Water mixing bench

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Good day all!

I am a 4th year mech engineering student. My final project is a water mixing bench where hot water from a hot reservoir supplies a heat exchanger which cools the water with air and the cool water flows back on the return line.

The hot water line consists of a centrifugal pump then a three-way actuated valve with allowing cold water to flow from the return line to lower the temperature. The hot water line then passes through a second centrifugal pump and another three -way actuated valve allowing cold water from the return line to mix with the hot line influencing the flow rate.

Basically the first valve is used for temperature control and the second valve is used for flow control.

The temperature can only be lowered due to the cold water.

Pumps operate at max set speed and do not have a vsd.

The temperature sensor reading is taken at the hot water outlet to the heat exchanger (desired set temperature) and another at the cold water outlet from the heat exchanger.

A pid controlls both actuators as said before - first actuator for temperature control and the second for flow rate control.

I am new to pid control and not sure what approach to take, because in the past, the pid controllers clashed. 

The temperature sensors and actuators are programmed into the plc already and all that needs to be done is the transfer function and constants for the controllers.

It is a Siemens Simatic S7-1200

If anyone could give me a basis on where to start and how I should go about calculating the constants for both controllers, or if the plc can do it for you?

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I am a touch confounded into selecting the correct stream's temperament influencing it is flowing a regime and for my situation, I have the Solid stream and particles is itinerant through two similar plates.

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