[Stoves] Energy loss into stove walls

Sat Aug 4 18:58:16 EDT 2007

Dear Paul

I hope this is helpful.

Penn has already given you the plan for getting the amount of heat in the
brick.  He could have said a lot more and I will give you a little to think
about, before you and others try to make a spreadsheet.  As my historian
brother always said, it is not quite that simple (as Penn is well aware :) .
Start with http://en.wikipedia.org/wiki/Specific_heat_capacity

>...and therefore call it 5.0 kg of "extra body" to be heated.

Perfect.

>The material is 2 inches (5 cm) thick, ...Inside there is a 6 inch 
cylindrical area 

Perfect.  The OD is about 10 inches.

>Height is 11 inches

Perfect.

Outside area is Pi*10*11= 346 sq inches.

This area disposes of heat.  Think of it this way: after heating up the
brickwork to a given temperature (which you did not measure) and things
continue in a constant way, there is heat passing into the walls from the
fire and through to the other side where it is radiated, conducted and
convected away from the surface (three losses).

If the system is in a stable condition, the heat loss is constant and
exactly the same amount of heat going into the hot side is leaving the cold
side.  In order to do that, the outside must be hotter than the air around
or else the temperature would go up until it can lose heat.  At some
temperature, the losses into the room are equal to the heat arriving through
the brick.

There are a couple of counter-intuitive things that must be kept in mind
when modelling this stove 'in your head'.

1.	The temperature of the outside of a stove does not, on its own, tell
you if it is losing a little heat or a lot.  The higher the temperature when
the heat flow is stable, the less heat you are losing....but....the more
heat you have to invest in the body because it is at a higher temperature.

2.	If the temperature is 120 deg and it is shiny silver, it is losing
about 40% as much heat as it would if it was 120 deg and it was a dark brick
colour (like an MCS).  Burning your hand doesn't tell you if it is losing a
lot of heat, it only tells you that it is hot.

3.	A cold outside temperature with a dark, rough surface (such as an
typical high-fired insulative brick) may still have a high heat loss.  If
the combination of air currents, colour, roughness and conductivity of the
outer skin of the brick are 'just right' the heat loss can be substantial.

This was discussed in detail on the list when I was in Malawi testing Andi
Michel's 5 or 6 ring stove made with the manual Ring Maker.

During that lab test it was shown that the heat losses from a reasonably
insulative brick were higher than with a solid, non-insulative brick.  The
reason was that the inside wall surface temperature of the solid brick was
far lower resulting in a much lower Delta T which more than compensated for
the increased thermal conduction.  As the outer layer was a smooth
galvanised sheet, the radiation was very low.

The fact is that these 'insulative ceramics' are not very insulative. If you
cut the thermal conductivity from 0.9 to 0.5 and increase the inside
temperature of the wall from 275 to 650, you definitely will lose more total
heat.  You will probably have a better CO figure (lower) but a lower overall
efficiency.

The simple answer to your question of whether or not it is better to have
insulative brickwork has no direct answer.  It depends.  If you are
interested in lifetime (which we have to be if we are in the cooking
appliance business and not the garage experimenting business) you have to
address the issue of thermal shock and its effect on the materials.

My point is that the amount of heat lost to the stove body has to be added
to the amount of heat lost _through_ the stove body during the cook.  In a
solid brick Lion Stove the amount of heat 'lost' to a substantial number of
face bricks is about 3% or less of that produced during cooking.  Heat
coming through the bricks is captured and recycled into the fire which does
not happen with your stove.  As the Lion Stove uses about 75% less fuel,
there is a large gain in efficiency without using light (and fragile)
brickwork.  The working life before cracking increases from 5 days to 5
years (I hope).

If you lower the conductivity from 0.9 to 0.03 and increase the wall
temperature from 275 to 800, you win big time, provided that the material
can take the physical stress of being quite a bit bigger on one side than
the other.  If you make a low thermal expansion brick, that stress can be
greatly reduced but you can't jump in and say 'go for it' until you
calculate the overall effect, the combination, of the changes. The thickness
of the wall also affects the performance.

Additional info to track into the future for future reference:

Something to keep track of is the Thermal Diffusivity.   "Diffusivity is
described as an index of the ease or difficulty with which [concrete]
undergoes temperature change and, numerically, is the thermal conductivity
divided by the product of specific heat and density. Typical diffusivity
values for [concrete] range from 0.03 square foot/hour for basalt concrete
to 0.06 square foot/hour for quartzite concrete."  [I lost the source]

This has not been discussed on this list as far as I know.

During a short cooking period, the thermal diffusivity is an important
issue, but much less so in large institutional cookers with a long service
time per day.  Retained heat cookers with fires in them some of the time
(Lanny's stoves) need to know consider thermal diffusivity.

>What about saying that 5.0 kg of mass was
>raised from ambient temperature (30 degrees C ) to an average temperature
of
>730 deg C.  (Net change of 700 degrees C.)   How much heat is needed for
that? 

The heat in a brick (specific heat of about 0.84
http://www.crest.org/discussiongroups/resources/stoves/Andreatta/Heatloss.ht
m ) is not related to its density, but its mass.  There is a little article
noting that increasing the thermal conductivity of a brick with iron powder
and simultaneously reducing its density gave a better insulation.  It will
of course hold more heat if the final thickness is greater and has the same
mass as the original.  Interesting, and covered in
http://www.freepatentsonline.com/4265845.html showing that increasing heat
capacity does not necessarily decrease insulation. Only sometimes.

>and what amount of wood would provide that amount of heat?

Too soon to know.

>Can we please get some numbers to work on, and then we can discuss the
merits
>and demerits of metal vs. insulative ceramic.

I welcome an open investigation and debate of this question.  That is what
science is about.  We don't have to come to a firm conclusion, or even
agree, but we can't over-simply the stove and then invest large amounts of
money in multiplying the products based on an inaccurate theoretical
representation of a partially understood truth.

> How much heat energy (and wood equivalent) is needed to raise 5 kg of
material 700 deg C?

Well, it is about 200 gm of wood (assuming no losses), but you forgot to
divide by two at one point.  The average temperature change was 350,
assuming the outside remained 30 (which it didn't).  So in the end, there is
not enough information to know the answer.

Regards
Crispin in Matsapha

PS The outside temperature of the average brick kiln in Lubumbashi is about
120 degrees and they lose 180 KW continuously into the night air!