When heat exchangers are connected to give parallel flow,
the criterion for flow distribution must be specified.
This is done in a direct way by entering a three-way valve in front of the two exchangers
and setting the relative mass flow rate for one of the outlets.
This is in in principle sufficient for a solution.
However, to introduce an automatic adjustment for varying heat exchanger load,
it is recommended to control the valve by a Valve control. In such a control,
a measurement is performed after one of the heat exchangers.
The three-way valve is controlled to achieve a specific temperature (secondary circuits)
or superheat/subcooling/vapour fraction (refrigerant circuits) at the measurement connector.
For refrigerant circuits it is commonplace to require the same conditions at exit
of the two heat exchangers (evaporators/condensers), one set by a valve control,
the other with an enthalpy control.
In the template Actual plant heating a laboratory
there are two valve controls.