[Stoves] Turn down by moving the pot

Crispin Pembert-Pigott crispinpigott at outlook.com
Fri Feb 28 17:10:09 CST 2014


Dear Paul and Other Stoves testers

 

>From Paul: "As I understand the WBT procedures, doing this would result in
more favorable efficiency numbers than if the pot was boiling vigorously on
the full impact of the fire, and losing much water."

 

The thermal efficiency is a stove when simmering, based on missing water, is
not a scientifically valid calculation so there is no point trying to
improve your score on it. For any given burn rate, the performance of a
'more efficient stove' (meaning heat transfer efficiency is better) will
perform worse when rated by the GACC-WBT method (please differentiate). The
better the stove is in real life the worse for the rating on emissions and
fuel consumption. 

 

The problem is the metrics chosen for rating performance. 

 

Can someone please confirm this for me.    And perhaps give an example where
the ONLY VARIABLE THAT CHANGES IS THAT THE POT BOILS OFF SIGNIFICANTLY LESS
WATER if the pot is placed to the side.   I am thinking of the difference in
the amount of water in the pot being even 2 or 3 liters less between the two
examples.



If you boil less water, the assumption is you have transferred less energy,
right? Well, that is no problem if the task at hand is to keep a pot hot. If
the performance is measured by evaporating water, then that is a different
task. I.E. set one task, but rated on another. Problem.


>Yes, these types of stoves could exist, as in an example of a TLUD without
any turn down of primary air and with a pot support that allows the pot to
be shifted to the side (such as on 2 pieces of rebar).



Turning down a stove by any means is advantageous in that you might have a
stove with a turn down ratio of 1.5 and the local community wants 3:1. Yes
it will waste fuel, but you might be really fuel efficient at other times
and make up for it.


When we have clarification about this, we can then discuss if moving the pot
should be a factor in stove testing.    And also if the amount of remaining
water after simmer should be a factor. 



Simmering (if asked to do it) is a task, an arbitrary one. There is no point
in measuring how much water is in a simmered pot - either it is simmering on
low power or it is not. The use of water before and after for 'simmering
calculations' is a red herring and has misled many people into thinking
there stove was doing worse or better. 

 

The only useful info you can get while simmering (apart from the fact of
being able to do it) is the emissions with time and the heat transfer rate,
which cannot be determined on the basis of missing water, only rising water
temperature from cold. That means using a cold pot at low power, not keeping
a hot pot boiling, almost. 

 

It is an easy test to do and provides useful information.

 

Regards

Crispin 

 

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