[Stoves] Test report of metal chulah

Crispin Pemberton-Pigott crispinpigott at outlook.com
Wed Dec 31 16:38:06 CST 2014


Dear Saloop

 

I have look again at the formulas used for calculating the efficiency and
you should be careful in drawing conclusions about the stove and its various
forms until you have corrected the calculations.

 

This formula (copied below) does not give the correct efficiency value.
Because you are interpreting the calculated results to make conclusions
about the design, it is really important that you use a slightly modified
test method and correct the formula.

 



 

There are two problems with this formula. 

The first is that you have counted the heat gained by the water before it
started boiling. No problem. Then you evaporated water, no problem.  Later
the water cooled and some of it evaporated. That is a problem.  The heat
driving that evaporation did not come from the fire (which is out) but from
the cooling of the water. Energy was transformed from hot water into hot
steam at a conversion rate of at least 540 kCal/kg (more, if the temperature
at the time was below the boiling point).

 

I mentioned this problem before. 

 

Here is your formula broken down:

 

{2 (99.8-31.5)                     [I left out the x1]

 

That is: water mass times Delta T. The result is the enthalpy change in the
water.

 

+(0.58 x 540}

 

That is: the enthalpy change from water to steam for 0.58 kg of water.

 

However you note that the test was finished and the water mass missing was
measured at the end of a cooling period. Is that correct? Your description
does not state explicitly that the water mass was determined at the end of
the gasifier experiment, though it does say so for the Chula experiment.  If
so there is a problem. 

 

You have not calculated the change in enthalpy for the pot cooling between
flame out and the time when you determined the mass of water evaporated.
This change in enthalpy is negative, not positive and is missing from the
formula.

 

It is the same as the first portion but with a negative number:

 

{? (36-99.8)     [in the case of Test 1]

 

{? (36-99.8)         What number should be in place of the '?'. It should be
the final mass.

 

1.134 (36-99.8) 

= 1.134 x -63.8 

= -72.34 Kcal heat lost from the hot water and turned into evaporation and
the heating of the local environment directly from the pot surface (which is
an unknown value).

 

Thus the reasonably correct total heat gained number is:

 

{2 (99.8-31.5) - 1.134 (36-99.8) + (0.58 x 540}

 

Water heating - water cooling + total water evaporated.

 

Second problem with the calculation:

You have allowed the pot to cool long after the fire has been removed from
under it. The pot loses heat into the air, not only from evaporation. You do
not know the amount of heat gained by the pot and water that was
subsequently lost to the surrounding air. You do not know how much heat was
lost from the pot when the water temperature was rising and or when it was
boiling. Letting the pot cool for so long has allowed a large amount of heat
to be lost from the pot 'uncounted' meaning, counting the lost heat that was
not turned into evaporation. The longer the wait (without fire) the greater
the unknown and the greater the error and the less confidence we can have in
the final number.

 

To correct this you need to know the mass of the water at the moment when
the gasification stops and stop the experiment at that time. You have
recorded the time, and the temperature, but not the water mass remaining. If
you know the mass of water remaining when the gasification is completed, you
can get a pretty good figure for the cold start system efficiency.

 

 

There are two problems with the test procedure.

The first is that it is not clear what 'efficiency' you are trying to
determine. An efficiency is a ratio and there are many ratios that can be
reported.  As I read the description, it seems to me you want to know the
system efficiency, that is, the % of useful heat available in the fuel that
is detected in the pot.

 

It would be a very unusual cook who was 'cooking' while the pot reduced in
temperature to 36 C. I think this is a reasonable assumption. So if you
could report the mass of water either when the flame goes out or after it
cools to 95 C (whether the flame is out or not -  otherwise known as the
Chinese Method) you can use the extended formula above that accounts for the
enthalpy of cooling.

 

To get the best answer, record the mass of water when the gasification is
finished, or when water drops to 95 C (you decide and report which you have
done).

 

The second problem with the procedure is that the pot was allowed to cool
while sitting on the stove which allows some of the heat retained by the
stove body to be transferred to the pot. This can be quite misleading when
trying to determine whether or not the change in architecture is affecting
the performance. The cooling energy calculation has such large unknowns in
it that decisions about results may be misguided because of something taking
place long after the real cooking has stopped. That should be avoided.

 

Regarding the secondary air inlets, my previous comments still apply. I
won't repeat them here.  The number of holes it too large and the diameter
too small to impel the air into the centre of the gas chamber. 

 

I agree with all Julien's comments. I believe a better seal between the
riser and the body would prevent hot air which is now rising past the
outside of the riser and into the flaming area at the top edge of the riser.
That air is surely cooling the flame and pot surface and interfering with
the transfer of heat to the pot.

 

Regards

Crispin

 

Respected sir,
        Please find the resized documents attached with this mail. It has
been sized down to 800 KB. Hope it is possible to post it.

Thanking you,

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