packageUsersGuide

User's Guide

Extends from Modelica.Icons.Information (Icon for general information packages).

Information

This package contains models for fans and pumps (movers). The same models can be used for fans or pumps.

Model description

The models consider the pressure rise, flow rate, speed, power consumption, and heat dissipation based on the user's specification. They can take pressure rise (head), mass flow rate, or speed (absolute or relative) as control signal, and compute resulting quantities based on user-provided performance curves.

While the models in the package Buildings.Fluid.Movers allow full customization, preconfigured models that use the same underlying physical equations are available in the package Buildings.Fluid.Movers.Preconfigured. The models in Buildings.Fluid.Movers can also be parameterized with the data records from Buildings.Fluid.Movers.Data.

A detailed description of the fan and pump models can be found in Wetter (2013). The models are implemented as described in this paper, except that equation (20) is no longer used. The reason is that the transition (24) caused the derivative

d Δp(r(t), V(t)) ⁄ d r(t)

to have an inflection point in the regularization region r(t) ∈ (δ/2, δ). This caused some models to not converge. To correct this, for r(t) < δ, the term V(t) ⁄ r(t) in (16) has been modified so that (16) can be used for any value of r(t).

Below, the models are briefly described.

Performance data

The models use performance curves that compute pressure rise, electrical power draw and efficiency as a function of the volume flow rate and the speed. The following performance curves are implemented:

Independent variable Dependent variable Record for performance data Function
Volume flow rate Pressure flowParameters pressure
Volume flow rate Efficiency
(hydraulic or motor)		 efficiencyParameters efficiency
Motor part load ratio Motor efficiency* efficiencyParameters_yMot efficiency_yMot
Volume flow rate Power** powerParameters power

Notes (applicable to Buildings.Fluid.Movers.FlowControlled_dp and Buildings.Fluid.Movers.FlowControlled_m_flow):

  • * The models will ignore this record if the nominal motor power is not provided and cannot be estimated from the pressure curve. This is because calculating the motor part load ratio requires knowing the nominal power.
  • ** The models will ignore this record if the pressure curve is not provided and the speed is unknown. This is because the models wouldn't be able to compute the elctrical power correctly using similarity laws without speed. In this case the user should consider using the preconfigured mover models in Buildings.Fluid.Movers.Preconfigured which will auto-populate a pressure curve from nominal flow rate and pressure.

These performance curves are implemented in Buildings.Fluid.Movers.BaseClasses.Characteristics, and are used in the performance records in the package Buildings.Fluid.Movers.Data. The package Buildings.Fluid.Movers.Data contains different data records.

Models that use performance curves for pressure rise

The model Buildings.Fluid.Movers.SpeedControlled_y takes as an input a control signal between 0 and 1. From this input and the current flow rate, they compute the pressure rise. This pressure rise is computed using a user-provided list of operating points that defines the fan or pump curve at full speed. For other speeds, similarity laws are used to scale the performance curves, as described in Buildings.Fluid.Movers.BaseClasses.Characteristics.pressure.

For example, suppose a pump needs to be modeled whose pressure versus flow relation crosses, at full speed, the points shown in the table below.

Volume flow rate [m3⁄s] Head [Pa]
0.0003 45000
0.0006 35000
0.0008 15000

Then, a declaration would be 

  Buildings.Fluid.Movers.SpeedControlled_y pum(
    redeclare package Medium = Medium,
    per.pressure(V_flow={0.0003,0.0006,0.0008},
                 dp    ={45,35,15}*1000))
    "Circulation pump";

This will model the following pump curve for the pump input signal y=1.

image

See Buildings.Fluid.Movers.Validation.PressureCurve for a small example that validates the pressure curve specification.

Models that directly control the head or the mass flow rate

The models Buildings.Fluid.Movers.FlowControlled_dp and Buildings.Fluid.Movers.FlowControlled_m_flow take as an input the pressure difference or the mass flow rate. This pressure difference or mass flow rate will be provided by the fan or pump, i.e., the fan or pump has idealized perfect control and infinite capacity. Using these models that take as an input the head or the mass flow rate often leads to smaller system of equations compared to using the models that take as an input the speed. The validation model Buildings.Fluid.Movers.Validation.ComparePowerInput demonstrates that the models with different input signals produce the same power consumption estimates.

These models can be configured for three different control inputs. For Buildings.Fluid.Movers.FlowControlled_dp, the head is as follows:

  • If the parameter inputType==Buildings.Fluid.Types.InputType.Continuous, the head is dp=dp_in, where dp_in is an input connector.

  • If the parameter inputType==Buildings.Fluid.Types.InputType.Constant, the head is dp=constantHead, where constantHead is a parameter.

  • If the parameter inputType==Buildings.Fluid.Types.InputType.Stages, the head is dp=heads, where heads is a vectorized parameter. For example, if a mover has two stages and the head of the first stage should be 60% of the nominal head and the second stage equal to dp_nominal, set heads={0.6, 1}*dp_nominal. Then, the mover will have the following heads:

    input signal stage Head [Pa]
    0 0
    1 0.6*dp_nominal
    2 dp_nominal

Similarly, for Buildings.Fluid.Movers.FlowControlled_m_flow, the mass flow rate is as follows:

  • If the parameter inputType==Buildings.Fluid.Types.InputType.Continuous, the mass flow rate is m_flow=m_flow_in, where m_flow_in is an input connector.

  • If the parameter inputType==Buildings.Fluid.Types.InputType.Constant, the mass flow rate is m_flow=constantMassFlowRate, where constantMassFlowRate is a parameter.

  • If the parameter inputType==Buildings.Fluid.Types.InputType.Stages, the mass flow rate is m_flow=massFlowRates, where massFlowRates is a vectorized parameter. For example, if a mover has two stages and the mass flow rate of the first stage should be 60% of the nominal mass flow rate and the second stage equal to m_flow_nominal, set massFlowRates={0.6, 1}*m_flow_nominal. Then, the mover will have the following mass flow rates:

    input signal stage Mass flow rates [kg/s]
    0 0
    1 0.6*m_flow_nominal
    2 m_flow_nominal

These two models do not need to use a performance curve for the flow characteristics. The reason is that

  • for given pressure rise (or mass flow rate), the mass flow rate (or pressure rise) is computed from the flow resistance of the duct or piping network, and
  • at zero pressure difference, solving for the flow rate and the revolution leads to a singularity.

However, the computation of the electrical power consumption requires the mover speed to be known and the computation of the mover speed requires the performance curves for the flow and efficiency/power characteristics. Therefore these performance curves do need to be provided if the user desires a correct electrical power computation. If the curves are not provided, a simplified computation is used, where the efficiency curve is used and assumed to be correct for all speeds. This loss of accuracy has the advantage that it allows to use the mover models without requiring flow and efficiency/power characteristics.

The model Buildings.Fluid.Movers.FlowControlled_dp has an option to control the mover such that the pressure difference set point is obtained across two remote points in the system. To use this functionality parameter prescribeSystemPressure has to be enabled and a differential pressure measurement must be connected to the pump input dpMea. This functionality is demonstrated in Buildings.Fluid.Movers.Validation.FlowControlled_dpSystem.

The models Buildings.Fluid.Movers.FlowControlled_dp and Buildings.Fluid.Movers.FlowControlled_m_flow both have a parameter m_flow_nominal. For Buildings.Fluid.Movers.FlowControlled_m_flow, this parameter is used for convenience to set a default value for the parameters constantMassFlowRate and massFlowRates. For both models, the value is also used for the following:

  • To compute the size of the fluid volume that can be used to approximate the inertia of the mover if the energy dynamics is selected to be dynamic.
  • To compute a default pressure curve if no pressure curve has been specified in the record per.pressure. The default pressure curve is the line that intersects (dp, V_flow) = (dp_nominal, 0) and (dp, V_flow) = (m_flow_nominal/rho_default, 0).
  • To regularize the equations near zero flow rate to ensure a numerically robust model.

However, otherwise m_flow_nominal does not affect the mass flow rate of the mover as the mass flow rate is determined by the input signal or the above explained parameters.

Electrical power consumption

All models compute the motor power draw Pele, the hydraulic power input Ẇhyd, the flow work Ẇflo and the heat dissipated into the medium Q̇. Based on the first law, the flow work is

Ẇflo = | V̇ Δp |,

where V̇ is the volume flow rate and Δp is the pressure rise. In order to prevent the model from producing negative mover power when either the flow rate or pressure rise is forced to be negative, the flow work Ẇflo is constrained to be non-negative. The regularisation starts around 0.01% of the characteristic maximum power Ẇmax = V̇max Δpmax. See discussions and an example of this situation in IBPSA, #1621.

The heat dissipated into the medium is as follows: If the motor is cooled by the fluid, as indicated by per.motorCooledByFluid=true, then the heat dissipated into the medium is

Q̇ = Pele - Ẇflo.

If per.motorCooledByFluid=false, then the motor is outside the fluid stream, and only the shaft, or hydraulic, work Ẇhyd enters the thermodynamic control volume. Hence,

Q̇ = Ẇhyd - Ẇflo.

The efficiencies are defined as

η = Ẇflo ⁄ Pele = ηhyd   ηmot
ηhyd = Ẇflo ⁄ Ẇhyd
ηmot = Ẇhyd ⁄ Pele

where η is the total efficiency, ηhyd is the hydraulic efficiency, and ηmot is the motor efficiency. From the definition one has

η = ηhyd   ηmot.

Hydraulic efficiency

The following options are used to specify how ηhyd is computed.

These options are validated in Buildings.Fluid.Movers.BaseClasses.Validation.HydraulicEfficiencyMethods and Buildings.Fluid.Movers.Validation.ComparePowerHydraulic.

The model uses EulerNumber as the default option unless a pressure curve is not provided. In this case, the model overrides it and uses NotProvided instead.

The user can use the same options to specify the total efficiency η instead by setting per.powerOrEfficiencyIsHydraulic=false. This changes the default constant value to η=0.49 and also imposes an additional constraint of ηhyd ≤ 1 to prevent the division ηhyd = η ⁄ ηmot from producing efficiency values larger than one. This configuration is validated in Buildings.Fluid.Movers.BaseClasses.Validation.TotalEfficiencyMethods and Buildings.Fluid.Movers.Validation.ComparePowerTotal.

Although the Euler number method is defined for ηhyd, this implementation applies it also to η and Pele as an approximation. The basis is that ηmot is mostly constant for motors larger than about 3.5 kW or 5 HP except when the motor part load drops below around 40%, (see the documentation of Buildings.Fluid.Movers.BaseClasses.Characteristics.motorEfficiencyCurve) which shows that η and ηhyd are roughly linear to each other for motors of this size.

Motor efficiency

The following options are used to specify how ηmot is computed.

  • Efficiency_VolumeFlowRate - This is same as the option for ηhyd with the same name.
  • Efficiency_MotorPartLoadRatio - The user provides an array of ηmot vs. motor part load ratio ymot=Whyd ⁄ Pmot,nominal. The efficiency is interpolated or extrapolated using Buildings.Fluid.Movers.BaseClasses.Characteristics.efficiency_yMot. See Buildings.Fluid.Movers.BaseClasses.Validation.MotorEfficiencyMethods as an example.
  • GenericCurve (default 1) - The user provides the rated motor power Pmot,nominal and maximum motor efficiency ηmot,max. The model then uses a generic motor efficiency curve as a function of motor PLR generated using Buildings.Fluid.Movers.BaseClasses.Characteristics.motorEfficiencyCurve. The ηmot,max is assumed to be 0.7 if not specified by user. If Pmot,nominal is unspecified, the model estimates it in the following ways:
    • If a power curve is provided,
      • If the curve refers to total electric power Pele,

        Pmot,nominal= Ẇmax,

        where Ẇmax is the maximum value on the provided power curve.
      • If the curve refers to hydraulic power Ẇhyd,

        Pmot,nominal= 1.2   Ẇmax,

        where the factor 1.2 accounts for a 20% oversize of the motor.
    • Otherwise, if only a pressure curve is provided,

      Pmot,nominal= 1.2   (V̇max ⁄ 2)   (Δpmax ⁄ 2) ⁄ ηhyd,p,

      where the factor 1.2 also assumes a 20% oversize and the assumed peak hydraulic efficiency ηhyd,p=0.7 unless a hydraulic peak value is available in the record.
    The model then computes the efficiency the same way as in the option of Efficiency_MotorPartLoadRatio.
  • NotProvided (default 2) - The information of this efficiency item is not provided. The model uses a constant value ηmot=0.7.

These options are tested in Buildings.Fluid.Movers.BaseClasses.Validation.MotorEfficiencyMethods.

By default, the model uses the GenericCurve to obtain more accurate results with variable ηmot. There are two exceptions:

  • When neither pressure curve nor nominal motor power is provided, the model overrides it and uses NotProvided instead.
  • When the user specifies that the provided power is total power instead of hydraulic power, i.e. per.powerOrEfficiencyIsHydraulic==false, the model uses NotProvided as default. The user can still mannually set it to GenericCurve, but this is not recommended. There are two reasons:
    1. Consider the following two equations:

      ηmot = f(Ẇhyd),
      Pele = Ẇhyd ⁄ ηmot,

      where f(⋅) refers to the curve of motor efficiency vs. motor PLR. When Ẇhyd is known (i.e. per.powerOrEfficiencyIsHydraulic=true), the unknowns are ηmot and Pele which can be solved explicitly. Otherwise, the unknowns are ηmot and Ẇhyd, and an iterative solution would be required which may not converge for some values.
    2. If the power data provided refer to the total electric power instead of the hydraulic power of the mover, it is likely that the mover comes with built-in motors. This is often the case for small pumps whose motor is inside the fluid stream (such as the pumps whose data are provided in Buildings.Fluid.Movers.Data.Pumps.Wilo). Because the models only use the two efficiencies to differentiate mover heat carried away by the fluid and dissipated to the ambient and now only a negligible amount of heat dissipates into the ambient, the separation of ηhyd and ηmot is then not important.
Start-up and shut-down transients

All models have a parameter use_riseTime. This parameter affects the fan output as follows:

  1. If use_riseTime=false, then the input signal y (or m_flow_in, or dp_in) is equal to the fan speed (or the mass flow rate or pressure rise). Thus, a step change in the input signal causes a step change in the fan speed (or mass flow rate or pressure rise).
  2. If use_riseTime=true, which is the default, then the fan speed (or the mass flow rate or the pressure rise) changes linear in time until it reaches the control input. The parameter riseTime, which by default is set to 30 seconds, determines how fast the speed changes. For example, if riseTime=30 seconds and the current speed is 0, then a step change in the fan input signal from 0 to 1 will cause the fan speed to increase its speed linearly to the full speed within 30 seconds. Similarly, if the fan speed is then reduced by changing the input signal from 1 to 0.5, it will take 15 seconds to achieve the new set point.

The figure below shows for a fan with use_riseTime=true and riseTime=30 seconds the speed input signal and the actual speed.

image

Although many simulations do not require such a detailed model that approximates the transients of fans or pumps, it turns out that using such a continuous change in speed can reduce computing time and can lead to fewer convergence problems in large system models, because a sudden change in control signal, such as when a fan switches on, is damped before it affects the air flow rate. This continuous change in flow rate turns out to be easier, and in some cases faster, to simulate compared to a step change. For most simulations, we therefore recommend to use the default settings of use_riseTime=true and riseTime=30 seconds. An exception are situations in which the fan or pump is operated at a fixed speed during the whole simulation. In this case, set use_riseTime=false.

Note that if the fan is part of a closed loop control, then the value of riseTime affects the transient response of the control. When changing the value of riseTime, the control gains may need to be retuned. We now present values control parameters that seem to work in most cases. Suppose there is a closed loop control with a PI-controller Buildings.Controls.Continuous.LimPID and a fan or pump, configured with use_riseTime=true and riseTime=30 seconds. Assume that the transient response of the other dynamic elements in the control loop is fast compared to the value of riseTime. Then, a proportional gain of k=0.5 and an integrator time constant of Ti=15 seconds often yields satisfactory closed loop control performance. These values may need to be changed for different applications as they are also a function of the loop gain. If the control loop shows oscillatory behavior, then reduce k and/or increase Ti. If the control loop reacts too slow, do the opposite.

Fluid volume of the component

All models can be configured to have a fluid volume at the low-pressure side. Adding such a volume sometimes helps the solver to find a solution during initialization and time integration of large models.

Enthalpy change of the component

If per.motorCooledByFluid=true, then the enthalpy change between the inlet and outlet fluid port is equal to the electrical power Pele that is consumed by the component. Otherwise, it is equal to the hydraulic work Whyd. The parameter addPowerToMedium, which is by default set to true, can be used to simplify the equations. If addPowerToMedium = false, then no enthalpy change occurs between inlet and outlet. This can lead to simpler equations, but the temperature rise across the component will be zero. In particular for fans, this simplification may not be permissible.

Differences to models in Modelica.Fluid.Machines

The models in this package differ from Modelica.Fluid.Machines primarily in the following points:

  • They use a different base class, which allows to have zero mass flow rate. The models in Modelica.Fluid restrict the number of revolutions, and hence the flow rate, to be non-zero.
  • For the model with prescribed pressure, the input signal is the pressure difference between the two ports, and not the absolute pressure at port_b.
  • The pressure calculations are based on total pressure in Pascals instead of the pump head in meters. This change was done to avoid ambiguities in the parameterization if the models are used as a fan with air as the medium. The original formulation in Modelica.Fluid.Machines converts head to pressure using the density medium.d. Therefore, for fans, head would be converted to pressure using the density of air. However, for pumps, manufacturers typically publish the head in millimeters water (mmH2O). Therefore, to avoid confusion when using these models with media other than water, we changed the models to use total pressure in Pascals instead of head in meters.
  • The performance data are interpolated using cubic hermite splines instead of polynomials. These functions are implemented in Buildings.Fluid.Movers.BaseClasses.Characteristics.
  • The efficiency calculation is different, in particular, the models in this package allow use of the Euler number to compute the hydraulic efficiency.

References

Michael Wetter. Fan and pump model that has a unique solution for any pressure boundary condition and control signal. Proc. of the 13th Conference of the International Building Performance Simulation Association, p. 3505-3512. Chambery, France. August 2013.

Hongxiang Fu, David Blum, Michael Wetter. Fan and Pump Efficiency in Modelica based on the Euler Number. Proc. of the American Modelica Conference 2022, p. 19-25. Dallas, TX, USA. October 2022. https://doi.org/10.3384/ECP2118619

EnergyPlus 9.6.0 Engineering Reference