.BuildSysPro.IBPSA.Fluid.Movers.UsersGuide

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

Notes (applicable to IBPSA.Fluid.Movers.FlowControlled_dp and IBPSA.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 can mitigate the error by providing other information for hydraulic efficiency. Compare validation models IBPSA.Fluid.Movers.Validation.PowerSimplified, IBPSA.Fluid.Movers.Validation.PowerExact, and IBPSA.Fluid.Movers.Validation.PowerEuler as an example.

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

Models that use performance curves for pressure rise

The models IBPSA.Fluid.Movers.SpeedControlled_y and IBPSA.Fluid.Movers.SpeedControlled_Nrpm take as an input either a control signal between 0 and 1, or the rotational speed in units of [1/min]. 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 IBPSA.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.

Then, a declaration would be

  IBPSA.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.

Models that directly control the head or the mass flow rate

The models IBPSA.Fluid.Movers.FlowControlled_dp and IBPSA.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.

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

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

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

• If the parameter inputType==IBPSA.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 IBPSA.Fluid.Movers.FlowControlled_m_flow, the mass flow rate is as follows:

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

• If the parameter inputType==IBPSA.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 IBPSA.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 IBPSA.Fluid.Movers.Validation.FlowControlled_dpSystem.

The models IBPSA.Fluid.Movers.FlowControlled_dp and IBPSA.Fluid.Movers.FlowControlled_m_flow both have a parameter m_flow_nominal. For IBPSA.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 P_ele, the hydraulic power input Ẇ_hyd, the flow work Ẇ_flo and the heat dissipated into the medium Q̇. Based on the first law, the flow work is

Ẇ_flo = | 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 Ẇ_flo is constrained to be non-negative. The regularisation starts around 0.01% of the characteristic maximum power Ẇ_max = V̇_max Δp_max. 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 = P_ele - Ẇ_flo.

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

Q̇ = Ẇ_hyd - Ẇ_flo.

The efficiencies are defined as

η = Ẇ_flo ⁄ P_ele = η_hyd   η_mot
η_hyd = Ẇ_flo ⁄ Ẇ_hyd
η_mot = Ẇ_hyd ⁄ P_ele

where η is the total efficiency, η_hyd is the hydraulic efficiency, and η_mot is the motor efficiency. From the definition one has

η = η_hyd   η_mot.

Hydraulic efficiency

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

• Efficiency_VolumeFlowRate - The user provides an array of η_hyd vs. V̇. If the array has only one element, η_hyd is considered constant. If the array has more than one element, the efficiency is interpolated or extrapolated using IBPSA.Fluid.Movers.BaseClasses.Characteristics.efficiency. See IBPSA.Fluid.Movers.Validation.PowerSimplified as an example.
• Power_VolumeFlowRate - The user provides an array of Ẇ_hyd vs. V̇. The power is interpolated or extrapolated using IBPSA.Fluid.Movers.BaseClasses.Characteristics.power. η_hyd is then computed from Ẇ_hyd. See IBPSA.Fluid.Movers.Validation.PowerExact as an example.
• EulerNumber (default 1) - The model uses a triple (η_hyd, V̇, Δp) corresponding to the operating point at which the peak efficiency is attained. It computes η_hyd and Ẇ_hyd using the package IBPSA.Fluid.Movers.BaseClasses.Euler. The model finds η_hyd by evaluating the following correlation:
  

  
  

  where x=log10(Eu ⁄ Eu_p), with the subscript p denoting the condition where the mover is operating at peak efficiency. The Euler number is defined as
  

  Eu=(pressure forces)/(inertial forces)

  from which one can derive the ratio of Euler numbers as
  

  Eu ⁄ Eu_p =(Δp ⁄ V̇²) ⁄ (Δp_p ⁄ V̇_p²).

  The peak point can be provided directly by the user or computed by calling the function IBPSA.Fluid.Movers.BaseClasses.Euler.getPeak. This function finds the peak point when both pressure and power curves are provided. When only the pressure curve is available, the function estimates the peak point to be at V̇=V̇_max ⁄ 2. Examples:

  • IBPSA.Fluid.Movers.Examples.StaticReset specifies the peak point directly.
  • IBPSA.Fluid.Movers.Validation.PowerEuler explictly calls the function.
  • IBPSA.Fluid.Movers.BaseClasses.Validation.EulerComparison implicitly calls the function when IBPSA.Fluid.Movers.Data.Generic is instantiated.

  For simplicity, the implementation does not directly use this method to estimate η_hyd at any operation point. Rather, it only computes a power curve at nominal speed and then uses similarity laws to estimate power at reduced speeds. Because the Euler number method does not account for the efficiency degradation along any curve Δp=kV̇², these two methods are equivalent. See the documentation of IBPSA.Fluid.Movers.BaseClasses.Euler.power for more details. Also see IBPSA.Fluid.Movers.BaseClasses.Validation.EulerReducedSpeed for demonstration.

  For more information on the Euler number method, see the documentation of IBPSA.Fluid.Movers.BaseClasses.Euler.correlation, EnergyPlus 9.6.0 Engineering Reference chapter 16.4 equations 16.209 through 16.218, and Fu et al. (2022)

• NotProvided (default 2) - The information of this efficiency item is not provided. The model uses a constant value η_hyd=0.7.

These options are tested in IBPSA.Fluid.Movers.BaseClasses.Validation.HydraulicEfficiencyMethods.

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 η_hyd ≤ 1 to prevent the division η_hyd = η ⁄ η_mot from producing efficiency values larger than one. This configuration is tested in IBPSA.Fluid.Movers.BaseClasses.Validation.TotalEfficiencyMethods.

Although the Euler number method is defined for η_hyd, this implementation applies it also to η and P_ele as an approximation. The basis is that η_mot 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 IBPSA.Fluid.Movers.BaseClasses.Characteristics.motorEfficiencyCurve) which shows that η and η_hyd are roughly linear to each other for motors of this size.

Motor efficiency

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

• Efficiency_VolumeFlowRate - This is same as the option for η_hyd with the same name.
• Efficiency_MotorPartLoadRatio - The user provides an array of η_mot vs. motor part load ratio y_mot=W_hyd ⁄ P_mot,nominal. The efficiency is interpolated or extrapolated using IBPSA.Fluid.Movers.BaseClasses.Characteristics.efficiency_yMot. See IBPSA.Fluid.Movers.BaseClasses.Validation.MotorEfficiencyMethods as an example.
• GenericCurve (default 1) - The user provides the rated motor power P_mot,nominal and maximum motor efficiency η_mot,max. The model then uses a generic motor efficiency curve as a function of motor PLR generated using IBPSA.Fluid.Movers.BaseClasses.Characteristics.motorEfficiencyCurve. The η_mot,max is assumed to be 0.7 if not specified by user. If P_mot,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 P_ele,

      P_mot,nominal= Ẇ_max,

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

      P_mot,nominal= 1.2   Ẇ_max,

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

    P_mot,nominal= 1.2   (V̇_max ⁄ 2)   (Δp_max ⁄ 2) ⁄ η_hyd,p,

    where the factor 1.2 also assumes a 20% oversize and the assumed peak hydraulic efficiency η_hyd,p=0.7.
  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 η_mot=0.7.

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

By default, the model uses the GenericCurve to obtain more accurate results with variable η_mot. 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:

     η_mot = f(Ẇ_hyd),
     P_ele = Ẇ_hyd ⁄ η_mot,

     where f(⋅) refers to the curve of motor efficiency vs. motor PLR. When Ẇ_hyd is known (i.e. per.powerOrEfficiencyIsHydraulic=true), the unknowns are η_mot and P_ele which can be solved explicitly. Otherwise, the unknowns are η_mot and Ẇ_hyd, 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 IBPSA.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 η_hyd and η_mot is then not important.

Start-up and shut-down transients

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

1. If use_inputFilter=false, then the input signal y (or Nrpm, 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_inputFilter=true, which is the default, then the fan speed (or the mass flow rate or the pressure rise) is equal to the output of a filter. This filter is implemented as a 2nd order differential equation and can be thought of as approximating the inertia of the rotor and the fluid. Thus, a step change in the fan input signal will cause a gradual change in the fan speed. The filter has a parameter riseTime, which by default is set to 30 seconds. The rise time is the time required to reach 99.6% of the full speed, or, if the fan is switched off, to reach a fan speed of 0.4%.

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

Although many simulations do not require such a detailed model that approximates the transients of fans or pumps, it turns out that using this filter can reduce computing time and can lead to fewer convergence problems in large system models. With a filter, any 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_inputFilter=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_inputFilter=false.

Note that if the fan is part of a closed loop control, then the filter affects the transient response of the control. When changing the value of use_inputFilter, 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 IBPSA.Controls.Continuous.LimPID and a fan or pump, configured with use_inputFilter=true and riseTime=30 seconds. Assume that the transient response of the other dynamic elements in the control loop is fast compared to the rise time of the filter. 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 P_ele that is consumed by the component. Otherwise, it is equal to the hydraulic work W_hyd. 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 IBPSA.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