Dear Crispin The question of "What is an acceptable Turn-Down Ratio?" (TDR) is not a simple one.
Fundamentally, the Cook has to control the heat to the pot for two reasons: 1: To cook it properly 2: To cook it efficiently When heating a "watery food" such as a soup, there would be little concern for burning the food, in that the large amount of water present, and its fluidity, would make burning or scorching very unlikely, and a small TDR would work for the Cook. At the other extreme, a "non-fluid" or "pasty" food requiring a long cooking time, like beans or a stew, should have a large TDR to avoid burning or scorching of the food. If the stove does not have an adequately high TDR, the Cook may be able to compensate by: 1: Adding extra water and allow it to boil off 2: Move the pot to a cool part of the stove top. In a test, like WB 4.2.1, the test is run with no cover on the pot. There can be a very significant heat loss from the pot, even though its temperature is below boiling, as a consequence of evaporative cooling. Thus, a TDR of perhaps only 2 may suffice with a non-covered pot, while a TDR of 6 may be required if the pot is covered. While an open top pot man enable a low TDR of say 2 or 3 might hold the pot contents within the simmering range, the foods being cooked may not be able to accept a water addition part way through the cooking process. The required TDR will depend on whether the cooking pot is covered or not. >From a fuel efficiency standpoint, the cooking is best done with a covered pot: a: heat the pot to boiling temperature on "High Heat" and then b:turn down the stove to the point where there is just an occasional wisp of steam escaping from under the cover. This is the way that most "pot cooking" is done in the real world. One notable exception would be when the Cook purposely wants to boil away excess water, to thicken the food. The nature of the pot, and ambient conditions have a big effect on the required turn-down ratio. Also, the maximum heat input rate to the pot, ie "pot power", will have an effect on the required TDR. For example, if a covered pot of water was heated to boiling, and if its heat loss rate was 500 watts at boiling temperature, then a stove with a maximum "pot power input" of 1 kw would be perfect with a TDR of 2. On the other hand, if the same pot was set on a stove with a maximum pot power of 3 kw, then the TDR would have to be 6. However, the first pot would be very slow to heat up to boiling temperature In summary: 1: For most efficient cooking, and to reflect most common cooking practises, the pot should be covered. 2: Heating pots with no cover will require more cooking fuel, but by addition of water, burning or scorching of foods can be prevented, with a lower TDR. This assumes that adding water during cooking is acceptable to the Cook. 3: .A stove heating a covered pot will require a larger TDR than a stove heating an open top pot with no cover. 4: A stove with "high pot power input" will require a larger TDR than a pot with a lower "pot power input." The actual TDR required can be measured with present day instrumentation in common use. Once the maximum "high pot power input" rate is measured, the fire can be turned down, or allowed to die down slowly, and the point where water temperature starts to fall can also be measured. Knowing the rate of fuel burning at that time enables calculation of the required TDR. It would also allow calculation of fuel efficiency at the lower required TDR. Best wishes, Kevin ----- Original Message ----- From: Crispin Pemberton-Pigott To: 'Discussion of biomass cooking stoves' Sent: Thursday, May 02, 2013 11:12 PM Subject: Re: [Stoves] Stove Definition - controllability Dear Friends I would like to remind everyone that it is pretty important to a cook to be able to control the fire in some manner. There are lots of precedents so I won’t repeat them. I would like to have a minimum control exerted over the cooking power in order to qualify as a ‘cooking stove’. There are many appliances which are used for heating water, showers (like the Geyser 2000 etc) or drying fish and so one and on. But in order to ‘cook’ the fire has to be controllable. For an electric or gas stove this is fairly easy. In order to start things off, what does everyone think about a turn down ration of 4:1 where the turn down is ‘willful’ meaning it is controlled by the cook be either removing fuel, controlling airflow or by some other means. The reason is that stoves are appearing which definitely burn fuel and provide heat but are not very controllable (or not at all controllable). While one car argue that by brilliantly fuelling the stove in just the right manner a fire and its burn can be exactly matched to a cooking need – agreed this is possible – but is it ‘cooking’? When sitting in the field with cooks it becomes obvious that most cooking involves controlling the power at some point. How much control should be applicable to a stove in order to qualify as a ‘cooking stove’? If I ask for a water heating stove, it would not have to have any controllability at all – it just needs to heat the water within a certain time after which it can go out – no one will mind. But if we want to present a ‘solution’ (a cooking alternative to an open fire or sheltered fire) it will have to be manageable ‘to a certain extent’. Thus if someone says, “Here is my new cooking stove,” I can say, “Prove it can cook.” If I ask for a maximum power of X and ask for a demonstration that it can be controlled to X/4 is that reasonable as a minimum standard of proof? Thanks Crispin ------------------------------------------------------------------------------ _______________________________________________ Stoves mailing list to Send a Message to the list, use the email address [email protected] to UNSUBSCRIBE or Change your List Settings use the web page http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org for more Biomass Cooking Stoves, News and Information see our web site: http://stoves.bioenergylists.org/
_______________________________________________ Stoves mailing list to Send a Message to the list, use the email address [email protected] to UNSUBSCRIBE or Change your List Settings use the web page http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org for more Biomass Cooking Stoves, News and Information see our web site: http://stoves.bioenergylists.org/
