Rule of thumb for resistance of heater - when is it dead?
Rule of thumb for resistance of heater - when is it dead?
(OP)
In the plastic industry we use a lot of heater bands and cartridge heaters and other simple resistance heaters. It's easy to do the numbers to figure out what the resistance of a heater should be, given voltage and wattage, but is there a rule of thumb for how far away from this value the heater can go before it's 'on the way out'? When several heaters are running in parallel on the same controller it gets tricky to efficiently figure out if they're all working properly.
Mike
Mike





RE: Rule of thumb for resistance of heater - when is it dead?
There are little clip on current idiot-lights you can get that would show at a glance a blown heater.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Rule of thumb for resistance of heater - when is it dead?
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Sometimes I wake up Grumpy.
Other times I just let her sleep!
RE: Rule of thumb for resistance of heater - when is it dead?
RE: Rule of thumb for resistance of heater - when is it dead?
RE: Rule of thumb for resistance of heater - when is it dead?
For conductively-coupled elements, there is less likely to be a uniform loss of mass along the length of the wire/ribbon, and more likely that a failure occurs at a local imperfection which forms a "hot spot". Also, the mass loss of the element has more to do with chemical reactions (oxididation) and diffusion of the metal into its surrounding insulation, which are more complex phenomena. Still, the failure mode should have a runaway curve just like the evaporating filament. Logging the voltage/current on a regular basis may help give you some empirical data to help find a good "endpoint" at which you should replace the heater.
RE: Rule of thumb for resistance of heater - when is it dead?
But, you could still apply the same approach to monitoring resistance trending against known values, which could probably be used to determine deterioration.
RE: Rule of thumb for resistance of heater - when is it dead?
I thank all respondents for their kind attention.
Mike
RE: Rule of thumb for resistance of heater - when is it dead?
RE: Rule of thumb for resistance of heater - when is it dead?
Mike
RE: Rule of thumb for resistance of heater - when is it dead?
When is it dead? When Bones comes along declares: "He's dead, Jim" (Star Trek original series)
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"It's the questions that drive us"
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RE: Rule of thumb for resistance of heater - when is it dead?
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Rule of thumb for resistance of heater - when is it dead?
RE: Rule of thumb for resistance of heater - when is it dead?
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Sometimes I wake up Grumpy.
Other times I just let her sleep!
RE: Rule of thumb for resistance of heater - when is it dead?
TTFN
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RE: Rule of thumb for resistance of heater - when is it dead?
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Sometimes I wake up Grumpy.
Other times I just let her sleep!
RE: Rule of thumb for resistance of heater - when is it dead?
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Rule of thumb for resistance of heater - when is it dead?
Suggestions: Use a better heater. Some use a higher voltage (and wattage) rated heater for increased longevity. Or protect each heater separately.
RE: Rule of thumb for resistance of heater - when is it dead?
RE: Rule of thumb for resistance of heater - when is it dead?
I suppose the only other option is to pull the wire boxes apart and try to get amperage readings on each heater and see if one seems to be drawing fewer amps than it should.
Mike
RE: Rule of thumb for resistance of heater - when is it dead?
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Sometimes I wake up Grumpy.
Other times I just let her sleep!
RE: Rule of thumb for resistance of heater - when is it dead?
RE: Rule of thumb for resistance of heater - when is it dead?
I would consider trending the currents of groups of heaters.
If one heater in a group is failing, it should impact the current of the group.
The control method for the heaters will influence the trend data.
ie: On-off, time proportional, PWM.
I would expect to see a slight drop in current before a failure. Failure may produce internal arcing, which may cause a fault to ground.
The normal failure mode will be a slight decrease in current followed by no current at failure.
In some heaters the physical layout internally makes it possible for the heater to short the incoming leads resulting in a low resistance and higher current and blown fuses at failure.
I like itsmoked's suggestion. Can you give us a link, itsmoked?
respectfully
RE: Rule of thumb for resistance of heater - when is it dead?
Bill; These are what I am referring to. You stick one on each heater leg. If it ever glows then current is flowing thru the wire it is clipped around. They work great. You can get them with remote LEDs or built-in LEDs.
See CURRENT INDICATORS.
http://dkc3.digikey.com/PDF/T073/P2249.pdf
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Rule of thumb for resistance of heater - when is it dead?
Mike
RE: Rule of thumb for resistance of heater - when is it dead?
Mike
RE: Rule of thumb for resistance of heater - when is it dead?
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Rule of thumb for resistance of heater - when is it dead?
Mike
RE: Rule of thumb for resistance of heater - when is it dead?
RE: Rule of thumb for resistance of heater - when is it dead?
When I refer to BX I'm talking about the armour on the leads to the heater band. Normally you can get the leads either with unshielded fibreglass wires, or those wires can be shielded with a stainless steel braided cover, or they can have an stainless 'BX-ish' armour on it. Maybe it's not correct to call it BX but that's what I've always called it for the sake of expedience since everybody knows what BX looks like and functions like, and this is just a version of that. I greatly prefer the BX armour rather than the stainless braid. The braid can kink at the drop of a hat... the armour is much, much more rugged which is quite important when you have the heaters mounted on a removable die, on an exposed flow tube, or on a screen changer plate where there is motion. For the sake of a couple of extra bucks per heater I always get the BX armour.
Mike
RE: Rule of thumb for resistance of heater - when is it dead?
RE: Rule of thumb for resistance of heater - when is it dead?
Mike
RE: Rule of thumb for resistance of heater - when is it dead?
RE: Rule of thumb for resistance of heater - when is it dead?
Mike
RE: Rule of thumb for resistance of heater - when is it dead?
RE: Rule of thumb for resistance of heater - when is it dead?
David is too polite to say 'Please improve the tone of your posts to a professional level or stop posting'.
Thanks.
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Sometimes I wake up Grumpy.
Other times I just let her sleep!
RE: Rule of thumb for resistance of heater - when is it dead?
RE: Rule of thumb for resistance of heater - when is it dead?
RE: Rule of thumb for resistance of heater - when is it dead?
My use of the megger on these heaters would be to test insulation to ground, and specifically in the case of mica heater bands, looking for cracks in the mica. It may well be that the megger would be useless for this purpose... I've never heard of anyone doing this before. However, I don't mind taking the road less travelled (or even the road untravelled) if it might get me to a desirable destination.
I'll give it a try and see what happens. I'll compare the readings of new heaters to the readings of old 'condemned' heaters that are still working, but not working well, and see how it goes. If I fry a couple of heaters along the way, it's no great loss... they're as cheap as chips, relatively speaking, and the potential gain is considerable.
Mike
RE: Rule of thumb for resistance of heater - when is it dead?
RE: Rule of thumb for resistance of heater - when is it dead?
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Sometimes I wake up Grumpy.
Other times I just let her sleep!
RE: Rule of thumb for resistance of heater - when is it dead?
Mike
RE: Rule of thumb for resistance of heater - when is it dead?
I come from Heater manufacturing company.
At first instance when a customer tells us that there element is failing the first thing we ask is, are you running the heater at rated voltage or otherway around is the element manufactured for your rated voltage. If the element is designed for 240V, the normal tolerence is 5% so you can barely use it 250V & if there any small positive fluctuation in your supply the elements will fail.
If this is what the problem is than if you want to make sure the elements dont fail in future get your manufacturer to manufacture element for 260V (+ tolerence)and use it at 250V instead this will give you reduced output but is a safe design practice.
If voltage rating is not the reason, than it must one of your control faults like earth faults.
Last but not least.... go for a low Watt Density Element, choose element length so that the minimum ohms/meter stay low, this effects actual element (Ni/Chr resistance) thickness. Higher ohmic value means the element gets thinner which means slight fluctuation fails the element.
I hope to have answered, otherwise please let me know.
Regards
Fasi Rahman
RE: Rule of thumb for resistance of heater - when is it dead?
The low watt density is something that I hadn't thought of before, but it makes perfect sense. I will be sure to specify that whenever possible.
Mike
RE: Rule of thumb for resistance of heater - when is it dead?
Fasi007 makes another good point about using the lowest watt density heater elements that will do the job. Just remember that lower heater watt density has the downside of longer heat-up time so this is a bit of a balancing act between longevity and performance.
I played around a bit in a worksheet, and made up a couple of tables showing why resistance measurements fall apart in highly paralleled zones, as well as a table with actual megohmmeter measurements (although for cast-in extruder barrel heaters, and not band heaters).
They are at http:
RE: Rule of thumb for resistance of heater - when is it dead?
The cast barrel heaters are very much on my list of things to check by megger, as they are pricey buggers are more importantly they are not something you can change on a whim. At least, not on the 4.5" and 6" screws that we use (and half of them are water-cooled, adding to the opportunities for ground problems, and making it much more irritating to swap them out).
I'm going to print out your worksheet at my first opportunity. Thanks again!
Mike
RE: Rule of thumb for resistance of heater - when is it dead?
You may want to reload the link http:
Cleaned it up a bit, and added results from the terminal resistance assay. Interestingly enough, two of the heater halves that showed low megohmeter readings gave unusually high and low terminal resistances as well.
Bob