Modifying woodheater for efficiency.....is this safe? 5

[tabletop]

I built my woodheater to heat my house and it has performed well for many years, however it requires repair. While it's offline I would like to implement some upgrades to gain efficiency. Glass door style woodheaters like this tend to only burn 'completely' and efficiently when the conditions are perfect-lots of red hot coals
Im toying with the idea of superheating the exhaust in the hope of re-igniting the gas to gain heat that would otherwise be wasted.
Once the fire is well and truely underway (1 hour or so)I would redirect the flu down a 3/4" pipe (should be large enough)and using a hot air pump, pump it through the very bed of the fire, back and forth through the coals...
...and here is where Im not sure. Firstly, is this safe? I imagine the smoke temp at this point is extreme. Is this situation potentially explosive??
If safe, from here Im hoping I have a flame(perhaps with pilot flame first), which I could further extract heat from and then exhaust to flu.

So, is it safe?
And, can I expect a flame at the end of this pipe??
btw, Im doing this from a hobby point of view, purely cos I enjoy it

 

[monaco8774]

No expert but the fire needs oxygen to burn, and the flue pulls a draught on the fire so that fresh air is drawn in, if you take some exhaust and put it back on the fire, you might take away some of the excess oxygen that is helping to get complete combustion, what is usually done for heat recovery is the hot flue is used to heat up the air coming in via some exchanger without actually mixing the two. On my wood fire I have a damper in the chimney that directs the exhaust over a grill that heats the room more efficiently (thereby heating the air coming in) you cannot close this damper untill the fire is going because in the beginning you need a lot of natural draught and (excess air) to get the fire started

 

[tabletop]

monaco, I didnt make it very clear,sorry. I dont intend on sending the exhaust back into the fire itself. What I meant was heating the exhaust(smoke) by "piping" it through the coals to heat it up, then back out of the firebox perhaps to a seperate chamber to "burn" the hot exhaust(where I guess I might need to add some fresh oxygen to assist the burn) It's a heat exchange situation, the exhaust is totaly contained in the pipe as it goes through the coals.

 

[JEB66]

You should not have very much combustible materials in the flue gas to burn.

If there is any CO in the flue, the amount of energy you are putting into "pumping" it will be more than you get out.

 

[tabletop]

JEB66, No I dont expect huge gains here, just that I always notice a constant slow flame dancing at the roof of the firebox well above the wood but only when the fire is well fueled and quite hot. Before and after this 'peak' I believe Im loosing some fuel straight up the flu.

No biggie really, happy to give it a go but still worried if there is potential danger.....anyone?

 

[25362]

Have you considered at what temperature whould the blower (hot air pump) be supposed to work ?

 

[tabletop]

Around 350 celsius, 400 max. Havnt given it much thought yet, tho I know it will be a hurdle. Going on the hunch that if turbochargers can do it.....
Im thinking I might start with an old car heater box blower impeller. These are quite robust. Would build a steel shell for it and drive with small external motor. Should last long enough to see if this whole idea floats.

 

[berkshire]

tabletop (Materials)
Most commercial boilers burning wood inject " over fire air" to add extra oxygen to burn the smoke. This is placed in the furnace at about the top of the flame.
B.E.

 

[tabletop]

Actually some of the better new woodheaters do this too usually with a atmosphere fed tube running across the roof of the fire box with small holes. I will be doing this also, only cos I have too much time on my hands ,I will be pre-heating this air too (from the exhaust). All about not cooling the fire with the oxygen you feed it.

Still this issue of whether super heating smoke is potentially explosive? Anyone?? ...c'mon now, dont make me find out the hard way

 

[DanEE]

What you are trying to accomplish was done in a stove called a Vermont Downdrafter that was on the market many years ago.. I still have one that I bought in 1978 and it is a very efficient stove..

I googled trying to find any documentation you could look at regarding this stove, but this discussion was all I found..

http://www.kountrylife.com/cgi-bin/topic.cgi?bd=forum&th=155405

You start the stove off on "updraft mode" (the flue exhaust at the top of the fire box is open), and once you have a bed of hot coals, you switch to down draft mode by closing the upper flue exhaust.. The smoke then can exit only by being pulled down below 2 internal firewalls (one on each side of the stove that terminates just above the V shaped grates filled with hot coals. These internal firewalls in essence make a stove within a stove with the outer combustion chamber wrapping around the inner combustion chamber on sides and rear of the stove.

The extra air to flare the gases comes from the same thermostaticly controlled air feed that feeds the V shaped grates at the bottom, with the excess air being pulled into the secondary burn chamber on the outside (of the inner firebox) and just above the V shaped grate.

And contrary to what is said above there is a significant difference in heat output, once it is running in downdraft mode and flaring off the unburned gases from wood..

With cleaner burning fuels that have a more controlled primary combustion process, I would guess the improvement would not be so great...but in my experience with wood combustion with this stove, there is..

When it's tuned just right you can see the flared gas burning with a bright blue flame.. (the stove has two small 1/4" holes so you can peek into the secondary combustion chambers)..This secondary chamber has firebrick at the critical points due to the high temps that occur in that vicinity. The wood stacked above the V shaped grates in the primary chamber is in an oxygen starved environment and is "coked" from the heat of the flared gases being burned below and in the side chambers.. The coked wood drops down and keeps the grates supplied with red hot coals.

One thing I've found is your wood needs to have very low moisture content. Otherwise excess steam coming out of the wood as it's heated before combustion will kill the whole process.

Hope this is useful information for what you are trying to accomplish.. this stove design might be the simplest way to do what you are trying to do without any external blowers, etc..

I believe I still have the original documentation around somewhere.. if you are interested I'll scan it and email it to you...

 

[rmw]

The smoke that you are trying to burn is unburned fuel which is carbon (visual smoke) or carbon monoxide. Either will burn at the same temperatures as the original flame that created them in the firebox. So you are not going to "superheat" it any more than it was when it was born.

If you actually did manage to torch off the smoke, then you would have temperatures approximating those in the firebox.

The one thing that can happen is if it burns unevenly it can 'puff' (which is the word often used when coal grinder mills explode). This occurrence can make rectangular duct work on coal boilers look like sausages. I don't think you will have that much volume of gas to torch off unless you have an awful long flue.

Basically, to summarize, what is going to happen to your smoke is that when it reaches ignition temperature, assuming your plan get it there, it will complete the combustion process if it has enough oxygen to burn.

Someone mentioned overfire air earlier. I have seen overfire air jets in wood boiler furnaces where the flame was the typical yellow wood fire color the blue color of a gas flame. That is where the smoke was getting enough oxygen to complete the combustion process.

I hope this information helps.

rmw