Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
The fundamental fan power calculation resides in the fan object and is based on user inputs for total efficiency, pressure rise, maximum flow rate, motor efficiency and motor in air stream fraction. These inputs, along with the flow rate entering the fan, and the flow ratio based on fan maximum flow rate, are used to calculate fan power.
PartLoadRatio = MassFlow / MaxAirMassFlowRate
FanRuntimeFration = PartLoadRatio / OnOffFanPartLoadFraction
FanPower = MaxAirMassFlowRate * FanRuntimeFraction * DeltaPress / ( FanEff * RhoAir )
As noted in the previous answer, there is additional information available to the Fan:OnOff object that can be passed from specific parent objects (namely the AirloopHVAC:Unitary*, and AirloopHVAC:UnitarySystems) which can account for fan curves (power vs speed ratio).
When the speed ratio curves is used with Fan:OnOff then:
PartLoadRatio = MassFlow / MaxAirMassFlowRate
FanRuntimeFration = PartLoadRatio / OnOffFanPartLoadFraction
FanRuntimeFraction = FanRuntimeFraction / SpeedRatio
SpeedRaisedToPower = FanPowerRatioAtSpeedRatio = f(SpeedRatio)
EffRatioAtSpeedRatio =FanEfficiencyRatioAtSpeedRatio = f(SpeedRatio)
FanPower = MaxAirMassFlowRate * FanRuntimeFraction * DeltaPress / ( FanEff * RhoAir )
FanPower *= SpeedRaisedToPower / EffRatioAtSpeedRatio
| | 2 | No.2 Revision |
The fundamental fan power calculation resides in the fan object and is based on user inputs for total efficiency, pressure rise, maximum flow rate, motor efficiency and motor in air stream fraction. These inputs, along with the flow rate entering the fan, and the flow ratio based on fan maximum flow rate, are used to calculate fan power.
PartLoadRatio = MassFlow / MaxAirMassFlowRate
FanRuntimeFration = PartLoadRatio / OnOffFanPartLoadFraction
FanPower = MaxAirMassFlowRate * FanRuntimeFraction * DeltaPress / ( FanEff * RhoAir )
As noted in the previous answer, there is additional information available to the Fan:OnOff object that can be passed from specific parent objects (namely the AirloopHVAC:Unitary*, and AirloopHVAC:UnitarySystems) which can account for fan curves (power vs speed ratio).
When the speed ratio curves is are used with Fan:OnOff then:
PartLoadRatio = MassFlow / MaxAirMassFlowRate
FanRuntimeFration = PartLoadRatio / OnOffFanPartLoadFraction
FanRuntimeFraction = FanRuntimeFraction / SpeedRatio
SpeedRaisedToPower = FanPowerRatioAtSpeedRatio = f(SpeedRatio)
EffRatioAtSpeedRatio =FanEfficiencyRatioAtSpeedRatio = f(SpeedRatio)
FanPower = MaxAirMassFlowRate * FanRuntimeFraction * DeltaPress / ( FanEff * RhoAir )
FanPower *= SpeedRaisedToPower / EffRatioAtSpeedRatio
