Flue Cube - any thoughts or experiences of this??

[WFWales]

Thanks Chaps.

Most Intriguing - please keep the comments coming

[Stereo]

Our esse w23 has a SS grill just above the fire which heats up and causes the gasses to burn off as they pass through apparently. But it's no more than 18 inches above the fire.

[Alycidon]

Our esse w23 has a SS grill just above the fire which heats up and causes the gasses to burn off as they pass through apparently. But it's no more than 18 inches above the fire.

 

ESSE call that the secondary burn,

 

A

[openspaceman]

Can someone explain to me how there is a secondary 'burn' on top of the chimney pot?! The word burn implies flame and if things are that hot up there I'd better have the Fire Brigade on standby.

 

If by burn you mean a fast oxidising process giving off heat then there are many instances of this where there is no flame. A flame is an area of combining gases and from that one can deduce that most flames from a woodfire will be secondary combustion. So yes if you allow fuel gases out of the stove/fire and up the flue then they can burn on top of the chimney pot when they meet air. The conditions would have to be right though and this means either some sort of flame holding or temperatures above the auto ignition point. I posted a little while back about a blue flame on the pot from CO being produced in a chimney fire.

 

In the absence of high temperature or flame holding, then you need something to drop the temperature threshold at which the reaction can take place (actually you can add a support fuel but that;s even more wasteful). This is what a catalyst does. Thermal oxidisers for reducing organic smells from factories or waste treatment facilities seem to be surface effect combustion using catalysts.

 

I think the videos show a rather simple effect, the before smoke is black, implying lack of air in the fire, the simple solution is to add more secondary air at the stove. I cannot see further oxidation taking place above the stack without it getting very hot and this would imply a colossal waste of fuel. The other thing is the device is shown above a chimney pot that already has some sort of air entry at its base.

 

If this device does do any good it's to do with controlling draught rather than any combustion effect.

 

The smoke indications are fairly simple:

 

If you see white smoke it's small droplets of condensed vapour, mostly water but if the fire is hot but smouldering then some pyrolysis products as well.

 

As it turns yellowish then you are getting pyrolysis products probably with some Products of Incomplete Combustion.

 

Brown to black and you are sending lots of PICs up the flue because the hot fire is pyrolysing the wood and the products for this are starting to burn in a secondary flame but are starved of air. Looking at the flame it will be purpley near the tip and tailing off into black.

 

As the air conditions get better the sooty PICs decline but are still emitted as a blue haze. Even with a good air supply it's difficult to do better than this with wet wood and natural draught because the flame is being quenched before it can complete.

 

No smoke indicates good combustion temperatures but not necessarilly complete combustion as a lot of CO may still be present.

 

I see there has been another death from CO poisoning from a barbecue left near a tent door which shows how insidious CO poisoning can be. The dangers from burning charcoal are exacerbated because all the normal warnings from woodsmoke, the acrid smell, are missing.

[nepia]

Thanks for that. You've lost me a bit in the detail but I get that in theory at least secondary burning of waste gases is possible well away from the primary burner; that comes as a bit of a surprise but I've learnt. Cheers.

[openspaceman]

Thanks for that. You've lost me a bit in the detail but I get that in theory at least secondary burning of waste gases is possible well away from the primary burner; that comes as a bit of a surprise but I've learnt. Cheers.

 

It shouldn't be much of a surprise, consider a car engine, it works at near a perfect mix of fuel and air but there are only milliseconds to complete the combustion. As there is not quite perfect consumption the hot gases ( about 500C) travel on to the catalytic converter. Now as I said previously the catalyst just seems to lower the threshold at which a chemical reaction takes place. So any surviving oxygen molecules and any remaining part burned fuel can have a second chance to meet on the catalyst and combine. Of courser then their energy is given up as heat in the catalytic converter.

 

A couple of points here, if one cylinder misfires persistently then all its fuel and oxygen end up burning in the cat, which will get very hot.

 

The other is that prior to cats being mandatory lean burn petrol engines were being developed, these were challenging diesel engines for economy. However a catalytic converter would be receiving far too much oxygen to fuel gases from a lean burn and hence legislation made put paid to their development.

 

Running on from this you'll begin to see that fuels burn in fixed ratios of fuel to air, e.g. one carbon atom requires 1 oxygen molecule (this is two oxygen atoms) but because air consists of 21% oxygen and 78% Nitrogen you have to supply 5 times more volume of air to get the right amount of oxygen. The correct ratio of fuel to air is known as the stoichiometric ratio. This stoichiometric mix is what is needed for a car engine.

 

Wood is more difficult to burn but we have much more space and time to complete the combustion, we don't want smoke ( which is products of incomplete combustion) so we increase the opportunity for an oxygen molecule meeting a fuel molecule. We do this by adding excess air and maximising the three Ts, retention Time for the reactions to complete in the burner, Turbulence to increase mixing and the chances of an oxygen molecule to meet the fuel and Temperature to provide the energy to dissociate the molecules and facilitate complete combustion.

[nepia]

My surprise is unabated because there isn't a cat in the flue cube is there but again I'm grateful for the effort; cheers.

[rxe]

The site suggests some sort of catalytic burn in the cowl. I don't see how you'd have the temperature to do this, and you wouldn't want it anyway. The last thing I'd want on my roof is some sort of uncontrolled secondary burn device.

 

The reduction in smoke? Easy - it is a diffuser. It takes cold air in at the bottom, mixes it with the hot smoky air and, ta-dah, less (apparent) smoke.

 

If they've genuinely found a way of oxidising pollutants at low temperature with a device that can sit on the roof for a decade, they'd be better off selling it to the car industry.

[Goaty]

Didnt these used to be £166? now they are £249!!!!!!!

[Alycidon]

I did have a long chap with one of their guys at the stove trade show last month, designed round a vortex principal. He did explain how it works, but I cant just remember the detail. For what it is it does seem costly and therefore probably only going to be used where flue problems have been found that standard cowls cannot alleviate.

 

A