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Rule of thumb for resistance of heater - when is it dead?

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

RE: Rule of thumb for resistance of heater - when is it dead?

Yep you're right hard to tell.  Usually it comes down to they open circuit.  I don't think any other parameter will work very well.  If you can see the current then detecting a bad heater in parallel is not hard.

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?

Most resistance heaters tend to be either OK or open circuit as Keith says, unless you have a heater which is either so hot that it is oxidising or one that is consumed by the process. Both the latter will experience a resistance increase due to metal loss. Unless it is factored in to the original design, metal loss usually leads to fairly rapid failure.
 

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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?

Well, if you use an Ohms Wheel you can determine it easy enough. If you know the rated wattage of the heater and you know the applied voltage and you measure the current flow, you will know if the heater has the correct resistance value or not. In other words, say you have a heater you know to be rated 100W at 250V. The resistance then should be E2/P or (120 x 120)/100 or 144Ohms. Then I = E/R so I = 120/144 = .83A. If the measured current is lower, the resistance must be higher, indicating a material loss as ScottyUK said.

RE: Rule of thumb for resistance of heater - when is it dead?

If it's a heater that reaches high temperatures, then the cold resistance will probably be quite different than the operating resistance. Makes the simple Ohms Law to power approach less than useful.

RE: Rule of thumb for resistance of heater - when is it dead?

We used a rule of thumb that 10% resistance increase over the as-built condition was a near end-of-life condition; this was for tungsten filaments operating at a constant voltage in vacuum (radiatively coupled).  The math for this is surprisingly easy, if you assume a uniform evaporation of the filament along its length.  As the wire loses mass, the diameter drops and the surface area of the filament decreases, which causes the temperature of the filament to climb, which increases the rate of mass loss, and ... well you can see how this leads to a "runaway" failure condition.

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?

Oh yeah VE1BLL, didn't think that one through.

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?

(OP)
  I respect the notion that these heaters are either good or bad.  However, I have had situations where a pair of heaters on one controller kept blowing the breaker, then when I replaced the higher-resistance heater everything went back to normal.  I don't have a solid explanation for why this should be the case, but I do know that it is what happened.  Since I have had odd situations occur with these heaters before, I was hoping for a 'magic bullet' solution to deciding if they were okay or not.  If no such solution exists then I will continue on as I have been.
  I thank all respondents for their kind attention.

Mike

RE: Rule of thumb for resistance of heater - when is it dead?

I suspect with blowing the breaker, the problem was a short to ground.  I believe firmly that a change in resistance will indicate nothing.  Most failures occur from necking and generate a hot spot in only one place.  Failure is very quick.  An overall increase in resistance is self protecting.

RE: Rule of thumb for resistance of heater - when is it dead?

(OP)
  Interesting point... the short to ground.  I didn't check that, and the heater in question is cast aluminum so it could easily short to ground.  Thanks for the tip!

Mike

RE: Rule of thumb for resistance of heater - when is it dead?

I agree with Operhouse, I have had a unit fail in a likewise manner that kept blowing fuses and verified it with a meter- element was shorted to ground.

When is it dead? When Bones comes along declares: "He's dead, Jim" (Star Trek original series)

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
"It's the questions that drive us"
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

RE: Rule of thumb for resistance of heater - when is it dead?

When it stops moving?

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Rule of thumb for resistance of heater - when is it dead?

(OP)
  I will start being diligent in testing the element to ground (or at least testing the element to its casing, since almost none of these units are grounded).  In fact, megger testing might be the magic bullet that I've been looking for...  Hmmm...  checking the insulation resistance against 500V might give me the answer that I need... or it might blow the heater, but at least that would sort out the sheep from the goats, so to speak.  The heaters run on 240V normally, so 500V would be a reasonable voltage for a megger test, I think.  It wouldn't blow anything that wasn't ripe for the blowing, so to speak.  I _could_ test them at 250V if anyone thinks that 500V would be too destructive, but IEEE standards use 500V to test 110V motors, so I would expect 500V would be okay for 240V resistance heaters.  Any thoughts on that?

RE: Rule of thumb for resistance of heater - when is it dead?

500V is the standard test for line-neutral on a UK electrical installation so it sounds reasonable to me. Also, don't forget that 240V AC has a peak of 340V, so a 250V DC Megger test wouldn't really prove much.
 

----------------------------------
  
Sometimes I wake up Grumpy.
Other times I just let her sleep!

RE: Rule of thumb for resistance of heater - when is it dead?

Don't forget that most resistance heaters are made from copper and have a tendency to work harden from flexing, which usually shows up as an increased resistance.  Even relatively gentle bending, if repeated enough can cause resistance increases or outright breakage.

TTFN

FAQ731-376: Eng-Tips.com Forum Policies

RE: Rule of thumb for resistance of heater - when is it dead?

Copper? Most heater elements are Nichrome or something similar over here. Do you mean copper sheaths on the elements, or are you thinking of something else like an induction heater?
 

----------------------------------
  
Sometimes I wake up Grumpy.
Other times I just let her sleep!

RE: Rule of thumb for resistance of heater - when is it dead?

I think the meggar test may not tell you what you need to know. When the breaker opens, you know you've had a short circuit at that particular point in the heating cycle. A megger test on the same heater after it has cooled may falsely indicate there's no problem.

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?

Oh, yeah.  A good measure of the life capability of a conductively-coupled heater is the "watt density" of the filament, i.e. the number of watts divided by the surface area of the wire element.  This is a backwards but effective way of knowing the wire surface temperature in operation, obviously hotter wire will degrade more rapidly, so a lower watt density is better.  Of course, you can't get this data from a McMaster catalog; you have to either get it from the manufacturer or cut apart some heaters and measure it for yourself (all you really need to know is the diameter of the conductors and alloy used, if you know the resistance and wattage in operation, you can calculate the length of wire required, its operating temperature and resistance).

RE: Rule of thumb for resistance of heater - when is it dead?

(OP)
  I'm hoping the megger testing will tell me if a heater is _going_ to fail, not that it already has.  I'm looking to add a predictive element to our preventative maintenance.  Right now it amounts to replacing heaters as they fail, but that could be hours or even days after the failure.  I don't always have spares, since the heater bands are of different diameters, different widths, and have different hole patterns.  Keeping spares for _all_ of them would be quite an undertaking.
  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?

My theory on failure mode of heaters which use a coiled nichrome element within a metallic sheath: the wire thins to breaking point due to metal loss from localised overheating and as it breaks the slight tension in the coiled element is released causing the live broken element to make contact with the earthed metal sheath, thus causing the fuse to blow or breaker to trip. No way a Megger is going to see that before the event. You need a crystal ball. smile
 

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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?

(OP)
  Most of the heaters in question here are mica heater bands.  They're semi-flexible, but the vast majority of them stay bolted in place permanently, so there is virtually no flexure in use.  It's fair to say that these bands will see no flexure at all once they've been heated once.  There are some exceptions but they are relatively rare.  Mica band heaters are formed by a resistance ribbon sandwiched between two sheets of mica for insulation.  I think maybe the megger will be able to detect cracks in the mica.  The ribbon shouldn't be able to ground out unless the mica has been compromised, so I think this method has promise.  Of course, I could be wrong...

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?

Looking to predict a heater failure is like the holy grail.  In the semiconductor manufacturing realm they have always wanted this.  A heater blowing out during a process can cost them a million dollars.  They are always looking for this.  It's come down to they want to know within a couple of power line cycles so they can alter the process or process flow to minimize product loss.  On three occasions I have had to deliver some solution specifically for this.  Often they use SSRelays and want to know the instant the heater or the SSR fails.

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?

(OP)
Itsmoked: I assume you would have to remove the shielding from the wire to use this current indicator device?

Mike

RE: Rule of thumb for resistance of heater - when is it dead?

(OP)
Waross: You're right of course that watching the draw of the whole batch of heaters should give you a base-line that you can trend.  The problem is that some of them have only a couple of amps of draw yet they all have been standardized with 50A gauges.  Seeing a variation is more than I can expect of the operation staff.  Still, I will start recording values after I check all of them individually with an amp meter (to make sure they are currently working before recording the 'good value').

Mike

RE: Rule of thumb for resistance of heater - when is it dead?

Nay.  You need one wire of a heater's power wiring run thru the center of the device.  It can be anywhere in that circuit.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Rule of thumb for resistance of heater - when is it dead?

(OP)
  If it needs to be one wire then I _will_ have to remove the shielding, since the pair of wires are run together in the BX shielding.  It would be worth it, though.

Mike

RE: Rule of thumb for resistance of heater - when is it dead?

BX has been illegal (against code anyway) for years and years.  Perhaps you have AC cable?  Certainly there is a junction box somewhere that the conductors are available.  If not, you will need a junction box, you can't just break the sheath in an open run.

RE: Rule of thumb for resistance of heater - when is it dead?

(OP)
Dave,
  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?

But it isn't BX and you wouldn't actually want BX even if it were available.  There's a very good reason it isn't available any more.

RE: Rule of thumb for resistance of heater - when is it dead?

(OP)
Dave, it may not be technically BX, but BX is a generic term for that kind of armoured cable, and has been for almost a century.  It's not BX, it's really AC cable.  If you're going to complain that my kleenex wasn't actually made by the Kleenex people but rather is a Scottie facial tissue, then that's your option, but you might want to look around to see if it's bothering anyone else.

Mike
 

RE: Rule of thumb for resistance of heater - when is it dead?

I've no idea where this "Dave" business came from, but that's not me.  BX and AC are (were) constructed differently.  Call it what you want, but communication is more complete if it is more accurate.  It isn't a matter of a Kleenex vs a Scottie, it would be a facial tissue vs a paper towel and you'd really  confuse people if you mean a paper towel but talk about a kleenex.

RE: Rule of thumb for resistance of heater - when is it dead?

(OP)
  So someone with a screen name of 'davidbeach' who doesn't sign his posts otherwise, is at a loss as to where I got the idea that his name might be 'David Beach', and at the same time he is chiding me for my imprecise communication.  Okay, I can handle the irony.  I'll try not to refer to AC cable as BX cable in your presence again.  I'll still call it that when I'm ordering cable, because if I don't then I'll get blank stares from people wondering what the difference is between cable for AC and DC.  They shouldn't re-use active acronyms, in my opinion, but that is, of course, only my opinion.

Mike

RE: Rule of thumb for resistance of heater - when is it dead?

My point was that I have never used "Dave" and never will, so I have no idea why anyone would think that "Dave" should be used in reference to me.  There is no way of parsing my handle of davidbeach and coming up with "Dave".

RE: Rule of thumb for resistance of heater - when is it dead?

Mike,

David is too polite to say 'Please improve the tone of your posts to a professional level or stop posting'.

Thanks.
 

----------------------------------
  
Sometimes I wake up Grumpy.
Other times I just let her sleep!

RE: Rule of thumb for resistance of heater - when is it dead?

Thanks, ScottyUK.

RE: Rule of thumb for resistance of heater - when is it dead?

You would not generally put a device back in service that had failed a meggar test. The test itself stresses the insulation, and I would expect a heater that had failed the test to fail to heat upon return to service. If the failure mode of the heater is an arcing ground fault insufficient to cause over current operation prior to actually open circuiting, this current has a distinct signature. Relays intended to detect high impedance faults come to mind.

RE: Rule of thumb for resistance of heater - when is it dead?

(OP)
  My understanding is that a megger test 'should' be non-destructive.  At least, that's my understanding as it applies to motors using IEEE Standard 43-2000 (R2006), “The Recommended Practice for Testing Insulation Resistance of Rotating Machinery.”  Now, if it fails the test, does the megger 'pave the way' for easier arcing in the future?  Could be, I suppose.  I use a megger on the motors regularly, and I suppose a non-infinity result would be considered a 'failure' in some respects... yet I wouldn't expect it to make them worse.  I don't necessarily have facts to back that up, you understand, it's just a belief based on what I have seen and heard.

 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?

Might be a better test if the heater was up to temperature when the megger test was done.  Clearance would be quite different when cold.  Or use a higher voltage, doesn't take much space for 500V.

RE: Rule of thumb for resistance of heater - when is it dead?

Excellent idea Operahouse.
 

----------------------------------
  
Sometimes I wake up Grumpy.
Other times I just let her sleep!

RE: Rule of thumb for resistance of heater - when is it dead?

(OP)
Operahouse: The heaters run at 240V nominal, so I would normally use 500V to test them.  You feel that 1000V would be more appropriate if they're cold?  Testing while they're hot is no problem, in general, though I'm sure I will find situations where it is inconvenient.

Mike

RE: Rule of thumb for resistance of heater - when is it dead?

Hi,

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?

(OP)
Fasi:  Thank you for the knowledgeable response.  We haven't had trouble with over-voltage so far.  The highest voltage I have seen on a 240V-nominal line at our plant is 247V.  I will keep that in mind, though, in case I find some on the shelf that are rated lower.
  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?

Waross makes a good point - in highly paralleled zones it is easier to determine element failure using historical current demand measurements than by resistance records. I record both current and resistance data since there are times (for instance, during a machine rebuild when power cannot be safely applied) where historical resistance measurements are the only game in town.

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://mysite.verizon.net/zootsuite/techteach/Control/extrusion/parallel_heater_commentary.html

RE: Rule of thumb for resistance of heater - when is it dead?

(OP)
Rawelk:   Fabulous info!  The low-density shouldn't be a problem (within reason) as we process HDPE, so we use a very long warm-up as it is (8 hours).  No fooling around with a polyolefin.  The plant runs 24/5, so warm-ups don't happen every day.
  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?

MetalworkerMike: Thanks for the kudos. I feel your pain;) ... extruder barrel heaters can be a chore.

You may want to reload the link http://mysite.verizon.net/zootsuite/techteach/Control/extrusion/parallel_heater_commentary.html

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


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