Hot water jugs v. kettles

[polygonum]

I didn't say 'really effective'!

OK - place an air gap of a few mm between the water and the inner surface - fine by me. By the way, how are you going to achieve that?

[John Rumm]

Quite probably - there may not be much copper in a flex, but an unconstrained arc flash in your living room could still be pretty undesirable I would have thought...

[John Rumm]

I suspect the energy loss angle is probably of little consequence...

Still I suppose its worth checking.

Take a jug kettle, and treat it as a 150mm diameter cylinder that is say

200mm tall. Lets ignore the losses through the better insulated top and base.

That gives us a radiating area of pi x 0.15 x 0.2 = 0.094 m^2

Lets take our final surface temperature as 100 deg C or 373K

Lets ignore the radiated gains absorbed from the surroundings.

So we get

p = sigma T^4 A

Where P is the power output, sigma is steffan's constant = 5.6703e-8

p = 5.6703e-8 x 373^4 x 0.094 = 103W

However that is for a perfect black body radiator. For a nice polished stainless kettle, that needs to be multiplied by the emissivity of the surface, so say 0.075. That gives us a gross radiation rate of just under 8W. The real figure will be substantially less since we are ignoring any radiated heat absorbed from the surrounds (small number) and also the fact that was the loss calculated at the final temperature and we spend relatively little time at that. So we can take the average loss as somewhat less than half that.

So in summary, from an energy saving point of view, an insulated kettle is pretty pointless. The main benefit is the lower risk of contact burns.

[Martin Brown]

I suspect that most of the heat loss from a kettle is by air convection and escaping steam anyway although the radiative losses are too large to ignore once the object is at 30C or more above ambient.

Indeed. Better insulation or a mirror finish on a teasmade teapot might slow down its tendency to lose heat too quickly though.

[John Rumm]

Yup sure, quite a different question if you are trying to get something to stay warm after heating...