.Buildings.Fluid.HydronicConfigurations.ActiveNetworks.Examples.InjectionTwoWayVariable

Information

This model is almost similar to Buildings.Fluid.HydronicConfigurations.ActiveNetworks.Examples.InjectionTwoWayConstant except that a cooling system is represented, and the consumer circuit is a variable flow circuit with a variable speed pump and two-way valves. The pump speed is modulated to track a constant pressure differential at the boundaries of the remote terminal unit.

For this circuit to operate as intended, it is critical that the secondary supply temperature set point be different from the primary supply temperature. Otherwise, the tracking error does not change sign and there is no overshoot that can desaturate the integral term of the PI controller. In other words, the controller output is fixed as soon as the measured value equals the set point. Therefore, the equilibrium point typically differs from the control intent which is a primary flow rate varying with the load. One can observe that behavior by setting TLiqSup_nominal=TLiqEnt_nominal and have_resT2=false. Such setting yields a fixed valve position with a primary recirculation and a flow reversal in the bypass whereas the control intent would be a slightly closer position ensuring a positive flow in the bypass. Note that this is nearly invisible from an operating standpoint since the set point and the loads are met. However, this is definitely detrimental to the overall performance as the primary circuit is operated at a higher flow rate and lower ΔT than needed. The system practically behaves as there was no control valve installed on the primary return line.

The fact that the load seems unmet at partial load (see plot #4) is due to the load model that does not guarantee a linear variation of the load with the input signal in cooling mode, see Buildings.Fluid.HydronicConfigurations.ActiveNetworks.Examples.BaseClasses.Load.

Revisions

• June 30, 2022, by Antoine Gautier:
  First implementation.