[Stoves] Turn down by moving the pot

Cookswell Jikos cookswelljikos at gmail.com
Mon Mar 3 12:14:59 CST 2014


Dear Kevin,

Thank you very much for this, I have been thinking about that quite alot in
regards how people in Kenya (and E.A) cook ugali (a polenta maize flour
type of food). It starts out liquid but ends up a solid - how much will the
size (diameter) of the source contribute to cold/hot spots during this
conversion? Is a graduated turn down function or a simple high/low better
for controlling this?

Thanks,

Teddy

*Cookswell Jikos*
www.cookswell.co.ke
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On Sat, Mar 1, 2014 at 5:47 AM, Kevin C <kchisholm at ca.inter.net> wrote:

>
> 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
>>
>>
>>
>
>
>
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