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I am currently trying to figure out how to set up an automatic controller in order to get a precise air temperature at a specific flow rate with a hot and fresh air mix system.

Picture the system like this: On both hot and cold air pipes, there is a control flap which allows to lower or increase the flowrate according to the desired resulting temperature.

Cold air is directly sucked from the environement. Its temperature varies along with the weather.

These 2 pipes meet at a T junction (a kind of mixing chamber) and are mixed on some length of pipe before being sucked by a fan which also moves both cold and hot gazes before the mix.

The system has been designed so that when the flaps are open at 50%, they will allow the fan to suck the targeted flowrate Q at the desired temperature T when fresh air temperature is 15°C.

Now, because fresh air temperature will vary over time, the flaps need to be adjusted to maintain the desired working conditions of the fan.

I am having a hard time to understand how the flaps control system should work because:

  • Cold air flap position change alone will affect the hot air flowrate, and vice versa
  • Changing the position of the flaps will not affect the flowrate linearily
  • The flowrate needs to remain constant

Which equation should I use for this? And, along with temperatures, which measurement is the most relevant? Static pressure? Velocity/dynamic pressure?

Any idea is welcome

Snite
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  • rather than equations, figure out sensors and feedback loops. you may not have sufficient control without introducing something active like a fan. – Abel May 12 '22 at 02:50
  • You will essentially have 2 control loops running in parallel. One to control the temperature, and the other to control the flow rate. The flow rate control loop output will go to both valves in parallel, and the temperature control output will go to them deferentially (opens one, closes the other). – Drew May 13 '22 at 19:49

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