[Stoves] Cajun Rocket Pot Tested

[Ronal W. Larson]

Dale and Crispin:

   This is to try to close the gap a bit.  Dale reports for his home gas range testing two differences in efficiency:  8.8 and 13.5  (the “knobby aluminum” over the steel).  Crispin says the efficiency should be strongly modified by the materials and weight differences - which I can see makes sense.

   My problem is that playing around with Dale’s stated power levels ("The power input to the regular pot was 1128 Watts, while that of the finned pot 1227 Watts, or 1.088 times as much."),  I can’t see from the given data, how Dale got those power numbers.  I think there should have been some water evaporated, but none is stated.  Did the 700 watt number come in some way?

   So,  Dale, can you show the computations you used to get 1128 and 1227.? Adding in the specific heat of the pots (if you didn’t - and you might have) should help with better understanding both this geometry and the importance of including what Crispin wants to include.

Ron
 




On Nov 18, 2013, at 4:57 PM, Crispin Pemberton-Pigott <crispinpigott at gmail.com> wrote:

> Dear Dale
>  
> What a welcome report on a promising technology!
>  
> I have several points that I’d like to raise which probably impact materially on your concluding numbers.
>  
> I am happy to see at the end a discussion of the thermal mass of the pots and the fact they have different materials and different masses. Because the tests were conducted to check the heat transfer efficiency and the overall fuel consumption (two different metrics) at high and low power (again, two different metrics) I feel it is important to put the thermal mass in context at the beginning then move to the other points.
>  
> The British, Indian, SeTAR and Indonesian heat transfer efficiency tests all consider the mass of the pot in the calculation. There may be many others – I have not read them all, of course. I would like to investigate the implications of this.
>  
> The work done heating the water (and the pot) is measured to the boiling point and includes heating the thermal mass of the water and the pot together. At the very end you mention the difference in in energy being 130,000 for the aluminum and 26,000 for the stainless steel pots. This 104,000 Joule difference should be applied to the portion of the test that involves changing the temperature, not the whole test because most of the time, there is no change in temperature.
>  
> The answers sought related to portions of the test, which are separately reported (good). If we take the change in temperature to be 80° C then the energy needed to heat the water (calculated on the same basis as the energy needed to heat the pot) is:
>  
> 5000 g x 4.186 x 80° = 1,674,400 J
>  
> The pots used respectively 26,000 and 130,000 (from your calculation)
>  
> Thus the two tests require 1,700,400 and 1,804,400 respectively for a difference of 1.061 in the heating work done. This means the improved pot was required to (and did) absorb 6.1% more heat during that portion of the test. Once the pot is hot, the impact of the pot mass disappears because the temperature is pretty much constant.
>  
> Because the pot mass was not considered in your calculation, the ‘additional heat gained’ number (the claim that all things considered, it gains heat more efficiently) drops from 16.1% to about 10%
>  
> 10/16.1 = 0.62, 1-0.62 = 0.38 = 38% under-reported performance
>  
> I think recalculating it to include the pot mass would materially affect the conclusions – i.e. that the difference in the calculated result is significant with a high degree of confidence.
>  
> With respect to the determination of thermal efficiency at low power, the things being measured – missing mass of water and energy consumption – are not strongly correlated because, as you clearly explained, the losses of heat from the pot by routes other than evaporating water are large compared with the energy used for evaporating water. Very small changes in the local circumstances strongly affect the calculated result.
>  
> For this reason, there was some time ago a general agreement that ‘simmering efficiency’ is not really a helpful indicator because the calculated efficiency of a perfect simmer is 0% which is counter-intuitive to the claim of the method applied. If you changed the low power fuel burn rate, you will get a different low power efficiency at each power setting because it is a method that does not actually report the low power heat transfer efficiency (or the fuel efficiency).
>  
> I predict, based on your numbers, that if you were to test the same pots using cold water on a low power flame, the heat gained by the finned pot would be of a similar order of magnitude larger than the standard pot as is shown above, i.e. the difference between 10 and 16.1.
>  
> The work done by the stove in heating the water can most accurately be measured when the water is not boiling for all the reasons you stated about conductive, convective and radiative losses. As we are testing the pot, not the stove or the fuel, it will be most accurate if the water and pot are heated from some temperature above ambient such as 30° (to ensure that the heat gain rate is continuous and stable) to about 70° above which point some evaporation can be expected, complicating the calculation and introducing the relative imprecision caused by a changing mass v.s. a changing temperature. Heat gain assessed by ΔT is about 500 times more accurate than assessing it by ΔM.
>  
> The presence of a lid ensures that the heat gained is collected and not lost to radiation or small amounts of water evaporating. The result, the temperature rise of a 20° pot from 30° to 70°, is a very good reflection of the gained by the pot and virtually eliminates errors.
>  
> I would be really interested to see the difference between these two methods reported for that very same pot, taking your set of experiments as a baseline. I predict that the improvement in heat gain, calculated from 30° to 70° and taking into consideration the pot material and mass, will show an improvement of about 16% for a steady state fire of any magnitude that you used.
>  
> Where my estimate will be wrong is the low power comparison because the baseline is not really a measure of heat transfer efficiency, but my estimate not it will not be as wrong as the baseline number.
>  
> As the finned pot is claimed to be for cooking with gas (as I understand it) only two need be need be performed for high and again for low power (one for each pot type).
>  
> Because the heat transfer efficiency is not dependent on the mass of water in the pot, the water does not need to be exactly 5000 g if the pot mass is considered. This is important for testers to realise.  This mass-independence was recently confirmed again by students at the China Agricultural University to a high degree of precision (four nines) across a wide range of ‘pot fullness’.
>  
> I find this knowledge really helpful for comparing stoves that do not deal well with such a large pot. Putting in 4 litres of 5, or 6 would not change the calculated heat transfer efficiency because it doesn’t measurably vary. But the pot mass must be considered if there is a change in water temperature.
>  
> If there are other experimenters in Stove Land who have finned pots it would be great to hear from them as well, comparing not only the different appliances, but the different calculation methods.
>  
> Best regards
> Crispin
>  
>  
> A few months ago we had a discussion of Cajun Rocket Pots, a series of pots with heat transfer fins on the bottom meant for cooking seafood in the Southern US.  Their claim was that their pots reduce fuel use by 50% and reduce time to boil considerably.  (The name “Rocket Pot” has nothing to do with the rocket stove.)  I gave the opinion that their design was excellent and that it would revolutionize our cookstove work.  All we had to do was test it on some stoves of the type we use and confirm that it works.
>  
> Alas, reality interfered.    I got one of their pots, the 8 quart size.  This is their smallest size and is appropriate for a 5 liter water boiling test.  The finned pot performed only marginally better than a regular pot of the same size, typically by about 10%.  Very disappointing.  It’s possible that for certain types of stoves with certain shapes of gas burners the pot really does perform well, but it did not perform particularly well on any of the stoves I tested. 
>  
> I’ve attached a report with the test details of a finned and a regular pot being tested on 7 different stoves.  Some stoves gave better results than others, but the 10% improvement is a typical number. 
>  
> Dale Andreatta
> <Cajun Rocket Pot Report.pdf>_______________________________________________
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