The System Model fan models fans of various types using a relatively simple engineering model. It can be used in variable air volume, constant volume, on-off cycling, two-speed, or multi-speed applications. It was designed as a replacement for all of the other fan types: Constant Volume, On/Off, and Variable Volume.
The electric power consumed by the fan can be directly input or autosized using one of three optional methods. For fans that can vary the volume flow rate the performance can be described using a separate performance curve.
A unique auto-generated name for the fan. Any reference to this fan by another object will use this name.
The type of the System Model fan is fixed as:
This is the total system design pressure rise experienced by the fan (in Pascals or inH2O) at full flow rate and altitude-adjusted standard density of dry air at 20°C drybulb.
Select from:
1-Autosize, where EnergyPlus will autosize the Electric power per unit flow rate using one of the methods listed below under Design power sizing method.
2-Manual input, where the Design electric power consumption is entered manually in the setting below.
This setting is available when the Design electric power consumption method is set to 2-Manual input. It is the electric power consumption at the full Maximum flow rate and Pressure rise (in W). The value entered in this field is used to determine the fan efficiency.
This setting is used when the Design electric power consumption method is set to 1-Autosize to select how the fan’s Design electric power consumption is sized . There are three choices:
1-Power per flow, where the value entered for Electric power per unit flow rate is used to size the Design electric power consumption. This method is useful during early-phase design modelling when little information is available for determining the Design pressure rise. Although the pressure rise is not used to size the Design electric power consumption it is still used to determine the heat added to the air stream as a result of the work done by the fan.
2-Power per flow per pressure, where the value entered for Electric power per unit flow rate per unit pressure is used to size the Design electric power consumption. This method takes into account the Design Pressure Rise when sizing the Design Electric Power Consumption.
3-Total efficiency and pressure, where the values entered for Fan total efficiency and Design pressure rise are used to size the power.
This setting is used when the Design electric power consumption method is set to 1-Autosize and the Design power sizing method is set to 1-Power per flow. This value has units of W/(m3/s) and is used to scale the Design electric power consumption directly from the Maximum flow rate. It is defined such that
Design electric power consumption = (Electric power per unit flow rate) * (Maximum flow rate).
This setting is used when the Design electric power consumption method is set to 1-Autosize and the Design power sizing method is set to 1-Power per flow per pressure. This value has units W/((m3/s)-Pa) and is used to scale the Design electric power consumption from the Maximum flow rate and the Pressure rise. It is defined such that
Design electric power consumption = (Electric power per unit flow rate per unit pressure) * (Maximum flow rate) * Design pressure rise).
The default is 1.66667.
This is the overall efficiency of the fan, i.e. the ratio of the power delivered to the fluid to the electrical input power. It is the product of the motor efficiency and the impeller efficiency. It is used when the Design electric power consumption method is set to 1-Autosize and the Design power sizing method is set to 3-Total efficiency and pressure. This value is used to determine the Design electric power consumption from the Maximum flow rate and the Pressure rise. It is defined such that
Design electric power consumption = (Maximum flow rate) * (Design pressure rise) / (Fan Total Efficiency).
The default is 0.7.
Enter the full load air volumetric flow rate (m3/sec or ft3/min) installed in the HVAC system. This volume flow rate will be converted to a mass flow rate using an altitude-adjusted standard density of dry air at 20 °C drybulb. This full-speed flow rate and serves as the upper limit for fans that can vary their flow rate.
This field can be autosized and is greyed out if the parent HVAC component (e.g. AHU) defines the design air flow rate.
The fan shaft power divided by the electrical power consumed. Must be between 0 and 1.
This is the fraction of the motor heat that is added to the air stream. The value must be between 0 and 1. A value of 0 means fan motor is located completely outside of the air stream and none of the motor’s heat is added to the air stream. A value of 1.0 means the motor is located completely inside of air stream and all of the motor’s heat is added to the air stream.
Note: Regardless of the value here there will be heat added to the air stream as a result of the work done to move the air. This setting only describes what happens to the heat generated as a result of the motor’s inefficiency. The heat lost from the motor that is not added to the air stream can be added to the surrounding thermal zone where the motor is located using the settings below.
The heat lost from the motor that is not added to the air stream can be added to the thermal zone where the motor is located by checking this checkbox and selecting the zone in the setting below. If the fan is outdoors, or the motor’s thermal losses are not to be modeled then leave this unchecked.
When the Motor loss added to zone checkbox is checked, you must select the zone in which the fan motor is located. In this case, the portion of the motor’s thermal losses that are not added to the air stream are added to the surrounding thermal zone as internal heat gains.
When the Motor loss added to zone checkbox is checked, you can define the split between thermal radiation and convection for the heat losses from the fan motor. If this field is left at the default zero value all the losses will be convective. Values should be between 0 and 1.
This setting is used to select how the fan speed can be varied. There are two choices:
1-Continuous, which indicates that the fan speed is variable and can change smoothly up to the Maximum flow rate.
2-Discrete, which indicates that the fan can operate only at specific speed settings and cannot be continuously varied.
This input informs the program how power should be calculated with Discrete control using time-weighted averaging and Variable control using flow-weighted averaging.
Tip: For constant volume or on-off fans use 2-Discrete with Number of speeds set to 1. For variable air volume fans use 1-Continuous, in which case, a fan performance curve must be selected for Electric power function of flow fraction curve.
This setting is only available when the Speed control method is set to 1-Continuous. It is used to describe how low a variable speed fan can be operated. This value is used to calculate the fan power at low flow rates but it does not enforce a lower end of the air flow during simulation. The value is a fraction of the Maximum flow rate and should be between 0 and 1.
Select the performance curve that describes how electric power consumption varies with air flow rate. The independent “x” variable of the performance curve is a normalized flow fraction defined as the current flow rate divided by the Maximum flow rate. The model actually uses the ratio of (moist) air mass flow rates with the numerator taking account of humidity and barometric pressure. The dependent variable that is the result of the performance curve or lookup table is a fraction that is multiplied by the Design electric power consumption to determine the electric power use as a function of flow rate. Any of the single-independent-variable curves can be used.
This field is available if the Speed control method is set to 1-Continuous. It is also used when the Speed control method is set to 2-Discrete and the Number of speeds is greater than 1 and Electric power fraction from curve is checked.
This numeric field is used to specify the number of different speed levels available when Speed Control Method is set to Discrete. This field and the remaining field sets are not used when the Speed control method is set to 1-Continuous. For a constant volume fan enter a value of 1.0. A value of 1.0 will use the fan’s maximum design and no additional field sets are needed. When set to a value greater than 1 then a pair of flow and power fraction inputs are provided for each speed in the remaining input fields.
Check this checkbox is available when Speed control method is set to 2-Discrete and the Number of speeds is greater than 1. In this case checking this checkbox indicates that EnergyPlus should calculate the electric power fraction for each discreet speed.
Enter a pair of values for the flow fraction and electric power fraction for each speed. The sets should be defined in increasing order so that the flow fractions become larger in subsequent field sets. Typically the highest speed level will match the design maximum and have fractions of 1.0.
This is the flow fraction for the fan speed. It is multiplied by the Maximum flow rate to obtain the flow rate when operating at this speed.
This is the electric power fraction for the fan speed. It is multiplied by the Design electric power consumption to obtain the power consumption when operating at this speed. This field is available only if the Electric power fraction from curve checkbox is unchecked. Otherwise, if it is checked, the power at this speed will be determined using the performance curve selected for Electric power function of flow fraction curve.
This is the Schedule that defines whether the fan can run during a given time period. A schedule value greater than 0 (usually 1 is used) indicates that the fan can be on during a given time period. A value less than or equal to 0 (usually 0 is used) denotes that the fan is off.
This setting is non-editable in cases where the operation availability schedule is defined by the parent HVAC component (e.g. AHU).