On 27/10/2020 10:40, Halmyre wrote:

We had our kitchen done 17 and a half years ago, with Neff units. Dishwasher has been replaced, although with hindsight it might have been a simple repair. Washing machine motor has been repaired, fridge/freezer replaced a couple of years ago and I've replaced the oven fan heater element three or four times.

Both the upper and lower grill elements went in the X-years old
Whirlpool double oven which was here when we moved in. That was after 5
years of our use.

What interests me is /why/ those elements failed. They have no moving
parts and are effectively just wire resistances. Any idea why yours
failed? It seems to me the elements must be designed that way; is there
something they do to the (nichrome?) wire which almost guarantees it
will fail after a few years?

--

Jeff

On Wednesday, October 28, 2020 at 4:14:28 PM UTC, Jeff Layman wrote:
On 27/10/2020 10:40, Halmyre wrote:

We had our kitchen done 17 and a half years ago, with Neff units. Dishwasher has been replaced, although with hindsight it might have been a simple repair. Washing machine motor has been repaired, fridge/freezer replaced a couple of years ago and I've replaced the oven fan heater element three or four times.
Both the upper and lower grill elements went in the X-years old
Whirlpool double oven which was here when we moved in. That was after 5
years of our use.

What interests me is /why/ those elements failed. They have no moving
parts and are effectively just wire resistances. Any idea why yours
failed? It seems to me the elements must be designed that way; is there
something they do to the (nichrome?) wire which almost guarantees it
will fail after a few years?


No idea. The failed units sometimes have a bulge or a roughened patch where the failure is. I imagine it's a pretty hard life they get. I sometimes see it glowing bright red through the back panel.

We never use the conventional (non-fan) heating so their elements are still original.

Jeff Layman wrote:
What interests me is /why/ those elements failed. They have no moving
parts and are effectively just wire resistances. Any idea why yours
failed? It seems to me the elements must be designed that way; is there
something they do to the (nichrome?) wire which almost guarantees it
will fail after a few years?

I assume it's the same reason lightbulbs fail. They run at high
temperatures. Eventually the material ages (in the case of lightbulbs due
to filament evaporation - not sure of the mechanism of a ceramic element but
possibly some kind of migration into the substrate) and a positive feedback
loop causes thermal runaway and catastrophic failure.

Theo

On 28/10/2020 20:32, Theo wrote:
Jeff Layman wrote:
What interests me is /why/ those elements failed. They have no moving
parts and are effectively just wire resistances. Any idea why yours
failed? It seems to me the elements must be designed that way; is there
something they do to the (nichrome?) wire which almost guarantees it
will fail after a few years?

I assume it's the same reason lightbulbs fail. They run at high
temperatures. Eventually the material ages (in the case of lightbulbs due
to filament evaporation - not sure of the mechanism of a ceramic element but
possibly some kind of migration into the substrate) and a positive feedback
loop causes thermal runaway and catastrophic failure.

But oven grills run at a lowish temperature - a dull red heat (around
700 - 800 deg C), e. which is about half the melting point of nichrome.
They are also supported along their length, unlike an incandescent
filament, which runs at yellow-white heat at around 1300 deg C. It also
seems to me that toasters run at a higher temperature, but they seem to
burn out at a much lower rate than oven grills. Our toaster, for
example, is around 25 years old, and is used much more than the oven grill.

--

Jeff

On 28/10/2020 22:29, Jeff Layman wrote:
On 28/10/2020 20:32, Theo wrote:
Jeff Layman wrote:
What interests me is /why/ those elements failed. They have no moving
parts and are effectively just wire resistances.Â* Any idea why yours
failed? It seems to me the elements must be designed that way; is there
something they do to the (nichrome?) wire which almost guarantees it
will fail after a few years?

I assume it's the same reason lightbulbs fail.Â* They run at high
temperatures.Â* Eventually the material ages (in the case of lightbulbs
due
to filament evaporation - not sure of the mechanism of a ceramic
element but
possibly some kind of migration into the substrate) and a positive
feedback
loop causes thermal runaway and catastrophic failure.

But oven grills run at a lowish temperature - a dull red heat (around
700 - 800 deg C), e. which is about half the melting point of nichrome.
They are also supported along their length, unlike an incandescent
filament, which runs at yellow-white heat at around 1300 deg C. It also
seems to me thatÂ* toasters run at a higher temperature, but they seem to
burn out at a much lower rate than oven grills. Our toaster, for
example, is around 25 years old, and is used much more than the oven grill.

What I don't understand is halogen radiant heaters and radiant rings (on
a vitroceramic hob).

Surely the point of the halogen gas is to enable white heat without the
filament burning out - heaters and rings are only red hot.

--
Max Demian

On 29/10/2020 10:54, Max Demian wrote:
On 28/10/2020 22:29, Jeff Layman wrote:
On 28/10/2020 20:32, Theo wrote:
Jeff Layman wrote:
What interests me is /why/ those elements failed. They have no moving
parts and are effectively just wire resistances.Â* Any idea why yours
failed? It seems to me the elements must be designed that way; is there
something they do to the (nichrome?) wire which almost guarantees it
will fail after a few years?

I assume it's the same reason lightbulbs fail.Â* They run at high
temperatures.Â* Eventually the material ages (in the case of lightbulbs
due
to filament evaporation - not sure of the mechanism of a ceramic
element but
possibly some kind of migration into the substrate) and a positive
feedback
loop causes thermal runaway and catastrophic failure.

But oven grills run at a lowish temperature - a dull red heat (around
700 - 800 deg C), e. which is about half the melting point of nichrome.
They are also supported along their length, unlike an incandescent
filament, which runs at yellow-white heat at around 1300 deg C. It also
seems to me thatÂ* toasters run at a higher temperature, but they seem to
burn out at a much lower rate than oven grills. Our toaster, for
example, is around 25 years old, and is used much more than the oven grill.

What I don't understand is halogen radiant heaters and radiant rings (on
a vitroceramic hob).

Surely the point of the halogen gas is to enable white heat without the
filament burning out - heaters and rings are only red hot.

I would expect "darker" rings to be more efficient, in that they don't
waste energy as visible light. However, I think that halogen hobs were
developed as they could be turned on and off almost instantly in a
similar way to gas. The problem with ordinary rings is that they are
thermally very slow. Do halogen hobs burn out their "elements" at the
same rate as ordinary rings?

--

Jeff

On Wed, 28 Oct 2020 22:29:52 +0000, Jeff Layman wrote:

What interests me is /why/ those elements failed.

I assume it's the same reason lightbulbs fail. They run at high
temperatures. Eventually the material ages (in the case of
lightbulbs
due to filament evaporation - not sure of the mechanism of a
ceramic
element but possibly some kind of migration into the substrate)
and a
positive feedback loop causes thermal runaway and catastrophic
failure.

Something like that but when our grill element failed it was just a
"hot spot". Long (inches) sections of the element had ruptured. I
guess that could have started from a hot spot then the live end of
the broken element wire shorting to earth via the casing blowing it
apart and consuming the wire. The cooker isn't on an RCD and the
resistance of the remaining element wire keeping the fault current
below that required to trip the 32 A MCB.

But oven grills run at a lowish temperature - a dull red heat (around
700 - 800 deg C), e. which is about half the melting point of nichrome.
They are also supported along their length, unlike an incandescent
filament, which runs at yellow-white heat at around 1300 deg C.

Make that 2,400+ C (colour temp of an tungsten filament lamp is
around 2700 K).

Hot spots certainly form in heating elements, I've seen the variation
in glow. That must stress that section more than else where if only
due to the differing expansion.

--
Cheers
Dave.

On Wednesday, 28 October 2020 22:29:57 UTC, Jeff Layman wrote:
On 28/10/2020 20:32, Theo wrote:
Jeff Layman wrote:
What interests me is /why/ those elements failed. They have no moving
parts and are effectively just wire resistances. Any idea why yours
failed? It seems to me the elements must be designed that way; is there
something they do to the (nichrome?) wire which almost guarantees it
will fail after a few years?

I assume it's the same reason lightbulbs fail. They run at high
temperatures. Eventually the material ages (in the case of lightbulbs due
to filament evaporation - not sure of the mechanism of a ceramic element but
possibly some kind of migration into the substrate) and a positive feedback
loop causes thermal runaway and catastrophic failure.

But oven grills run at a lowish temperature - a dull red heat (around
700 - 800 deg C), e. which is about half the melting point of nichrome.
They are also supported along their length, unlike an incandescent
filament, which runs at yellow-white heat at around 1300 deg C. It also
seems to me that toasters run at a higher temperature, but they seem to
burn out at a much lower rate than oven grills. Our toaster, for
example, is around 25 years old, and is used much more than the oven grill.

The insulation is hygroscopic. Keeping the ends well sealed when it cycles from nothing to red hot is difficult, or should I say ultimately impossible. Water vapour is absorbed, flashover occurs from element to case et voila, something melts or vapourises.


NT

On 29/10/2020 18:38, Jeff Layman wrote:
On 29/10/2020 10:54, Max Demian wrote:
On 28/10/2020 22:29, Jeff Layman wrote:
On 28/10/2020 20:32, Theo wrote:
Jeff Layman wrote:
What interests me is /why/ those elements failed. They have no moving
parts and are effectively just wire resistances.Â* Any idea why yours
failed? It seems to me the elements must be designed that way; is
there
something they do to the (nichrome?) wire which almost guarantees it
will fail after a few years?

I assume it's the same reason lightbulbs fail.Â* They run at high
temperatures.Â* Eventually the material ages (in the case of lightbulbs
due
to filament evaporation - not sure of the mechanism of a ceramic
element but
possibly some kind of migration into the substrate) and a positive
feedback
loop causes thermal runaway and catastrophic failure.

But oven grills run at a lowish temperature - a dull red heat (around
700 - 800 deg C), e. which is about half the melting point of nichrome.
They are also supported along their length, unlike an incandescent
filament, which runs at yellow-white heat at around 1300 deg C. It also
seems to me thatÂ* toasters run at a higher temperature, but they seem to
burn out at a much lower rate than oven grills. Our toaster, for
example, is around 25 years old, and is used much more than the oven
grill.

What I don't understand is halogen radiant heaters and radiant rings (on
a vitroceramic hob).

Surely the point of the halogen gas is to enable white heat without the
filament burning out - heaters and rings are only red hot.

I would expect "darker" rings to be more efficient, in that they don't
waste energy as visible light. However, I think that halogen hobs were
developed as they could be turned on and off almost instantly in a
similar way to gas. The problem with ordinary rings is that they are
thermally very slow. Do halogen hobs burn out their "elements" at the
same rate as ordinary rings?

But why halogen? Why not argon/nitrogen like the old fashioned tungsten
lamps?

--
Max Demian