Buildings.DHC.BaseClasses.Steam

Package for steam systems using the split-medium approach.

Information

This package contains base class models for steam systems. These models use the split-medium approach to allow various water/steam models to be coupled with a numerically-efficient liquid water model. This can greatly improve the computing performance by decoupling the energy and mass balance equations.

Extends from Modelica.Icons.VariantsPackage (Icon for package containing variants).

Package Content

Name	Description
PartialSaturatedControlVolume	Partial control volume for evaporation/condensation processes
PartialTwoPortTwoMedium	Partial model with two ports with two separate medium models without storing mass or energy

Buildings.DHC.BaseClasses.Steam.PartialSaturatedControlVolume

Partial control volume for evaporation/condensation processes

Information

This model represents a partial control volume for either condensation or evaporation processes of water with liquid and vapor phases in equilibrium and at a saturated state. Models that extend this base class need to assign the mass flow rate at each port and the enthlapy at each port, as exemplifed in the evaporation and condensation models listed below. The volume can exchange heat through its heatPort when configured with dynamic mass and energy balances. In steady state, the heat port is conditionally removed in order to maintain a consistent set of equations.

This model is similar to Modelica.Fluid.Examples.DrumBoiler.BaseClasses.EquilibriumDrumBoiler with the following exceptions:

• Rather than a two-phase medium, fluid mediums are modeled as two single-state fluids, with liquid water at the up-stream port(port_a), and steam vapor at the downstream port (port_b) for instances of this base class that model evaporation (the opposite for condensation);
• The metal drum is excluded from the mass and energy balances;

Implementation

This model is configured to allow both steady state and dynamic mass and energy balances. The heat transfer through the heatPort is disabled in steady state balance. This is required because the fluid is restricted to a saturated state; thus, the heat transfer rate is a function of mass flow rate only if the volume is steady. The fluid mass m in the volume is calculated as

m = ρ_sV_s + ρ_wV_w

where ρ is density,V is volume, and subscripts represent the steam and liquid water components, respectively. The total internal energy U is

U = ρ_sV_sh_s + ρ_wV_w − pV

where h is specific enthalpy, p is pressure, and the total volume of fluid V=V_s+V_w.

The steady state mass balance is given as

ṁ_s + ṁ_w = 0,

while no additional equation is given for the steady state energy balance, since the heat flow rate into the water must be removed from the system in which the control volume is used.

The dynamic mass and energy balances are given as

dm/dt = ṁ_s + ṁ_w
dU/dt = Q̇ + ṁ_s h_s + ṁ _w h_w

where ̇ṁ_s and ṁ_w are the mass flow rates of steam and liquid water respectively; Q̇ is the heat flow rate into the control volume; h_s and h_w are the specific enthalpies of steam and liquid water, respectively. Note that with an evaporation process, the liquid phase (water) is always assigned at the port_a (inlet), while the vapor phase (steam) is always at the port_b (outlet). The opposite holds for a condensation process.

Assumptions

Three principal assumptions are made with this model:

• The fluid within the volume is wet steam.
• Liquid and vapor subcomponents are at equilibrium; and
• Fluid is discharged from the volume as ei ther saturated liquid or saturated vapor.

Models that extend this base class include Buildings.DHC.Plants.Steam.BaseClasses.ControlVolumeEvaporation and Buildings.DHC.Loads.Steam.BaseClasses.ControlVolumeCondensation.

Reference

Hinkelman, Kathryn, Saranya Anbarasu, Michael Wetter, Antoine Gautier, and Wangda Zuo. 2022. “A Fast and Accurate Modeling Approach for Water and Steam Thermodynamics with Practical Applications in District Heating System Simulation.” Preprint. February 24. doi:10.13140/RG.2.2.20710.29762.

Extends from Buildings.BaseClasses.BaseIcon (Base icon), Buildings.DHC.BaseClasses.Steam.PartialTwoPortTwoMedium (Partial model with two ports with two separate medium models without storing mass or energy).

Parameters

Connectors

Modelica definition

Buildings.DHC.BaseClasses.Steam.PartialTwoPortTwoMedium

Partial model with two ports with two separate medium models without storing mass or energy

Information

This partial model defines an interface for components with two ports and separate medium definitions at each port. The component transports fluid between two ports without storing mass or energy. The treatment of the design flow direction and of flow reversal are predefined based on the parameter allowFlowReversal.

This model is intended for steam heating applications, where phase change is inherently present. The split-medium approach enables a numerically-efficient liquid water model (i.e., Buildings.Media.Specialized.Water.TemperatureDependentDensity) to be implemented alongside various water/steam models for other phases. For most applications, an efficient model (i.e., Modelica.Media.Water.StandardWater is suitable as it covers the largest range of pressure-temperature conditions through its implementation of the IAPWS-IF97 water/steam formulation. If a reduce pressure-temperature range is applicable, Buildings.Media.Steam) provides a more efficient implementation. Through the split-medium approach, pressure and density calculations are decoupled, eliminating costly nonlinear systems of equations. This interface model also includes parameters for mass and energy dynamics as well as initialization.

Reference

Hinkelman, Kathryn, Saranya Anbarasu, Michael Wetter, Antoine Gautier, and Wangda Zuo. 2022. “A Fast and Accurate Modeling Approach for Water and Steam Thermodynamics with Practical Applications in District Heating System Simulation.” Preprint. February 24. doi:10.13140/RG.2.2.20710.29762.

Parameters

Connectors

Modelica definition