Dear Crispin Thanks for your comprehensive reply! I fully agree with all your comments, except for those which I extract as follows:
1: >Also, the maximum heat input rate to the pot, i.e. "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. # What I was getting at here is if the stove was "high powered" to rate well in a "Water Boiling Test", it may be "over-powered" for what is needed to cook most foods. Are we "on the same page" now? :-) 2: > 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. # Consider two WBT Tests... one with a cover, and one with no cover, on the same stove. The same amount of "power to the pot" will show a higher "pot power" when covered, than when not covered, because of extra losses from the uncovered pot. If the stove was subsequently modified so that "covered pot" had the same "time to boil", its actual "heat to the pot" would be less than the "heat to the uncovered pot." Thus, the covered pot would be less likely to be "overpowered, in relation to the actual energy required for maximum cooking. Accordingly, the covered pot would require a lower turn-down ratio than would an uncovered pot, in that most cooking is done with a covered pot. Does that make sense? 3: >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. # I can see why you found it "debateable" and why you were "unconvinced"... I think I had my conclusions reversed, as per explanation in 2: above. I presently feel the correct statement should be: "A stove heating a covered pot will require a LOWER TDR than a stove heating an open top pot with no cover." Does that make better sense now? 4: >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. # I think my >4 statement above is correct, based on the fact that 'high powered" stove will have a greater excess of power above that required for normal "maximum required cooking power." The presumption is that "high power is good" for fast boiling, but that it is bad because of providing excess power above that required for maximum cooking effect. In your note to Paul and Lanny on 4 May, it appears that: * 2 watts/square cm. is good for a maximum water boiling requirement * 1 watt/square cm is the "normally required maximum power input, for actually cooking" * 1/2 watt/square cm is an acceptably "low cooking heat" This being the case, why not simply do the test with whatever pot diameter is chosen, and then calculate back to "Actual watt power per square cm into the pot", and compare "Actual" with the Absolute Standards" of 2,1,and 1/2 watts/square cm, apparently required for "good cooking and good cooking control"? Tests would be run to determine: a: Stove performance at maximum power input to the pot when boiling water b: Stove performance at 2 watt/square cm input to the pot when boiling water c: Stove performance at 1 watt/square cm "normal maximum food cooking power" d: Stove performance at 1/2 watt/square cm "normal minimum food cooking power" Input fuel consumption, health parameters, and safety parameters would also be measured. If the 2,1, and 1/2 numbers were a bit off, they could easily be changed in the future, with the same test protocol. 5: >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. # If you knew the "total energy in the fuel charge, at the start", and the energy remaining in the fuel after the test is complete, there may be an error in the absolute sense as a result of pro-rating energy consumed over the burn time, but the error would be consistent. It would thus be fair to all stoves, and should thus give a repeatable ranking of all stoves. Is there a better way to do things than weighing the fuel as it disappears, and assuming that "energy release is proportional to loss of fuel weight"? Thanks for your insight into what goes into configuring a good stove test. Best wishes, Kevin ----- Original Message ----- From: Crispin Pemberton-Pigott To: 'Discussion of biomass cooking stoves' Sent: Saturday, May 04, 2013 9:01 PM Subject: Re: [Stoves] Stove Definition - controllability 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/
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