Just wondering - Heat Transfer

[DerbyBorn]

Are immersion heaters as efficient as they could be? Could the tubular element be improved upon - or doesn't it matter?

[Bob Minchin]

100% efficient. There is nowhere else for the heat to go but into the water.

[dennis]

They convert 100% of the electricity to heat. What efficiency would you like to improve?

[DerbyBorn]

dennis@home wrote in news:585e54ca$0$1285$b1db1813 $ [e-mail]:

I sort of knew all that - but could the dissapation be improved - I imagine there is incredibly hot water next to the element - and the element itself will be very hot. Spreading the 3kW over a larger area - could this result in the element running cooler and the heat distributing better.

[Tim+]

One way might be to surround the element with something fluid that could redistribute the heat by convection. Don't suppose it'll ever catch on though...

Tim

[newshound]

If you put fins on the cladding the heat transfer would improve, and the resistive element would run a bit cooler. That might improve its life? Also, a cooler element would have lower resistance, hence slightly higher current and output.

[Dave Plowman (News)]

Since the only thing generated by the electricity they use is heat, where can it go other than into the water?

(It might also produce 'light' like UV or infra red - but in such a tiny amount as can be disregarded.)

[Graham.]

Great. No need for that bulky insulation round the cylinder then ;-)

[spuorgelgoog]

Might that help reduce scaling on elements in hard water areas?

Owain

[DerbyBorn]

Nice one!

[T i m]

I think the typical installation orientation of nearly vertical would not help with any 'finning' that would in turn help the conduction and the convection that supports it.

If the (typically orientated) element had some longitudinal 'finning', maybe in the form of a cross, that may increase the surface area sufficiently and not interfere with the convection too much?

Interesting thought though. ;-)

Cheers, T i m

[mechanic]

Who has those these days?

[DerbyBorn]

I was thinking that if it was twice the area the surface temp could be proportionally lower - but getting the heat from the resistance wire to the surgace could become and issue.

[ARW]

And a little bit of noise on some of them.

[Huge]

That'll be why the element head gets hot, will it?

[Andy Burns]

You could put fins on the element, it would speed the recovery time.

[Chris Hogg]

The core of the element will heat up until the heat (energy) taken away into the water by conduction and convection balances the electrical energy dissipated in the core of the element. At that point, equilibrium is established. The water itself won't get much hotter that 100C (there's sometimes a slight head depending on where the hot tank is relative to the header tank). When the power is switched off, any residual heat in the element will be rapidly taken up by the water. The elements consist of a nichrome heating wire surrounded by magnesium oxide powder insulation; they are designed to get quite hot, just like the elements on an electric cooker or in an electric kettle. They've been around for a very long time and usually last a very long time IME. I don't see how you could improve on them significantly.

[Andrew]

If you live in hard water area, the element might be furred up, but that would only slow down the rate of heat transfer.

A decent amount of insulation around the tank is more important if electric is the main heating source.

[T i m]

I think the OP was considering the transfer of energy between the element and the water more than the efficiency of the element itself.

Ignoring any losses from the cylinder and surrounding pipework, isn't the transfer of energy between the element and water a function of the temperature / power of the element and the ability of the water to absorb said energy?

So, if the element was just a single 30cm 10mm diameter 'loop' and was rated at say 10kW, wouldn't the water around the element boil (and I'm not sure how that would affect the energy transfer / efficiency)?

The same element rated at only 1kW would just take longer to heat the water.

So, if you took the 10kW element and increased it's surface area, wouldn't the element be able to transfer the heat energy to the water more efficiently (no 'kettling')?

After all, it's just the same process as a heatsink but in reverse, the only question is the science of the temperature differential between the element and the water and how that effects heat transfer efficiency?

Cheers, T i m

[John Rumm]

It does not really make any difference. If you stick 3kW in, then that is what must come out. The harder you make it to get out, the hotter the element will run, but eventually equilibrium will be reached.

Convection in the cylinder will take care of mixing the heated water.