modelInjectionTwoWayVariableReturn

Information

This model illustrates a configuration that is not recommended, that is an injection circuit with a two-way valve serving a variable flow consumer circuit, and controlled based on the return temperature. When comparing this model to Buildings.Fluid.HydronicConfigurations.ActiveNetworks.Examples.InjectionTwoWayConstantReturn one can notice that the design load is not met (see plot #4 between 6h and 8h) despite the return temperature set point being met (see plot #1) and the consumer circuit being operated at design flow rate (see plot #2). This is because, for the specific sizing of the cooling coil and for certain operating conditions the "process characteristic" is not monotonously decreasing as expected. This is illustrated by the simulation of the load model with open loop control (see plot #9). That simulation shows that for a constant load, an increasing supply temperature yields a decreasing return temperature. However, the control logic is based on the consideration that a decreasing return temperature is the signature of a decreasing load. It thus triggers the closing of the control valve, which in turn yields an increasing secondary flow recirculation, so an increasing supply temperature that further decreases the return temperature. The result is that the equilibrium point differs from the control intent, here with a supply temperature much higher than the design value (6.6 °C instead of 4.4 °C).

Parameters

Components

Revisions

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