autoclave heat control

[g.robert]

Hello everyone. I've been away for quite awhile... My questions is more logic based than PLC programming per say.

I'm using a Do-more processor with C-more HMI. I have PID to time proportional control to dc Watlow Din-a-mite SCR contactors. I've used this set up many times for platen heat control on rubber presses. I bypass the PID with heat at 100% until I reach 90% of set point then PID takes over cycling heat to reach / hold the set point.

We are controlling 4 x 12KW heat boxes located on a 40' x 5' autoclave. A blower circulates the internal air 100% of the time heat is on.

Problem is we have burnt up multiple heat elements. All wiring and voltages have been triple checked and verified. The TC have been certified to our display in the HMI. Some are saying we need to cycle the heat elements so they can't sit at 100% for "too long". I have never (10years +) had to program this into any heat control. We have no criteria on what is "too long". We have no way to check the heat (currently) directly at the element. Heat is currently checked via 2 type J TC. The theory was the heat would be relatively close throughout the chamber via the constantly circulated air.

FYI it takes about 2 hours for the heat to go from room to 300F. From there the heat would reach / hold set point via PID.

How could the elements get "too hot" with a set point of 330F and a chamber temp reading of <300F? Specifically has anyone done any large autoclave specific programming / heat control? Everyone is pointing fingers now... I greatly appreciate any educated suggestions.

 

[Steve Bailey]

Thermal shock? Maybe you need to bring a cold element up to temperature gradually. Similar to putting a soft start on a motor instead of an across the line starter.

Do the elements always fail at the same location? If so, perhaps there is something in the way the elements are mounted.

Have you done any thermodynamic analysis of the system? Is 2 hours a reasonable time to go from room temperature to 300 degrees?

 

[jrwb4gbm]

Make sure that the supply voltage is at the very bottom of the specifications for the elements, even if it means adding a voltage dropping transformer. I would suggest you go 10% below rated voltage of the elements, this should improve the element life. Don't do this if you can't achieve the necessary setpoint temperature.

 

[g.robert]

Thermal shock? Maybe you need to bring a cold element up to temperature gradually. Similar to putting a soft start on a motor instead of an across the line starter.

Do the elements always fail at the same location? If so, perhaps there is something in the way the elements are mounted.

Have you done any thermodynamic analysis of the system? Is 2 hours a reasonable time to go from room temperature to 300 degrees?

Typically the soft-start is to limit starting current on the electrical supply not the motor. None of it makes sense. Your oven, water heater etc all start at room temp, reach temp then shut off. There's something bigger going on here. I too was thinking the way the elements are mounted...

These are industrial heaters rated 480VAC 4KW connected in delta with 3 phase in. Your actual voltage will vary based on the utility supply and community load. We are talking multiple failures in a 8 hour test period...

What I really hope is someone that has or does autoclave heat control might chime in with 1st hand experience...

 

[Steve Bailey]

Sometimes you install a soft start to save wear and tear on the drivetrain.

My only experience with an autoclave is on a steam-heated unit. I was called in because it couldn't reach setpoint temperature. The PLC was controlling a 4 - 20 mA signal to the steam valve. The customer was convinced that the analog output was bad. Turned out that when the steam valve got to about 50% open it started venting more steam than it delivered to the autoclave.

Knowing nothing about the heating elements I can't offer any suggestion about what to look for in the mounting other than the obvious that the have to be mounted in such a way as to accommodate the thermal expansion. Too rigid a mounting will lead to premature failure.

 

[jraef]

What kind of heater element is it? Some have a high Positive Temperature Coefficient (PTC) of resistance, meaning the colder the element is, the LOWER the resistance is so when you first turn them on, there is a high current surge, then after hey heat up the resistance increases. For that type of heater element, you must use a Phase Angle SCR controller and soft start it to avoid damaging the element.

 

[tarik1978]

It could be overheating since you dont have a Temp sensor for each element you dont have any idea what temperature is.
In my opinion I will put a Temp sensor for each heating element and use PWM to control the heating.

 

[Ronnie Sullivan]

Is the term 'Autoclave' different in the US?
I have done a few and the first action when the doors are sealed is to evacuate all the air via a vacuum pump.
This is monitored by a transducer and when the air is all gone - then heat is introduced by live steam.

If there is air in yours then it's really just a circulating oven ?

So....... are you sure there is adequate airflow across the elements?

 

[Patrix]

We use in our company similar ovens.
These are just on/off controle and is also air circulated.
We use a higher rage of temperature, 320F to 392F (160C to 200C)
We change the elements once in a few years, about 3 years.

Are the elements suited for open air heating?

We also use rubber presses with heating plates, these presses have different kind of heating elements, if we use those elements in open air they would "burn up" within a few hours, so for open air usage, we use different type of heater elements.

 

[g.robert]

Sometimes you install a soft start to save wear and tear on the drivetrain.

My only experience with an autoclave is on a steam-heated unit. I was called in because it couldn't reach setpoint temperature. The PLC was controlling a 4 - 20 mA signal to the steam valve. The customer was convinced that the analog output was bad. Turned out that when the steam valve got to about 50% open it started venting more steam than it delivered to the autoclave.

Knowing nothing about the heating elements I can't offer any suggestion about what to look for in the mounting other than the obvious that the have to be mounted in such a way as to accommodate the thermal expansion. Too rigid a mounting will lead to premature failure.

We use in our company similar ovens.
These are just on/off controle and is also air circulated.
We use a higher rage of temperature, 320F to 392F (160C to 200C)
We change the elements once in a few years, about 3 years.

Are the elements suited for open air heating?

We also use rubber presses with heating plates, these presses have different kind of heating elements, if we use those elements in open air they would "burn up" within a few hours, so for open air usage, we use different type of heater elements.

Is the term 'Autoclave' different in the US?
I have done a few and the first action when the doors are sealed is to evacuate all the air via a vacuum pump.
This is monitored by a transducer and when the air is all gone - then heat is introduced by live steam.

If there is air in yours then it's really just a circulating oven ?

So....... are you sure there is adequate airflow across the elements?

This autoclave is pressurized to 70PSI. The heat is electric not steam. Its really just a large industrial convection oven...

Patrix seems to have answered my question in that nothing special is typically done...heat on / off to set point.

The builder swears up and down these elements are correct and have several other "old school" (relay logic) autoclave with them that have worked like a champ for years. We come in and add PLC, HMI & automation and burn up 1/2 the elements on the 1st go around...not exactly the results we had hoped for...I'm thinking the next logical step is to talk directly with the heat element supplier regarding the suitability of elements selected for this application.

 

[James Mcquade]

I installed a 66 kw heater unit several years ago and purchased everything from the supplier. We installed the system and within 3 weeks, we burnt up 4 scr controllers. we used a megger to test everything but the kitchen sink and found no issue.

I made the decision to switch over to Mercury contactors and the system has not stopped since. it has to do with the utility power and I never could understand what they were saying.

Since there are several old school relay units there, may I suggest that you replace the scr units with mercury contactors as your first step.
regards,
james

 

[g.robert]

I installed a 66 kw heater unit several years ago and purchased everything from the supplier. We installed the system and within 3 weeks, we burnt up 4 scr controllers. we used a megger to test everything but the kitchen sink and found no issue.

I made the decision to switch over to Mercury contactors and the system has not stopped since. it has to do with the utility power and I never could understand what they were saying.

Since there are several old school relay units there, may I suggest that you replace the scr units with mercury contactors as your first step.
regards,
james

Problem is we are loosing heating elements not SCR. 480 volt 12KW heat elements burning in two. We are thinking the guards over the elements may be restricting the circulating air enough to allow for excessive heat buildup at the element or something compromised the integrity of those elements (moisture, damage etc..)

 

[Steve Bailey]

How well balanced is the fan? Could the heating elements be vibrating? Can you change the fan speed to see if that's a possible cause?

 

[James Mcquade]

Agreed, you are loosing heater elements.
The point I was trying to get across is what has changed.
Scr controls and plc controls.

Maybe looking at it that way can help you find something you missed.

james

 

[Ronnie Sullivan]

In my opinion, when heating elements fail it's down to only a few things.
1, they perish over time through expansion and contraction.
2, the wrong voltage
3, the heat is not allowed to escape adequately.
4, electrolytic action of dissimilar metals at the terminals
5, hot spots on the elements.