Just wondering - Heat Transfer

[The Natural Philosopher]

because its cheaper.

Tim Tim, nasty and dim.

[rick]

I suppose you could improve on the design to allow it to get the heat away quicker - .... by having a load of fins - and therefore create surface area in contact with the water. Wouldn't make conversion of electricity to heat any more efficient.

Cost involved probably not worth any gain.

[T i m]

Bingo. And thanks for playing (I hopped you would get there in the end). ;-)

Cheers, T i m

[T i m]

So you say?

So you say? Killfile stopped working? Could you fix it please as it was much better having to hear all your confused attempts at replies to questions no one asked. ;-(

Cheers, T i m

[T i m]

So, are you saying that a 'kettling' element *will* be transferring it's energy to the water as effectively as one with say twice the surface area that wasn't kettling? (straight question).

So we aren't talking directly about a loss of energy here, but a loss of efficiency (because of kettling and other variables).

Cheers, T i m

[T i m]

Do you realise it's a sign of weakness if you are unable to explain things properly (not that you ever could now you have dug such a big hole) without resorting to expletives and insults?

See, if you weren't such a 'perfect engineer' you would have realised that the OP's question was *nothing* to do with overall system losses but *all* to do with how efficiently the heating element transferred it's energy to the water.

In fact, let's recap exactly what he said and see if the penny drops with you yet ... "Are immersion heaters as efficient as they could be? Could the tubular element be improved upon - or doesn't it matter?

Go on, have another go. ;-)

Cheers, T i m

[T i m]

That is what I was reflecting.

So, if we put a 100kW heater in there and had it on for 1 second, would the water end up as hot as if we put a 10kW heater in there and had that on for 10 seconds? I'm not talking 'theoretically, I'm talking real world here?

The (/my) 'big question' is that if (say) the heater element get's sufficiently hot as to start to boil the surrounding water, *does* the heat get carried into the water as effectively as if it wasn't boiling?

If the answer is no, then you could see how if you increased the surface are of the heater you decrease the energy / area and could take it down to a level where there was no kettling.

No, quite, but that wasn't really part of the question. ;-)

Cheers, T i m

[Chris Hogg]

Bubbles of steam on the surface of the element would certainly reduce the amount of heat being transferred to the water, effectively acting as insulation. But if you're actually putting in 100kW for 1 sec, all that will happen is that the element itself will get hotter under those circumstances than the 10kW/10 sec situation, and at the end of that 1 second, it will lose that extra heat to the water and the overall result will be the same. Assuming the 100kW heater was designed for that sort of duty*, and wasn't an over-driven 10kW one, I can't see any significant benefit in running it at a lower power.

*AIUI kettling can cause localised pitting and corrosion, but not if the element was designed to cope.

[dennis]

They will both transfer the same amount of heat for the same power consumption/

Of course you can make them better for some values of better.

Like you could use diamond as an insulator as it conducts heat very well.

You could put stainless steel tails on as they don't conduct heat very well.

etc.

However they won't happen until they are cheap.

[John]

Yes, because there is nowhere else for the heat to go.

Just as well, I don't like bent questions.

That isnt loss of efficiency, all the heat still ends up in the water.

[Huge]

[31 lines snipped]

And into the air in the airing cupboard.

[bert]

Collapsible to get them through the small hole.

[bert]

Via the water.

[bert]

In article , T i m writes

No Go watch a saucepan of water coming to the boil.

[mechanic]

Aren't the hot water cylinders supplied with integrated insulation round them, as one package? No separate 'bulky' insulation.

[T i m]

I'm not sure 'collapsible' and 'good thermal conductivity are normally found in the same sentence but if the heater was left at the same basic extended 'U' design, you could still increase the surface area by simply joining the two 'runs' together with a flat sheet (and the stat will simply run down the side of that) and so it will easily fit down the same hole in the cylinder.

Cheers, T i m

[T i m]

;-)

Ok, but only the energy that is available?

So, no kettling, good thermal conductivity with the water, plenty of time for the water to convect and take the heated water round the cylinder. Heater element stays reasonably cool and therefore low resistance therefore runs at full marked power.

Overdriven element (for the surface area), kettling, decreased contact with the water (steam insulating the element from the water), increased element temperature, higher resistance, reduced energy consumption from rated, lower actual output?

Now, it may not make much of a difference ITRW but we were just 'wondering' about such things here. ;-)

Cheers, T i m

[T i m]

Ok.

Ok, fair enough.

Other than when the element loses (or reduces) it's ability to transfer it's heat to the water, the internal resistance goes up and the power consumption goes down and so the output energy goes down?

And this would be over a sliding scale, from a single bubble to the entire element covered.

I believe these were the sort of lines the OP was thinking re his initial question (but not kettling specifically).

I know it's not quite the same (because it's not a closed system) but try to accelerate a tyred vehicle past it's ability to maintain traction and that extra power (and some of the previous power) is just wasted.

OK.

Cheers, T i m

[The Natural Philosopher]

Yes, Go watch a saucepan of water coming to the boil.

With a lid on it.

[The Natural Philosopher]

No, we are not wondering. You are wondering, because you are naive and ignorant.

Everyone else knows that is all bollocks. Its almost impossible to make an inefficient kettle in terms of electricity in versus heat transferred to the water.

And the amount of electricity used is not enough to seriously warrant any time being spent on making it marginally better.

I suggest you direct your limited intellect to more useful issues, like whether green cars are faster than white ones.