[Stoves] Black radiating surfaces

Sun May 25 10:02:16 CDT 2008

Dear All!

Following that discussion over some days i get the impression that the
motive is to get as much heat as possible from a heating source for the room
heating.

We have a source which is small in volume but high in temperature and want
to get the most of this produced energy into useful room heating energy.
Room heating has a very small temperature range where we feel good and its
normally between 18 to 27 degrees Celsius or better between 22 and 26
degrees Celsius. But it is not only the temperature it is as well the air
humidity. Winter dry air is around 15 to 35 % relative humidity, summer or
rain season air can be between 60 and 98 % humidity. Optimal would be around
60 to 65 % humidity. Winter season because of heat loss avoidance implies
normally low air exchange volume and consequently higher dust and and higher
Co2 levels in living area.

Why not rise usage efficiency of the heat source by using water as the
medium.Store heat at a temperature level of 30 to 35 degrees Celsius ( high
delta T, good efficiency in recovery of heat produced). Have a big water
storage in your home and pump this water over a open wall like structure and
you can capture dust, Co2 and you get the heat into the room. As well the
humidity is higher in your room. Isolate your outer walls and use 2 sheet
vacuum glasses at your windows and you dont have condensation bridges.

With this your heating is efficient, your temperature range optimized, your
breathing air clean and low in Co2 and your comfort level in the best range
thinkable. Take a good part fresh water and use the Co2 charged water for
your washing and toilet. ( Co2 saturation of water).

This works specially well in rooms where you normally use air conditioning
as cool and heat source.( look at the huge bills in electricity for air
conditioning and heating in the southern states of US and the most of Latin
America from Mexico to Argentina)

Well i recognize thats not the solution for third world-poor-people
circumstances but what ever we save in our circumstances will as well lower
energy needs and has an influence on the global energy consumption for
heating as well.
( I always hated this hot small stove at red heat and the noisy long dust
collecting chimney crossing the room and radiating near red heat, the front
of my body nearly cooked and the back freezing cold)

Best regards Nikolaus

On 5/25/08 10:28 AM, "IPC" <ipcipc at mweb.co.za> wrote:

> Dear John
> 
> It helps to know that the emissivity does not vary much with temperature.  A
> few examples from a table in Perry's Chemical Engineers Handbook (Table
> 10-17, 6th edn):
> Polished aluminium 440-1070 deg F 0.039-0.057
> Oxidised aluminium 390-1110  0.11-0.19
> Polished wrought iron 110-480  0.28
> Polished cast steel 1420-1900  0.52-0.56
> Cast iron oxidized at 1100 390-1110 0.64-0.78
> Steel oxidised at 1100 390-1110  0.79
> Red brick   70   0.93
> Plaster   50-190  0.91
> 
> So if it is high emissivity at room temperature it will probably be high
> emissivity when hot enough to be red.
> 
> Hope that helps,
> 
> Philip Lloyd
> 
> 
> -----Original Message-----
> From: stoves-bounces at listserv.repp.org
> [mailto:stoves-bounces at listserv.repp.org] On Behalf Of John Davies
> Sent: 25 May 2008 10:27
> To: Discussion of biomass cooking stoves
> Subject: Re: [Stoves] Black radiating surfaces
> 
> USING A COMBINATION OF RADIATION AND CONVECTION FOR SPACE HEATING
> 
> Stovers,
> 
> Crispin 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.
> 
> Observing the heat radiation of the air preheating jacket leads to the
> following perceptions:
> 
> 1. With matt silver internal and external surfaces ( oxidized galvanizing )
> a large amount of heat is radiated despite the air heating duty.
> 
> 2. When the external surface is blackened the radiation is increased, and
> appears to be similar to that of the exposed red hot tube.
> 
> 3. Are these just perceptions, or are we genuinely extracting as much
> radiated heat as in 2 ?
> 
> Now If we added a convection tube as suggested by Crispin, which had a black
> surface both internally and externally,the following may be true.
> 
> 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.
> 
> 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.
> 
> 3. The air gap between the combustion and convection tube will determine the
> ratio of heat extracted by each energy form. I would start with a 25mm gap (
> 1" ) with a convection tube with a height of 240 mm ( 9.5 " ) and take it
> from there. This would add a safety feature in the event of the combustion
> tube failing. The combustion tube does become brittle and crack after a
> period of time.
> 
> 4. IF maximum heat was required at the top of the stove, e.g.. powering a
> water heater, the air movement could be blocked off forming a sealed jacket,
> which if given  polished silver surfaces, would reduce radiation
> significantly. This would give the inverse of 1 and 2. , but might be
> detrimental to the metal of the combustion tube. In this case a silica based
> combustion tube might be required.
> 
> This brings us to the point where careful control of radiation and
> convection losses could be highly beneficial to the heat efficiency of a
> cooking stove or space heater. Do we want maximum or minimum heat loss from
> the unit. ?
> 
> Your discussion on the above will be greatly appreciated.
> 
> Thanking you,
>  John Davies
> 
> 
> 
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