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.