[Stoves] Turn down by moving the pot

Mon Mar 3 14:22:35 CST 2014

Dear Teddy
  ----- Original Message ----- 
  From: Cookswell Jikos 
  To: Discussion of biomass cooking stoves 
  Sent: Monday, March 03, 2014 2:14 PM
  Subject: Re: [Stoves] Turn down by moving the pot

  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? 

  # In my opinion, an adequate "turn-down ratio" is absolutely required for such cooking.

  # There is an "upper limit" to the temperature in the pot, which is the temperature beyond which the ugali will scorch or burn to the bottom of the pot. There is also a "lower limit" to the temperature in teh pot, below which the ugali will not cook adequately. As long as the stove can be operated in a manner to ensure that the temperature within the pot is always within this "window", then the stove system can accomplish its desired function, which is, of course, cooking food.

  # At the early stage, when the food is in a "fluid" state, it can be heated intensely, because the "fluid food" covects, and carries teh heat to the remainder of teh pot by convection or actual movement of the fluid. Under these conditions, the temperature of the inside face of the pot bottom is likely no more than about 5 degrees C above the boiling point of water at that location. 

  # In the latter stages of cooking, when the ugali has thickened, it is less able to move by convection, and the heat transfer mechanism changes over to heat transfer by conduction... heat is transferred from teh pot bottom, through the cooked ugali on the bottom of the pot, to nearby ugali, which is not yet cooked.  At this point, the "heat to the pot" must be reduced, because its rate of transfer from the inside of the pot is greatly reduced, to prevent burning or scorching of the food. Obviously, if heat is being applied to theoutside face of the pot at a rate of say 3 watts per square centimeter, and if it is being removed from the insde face of the pot at a rate of 1 watt/cm^2, then the temperature of the inner pot face will increase, and burning is likely.

  # Here would be a neat experiment for you to try...
  ==> Mix up teh ingrredients for a batch of ugali, and put some of it in a large frying pan that it too big for the heat source, only to a depth of say 2.5 cm. Place the oversized pan of "raw ugali" over teh heater, so that only one side of teh pan is heated, and turn on teh source of heat, and watch what happens. Eventually, you will see a zone of burned ugali, surrounded by a zone of "over-cooked" ugali, then a zone of "propertl cooked ugali", followe by a zone of "undercooked ugali", followed by a zone of "raw ugali." If you noted the temperatures and times during the test, you could learn a great deal about cooking ugali.
  ==> This experiment should vividly demonstrate the need for a "good" turn-down ratio.

  # A "graduated turn-down", ie, "infinitely variable", like a propane stove, is clearly superior to a "stepwise turn-down" capability, in that the former allows the the inside of the pot surface to be kept at  the maximum temperature below which the ugali will not burn, and thus will lead to the shortest possible cooking time. 

  Hope this helps.

  Kevin

  Thanks, 

  Teddy 

  Cookswell Jikos
  www.cookswell.co.ke
  www.facebook.com/CookswellJikos
  www.kenyacharcoal.blogspot.com
  Mobile: +254 700 380 009 

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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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