[Stoves] Energy loss into stove walls

[Robert Penn Taylor]

Paul,

The energy you're looking for is the heat energy stored in the stucture, 
yes? Then E = m*c*(Tfinal-Tinitial), where m is the mass of the 
structure, c is the specific heat capacity of the material, and Tfinal 
and Tinitial are the obvious. So E = 5kg * c J/(kg*K) * 730 K. Your 
problem is then to find the specific heat capacity, c, of the insulative 
ceramic. If you already know this value from some other source, you're 
set. Same thing works for the metal version, and you can certainly look 
up the heat capacity of all the many alloys on the internet.

If you don't know the specific heat capacity of the ceramic, and don't 
know how to determine it, email me and I'll send you a step-by-step 
procedure.

-Penn

-- 

Robert Penn Taylor
Graduate Research Assistant
Department of Mechanical Engineering
Iowa State University
(515) 294-5311


Paul S. Anderson wrote:
> Stovers,
> 
> Some calculation assistance please.
> 
> At Stove Camp I used an insulative ceramic 6-brick chamber for my updraft
> gasifier (AVUD style) (  http://www.hedon.info/docs/BP53-Anderson-14.pdf  )
> with a single-pot application above it to burn the gases.  My fuel consumption
> was surprisingly high, and I want to know how much energy (and the
> corresponding amount to woody fuel) was absorbed into the walls.
> 
> Total weight of the gasifier (not the rest of the stove structure) is 7 kg.  I
> will allow for 1 kg of excess weight of construction and another 1 kg for the
> equivalent device made of steel, and therefor call it 5.0 kg of "extra body" to
> be heated.  The material is 2 inches (5 cm) thick, and it does get rather hot on
> the outside (but not measured).  Inside there is a 6 inch (15 cm) diameter
> vertical cylindrical area for the hot gasification.  Height is 11 inches (28
> cm).
> 
> This is not a TLUD with pyrolysis around 400 deg C in a small zone of pyrolysis.
>  This is a true updraft unit with red-hot char in the base of this ceramic mass,
> and very hot throughout the whole mass.  I will estimate that it was 900 deg C
> or perhaps even higher, but that also is not known for sure.
> 
> Help please.  Can we simplify this?  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? 
> and what amount of wood would provide that amount of heat?
> 
> Basically, the issue is:  A water boiling test takes 25 minutes to boil and 45
> minutes of simmer (total 70 minutes, but we could make it easier to just say
> one hour of operation, as if boiling was done in 15 minutes).  During this time
> the stove body radiates heat (wasted through the sidewalls) and also takes in
> heat that is held in the sidewalls (also wasted as far as the water heating is
> concerned).  An equivalent metal version weighs 1 kg and the ceramic version
> weighs 6 kg (mine was 7 kg, but I allow 1 kg for excess and then another 1 kg
> as the equivalent weight of the metal version, leaving the 5 kg mentioned at
> the start.)
> 
> Can we please get some numbers to work on, and then we can discuss the merits
> and demerits of metal vs. insulative ceramic.  How much heat energy (and wood
> equivalent) is needed to raise 5 kg of material 700 deg C?
> 
> Thanks
> 
> Paul