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</o:shapelayout></xml><![endif]--></head><body bgcolor=white lang=EN-CA link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='color:#1F497D'>Dear Paul and All Water Boilers<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><div><p class=MsoNormal><span style='color:#1F497D'>></span>I appreciate and understand Crispin's comments. First a question, and then a reply.<br><br><span style='color:#1F497D'>></span>Question: Does this make any difference? I think it does. I have conducted enough WBT with the waiting for the final degree (local boiling point) to be reached to know that there there can be even minutes of waiting. And it is especially long if the fire is low/small/weak.<span style='color:#1F497D'><o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>There are two things at play in your ‘moment of delay’. The first is the unknown amount of heat invested water that has not yet boiled but which did not change temperature. The second is what the ‘tax’ on the heat invested is in terms of losses from the pot that are not being quantified.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>You might recall dimly a few years ago Grant Ballard-Tremeer and I were discussing something he had noticed which was a ‘reproducible effect’ in the improvement of the ‘thermal efficiency’ of simmering a pot with the lid on by taking the lid off, all other things being equal. The comment at the time was that there is heat passing through the pot lid that is not being measured.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>The point is that there is only one loss of heat from a pot that is counted: missing water. Other radiative, conductive and convective losses are ignored. When you have a lid off the pot, you add a 4<sup>th</sup> loss which is radiation from the water (which has a very high emissivity even though it is transparent). I have for years recommended using lids on the pots when assessing stove performance for a number of reasons but the fixed loss is the one I will concentrate on today because it is relevant.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>When the fire is ‘small/weak’ and the lid is off, then the losses nearly match the heat input and the energy gain in the water and material of the pot is slow. Thus the addition of subtraction of 100 watts of firepower makes an inordinate difference to the ‘time to boil’. When the fire is much larger than the loss, there is no nearly no impact. This means that the metric ‘time to boil’ is strongly, (grossly?) affected by firepower when it is well matched to a cooking pots with lids on. The loss of additional heat from a pot is about 80 watts or so. If that 80 watts is 50% of the heat being gained in the pot, then removing the lid cripples the stoves and greatly extends the time needed to bring the pot to a boil.</span><br><br>Reply: Why not define the "End-of-test boiling point" to be either 1 or 2 degrees C less than the local boiling point? <span style='color:#1F497D'><o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Well, the real question should be, “What are you trying to assess about the stove’s performance?” The time to boil is not a particularly informative metric. In fact it can be calculated from the average thermal efficiency and the firepower but that is not the point. First ask, “What are we trying to understand about the stove’s performance?”<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>We as designers want to know the effective heat transfer efficiency from fuel burning to hop pot. That is a valuable number telling us something about the stove structure. When we want to change the structure, we want a very accurate determination of the performance on that score because we want to know if we made a difference or not, then track down why that difference occurred. If you take a ‘time to boil’ metric it is so full of variation that it is not helpful in most cases. If the fire was completely controllable (as with an LPG or kerosene stove) and there was no leftover char, it might be useful. But the question it is answering is not really about boiling, it is about heat transfer. If the heat transfer efficiency is mis-reported by a large variation in the amount of heat contained in the pot of water, then it wrecks what would otherwise be a valuable number.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>></span>So when the test pot water temperature reaches that number and remains steady (or increases) for 10 seconds, the test is over. <span style='color:#1F497D'><o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Taking a reasonable large variation in water temperature, say from 25 to 65 degrees, you can get a thermal efficiency with quite a bit of accuracy (better than 10% and in many cases 5%) and with good precision (better than 6% and in many cases better than 3%). If you made a change to a system that improved the thermal efficiency by 10% and use a test (one that crosses the boiling point) you can have errors of 25%. Why complicate things with problems like that?<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Using a pot that is already boiling is also pretty accurate. If you have a lid on to minimise unmeasured losses and taking the evaporation rate as the dependent variable, burning a given mass of fuel will provide an accurate heat transfer efficiency (fire to pot, not hot gas to pot which is always a higher number). In other words the fire-to-pot scenario is not the heat transfer efficiency (which is hard to measure) but a proxy for the heat transfer. We are interested in both but can only really access one of them so we use it.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>></span>Latent heat of 2257 Joules per gram is a significant amount when compared with the energy to raise one degree when well below the boiling point.<br><br><span style='color:#1F497D'><o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>And that is my point. Why introduce an error that is several times larger than the thing we are trying to measure?<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Paul, if you run the heating test to near the boiling point you start to lose water mass and that complicates things. There is simply no need. The heat transfer efficiency at different pot temperatures has been studied closely by the Indians and there is nothing to worry about. The heat transfer efficiency from 30-65 is extremely safe in that virtually no water evaporates except in ridiculous circumstances (those ‘racing stoves’ we had years ago). Over 70 C the water starts to disappear which makes testing more complicated. <o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>A very simple and very accurate and very precise test of thermal efficiency is to heat a closed pot or water on a fire from below 30 to above 60 C, marking the time interval and the mass of fuel burned. If you replace the pot with another and carry on for as long as you like, you will end up with a total energy released figure (char compensated if appropriate) and a total mass of water heated a known number of degrees. If you ignore the charcoal, you get the system efficiency in terms of fuel consumed and work done.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Both are valuable and repeatable metrics.<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Regards<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>Crispin<o:p></o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><br>As Crispin points out from Prof. Lloyd's work, even the position of the temperature probe makes a difference. <br><br><span style='color:#1F497D'><o:p></o:p></span></p><p class=MsoNormal>On 1/6/2013 6:45 PM, Crispin Pemberton-Pigott wrote:<o:p></o:p></p></div><blockquote style='margin-top:5.0pt;margin-bottom:5.0pt'><p class=MsoNormal><span style='color:#1F497D'>Dear Tom</span><o:p></o:p></p><p class=MsoNormal><span style='color:#1F497D'> </span><o:p></o:p></p><p class=MsoNormal><span style='color:#1F497D'>Something further on the subject, Tom, is the work by Prof Philip Lloyd in Cape Town on water boiling tests (meaning the boiling of water, not WBT’s).</span><o:p></o:p></p><p class=MsoNormal><span style='color:#1F497D'> </span><o:p></o:p></p><p class=MsoNormal><span style='color:#1F497D'>He noticed that as the temperature of water goes up at a steady rate, you can project the time at which the pot should boil, but the targeted time is always missed – the temperature levels off just as the moment of boiling arrives. </span><o:p></o:p></p><p class=MsoNormal><span style='color:#1F497D'> </span><o:p></o:p></p><p class=MsoNormal><span style='color:#1F497D'>I believe that the cause is the investment of energy in the water at 100 degrees without reaching the point of evaporation which is 100 C + 2257 Joules per gram.</span><o:p></o:p></p><p class=MsoNormal><span style='color:#1F497D'> </span><o:p></o:p></p><p class=MsoNormal><span style='color:#1F497D'>It only take 244 Joules to raise water from 0 to 100 C. That means water which is in the pot and heated to 100 and ‘halfway’ to becoming steam holds quite a bit of energy that is not being measured by the thermometer. Consider the error involved:</span><o:p></o:p></p><p class=MsoNormal><span style='color:#1F497D'> </span><o:p></o:p></p><p class=MsoNormal><span style='color:#1F497D'>Water at 90 C has a total enthalpy of about 1520 Joules/g. Between 90 and 100 + half way to becoming steam is another 1170 Joules. But only 42 of them show up as ‘a temperature increase’.</span><o:p></o:p></p><p class=MsoNormal><span style='color:#1F497D'> </span><o:p></o:p></p><p class=MsoNormal><span style='color:#1F497D'>Thus as the boiling point is reached, a lot of energy disappears into the mass of the water and does not change the temperature much. This is easily seen on a temperature:time plot.</span><o:p></o:p></p><p class=MsoNormal><span style='color:#1F497D'> </span><o:p></o:p></p><p class=MsoNormal><span style='color:#1F497D'>For this reason if you wanted to determine, for example, the heat transfer efficiency of a stove design working with a certain fuel, pot size and firepower, it should be done without crossing the boiling point. Either it should be measured when the water is below perhaps 70 C (as per SeTAR and Indian methods) or when it has a fully developed rolling boil (as recommended by Piet Visser). Trying to determine the heat transfer efficiency or a proxy of it while crossing the boiling point pretty much guarantees a large error because it is impossible to tell how much energy has entered the water <i>at</i> the boiling point.</span><o:p></o:p></p><p class=MsoNormal><span style='color:#1F497D'> </span><o:p></o:p></p><p class=MsoNormal><span style='color:#1F497D'>While this observation has been called ‘speculative physics’ that fact remains that using a thermometer/thermocouple to get stove metrics is not quite as simple as it first appears.</span><o:p></o:p></p><p class=MsoNormal><span style='color:#1F497D'> </span><o:p></o:p></p><p class=MsoNormal><span style='color:#1F497D'>Using a clock at the same time one can get Time to Boil, with a scale: Fuel to Boil, ditto for different pot sizes, but not get an accurate measure of thermal efficiency, heat transfer rate or true energy content of the pot. One of the proofs of this error is the calculation of the apparent energy efficiency during the heating of a pot water and the same calculation to a boiling pot. The latter efficiency is always a lower number. If you calculate the difference, it is the heat invested in the water at 100 C without boiling it away.</span><o:p></o:p></p><p class=MsoNormal><span style='color:#1F497D'> </span><o:p></o:p></p><p class=MsoNormal><span style='color:#1F497D'>When it comes to temperature, measure with care!</span><o:p></o:p></p><p class=MsoNormal><span style='color:#1F497D'> </span><o:p></o:p></p><p class=MsoNormal><span style='color:#1F497D'>Regards</span><o:p></o:p></p><p class=MsoNormal><span style='color:#1F497D'>Crispin</span><o:p></o:p></p><p class=MsoNormal><span style='color:#1F497D'> </span><o:p></o:p></p><p class=MsoNormal><span style='color:#1F497D'> </span><o:p></o:p></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Times New Roman","serif"'><br><br><br><o:p></o:p></span></p><pre>_______________________________________________<o:p></o:p></pre><pre>Stoves mailing list<o:p></o:p></pre><pre><o:p> </o:p></pre><pre>to Send a Message to the list, use the email address<o:p></o:p></pre><pre><a href="mailto:stoves@lists.bioenergylists.org">stoves@lists.bioenergylists.org</a><o:p></o:p></pre><pre><o:p> </o:p></pre><pre>to UNSUBSCRIBE or Change your List Settings use the web page<o:p></o:p></pre><pre><a href="http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org">http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org</a><o:p></o:p></pre><pre><o:p> </o:p></pre><pre>for more Biomass Cooking Stoves, News and Information see our web site:<o:p></o:p></pre><pre><a href="http://www.bioenergylists.org/">http://www.bioenergylists.org/</a><o:p></o:p></pre><pre><o:p> </o:p></pre></blockquote><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Times New Roman","serif"'><o:p> </o:p></span></p></div></body></html>