Why Do 4-Coil Heating Elements Fail More Than 3-Coil Ones?

[wolram]

I hope some one can help with this.

2 different but identical rated elements are used for exactly the same purpose, the only difference between the 2 is, one has the element bent into three outer coils and three inner coils, the other has four outer coils and four inner coils, the one with the four coils all ways fails before the one with three coils (blows between adjacent coils), does this sound like a purely mechanical ie bend radius fault or could the different coil winding cause some electrical difference?

 

[TVP45]

I can't picture what this looks like, but a quick guess is that, if you have more coils in a similar space, you're running into expansion shorting.

 

[Danger]

Were the elements identically rated before or after bending? Since bending alters the internal structure, the one with more bends might have more resistance. Just a thought; this isn't my area.

 

[Astronuc]

Woolie, have you measured and compared the resistance of both cools. If one fails before the other, perhaps it has smaller cross-sectional area in the cools, and possibly higher residual stresses. The smaller cross-sectional area will likely have a shorter time to failure because the same crack/flaw size reaches a critical length (based on the ratio of crack/flaw size vs cross-sectional distance) in a shorter time.

Are the coils fabricated of the same material? Are they the same size?

 

[wolram]

Woolie, have you measured and compared the resistance of both cools. If one fails before the other, perhaps it has smaller cross-sectional area in the cools, and possibly higher residual stresses. The smaller cross-sectional area will likely have a shorter time to failure because the same crack/flaw size reaches a critical length (based on the ratio of crack/flaw size vs cross-sectional distance) in a shorter time.

Are the coils fabricated of the same material? Are they the same size?

The elements are both 5mm dia, we have three of them in delta 3ph 440v they both measure 35 ohms, and seem interchangeable as any time we order them we can get either one, so far no one knows why the four coil one blows across coils, can these things vibrate
in use and wear through the outer copper?

 

[wolram]

I can't picture what this looks like, but a quick guess is that, if you have more coils in a similar space, you're running into expansion shorting.

The coils are wound very close, is this like friction between the coils?

 

[TVP45]

The coils are wound very close, is this like friction between the coils?

If what I'm visualizing is correct, you now have a closer space between the coils and, once they get hot, they are apt to vibrate a bit and possibly hit an adjacent coil. Again, I'm guessing at the construction, but I've seen something similar in gas analyzer heaters.

 

[TVP45]

Oh, wait a minute. Forgot this since it was way in the back of my now largely useless head (I moved all the neurons years ago). Have you shifted the voltage across adjacent turns, ie.e., by going to 4 coils, do you now have the situation where you have, say, 60 V side by side whereas with 3 coils, you maybe had 38 V? Just a thought.

 

[wolram]

Oh, wait a minute. Forgot this since it was way in the back of my now largely useless head (I moved all the neurons years ago). Have you shifted the voltage across adjacent turns, ie.e., by going to 4 coils, do you now have the situation where you have, say, 60 V side by side whereas with 3 coils, you maybe had 38 V? Just a thought.

No, i think your first idea sounds right, current draw on both types of element is the same,
well within half an amp or so.

 

[wolram]

If you imagine winding a coil, one end is bent around then wound around the inner coil, there is a gap of about .25 inches between inner and outer, but the coil windings all most touch.

 

[dlgoff]

It sounds to me like the one with 4 inner & 4 outer is creating a different temperature gradiant across the elements causing maybe a hotspot?

 

[wolram]

It sounds to me like the one with 4 inner & 4 outer is creating a different temperature gradiant across the elements causing maybe a hotspot?

It is possible but the manufacturer, (a company that specialises in making elements) tells us we are the only company that has such problems, so why make two different types? it makes no sense to me.

 

[Danger]

I can see that we're going to have a bit of fun trying to figure this out. In the meantime, order only the 3 coil units.