[Stoves] Turn down by moving the pot

[Kevin C]

Dear Paul

I've seen Crispin's reply, and mine comes from a different angle...  
hopefully, it will hve an element of "contribtory helpfulness".

There are two fundamental "cooking tasks", in the sense that:
1: The product being cooked/heated is perfectly fluid, such that heat  
applied to one part of a pot will be transferred by "convective  
stirring", such that the entire pot contents are virtually at the same  
temperature. Examples would be heating/boiling water, making tea, or  
perhaps a "watery soup", boiling an egg, cooking whole potatoes, etc
2: The product being heated or cooked is viscid and does not "move  
around" by convection in the pot. Examples would be cooking rice,  
frying an egg, cooking a stew.

In the former case, where the product can move by convection, and is  
not subject to burning, then moving the pot to the side of the stove  
will reduce the total heat energy picked up by the pot in a given  
period of time. For example, if the stove was operating at a constant  
heat input rate equivalent to 3 watts per square cm. of pot bottom  
area, then moving the pot so that only half the pot was in the heat,  
it is clear that the "heat to the pot" would be cut in half. The  
energy would be supplied at the same intensity, but to only half the  
area. The desired heating task would be accomplished. However, since  
the fuel burn rate was the same, and "half the cooking work" was being  
done, fuel utilization would be halved. (When cooking an egg or  
potatoes by boiling, the food does not move, but the water can  
circulate freely.)

In the second case, where the food could not move by convection, when  
the pot was moved partially off the stove heating area, with the same  
heating energy density of 3 watts per square cm, a "cooking failure"  
is likely, in that the food in the "hot area" would be cooked and  
possibly burned, while the food that was not above the heated area  
would be partially cooked or raw. The system would fail as a "cooker".  
On the other hand, if the heat input rate could be modulated, all of  
the pot bottom could be receiving heat, and there would be no areas of  
"overcooking or burning" and no areas of "raw or partially cooked food."

A "good turn-down ratio" is necessary for the stove to function  
properly, when attempting to cook foods that cannot rely on convection  
of water throughout the pot, for proper cooking.

To see the problem vividly, try to cook rice, or stew, or a fried egg  
on an electric stove operating on "High Power", and then attempt to  
control heat input to the pot by removing part of the pot from the  
burner.

If a stove does not have an adequate "built in turn-down ratio", the  
problems may be avoided by elevating the pot above the stove, allowing  
the inflow of "side air" to dilute the intensity of the stove heat  
reaching the pot. That would work, but it would be bad for stove  
efficiency.

Best wishes,

Kevin

Quoting Paul Anderson <psanders at ilstu.edu>:

> On 2/26/2014 2:25 PM, Crispin Pembert-Pigott wrote:
>> There are many ways to control the power getting into the pot --  
>> not only lowering the fire. But one way or another control needs to  
>> be exercised by the cook.
>>
> Stovers,
>
> One way to have less heat entering the pot is to move the pot off to  
> the side so that only part of the heat has any contact with the pot.  
>    The results of this are:
>
> 1.  The SAME amount of energy / fuel is released in the combustion  
> chamber, AND
>
> 2.  LESS water is boiled away from the simmering pot.
>
> 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.
>
> 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.
>
> 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).
>
> 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.
>
> Paul
>
> Doc  /  Dr TLUD  /  Prof. Paul S. Anderson, PhD
> Email:  psanders at ilstu.edu
> Skype: paultlud      Phone: +1-309-452-7072
> Website:  www.drtlud.com
>
>