What is the best logic to control the flow and temperature?

[Elyas_gh]

Greetings In my project, there are three control valve One of the hot water line with a diameter of 200 Another on the cold water line with a diameter of 100 And a third on the way mixing cold and hot water with diameter 200 installed. after the third valve One flow meter is installed And at the end of a temperature transmitter installed. What is the best logic for control flow and temperature? Thanks NewPdl2.pdf

[cHud]

I would use a pid controller for each valve. The hot water valve and cold water valve's pid block would control the temperature and the 3rd valve (mixing valve) would control the flow

[RDA]

sorry for my english, most likely it will not work well from the point of view of plant mechatronic ideal is to have a 3-way valve that works in mixing with one pid regulator that controls the output mix.

[JRoss]

I agree with cHud. If you use three 2-way control valves, the hot and cold valves could each have their own PID controller, one in heating mode the other in cooling mode. The water would then mix in a manifold, supplying the correct temperature. Then a flow control valve controlled by another PID controller could operate independently to control the flow rate of the mixed water. Fairly simple to set up and it should give you good performance. With your method, a 3-way mixing valve would give you control of the temperature, but no control of the flow. In that scenario, the flow would be adjusted by opening and closing the supply valves, but since you have two pipes of different diameter, and a constantly changing mix (from the temperature controller), it will be difficult. You would have to use cascading PID controllers, where the control output of the temperature PID is used to adjust the control output of the flow PID. The PIDs would be fighting each other. If your flow doesn't need to be too tightly controlled, you could possibly just take the output of the temperature PID, and calculate the hot and cold supply valve positions without using a flow PID. But since the supply pipes are different sizes, it wouldn't be a straight-forward calculation.