The model recieves an input signal Q_in which is
the heat flow rate that has to be extracted from the fluid. Before
the heat flow rate is extracted, the input signal passes through a
limiter block with an upper and lower limit.
The upper limit ensures that the return temperature does not
drops down to a value under a minimum temperature
TDHNMin defined from outside.
The lower limit is in general set to 0 in order to avoid the block to cool the fluid.