I know a shoping centre that has a common chilled water pipe header in the plantroom.
approx dimensions
diameter 1200 mm
length 10m
There is a hot end and a cold end.
On the primary side, a number of packaged chillers draws water from the hot side and returns it to the cold side.
On the secondary side, chilled water is pumped from the cold side to the shopping centre and returned to the hot side.
Nothing stops the mixing of hot and cold fluids!!
I can see that the benefit of such a design is that you dont have to worry about balancing the primary and secondary flows.
What about the energy lost due to mixing in the header?