Induced Draft Fans

Induced Draft Fans

PostBy: DavidW1820 On: Tue Dec 14, 2010 10:51 pm

Has anybody here had any experience using an industrial duty centrifugal exhaust fan with radial blades to maintain an ideal draft in a boiler firing soft coal. My thought was to install it next to the chimney and use a standard draft control inline between the fan and the boiler to prevent excess draft. I figured there would be many benefits. Much smaller chimney required, much stronger draft in any weather condition, no puffbacks when loading. Only downside I see would be the periodic cleaning of the housing and fan blades and the cost of the fan and electricity to run it. They can be had with temperature ratings to 800 degrees which should be far higher than the flue temp of a typical 200,000 btu boiler, hand fired. It would seem to solve many of the problems people have with burning bituminous coal in smaller hand fired boilers. I have not seen any commercially available powerventers specifically designed for solid fuel use. They are all designed primarily for gas combustion.

Dave
DavidW1820
 
Stove/Furnace Make: Burnham
Stove/Furnace Model: 4N-240

Re: Induced Draft Fans

PostBy: BigBarney On: Wed Dec 15, 2010 12:40 am

What happens when you've just loaded coal and the power goes out?


Even with normal operation this is not a safe practice .


On a hand fed boiler the only safe way is a normally

aspirated or blower fed boiler to a good chimney.

You cannot depend on an electrically powered device to

exhaust the flue,it can be done with a stoker designed to

have a small amount of coal on the grate which will burn

out quickly without the draft.


To prevent excessive draft a barometric damper is always

suggested in the flue to even out the draft due to heavy

air, wind gusts and changes in barometric pressure.


BigBarney
BigBarney
 

Re: Induced Draft Fans

PostBy: DavidW1820 On: Wed Dec 15, 2010 4:32 am

I understand your point. If in the unlikely event that happened, there would still be a fully functioning chimney to fall back on. I'm not advocating a dryer vent poking out of the side of the house. There would be a 30 foot class A lined chimney to exhaust the burning coal. I would imagine that with a sudden power loss and a full load, the fire would gradually die down and go out. You could easily use battery back up or a generator too to keep it going. That's what large industrial and utility boilers do.
DavidW1820
 
Stove/Furnace Make: Burnham
Stove/Furnace Model: 4N-240


Re: Induced Draft Fans

PostBy: Yanche On: Wed Dec 15, 2010 11:00 am

DavidW1820 wrote:Has anybody here had any experience using an industrial duty centrifugal exhaust fan with radial blades to maintain an ideal draft in a boiler firing soft coal. My thought was to install it next to the chimney and use a standard draft control inline between the fan and the boiler to prevent excess draft. I figured there would be many benefits. Much smaller chimney required, much stronger draft in any weather condition, no puffbacks when loading. Only downside I see would be the periodic cleaning of the housing and fan blades and the cost of the fan and electricity to run it. They can be had with temperature ratings to 800 degrees which should be far higher than the flue temp of a typical 200,000 btu boiler, hand fired. It would seem to solve many of the problems people have with burning bituminous coal in smaller hand fired boilers. I have not seen any commercially available powerventers specifically designed for solid fuel use. They are all designed primarily for gas combustion.

Dave


The AHS and AA boilers have internal combustion inducing fans. Both use Anthracite coal exclusively. Neither has a feedback control system to vary the blower speed based on some measurement. While such a system would get more BTU's out of the fuel the cost would be very high. Ideally you would measure the flue exhaust gases and compare what you measure to the ideal coal combustion chemistry equation and adjust the air as needed. It's done for power plant sized boilers but for even large residential boilers it's just a pipe dream. Designing a reliable measuring system for the corrosive flue gas environment would be quite an engineering challenge.


DavidW1820 wrote:I understand your point. If in the unlikely event that happened, there would still be a fully functioning chimney to fall back on. I'm not advocating a dryer vent poking out of the side of the house. There would be a 30 foot class A lined chimney to exhaust the burning coal. I would imagine that with a sudden power loss and a full load, the fire would gradually die down and go out. You could easily use battery back up or a generator too to keep it going. That's what large industrial and utility boilers do.


But such large industrial and utility boiler installations aren't living spaces. And they aren't designed for sleeping, i.e. long periods were a failure goes unnoticed.
Yanche
 
Stoker Coal Boiler: Alternate Heating Systems S-130
Coal Size/Type: Anthracite Pea

Re: Induced Draft Fans

PostBy: BigBarney On: Wed Dec 15, 2010 1:21 pm

The reason I posted my remark is that you said a smaller chimney

would be used, with an 8" - 30' clay tile lined chimney you don't need any

induced draft.

This type of chimney will draft all you need to burn coal.

Puff backs happen when loading in a manner that causes the

burning mass to be covered too much and exposing the new fuel to

much of the glowing coal,the volatiles can not be burned fast

enough so a puff back happens.




BigBarney
BigBarney
 

Re: Induced Draft Fans

PostBy: DavidW1820 On: Wed Dec 15, 2010 6:54 pm

I'm aware of the fans that the AHS and AA boilers use to induce draft and essentially I'm suggesting the same thing, only located outside the boiler housing. The big advantage as I see it is to better control the combustion without any worry of the weather and, to minimize the size and height of the aforementioned chimney. The exhausts of the AHS and AA et al, exit the boiler under pressure of the fan so therefore they need little to no draft from the chimney except to make sure it all safely leaves the premises. This is what I'm thinking. The size and speed would have to be carefully calibrated to generate just the right amount of pull. If it stopped working the flue gases would still pass right through it up the chimney until the fire went out.
DavidW1820
 
Stove/Furnace Make: Burnham
Stove/Furnace Model: 4N-240

Re: Induced Draft Fans

PostBy: Berlin On: Wed Dec 15, 2010 10:45 pm

it could be done, but why minimize the size of the chimney? a proper chimney setup will provide ample draft under any conditions and do it for free (no electricity) or wearing out (unless, of course you're using stainless). In addition, a proper chimney looks good and keeps the flue gasses high enough so that there is no smell or smoke at ground level and serves to dilute those gasses before they enter the area where one might be breathing. If you want better control of draft or the fire, use a baro or a thermostatically controlled air control for the appliance.
Berlin
 
Hot Air Coal Stoker Furnace: Will-Burt Combustioneer 77B
Coal Size/Type: Ohio BITUMINOUS pea stoker coal

Re: Induced Draft Fans

PostBy: DavidW1820 On: Thu Dec 16, 2010 2:41 am

Berlin:

I get the benefits of a good chimney and have one. It's not as tall and wide as it should be though. A fan would make the gases exit at a fast rate and go straight up. You lose a lot of heat through a big chimney to get a strong draft. It would be cheaper to add a fan than rebuild the chimney.
DavidW1820
 
Stove/Furnace Make: Burnham
Stove/Furnace Model: 4N-240

Re: Induced Draft Fans

PostBy: BigBarney On: Thu Dec 16, 2010 3:37 pm

David

What is the height and size of your chimney?

Are you using a barometric damper?

You don't lose much heat up the chimney,the

barometric damper handles that task,and lets the

appliance have just the draft it needs for efficient

combustion. The air bypassed in the Baro is low

temperature room air ~ 70* not the ~ 200* air in

the stove.


BigBarney
BigBarney