[Stoves] Energy loss into stove walls

Sat Aug 18 19:58:40 EDT 2007

DEAR PAUL AND ALL:

I have long favored metal (and sometimes tincanium) for stoves, 
particular experimental ones becasuse

    * Metal stoves have a very low heat capacity compared to ceramic
      bricks, so start right up and run constantly, then cool quickly
    * Metal does not crack
    * Clean metal can reflect heat back to the charge that ceramics won't.
    * Metal stove walls keep themselves cooler than the fuel, since they
      are typically free to radiate to the outside and conduct the heat away
    * Metal stoves can be insulated with aluminum foil tubes outside
      which reflect the heat back to the surface
    * Ceramic bricks are not very good insulators, but ceramic fibre
      materials are excellent, either on the inner surface of the metal
      (metal stays cool) or outside the metal (better use stainless)
    * Metal is easy to fabricate in a wide variety of shapes
    * Metal is often available preformed
    * Metal shapes are easy to manufacture

Paul and I have both made hundreds of stoves from metal. 

TOM REED 

Paul S. Anderson wrote:
> Dear Crispin and Penn and all,
>
> Thank you to both of you for your assistance.  Crispin has stimulated 
> discussion
> about ceramic vs metal stoves.  I hope that Tom Reed will state his pro-metal
> arguements.
>
> About Crispin's message and my specific stove question, I was aware of several
> of your key points, but it was instructive to read and think about them again,
> plus the additional concepts.  I was assuming that the heat lost through the
> sides of the ceramic gasifier was about the same as from a metal one.  Big
> assumptions, but I am just after some preliminary findings.  Also, I did not
> mention before:  The 6 insulative bricks were held together inside a very
> thin-walled metal can (the kind in which you get 3-flavors of popcorn in the
> USA at Christmas time.)
>
> Crispin wrote:
>   
>> 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.
>>     
>
> Thanks for the approx number.  I did not forget to divide by two.  The inside
> temperature is well over 1000 deg C and the outside temp was much more than 30
> C, so that is why I used the 700 deg C number.
>
> I note and agree with your use of 0.84 because of what Wikipedia says is the
> specific heat capacity of many construction materials (cement, soil, sand) as
> being in the 0.8 to 0.9 with the units expressed as  J * 1/g * 1/K    or    J
> g-1 K-1  or as Penn wrote:  J/(kg*K) .   That gives the heat at about 3000 J,
> which is 200 gms of wood because we get about 15 MJ per kg of wood.
>
> Nordica has not yet sent me the data sheet for my test of this stove, but from
> the Report on Stove Camp recently posted by Tom Miles, the stove used 
> 231 gm of
> wood per liter, time 5 liters = 1155 gms.  That was above the 850 gm benchmark
> for 5 liters.  Subtracting 200 g would put it at 955 gms.  And there were
> several other factors.
>
> A.  I used about 50% of the fuel as Illinois woodchips that have a higher
> moisture content (MC) estimated to be about 15 to 20 % compared to the kiln
> dried wood (MC = 5 %) that Apro uses in the official testing.  An increase of
> 10% of MC for half of my fuel means 75 gm of water (meaning less fuel value)
> and that the additional water must be raised from 30 C to 100 C and boiled off
> from the fuel (consuming energy that could not heat the pot).
>
> B.  Plus issues of the pot being too high from the fire and
>
> C.  having a pot-skirt that was with a large gap, promoting a rather 
> rapid fire
> during the simmer stage.
>
> Remember that this was done at Stove Camp and the stove configuration was only
> finalize while at camp.  I had only tested other aspects (primary air inlet,
> etc.) of the ceramic AVUD gasifier (or continuous feed TLUD) with 2 lightings
> prior to arrival at Stove Camp.
>
> I only had one chance to run a test of this stove under the emissions 
> hood, so I
> was not able to get all the variables fully controlled nor maximized with
> repeated testing.  I was glad that I got the one test run of this stove and
> that the CO and PM were wonderfully low!!! ( CO  =  1.2 g per liter,   and
> PM = 63 g per liter.)   But fuel usage was high, and I am trying to understand
> it and correct the situation.  I hope to drop the CO and PM even lower when I
> am using only half the amount of fuel.
>
> In conclusion, the updraft gasifier (continuous feed TLUD that becomes an
> updraft AVUD gasifier for continuous feeding) is extremely hot, and I believe
> that ceramic protection is important.  However, Crispin has help me realize
> that a relatively thin but dense ceramic liner inside a sturdy steel cylinder
> might be just fine, especially if it can be easily patched if cracks cause
> parts to fall off.  That could be more appropriate than getting heavily
> involved with the rather fragile insulative ceramics and the need to fire them
> beforehand.
>
> [NOTE:  If you are going to shift to a discussion of ceramics (and not 
> continue
> about the energy loss into stove wall and/or my stove test), please change the
> Subject of your reply message.  Personally, I am annoyed when we have weeks of
> discussion with one Subject line that has long been forgotten.  
> Maintaining the
> "threads" can be important, but so is having a Subject that reflects 
> the content
> of the message/thread.]
>
> I love this Stoves Listserv!!!!!!!!!  Thanks for being so helpful.
>
> Paul
>
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