Need your help to solve this

[napoli1]

I have a chilled water line supply at 42 deg F and a by pass return temp varies. The return line and the by pass both have contro valves. If I set up the RTD to the return line as master to mantain the tempreatur at 54 Deg f How I will control the bypass line? I know I need a ratio controller that will work off the first PID but I am having problem to visualize this loop
the system is for building chiled water supply.
42 Deg F
TE--------------supply

the bypass X
----------------X return

this is what the sysstem look like. I have one TE for final temperature and the setpoint is at 54 Deg f. The X is the return valve and the by-pass valve, no flow meter. the final temperature is 54 Deg f the bypass is a normally open valve and the temperature range from 54 to 110.
I was thinking to add two more RTD to better control the mix temperature. If I set up the PID on the return valve then I could calculate the position of the by-pass based on:

CO(pid)*inlet temp+co(by-pass)*by-pass temp= final temperaure
so if the resturn valve is at 70% with the inlet at 42 F and the by-pass temp is at 82 I can calculate the by-pass valve position to CO2=((54-(co1*42))/82) =30%.
Is this correct or is there a better solution?? such as set up a PID on the main valve and a ratio on the second? but what should i use for PV, sp,?

 

[itsmoked]

Wouldn't you just monitor the temperature returning from the air handlers and use that via a PID to control the bypass valve? The colder the return to more you OPEN the bypass.

Keith Cress
kcress -

http://www.flaminsystems.com