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Old January 13th, 2013, 04:23 AM   #1
daniyal
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Cooling Tower Temperature and Vacuum Control

Dear All,

Its being very tough for me to control the cooling water temperature and vacuum by PID loop. attached is the control routine which i am using to control the temperature an Vacuum.
The system is consist of 8 cells of Cooling tower Fans in Power Plant that is operation by 8 PF755 VFD's by increasing and decreasing the speed of all drives at same time with PID controller to control the Elements.
Normally Vacuum Controller activated when the temperature is greater the Setpoint, and when the temperature is below down to the setpoint the control switches over to the temperature controller and decreasing the speeds to achieve the setpoint. But the fan speed decreases untill the temperature does not reached at the setpoint and it makes the oscillation for the drives. I have tuned the PID loop on very slow response to stable the speed at which temperature again rise but temperature goes down very rapidly.
Kindly tell me is there any better and accurate way for controlling or it need to be tune it again for nullify the oscillation and smooth the controlling.

Looking forward for your kind response
Thanks
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Old January 17th, 2013, 07:17 AM   #2
daniyal
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Nobody have the Answer
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Old January 17th, 2013, 07:37 AM   #3
shooter
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please give a nice diagram and what type of control you are using.
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Old January 17th, 2013, 09:07 AM   #4
Tom Jenkins
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This is one of those broad, open ended questions that are almost impossible to answer on a forum like this:

We don't know what the physical system looks like - all cooling towers aren't the same

You don't identify if the problem occurs all of the time, or only at extremes of loading or ambient temperature

You don't identify if all of the towers have this problem, or only some.

You don't tell us how tight you want to control the temp or how tight you need to control the temp

You don't tell us why you want to use a single PID loop to control what are probably not 100% identical towers in terms of performance, water distribution etc.

And, personally, I wouldn't use PID for this.

Remember, the guys on this forum get paid to do what you are asking them to do for nothing. They are helpful, but there is a limit to how much time they can and should devote to a problem you are probably getting paid to solve!

It doesn't help to post snarky comments like "Nobody have the Answer" either. (Especially if you can't be bothered to check your grammar!)

Last edited by Tom Jenkins; January 17th, 2013 at 09:20 AM.
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Old January 17th, 2013, 03:25 PM   #5
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Here are some ideas:

(1) Sounds like there maybe a discontinuity transitioning from temperature to vacuum control. The process dynamics may be radically different when the vacuum comes on. Try to decouple this by turning on the vacuum control a little later say when T > Tsp by 5 degrees. This will give temperature control more time to catch a disturbance.

(2) Tuning of Temperature loops is difficult. The process is often 2nd order with considerable deadtime. If you have any PV filtering take it out, you don't need it with temperature. For temperature loops, the P&I parameters are much less aggressive then typical defaults often having a controller gain of .1 or less and having 3 to 5 minutes between repeats.

(3) This process is relatively non-linear particularly at the dew point. You may have a high degree of interaction with Pressure and temperature ( I refer to some dew point tables ). See if you can operate on linear part of the curve(s). Otherwise it could be very sensitive and often difficult to control.

(4) Could be your process gain is too high ? Try staging the start of your fans to lower process gain.


Hopefully this helps. Like Tom said we obviously need the whole picture here. I recommend that you hire an expert in cooling tower control to audit your system.



John
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