modelSteamHumidifier_X

Steam humidifier with leaving water mass fraction as input

Extends from Buildings.Fluid.HeatExchangers.BaseClasses.PartialPrescribedOutlet (Ideal heater, cooler, humidifier or dehumidifier with prescribed outlet conditions).

Information

Model for a steam humidifier with a prescribed outlet water vapor mass fraction in kg/kg total air.

This model forces the outlet water mass fraction at port_b to be no lower than the input signal X_wSet, subject to optional limits on the maximum water vapor mass flow rate that is added, as described by the parameter mWatMax_flow. By default, the model has unlimited capacity.

The output signal mWat_flow ≥ 0 is the moisture added to the medium if the flow rate is from port_a to port_b. If the flow is reversed, then mWat_flow = 0. The outlet specific enthalpy at port_b is increased by the enthalpy of steam at 100°C times the mass of steam that was added. Therefore, the temperature of the leaving fluid is slightly above the inlet temperature.

The outlet conditions at port_a are not affected by this model, other than for a possible pressure difference due to flow friction.

If the parameter energyDynamics is different from Modelica.Fluid.Types.Dynamics.SteadyState, the component models the dynamic response using a first order differential equation. The time constant of the component is equal to the parameter tau. This time constant is adjusted based on the mass flow rate using

τ_eff = τ |ṁ| ⁄ ṁ_nom

where τ_eff is the effective time constant for the given mass flow rate ṁ and τ is the time constant at the nominal mass flow rate ṁ_nom. This type of dynamics is equal to the dynamics that a completely mixed control volume would have.

Optionally, this model can have a flow resistance. Set dp_nominal = 0 to disable the flow friction calculation.

For a model that uses a control signal u ∈ [0, 1] and multiplies this with the nominal water mass flow rate, use Buildings.Fluid.Humidifiers.Humidifier_u

Limitations

This model only adds water vapor for the flow from port_a to port_b. The water vapor of the reverse flow is not affected by this model.

Parameters

Type	Name	Default	Description
Boolean	homotopyInitialization (from PartialPrescribedOutlet)	true	= true, use homotopy method
Modelica.Units.SI.MassFlowRate	mWatMax_flow	Modelica.Constants.inf	Maximum water mass flow rate addition (positive)
Assumptions
Boolean	allowFlowReversal (from PartialTwoPort)	true	= false to simplify equations, assuming, but not enforcing, no flow reversal
Nominal condition
Modelica.Units.SI.MassFlowRate	m_flow_nominal (from PartialTwoPortInterface)		Nominal mass flow rate
Modelica.Units.SI.PressureDifference	dp_nominal (from TwoPortFlowResistanceParameters)		Pressure difference
Advanced
Modelica.Units.SI.MassFlowRate	m_flow_small (from PartialTwoPortInterface)	1E-4*abs(m_flow_nominal)	Small mass flow rate for regularization of zero flow
Advanced › Diagnostics
Boolean	show_T (from PartialTwoPortInterface)	false	= true, if actual temperature at port is computed
Flow resistance
Boolean	computeFlowResistance (from TwoPortFlowResistanceParameters)	true	=true, compute flow resistance. Set to false to assume no friction
Boolean	from_dp (from TwoPortFlowResistanceParameters)	false	= true, use m_flow = f(dp) else dp = f(m_flow)
Real	n (from TwoPortFlowResistanceParameters)	2	Flow exponent, n=1 for laminar, n=2 for turbulent
Boolean	linearizeFlowResistance (from TwoPortFlowResistanceParameters)	false	= true, use linear relation between m_flow and dp for any flow rate
Real	deltaM (from TwoPortFlowResistanceParameters)	0.1	Fraction of nominal flow rate where flow transitions to laminar
Dynamics › Conservation equations
Modelica.Fluid.Types.Dynamics	energyDynamics (from PartialPrescribedOutlet)	Modelica.Fluid.Types.Dynamics.SteadyState	Type of energy balance: dynamic (3 initialization options) or steady state
Dynamics
Modelica.Units.SI.Time	tau (from PartialPrescribedOutlet)	10	Time constant at nominal flow rate (used if energyDynamics not equal Modelica.Fluid.Types.Dynamics.SteadyState)
Initialization
Modelica.Units.SI.MassFraction[Medium.nX]	X_start	Medium.X_default	Start value of mass fractions m_i/m

Connectors

Type	Name	Description
Modelica.Fluid.Interfaces.FluidPort_a	port_a (from PartialTwoPort)	Fluid connector a (positive design flow direction is from port_a to port_b)
Modelica.Fluid.Interfaces.FluidPort_b	port_b (from PartialTwoPort)	Fluid connector b (positive design flow direction is from port_a to port_b)
Modelica.Blocks.Interfaces.RealInput	X_w	Set point for water vapor mass fraction in kg/kg total air of the fluid that leaves port_b
Modelica.Blocks.Interfaces.RealOutput	mWat_flow	Water added to the fluid (if flow is from port_a to port_b)
Modelica.Blocks.Interfaces.RealOutput	Q_flow	Heat flow rate added to the fluid (if flow is from port_a to port_b)

Components

Type	Name	Default	Description
Modelica.Units.SI.MassFlowRate	m_flow (from PartialTwoPortInterface)	port_a.m_flow	Mass flow rate from port_a to port_b (m_flow > 0 is design flow direction)
Modelica.Units.SI.PressureDifference	dp (from PartialTwoPortInterface)	port_a.p - port_b.p	Pressure difference between port_a and port_b
Medium.ThermodynamicState	sta_a (from PartialTwoPortInterface)	if allowFlowReversal then Medium.setState_phX(port_a.p, noEvent(actualStream(port_a.h_outflow)), noEvent(actualStream(port_a.Xi_outflow))) else Medium.setState_phX(port_a.p, noEvent(inStream(port_a.h_outflow)), noEvent(inStream(port_a.Xi_outflow)))	Medium properties in port_a
Medium.ThermodynamicState	sta_b (from PartialTwoPortInterface)	if allowFlowReversal then Medium.setState_phX(port_b.p, noEvent(actualStream(port_b.h_outflow)), noEvent(actualStream(port_b.Xi_outflow))) else Medium.setState_phX(port_b.p, noEvent(port_b.h_outflow), noEvent(port_b.Xi_outflow))	Medium properties in port_b

Revisions

March 8, 2022, by Michael Wetter:
Renamed parameter massDynamics to energyDynamics for consistency with other models.
May 10, 2017, by Michael Wetter:
First implementation.