Dear Kevin
>The question of "What is an acceptable Turn-Down Ratio?" (TDR) is not a simple >one. Actually, the cooks make it quite simple. If they cannot turn it down far enough to stop burning their food, they won’t use that stove for that purpose. Obviously the food preparation methods vary widely around the world so there is no ‘standard’ TDR which will satisfy everyone. That is we I consider the local comparative performance test the most relevant for any stove programme. That said, it is worth discussing the idea of a minimum ratio for performance reviews. >Fundamentally, the Cook has to control the heat to the pot for two reasons: >1: To cook it properly >2: To cook it efficiently I agree these are the considerations the cook applies. >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. The methods reported are: Taking off the lid which reduces the effective heating power by creating a large additional loss of heat from the whole pot. Adding cold water. Moving the pot away from the fire laterally. Elevating the pot/food above the fire (common with roasted meat in a standard BBQ) Stopping any fanning of the fire Removing fuel (fuel metering) Closing air vents (air metering) Splashing water on the fire (also common with a BBQ) Adding a wet piece of fuel Removing a heat transfer increasing device like a pot skirt Moving the pot to a second or third (etc) hole on the stove where the heat is less intense (common in Indonesia) Venting ambient air into the chimney to reduce the draft (arguably a form of air metering – used by John Davies in his packed bed coal gasifier) Inserting a plate between the heat source and the cooking vessel Transferring the pot or the contents of the pot to a retain heat cooker. >…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 I understand your point, the standardisation of a minimum requirement would consider dominant cooking methods and allow the market to sort out the different between, say, 4:1 and 6:1 products. >The required TDR will depend on whether the cooking pot is covered or not. I believe the cook will have a practical approach: if the food is going to cook adequately (meaning not at a particularly high power which another stove may have) there should be a minimum cooking power and a minimum turn down. I do not like the idea that it has to be a particular number of Watts that the stove has to yield or turn down to. The reason for this is there are a large number of cooking appliances for a wide range of things and any stated limit will immediately rule out stoves that are perfectly capable of delivering a needed cooked product. In order to have a viable and broadly applied standard, it will have to be relative to some stated ratios rather than ranges. It is reasonable for a national standard for domestic stoves to have an upper power limit on what is considered ‘domestic’. If a pot lid is off by local convention, the net cooking power will necessarily be reduced for any given firepower. I cannot state with certainty that it will allow the proper cooking of all foods with a lower TDR. It might even have to be higher. Remember that the only viable method of determining the TDR is to test the amount of heat getting into the pot and even with the lid off, at low temperatures that rate is pretty much the same as a pot with a lid on. In other words the ΔT and the water evaporated are still going to give a realistic value of heat gained, even if more of that heat is passed into the air in the form of water vapour. I am not convinced the reduced cooking efficiency (net heat in water) is lower or higher. >…One notable exception would be when the Cook purposely wants to boil away >excess water, to thicken the food. I feel this does not impact the TDR. >Also, the maximum heat input rate to the pot, ie "pot power", will have an >effect on the required TDR. Aha. Well this is not the case. The definition of the turn down is not the firepower, it is the heat gained by the pot. Interestingly, there are major outcomes from this conceptual difference. The WBT’s classically observed the fire but people do not cook with the heat of the fire, only the part which gets to the pot. As you have observed, the heat transfer efficiency varies with firepower and therefore stating a minimum TDR does not really say anything about the firepower involved, only the cooking power. In summary: 1: For most efficient cooking, and to reflect most common cooking practises, the pot should be covered. That is not relevant to the TDR, though an important programmatic consideration. >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. That is a cooking technique, not a TDR. >3: .A stove heating a covered pot will require a larger TDR than a stove >heating an open top pot with no cover. This is debatable and so far I am not convinced. 4: A stove with "high pot power input" will require a larger TDR than a pot with a lower "pot power input." This is a no-no because to embed the concept into a Standard means interfering with preferences of the manufacturer and customer. The deal is between the buyer and the provider, not the regulator. The regulations are there to protect the customer and to label a product as being able to perform certain things, such as to deliver a locally acceptable water boiling rate in a pot with a diameter of 150mm or 300mm (as per the label on the product). The customer knows full well that a stove that cooks ‘well’ with a 250mm diameter pot will have problems getting the same cooking experience out of a 400mm diameter pot. What is not permissible is to have a manufacturer claim that the product ‘cooks well’ with a 400 when a test of the heat transfer rate shows it will only do so if the pot is about 250 in diameter. With respect to controllability, a cooking stove needs some form of power regulating in terms of the heat getting to the pot. How that is achieved is up to the manufacturer. I have seen one stove that could turn down only about 10% and the mechanism used was to stop producing so much CO2 and produced CO instead. It worked…..! >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. Unfortunately that turns out to be imprecise. It is best to determining the heat transfer rate using cold pots of water, swapping them when the water reaches 65 to 70 deg C( before any evaporation takes place). It is quite easy to get a 2-3% precision on the whole operation (depending on the fuel type and the care take to measure it). >Knowing the rate of fuel burning at that time enables calculation of the >required TDR. The rate of mass loss is one thing, what is burning is quite another. Frank and I have been discussing how to refine that! It is not so simple with biomass, and is much more difficult it there is a lot of char involved, but not impossible. Thanks so much for your thoughtful inputs. Regards 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/
