Watching pots boil

My previous article about kettles left me wondering: Can gas hobs really waste more than half of the calorific energy in the gas? I decided to try a few more experiments and finally I think I have an answer: ‘Yes’. Gas hobs really do fail to transfer a great deal of the calorific energy in the gas to the pan or kettle they are heating.

Experiment#1: Heating different amounts of water in the same pan

Experiment#1: Heating different amounts of water in the same pan

Experiment#1 Rather than measuring the total time to reach 100 °C, I measured the rate of temperature rise. Because the heat capacity of water is well known, this allowed me to estimate how much thermal power was entering the water. So I spent a happy hour or so heating up various amounts of water: first 200g, then 400 g, 600g and finally 800g and I measured the temperature every 20 seconds.

The temperature rise versus time of four different amounts of water on a 1.75 kW burner

The temperature rise versus time of four different amounts of water on a 1.75 kW burner.

I knew the burner power was 1.75 kW, and after a little jiggery pokery with a spreadsheet I estimated the power entering the water as a function of temperature rise.

The temperature rise versus time of four different amounts of water on a 1.75 kW burner

The estimated amount of thermal power entering the water for water on a 1.75 kW burner.

The data is a little noisy but it shows that initially only about 850 W of the burner power reaches the water. Since this is 1750 W (nominal) burner, this raises the question of where the remaining 900 W of power is going! It is also interesting that rate of heat input to the water decreases with temperature rise such that the rate of heat input at the boiling temperature (roughly 80 °C temperature rise) is barely more than half the initial rate of heat input. I have calculated the radiated heat from the upper surfaces of the kettle and it cannot account for this.

Experiment#2: Heating the same amount of water in the four different size pans

Experiment#2: Heating the same amount of water in the four different size pans.

Experiment #2 Now I fixed the amount of water (0.5 kg) but I used four different pans: a baby pan (0.487 kg: diameter 15 cm); a mother pan (0.601 kg: diameter 17 cm); a daddy pan (0.834 kg: diameter 19 cm) and a big daddy pan (1.8 kg: diameter 24.5 cm). I measured the water temperature at the start, heated for 60 seconds and measured the temperature again, and then heated for another 60 seconds. The results are shown in the chart below.

The temperature rise after 60 seconds and 120 seconds of 500 g water heated in four different size pans.

The temperature rise after 60 seconds and 120 seconds of 500 g water heated in four different size pans.

The main observation is that the water heats significantly faster in the largest pan. After 60 seconds the temperature of the water in the small pan had risen 20 °C whereas the water in the largest pan was 5 °C hotter – despite the extra 0.347 kg of steel that needed heating. This is consistent with the idea that a great deal of the heat energy is lost because the hot gas from the combustion does not remain in contact with the base long enough to transfer its heat. On small pans, hot gases escape around the edge of the pan.

Estimated efficiency of heat transfer to a pan versus the area of the base of the pan.

Estimated efficiency of heat transfer to a pan versus the area of the base of the pan.

I estimated the heat capacity of the pans and then calculated the fraction of the energy of the gas that the [pan + water] combination had captured. The data show a linear dependence on area with the largest pan capturing a plausible 83% of the calorific energy of the fuel.

Two images of a kettle on a hob taken using infra red light. The image on the left shows the general situation of the kettle - notice the hot handle! The thermograph on the left shows details of the base with an estimated temperature of approximately 224 C

Two images of a kettle on a hob taken using infra red light. The image on the left shows the general situation of the kettle – notice the hot handle! The thermograph on the right shows details of the base with an estimated temperature of approximately 224 C.

Experiment #3 It occurred to me that the underside of the kettle (or pans) might become extremely hot and radiate a significant amount of energy. However borrowing a thermal camera from work, the hottest features I could see were only just over 200 °C. Even accounting for the considerable uncertainties in this estimate, the radiated energy from the base of the kettle is only a few tens of watts at most, and can’t account for the energy losses of hundreds of watts that I have observed. A picture of the kettle on the hob shows the strong heating of the base of the cooker and the handle – which was indeed too hot to hold.

Summary

So now a general picture emerges: although cooking with gas uses a primary fuel, typically half of that energy is lost on the hob. Using a large pan on a small flame, I managed to capture as much as 83% of the primary energy, but for a kettle, 50% is probably more typical. Using a large flame on a small kettle will easily waste more than 50% of the energy.

And I can now answer my initial question: Should I use an electric kettle (in which 60% of the primary energy is discarded at the power station, but which is essentially 100% efficient in my kitchen) or a gas kettle (in which nearly all the energy of the primary fuel is delivered but which is only rough 50% efficient in my kitchen)? My answer is that it makes very little difference. It is best to use whichever device allows you to easily boil the correct amount of water – and not to boil water which is then unused.

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6 Responses to “Watching pots boil”

  1. telescoper Says:

    Reblogged this on In the Dark and commented:
    As our Departmental Christmas lunch is looming I only have time for a brief reblog of this nice discussion of boiling water in pots. It might strike you as as a bit obsessive to write about the physics of such an everyday phenomenon, but I think a bit of an obsession about physics is a very good thing indeed.

    P.S. As a fully paid-up member of Pedants Anonymous I couldn’t resist drawing attention to the metonymic shift involved in the title “Watching pots boil”. Of course the pot doesn’t boil – the water in it does….

  2. teddnet Says:

    I think this is actually really important- the correct choice of kettle for millions of people actually results in non-trivial carbon savings. So, carrying on from the last post, I don’t think the extra water vapour from burning gas can be used as a counter argument to the need to consider the house-heating effect of the lost energy in the gas kettle. The old concept of Aga cookers was based on this- that the waste heat from inefficient cooking process was recycled in heating the home. The lost gas energy going around the kettle or pan is not really lost in winter. It does heat the house, and is therefore recovered by the thermostat and reduced central heating cost. In summer of course this effect doesn’t apply. And since you are measuring things so accurately and carefully, I think it needs to be addressed or refuted. My guess is that the extra humidity generated by boiling a kettle on gas by the gas itself is minute compared to the direct effects of boiling water, the effects of people walking around with wet lung membranes the area of tennis courts, and the showers taken by people trying to save energy by not taking baths. Personally I find the house if anything too dry in the winter, so a little extra humidity is welcome.

  3. David Howey (twitter @davidhowey) Says:

    Love it!! Turns out a bunch of my colleagues have been working on this too: http://www.isis-innovation.com/licensing/9280.html

  4. m.h. sotudenia Says:

    it was very good, i suggest the ratio of the diameter of flame circle to pan diameter is useful,thank you

  5. Candle Mass and Candle Power | Protons for Breakfast Blog Says:

    […] energy was being delivered as useful heat. This seems reasonable given my previous experience heating water with flames, and considering that some of the wax may go un-burned (to make the soot in the […]

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