Suitability od PID controler


#1

Dear Sirs,

I would like to ask you for
you opinion and help.

We use Gasketed plate heat
exchanger for cooling the medium with cooling water. We use feedback control
with PID regulator, but I am not sure if PID regulation is the bad choice for
this system or the P,I,D constants of regulator is poorly set up. In table I
summarize the streams properties.

Temperature (°C)

Flow rate (kg/min)

Cooling water IN

8-10

n/a

Cooling water OUT

n/a

Medium IN

20-27 (>25 in rare cases)

90 - 300

Medium OUT

Requested 20

We are sure, that flow of
cooling water is sufficient in all cases, even dont have information about the
flow rate. Medium flow rate change by steps and is constant for some time (see
figure below typical situation).

According my knowledge (I am
not specialized in regulation) PID controller is not good option if the medium
flow rate changes in so high peaks, as „ideal“ P,I,D constant for one flow rate
(say minimal = 90 kg/min) can be very poor for other flow rate (in our case maximum
300 kg/min). In some cases the temperature of medium is stable, but in some
cases is sinusoidal (see figure) with difference between minimal temperature
and maximal 2-2,5 °C. This difference is for our process too high, we request
to have temperature stable in 0,5 °C, maximum 1°C range.

So my question: is PID
regulator for this situation good choice and we need to tune up P, I,D
constants or we need to modify our regulation systém.

Thanks in advance for your
replies, and if needed more information I will answer them.

J.K.

Update: sketch of system added

**as new member I can not attach files, so if you are willing to take a look, please send me a email and I will send you the figures.