Tubular cartridge heater wattage

[AligatorAmy]

Hi,
I have a machine equipped with 2000mm long tubular cartridge heater (230V, 1600W, 4mm diameter). This tubular heater is sandwiched between the two steel plates (2000mm long) i.e. the heater keeps the steel components hot. My problem is that the temperature of the steel components varies along the heater +/-15degC (nominal T=190degC).

In order to maintain more uniform temperature along the steel components, I intend to replace the exisitng heater, 2000mm long, with 6 heaters, 325mm long each (there wil be approx. 8mm gap between each nearby 2 heaters). Each 325mm long heater will be powered and controlled separately. But all 6 heaters will be controlled in the way to maintain the same temperature in the steel based on the feedback from 6 thermocouples. The total power (wattage) of all 6x325mm long heaters will be the same as in case of 2000mm heater, i.e. 1600W.
Please also refer to the attached draft.

My question is:
Question 1
Does it mean that I would need to basically divide the power of 2000mm long heater, i.e. 1600W, by 6? What gives me approx. 270W per heater.
(If yes, I would then add perhaps 20% of power to each heater for some headroom, i.e. 270W+20%= 324W).
I have some doubt in regards to this solution because 6 heaters will be spread on the length of 2000mm and I am not sure whether simple dividing 1600W by 6 is adquate here.
Question 2
Do I maintain the in-line control on a single 325mm long heater temperature basically through varying the input voltage delivered to it?
If yes, my requirement to the heater supplier would be the condition that 325mm long heater must be capable to be operational at varying input voltage, say 180V-280V (with 230V nominal).

Please help.

Regards

 

[danw2]

With regard to temperature control, resistance heater control is generally done not by varying the voltage, but rather with PID control that uses "time proportioning", where the PID algorithm calculates the ON time in proportion to the Off time for a specified time interval.

For example, if the time interval were 5 seconds, and PID calculated it needed 38% output to maintain the temperature, the line voltage would be switched on for 1.9 seconds, and held off for the remaining 3.1 seconds of the 5 second interval.

Solid state relays are used to switch the line power to the elements because electromechanical relays wear out when cycled at a high frequency. The solid state relays are sized by the line voltage and the current needed to get the wattage required. Heat sinking a solid state relay is what gives them their long lives. Lack of heat sinking lets them die an early death.

With PID control, having an oversized heater element doesn't really matter, the controller provides heat as needed for temp maintenance. But no controller can make up for too little heat capacity from the element.

There are multizone controllers or multiple single loop controllers, which nowadays are almost a dime a dozen. Get something with a solid state output on the controller that drives the control input of the solid state relay for the longest life.

 

[EdStainless]

You need the same power over the same area, so watts/inch (actually square inch) is what you need.
And often cartridge heaters are controlled based on their internal temp.
If that does not give the desired results then offsets are applied.
One reason for this is that TC placement can have a major impact on the results, and it is often hard to get correct.

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P.E. Metallurgy, Plymouth Tube

 

[IRstuff]

As a first approximation, that's the general idea. You'll likely find that the two end heaters will run slightly higher wattage to account for the fact that they are exposed to the ends of the enclosure, while the middle 4 are not. Based on your description, the end heaters would need to be 30 degrees hotter than the middle two, and the two in between probably need to be on the order of 15 degrees hotter than the middle. These are only guesses, as getting the outside heaters hotter will likely bring up the temperature of the middle as well.

TTFN (ta ta for now)
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[AligatorAmy]

Thank you for the responses to everyone.

@danw2
Thank you for the outlined temperature control method. I am very graful.

@EdStainless
Indeed proper control over the system depends on correct TCK placement. Unfortunately, the steel plates are not the only parts of the assembly. There are also other components there as well as molten polymer and hot compressed air flowing through it, etc. Therefore, I need to place the TCK elsewhere than in the heater.

@IRstuff
Indeed, the temperature at the far ends of the plates is several degrees higher than in the middle. I think that to a specific extend the 6 heaters will do the job. I am also going to implement better thermal insulation of these plates (improved enclosure) as this can be done easily.

 

[MintJulep]

My problem is that the temperature of the steel components varies along the heater +/-15degC

Do you know why?

How have you managed to design a solution to your problem when you don't yet understand why the problem exists.

Six independent heaters and six independent controls to try and control the temperature of one thing is not likely to work well.

 

[EdStainless]

Amy, The problem with using anything other than the heater internal temp for control is that you have to have TC locations that minimize cross talk between heaters. Otherwise the controllers will fight with each other.
We used a 12 heater system and then set internal temps on the heaters, then when we surveyed the equipment we developed offsets to get a more uniform working environment. It controlled very well.
If currently the ends are hotter then you must have much more heat loss in the middle.
Do you have a good map of the actual temps?
You are usually better off having long heaters for uniformity, and then shorter ones in areas that need extra (or less) heat.
You also have to make sure that they are well seated in the holes, using plenty of MgO to assure uniform thermal conduction. (yes, Milk of Magnesia works fine if you don't mind the smell the first time you heat it)

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P.E. Metallurgy, Plymouth Tube

 

[IRstuff]

Oddly enough, I worked in a semiconductor fab, with diffusion furnaces around that length. A typical manual:

http://mfc.engr.arizona.edu/documents/LPCVD Manuals/M320_01 Furnace.pdf

states that the "flat" zone is only about a meter, at best, but that's with a 3-zone heater.

TTFN (ta ta for now)
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[EdStainless]

IR, I have built many tube furnaces.
Ours only had heated zones about 8' long.
Not only were they 5 zone but the spacing of the glowbars was closer in the end zones.
We could get 4' that was +/-10F at 2050F

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P.E. Metallurgy, Plymouth Tube