash" when a new load of coal is put in.
Tom
rouxzy
 
Hand Fed Coal Stove: Harman Mark III
Coal Size/Type: Stove, Nut / Anthracite