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Join Date: Feb 2019
Posts: 19

Cartridge Heater Failure

08/12/2019 7:29 AM

Hello, I have an array of cartridge heaters that each have a built-in thermocouple. This matter pertains specifically to the first few heaters in the array. The temperature for all heaters is set to "x". All of the heaters besides these first few will reach temp. and fluctuate +/- a few degrees. The first few heaters will fluctuate +/- 30 degrees or more. I have replaced all of them, and this doesn't solve the issue. Any ideas?

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#1

Re: Cartridge Heater Failure

08/12/2019 9:21 AM

If you are heating a cooler gas stream, it could just be natural fluctuation in reaching temperature....what you need is amp draw history of these heaters...

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#2

Re: Cartridge Heater Failure

08/12/2019 1:53 PM

Presumably the heater are all the same capacity (wattage).

If the air stream to be heated passes over the heater cartridges in series (over one heater after another) then as the air progresses it is somewhat heated before it hits the latter heaters. Those latter heaters presumably to not have to supply as much heat as the 'early-on' heaters do to get the air to setpoint.

The deadband for ON-OFF control or the tuning constants for PID control would be need to be different for early vs later heaters.

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#3

Re: Cartridge Heater Failure

08/13/2019 12:46 AM

It does sound like the first heaters are trying to do all the work. Draw a diagram that shows the heating element, position of the thermocouple, position of the outside of the cartridge and the things in between ( metal case, ceramic filler, etc.). Then think about what is happening heat-conduction-wise between these parts. Consider the high heating demand of the early heaters with the cool incoming fluid, and how the control is unstable because of the positions of the heater, sensor and fluid, and the capacity and ability of the first heaters to get their energy to the fluid. Can you monitor the temperature of the fluid (gas whatever) at different points in the path from start to finish. One thing you could try is to lower the setpoint of the early heaters to share the load on each heater. Maybe even make it dependent on the incoming temperature if that varies a lot. Make successive heaters' set points successively higher, etc. Also, to the previous poster's good suggestion, change the tuning of the different heaters, including spreading out the setpoints to share the load. If proportional heater control is included, try limiting the output power of the first heaters, or their % on-time if an one-off controller. This will do the same thing as progressively increasing the setpoints. And save those first heaters from burning out, giving their lives trying to do all the work themselves!

Good luck,

Hugh

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#4

Re: Cartridge Heater Failure

08/13/2019 4:53 AM

I would suggest that instead of setting the desired temperature of the early heaters you could measure the incoming air temp and try to achieve an offset temp of a portion of the desired temperature. If you have a PLC controlling the array then the offset temp of the first heater becomes the set point for the second heater and do this a third time to achieve a rising temp gradient across the heaters.

So first heater setpoint = input + 1/3 delta input to output temp, then do the same for the next two banks. The remaining heaters can be set to the desired temperature out you wish to achieve.

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#5

Re: Cartridge Heater Failure

08/13/2019 5:33 AM

What is the actual problem? Is it temperature control for the process - or some heaters getting too hot - burning out ?

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#6

Re: Cartridge Heater Failure

08/13/2019 9:50 AM

What is the control system? If it's a PID you should be able to us the "auto-tune" function and control to a tighter tolerance.

Not knowing how your array is laid out, I would expect the heaters on the exterior to work harder since there is less mass to absorb and reflect heat, making it harder to control.

Are all of the heaters exactly the same, just in different locations of the array? You may have too much wattage for the application also. Too much wattage is harder to control with an in-built TC; due to the watt density on the wire, it will react faster and then the TC and PID can't keep up.

More information would help in forming a more accurate response.

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#7

Re: Cartridge Heater Failure

08/13/2019 11:44 AM

las59036, with any heater element the most important factor is heat dissipation. There must be a medium that are in contact with the cartridge to carry the heat away. If not, they will work like a glow plug and destroy themselves or the thermo couple with thermal runaway. Firstly there must be contact.

The second important point in heater processes is that the system must be calibrated. That means that if you measure a calibration jig at a specific temperature with any of your cartridges/thermo couples it will give you a result that is exactly what is expected.

Start with a calibration Jig and calibrate each controller for the cartridge that will be used with it. If one burn out and get replaced you need to re calibrate its controller for the new Cartridge. There is no short cuts.

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#8

Re: Cartridge Heater Failure

08/15/2019 7:32 AM

Yep.

Share some drawings of the installation with a local Controls Engineer. Without process and control drawings, the forum is ill-equipped to go further.

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