[Stoves] Comparable heat outputs of stoves

[psanders at ilstu.edu]

Thank you, Andrew.  Your comments are helping us get off to a good start.  I
hope that Jigme relates this to his students and we can utilize the SI
measurements, with some notes about the Imperial measurements also.

Andrew states:
>  Denver dry wood has 15% mc wwb and yields .85*18.6 minus .15*2.7 MJ/kg
> =15.4MJ/kg

Therefore, since most wood has even a little more moisture than 15%, I suggest
that we (Jigme and I) use 15 MJ/kg instead of 16 MJ/kg.  We have not
justification to round up to 16 MJ/kg, and to go less than 15 MJ/kg would be
unnessarily complicating the calculations.

So, there are ~15 MJ/kg of energy in "typical, seasoned, sundried, 
regular woody
biomass."   Just knowing that "fact" goes a long way for understanding the
energy levels of woody fuel used in our cookstoves.

For the record, my interest in this started when the baker friend in 
India made
comments about oven stoves.  He specifically said   50,000 kcal =~ 
200,000 BTU.
No mention of the SI (metric) units!!!!!  And he was referring to an oven made
in Italy.

Also, worldwide it seems that pipe diameters are given in inches because the
standard pipes (ducts and casings) are 6 inch, 8 inch, etc.  Old 
traditions die
hard, even in countries where metric measurements are the norm.

Side note:  I was trained in aerial photography intrepretation, and there the
focal length of the camera is extremely accurately measured to the hundredths
of millimeters, as in 152.xx millimeters.  But the lense is a 6 inch 
lense from
the beginning of manufacturing through today.  But with such precision of
measurement, who would want to work with measurements like   5.9983 inches or
6.0024 inches!!!!!

Perhaps our tables and reference sheet that Jigme is working on will assist us
in getting more people to be comfortable with the SI metric measurements when
they see the equivalents of the old-style units.

General guideline????  Let's keep the numbers to no more than 3 significant
digits, and even better if 2 significant digits can be sufficient.  And when a
single digit (  like 4 BTU per 1 kcal  ) can be used, that is very easy to
remember.  I hope we can find some nice single and 2 digit conversions between
the SI metric and Imperial systems!!!

Paul

Quoting AJH <list at sylva.icuklive.co.uk>:

> On Thu, 01 Jun 2006 22:37:25 -0500, psanders at ilstu.edu wrote:
>
>> So, if we allow that most people are using 15% to 20% moisture wood,
>>>> the 18.6
>>>> MegaJoules per kilo must come down by that amount, so the actual 
>>>> MG would be
>>>> about 16 or 15 MJ per kilo of common moisture (seasoned or air-dried? or
>>>> "regular") woody biomass.
>>>>
>>>> Do we need to also allow that some of the fuel must be used to 
>>>> boil away the
>>>> moisture, resulting in less combustibles available for making high
>>>> temperatures?
>
> Yes we do but I don't think we should get overly complicated. Whilst
> we should bear in mind that wetter wood tends to need higher excess
> air, which greatly affects heat transfer efficiency, I use a ball park
> figure.
>
> I derive this from the fact that the moisture in the wood is boiled
> off and rejected as steam at flue temperature. The latent heat of
> steam is about 2.3MJ/kg and steam mass has to be raised from ambient,
> through boiling, to the flue temperature. I think subtracting 2.7MJ
> for every kg of water in the wood is close enough but stand open to
> argument on that.
>
> So oven dry wood has no moisture and yields ~18.6MJ/kg
> Denver dry wood has 15% mc wwb and yields .85*18.6 minus .15*2.7 MJ/kg
> =15.4MJ/kg
> Freshly felled willow has 60% mc wwb and yields .4*18.6 minus .6*2.7
> =5.82MJ/kg and is next to impossible to burn cleanly without
> sophisticated burning.
>>>>
>>>> My original objective was to get some ball-park comparisons.  Here
>>>> is a start:
>>>>
>>>> 4 BTU =~ 1 kcal
>>>> 1 BTU =~ 0.25 kcal
>>>>
>>>> 50,000 kcal =~ 200,000 BTU
>>>>
>
> I've posted my feeling on this, I don't think any system is
> fundamentally better or more accurate but this is an international
> list and, as was pointed out, even the US preference for imperialism
> is contrary to their agreed position to prefer SI units.
>
> My preference would be to go with Joules, Watts and kWhr rather than
> calories, even though they are the original heat units. To help people
> more familiar with imperial measure these could be parenthesised after
> the SI units.
>
>>>> 1 kg reg-wood =~ 16 MJ =~ 3800 kcal =~ 15,000 BTU =~ 4.5 kWhr(t),
>>>> which means that a nicely sized residential cookstove that takes 
>>>> one hour to
>>>> fully consume 1 kg of regular woody biomass is operating in the 4 
>>>> to 5 kWhr
>>>> range.  Because all of this energy is expressed as being in one hour of
>>>> constant burning, we can say that the power is a 4 to 5 kW stove.
>
> You've got it.
>
> AJH
>
>
>
>
>



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