Rule of thumb for resistance of heater - when is it dead? 1

[MetalworkerMike]

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

 

[ScottyUK]

Excellent idea Operahouse.


----------------------------------

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

 

[MetalworkerMike]

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

 

[Fasi007]

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

 

[MetalworkerMike]

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

 

[rawelk]

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

 

[MetalworkerMike]

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

 

[rawelk]

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