[Stoves] Black radiating surfaces

[andrew]

On Sunday 25 May 2008 09:27:15 John Davies wrote:

> > Discussing this with Nigel for a minute, it seems that you might
> > get more heat off the hot surface by putting an open ended
> > vertical convection tube over the hot part to increase the air
> > speed past the surface. This will pull
> > off more heat than the radiation will, though it will deliver it
> > in the form
> > of hot air instead of radiation which may give the perception of
> > 'not being
> > as hot' to someone sitting nearby.

This of course is the difficulty in the absence of a circulation fan.

One of John's problems was that the room is uninsulated, to a 
circulating current will exist but will be driven by hot air being 
cooled and running down the walls and then becoming a cold draught 
as it then runs along the floor to the furnace.

<snipped some questions I don't yet have a view on>
>
> 1. Additional convected hot air will enter the room, having a
> cooling effect, of the red hot surface, but not so much, as heat
> from the red hot coals will flow faster into the cooled surface,
> heating it to nearly the same temperature as before.

That entirely depends on the surface area and the amount of air 
passing it but at this stage do you have much effective use for the 
warmed air, which is moving up and away from your target audience?
>
> 2. Radiated heat will be well absorbed by the convection tube and
> radiated outwards at nearly the same rate as the exposed red hot
> tube.

I cannot see this as radiant energy is dependant on the fourth power 
of the temperature.

What's more of an issue is the comfort zone of the target audience, 
red heat, apart from being more damaging to the stove materials, 
will be too intense if close by, as the effect diminishes with the 
square of the distance to the observer it all gets quite 
complicated. As I said I suspect the compromise will be below red 
heat.

>
> 4. IF maximum heat was required at the top of the stove, e.g..
> powering a water heater, 

If the water heater has a demand all the time that the radiant room 
heating is required then you're in a position to extract most of the 
heat, also, because you're burning coal, you can cool the flue gases 
to a lower temperature than you could with a wood burner. The only 
drawback I can see is that the water heat exchanger needs to be 
slightly larger than if it were a water heater alone.

I can see this will be unduly complicated but a closed circuit 
Thermosyphon  could take heat from the insulated boiler at as low as 
50C and pass it down an insulated pipe to under the floor to, say, a 
buried car radiator over which people are seated and then back to 
the boiler. It would need large bore pipes and F&E tank though. I'm 
assuming no power for pumping being available.

In UK hydronic underfloor heating is one of the best ways of keeping 
efficiency high in a natural gas condensing boiler.

AJH