[Stoves] Cajun Rocket (finned) Pot, and Eneron Finned Pot

Crispin Pemberton-Pigott crispinpigott at gmail.com
Wed Nov 20 17:45:12 CST 2013


Dear Dale

 

Much appreciated the test report for the other pot – that is a really big difference and more in line with the goal of your earlier and very interesting tests on finned pots. 

 

For those of you who have not seen Dale’s work look through the ETHOS website and you will find a couple of papers there. 

 

I can add a little to the discussion below. Dale points out that as the pots reach the boiling point, there is a change in the ratio of heat gain by the two different pots compared with the same ratio calculated for a lower temperature.

 

Provided that the temperature in question is below, not at the boiling point (when other issues arise) this difference, noted as 23% instead of (10%, I guess) is entirely caused by not including the thermal mass of the pot. This difference in ratios does not arise if the pot is considered as well.

 

Why? As the temperature nears boiling, and evaporation starts to really kick in, the % of total heat that is devoted to changing the temperature of the pot material starts to diminish. The temperature change rate decreases and most of the energy goes into evaporation. As I mentioned earlier, once the temperature stops changing the difference in heat transfer efficiency shows up in different evaporation rates (provided the heating power remains high so unmeasurable losses are relatively small).

 

Interesting, yes? It shows clearly that the pot material and mass must be considered to be able to accurately determine the heat transfer efficiency (or any other efficiency that includes the pot) across any temperature range.

 

The effect of asking ‘the same question’ under different circumstances can best be understood conceptually by creating a heat flow diagramme that shows where all the heat is going, and what portions of it are measured and compared. Dale has shown that asking the same question at low temperature and near the boiling point gives a significantly (measurably) different answer if the pot material is not considered. 

 

In the same way trying to get the ‘efficiency’ while simmering a hot pot at low power gives basically no useful result. What you can measure does not deliver useful information.

 

Regards

Crispin

 

PS here are two slides on the matter: Low power hot pot v.s. high power cold pot.

 





 

+++++++++

 

All of Crispin’s comments are correct, and I suppose I should have included the mass of the pot and its energy gain to get the true energy transferred.  Still, my view is that whether the Cajun pot is 16% better or 10% better than a regular pot, that’s not enough to get excited about.  If someone want to redo the calculations I can send them the Word version of the report and they can edit it so that all the numbers are there.

 

I’ve attached a report about another finned pot, the Eneron Pot.  This report covers 3 types of gas burners of the type used in restaurants.  I chose to test the Cajun pot rather than the Eneron because I believed the Cajun pot would be very good, and the Eneron not as good.  The first of these beliefs was wrong, so perhaps I should have tested the Eneron pot.  Perhaps someone else would like to test it?  With a couple quick tests you could probably determine whether it is promising or not.  I might do that some day, as well as test variations of the Cajun pot.  

 

With regard to Ron’s questions, in my calculations I always included the evaporated water (except in the simulated open fire test where there was virtually no evaporation) but never the mass of the pot.  The formula for energy gained by the water would be:

 

(Mass of water times temperature gain times 4.186) + (Mass lost through evaporation times 2250)

 

Divide this number by the number of seconds elapsed during the test period gives you the power into the water in Watts.  This works for high or low power, or the overall test, or any other length of time.  

 

The estimated 700 Watts lost from the pot was not included in any calculations.  I gave that number as a way of explaining why the ratio of times to boil was greater than the ratio of heat transfer efficiencies.  As you approach the boiling point the net heat gain by the regular pot is about 1128 – 700 = 428.  The net heat gain by the finned pot is 1227 – 700 = 527, or about 23% more.  If you had a pot that produced 701 W of heat transfer it would approach the boiling point quickly and then take a reeeeeeeeeeealy long time to fully reach boiling.  (It seems that half of my tests go this way!)

 

Dale

 

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