This module impliments a heat exchanger model with a built-in
PID controller to control the outlet temperature at
port_b2 if set parameter
use_controller=true . Otherwise, if set
use_controller=false, the PID controller and the
three-way valve are removed and the outlet temperature at
port_b2 will not be controlled.
Note that if the three-way valve is activated, it'll have the same differential pressure as the heat exchanger.