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At work we got a new kettle recently. It's a fancy one with a water filter in it. You fill up the top half of the jug and it goes through a Brita filter into the bottom half of the jug where the heating element is.

It got me wondering what would be faster to boil the water.

Given a fixed volume of water and a kettle that provides a fixed amount of energy over time...

Would the water boil quicker (I.e. Reach 100 degrees and turn the kettle off) if you were to wait for all the water to filter through before turning the kettle on. Or if you were to turn the kettle on as soon as the water started filtering.

By quicker here I mean the time between turning the kettle on and it turning itself off. Not bothered about the time before turning the kettle on.

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    $\begingroup$ This is an interesting question. I'm having trouble coming up with a solid analytical answer though. On paper obviously you need to add the same amount of energy to heat the same mass of water up to the same temperature. Since we have a constant energy source, they should take the same amount of time. That said, the reality is it will not evenly heat. Adding the water slowly would potentially force mixing as you go. It also may interrupt the natural convection of the fluid. I'm not sure if the forced mixing would help or hurt the convective currents (probably help, forcing cool water... $\endgroup$
    – JMac
    Mar 22, 2017 at 10:43
  • $\begingroup$ @JMac that's exactly my thought process with this. Will the cold water cool down the hot water more than the hot water will heat up the cold water? Will the mixing affect things? etc... $\endgroup$
    – Fogmeister
    Mar 22, 2017 at 10:44
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    $\begingroup$ ... downwards. But as I said, there would be a lot to this analysis if you want to say one is quicker than the other. This is kinda a mix of answer and comment, but since I can't really come up with an answer I'll just leave to here to highlight some difficulties. $\endgroup$
    – JMac
    Mar 22, 2017 at 10:44
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    $\begingroup$ To address that a bit Fogmeister, if the cool water cools down the hot water, it itself would be heating up (by the same amount ignoring slight losses to the air). You actually want to maximize the amount of cool water near the heating element for the maximum temperature difference, so constantly pouring into the kettle might help force it. I think I have enough to outline an answer now actually, I'm going to do that. $\endgroup$
    – JMac
    Mar 22, 2017 at 10:48

2 Answers 2

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I haven't done a thorough analysis, I am basing this more on a qualitative analysis (and honestly, in this case I don't think that's going to be the best, any differences are going to be minimal compared to the time scale of the heating, so in reality any small factor I forget to consider could completely change the answer).

Hopefully this will address some of the complexity of this.

On paper obviously you need to add the same amount of energy to heat the same mass of water up to the same temperature. Since we have a constant energy source, they should take the same amount of time.

That said, the reality is it will not evenly heat. Adding the water slowly would potentially force mixing as you go. It also may interrupt the natural convection of the fluid. I'm not sure if the forced mixing would help or hurt the convective currents.

My thought is that slowly pouring the water will help. Mainly because the natural convective currents are trying to force the new cool water downwards, the same as the pouring water would be doing. This helps to maximize the temperature difference at the heating element for better heat transfer.

The other slight advantage of pouring water is you would be constantly generating heat from the fluid friction.

For those reasons I'd say it's marginally better to heat while pouring.

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Kettle's fuse goes off in the very beginning, where is not enough water and the kettle heats up too much.

Since this is not a response you expected, I have a second one. In principle - boiling should take a little more time with the water slowly pouring through the filter, because of a higher heat loss

Here is a small simulation that can be actually found at http://nbviewer.jupyter.org/urls/dl.dropbox.com/s/306bc6dwa59ea6d/konvice.ipynb (if you find an error, we can correct it)

enter image description here

The x-axis is time, y-axis is temperature, red line is with slowly pouring water, blue line is a full kettle from the beginning.

Check the (red) stable plateau of the temperature for 50 sec. - it is a balance until all water gets inside the kettle (1 litre here). This part can influence the boiling time, because just here there are higher losses.

For curiosity, I have added Stefan-Boltzman law for heat irradiation losses, but no real difference is visible. There could be also losses by air-cooling, I dont think they would be also important, it is just a limited time at 45 degrees Celsius.

Rule of thumb - mixing is so good, that temperature inhomogenities I dont think they play any role.

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  • $\begingroup$ Ooh, excellent. I will try to understand that simulation you created :D Thanks very much $\endgroup$
    – Fogmeister
    Mar 22, 2017 at 14:12
  • $\begingroup$ I'm very confused by what your graph shows. How do you have water at 45° at time 0? Also, where are you losing heat? It would be nice if you explained how you got that model instead of just plopping down a graph and saying it's slower. $\endgroup$
    – JMac
    Mar 22, 2017 at 16:52
  • $\begingroup$ check the link - there is a source in python. You can check for an inconsistency there, yes, could be. But it seems to me ok, you get small water volume and power 2kW, so you heat it very fast in the beginning and some equilibrium seems logical. Change the printout to every second, for curiosity. $\endgroup$
    – jaromrax
    Mar 23, 2017 at 7:58

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