temperature drop

Hi folks,
I have a steriliser that have three temp.control cycles ie sterilisation,intermidiate and production.The set points are 130C,120C and 115C respectively.
The problem is when the sterilisation cycle is complete,and the system shift to the intermidiate step,the temp drops to about 100C against the set point of 120C and the steam control valve remain at 100%open position,the end loop temp drops to 94C.When the production cycle is selected,the the temp. controls well against the 115C setpoint and all the temps. reads normal.

What could be wrong in the intermidiate step?I shall apreciate any feedback.

I would suspect it has to do more with the cooling process than it does the control loop. The dynamics for heating and cooling are different enough that when cooling integral gain can accumulate and make your control value stay low for longer than usual. This happens because you do not have active cooling so your system takes a long time to cool. During cooling integral error is accumulating and pushing you control value lower and lower. One of 2 things must happen for your control value to start moving you control back up, you must be below your set point at a large enough magnitude and for a long enough time to overcome the integral error or the combination of your p and d gain and error must overwhelm the integral gain and error. What happens when you go from the 120C to 115C? Does something similar happen? I might suggest turning down the integral gain, particularly when entering the intermediate step or using a ramp/soak function so that the system moves more slowly and integral gain does not accumulate.

-
Al

http://theplchub.com

Please confirm whether I understand this.

The sterilization cycle runs OK at 130C.

And the production cycle runs OK at 115C.

But the intermediate stage cannot heat up to 120C, even though the steam valve is wide open at 100%?

danw said:

Please confirm whether I understand this.

The sterilization cycle runs OK at 130C.

And the production cycle runs OK at 115C.

But the intermediate stage cannot heat up to 120C, even though the steam valve is wide open at 100%?

Click to expand...

A wiring and plumbing diagram would sure help to diagnose this situation.

Does the unit have cooling capacity? Is cooling capacity higher than heating capacity ie can I remove more BTU by cooling than I can put in with steam?

Are you really getting 100% steam flow in intermediate cycle - are you sure about that? Just because the lite says the valve is open does not mean the valve is really open.

If this unit ran OK and the programming has not been changed and now you are having problems the last place to look is in programming - you have a sensor or an actuator problem.

Have there been any wiring or component changes. Reviewing the last thing you did may find the problem. Just because something is brand new does not mean it will work OR is properly calibrated.

Dan Bentler

danw said:

Please confirm whether I understand this.

The sterilization cycle runs OK at 130C.

And the production cycle runs OK at 115C.

But the intermediate stage cannot heat up to 120C, even though the steam valve is wide open at 100%?

Click to expand...

Exacty.

kamau said:

Hi folks,
I have a steriliser that have three temp.control cycles ie sterilisation,intermidiate and production.The set points are 130C,120C and 115C respectively.
The problem is when the sterilisation cycle is complete,and the system shift to the intermidiate step,the temp drops to about 100C against the set point of 120C and the steam control valve remain at 100%open position,the end loop temp drops to 94C.When the production cycle is selected,the the temp. controls well against the 115C setpoint and all the temps. reads normal.

What could be wrong in the intermidiate step?I shall apreciate any feedback.

Click to expand...

Sounds as if you have a steam capacity problem, or cooling is happening way too fast after sterilization.

The problem is when the sterilisation cycle is complete,and the system shift to the intermidiate step,the temp drops to about 100C against the set point of 120C and the steam control valve remain at 100%open position,...

Click to expand...

You do realize that when your steriliser goes from Step 1 at 130 degrees to Step 2 at 120 degrees, it is in cooling mode, and not heating, so keeping the steam valve at 100% open will simply slow down the cooling. If the temperature drops to 100 degrees instead of the desired 120, then you are simply leaving it in Intermediate mode too long, or cooling too much.

The point is that in going from 130 to 120, you should not need the steam valve to be open at all. If it is open, then something is wrong with your control logic.