Fan - On/Off

The On/off fan models a constant air volume fan that is intended to cycle on and off in tandem with a cooling or heating system (i.e., AUTO fan control mode). The fan can also operate continuously like a Constant volume fan. If modelling continuous operation and this object is used as part of a system that utilizes Coil:Heating:Gas, Coil:Cooling:DX:SingleSpeed or Coil:Heating:DX:SingleSpeed, the user should confirm proper air flow rates (coil and fan max load fraction is less than or equal to 1 for all values of coil part-load ratio).

Multi-speed fan operation can also be modelled when the On/off fan is included as part of a compound object that allows multiple fan speeds (e.g.,Unitary Heat Cool, PTAC, etc.). In this case, the ratio of the compound object air flow rate to the fan’s maximum air flow rate is used to determine the power at alternate fan speeds. The input for Fan Power Ratio Function of Speed Ratio Curve Name must be entered to model multi-speed fan operation. An optional fan total efficiency ratio curve is also available to model efficiency differences at alternate fan speeds.

Name

A unique system assigned name for an instance of an On/off fan. Any reference to this fan by another object will use this name.

Fan total efficiency

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. The motor efficiency is the power delivered to the shaft divided by the electrical power input to the motor. The impeller efficiency is the power delivered to the air divided by the shaft power. The power delivered to the air is the mass flow rate of the air multiplied by the pressure rise divided by the air density. This efficiency must be between 0 and 1.

Pressure rise

The pressure rise (in Pascals or inH2O) at full flow and standard (sea level) conditions (20°C and 101325 Pa).

The required pressure rise across an AHU fan is dependent on the duct network supplied by the AHU. For example, if a duct network was sized using a constant pressure loss of 1 Pa/m and the index run of the network (route of highest pressure drop) was say 300 m and the fitting losses were say an additional 20%, the required pressure rise across the fan would be 300 x 1 x 1.2= 360 Pa.

End-use subcategory

Allows you to specify a user-defined end-use subcategory, e.g., "Central System", etc. A new meter for reporting is created for each unique subcategory (ref: Report Meter). Subcategories are also reported in the ABUPS table. If this field is omitted or blank, the fan will be assigned to the "General" end-use subcategory.

Maximum flow rate

The full load air volumetric flow rate (m3/sec or ft3/min) at standard temperature and pressure (dry air at 20°C dry-bulb). EnergyPlus uses local barometric pressure to account for altitude using equation for "standard atmospheric" pressure on p 6.1 of the ASHRAE 1997 HOF (SI edition) to initialize the air systems being simulated.

p=101325*(1-2.25577E-05*Z)**5.2559

where p=pressure in Pa and Z=altitude in m

Motor efficiency

The shaft power divided by the electrical power consumed. Must be between 0 and 1.

Motor in airstream fraction

The fraction of the motor heat that is added to the air stream. A value of 0 means that the motor is completely outside the air stream. A value of 1 means that all of the motor heat loss will go into the air stream and act to cause a temperature rise. Must be between 0 and 1.

Fan power ratio function of speed ratio curve

Select an Exponent Performance curve. This curve is used to simulate multi-speed fan motors, representing the ratio of actual fan power to rated fan power when a change in fan speed occurs.

Fan efficiency ratio function of speed ratio curve

Enter the name of a Quadratic or Cubic Performance curve. This curve is used to simulate multi-speed fan motors, representing the ratio of actual fan total efficiency to rated fan total efficiency when a change in fan speed occurs.

Operation

Availability schedule

Select 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.

Algorithm

The on/off fan model is similar to the simple constant volume fan model with the exception that the on/off fan may cycle on and off during a simulation time step. The cycling rate of the fan is known as the run time fraction. The calculation of run time fraction accounts for the part-load losses of other equipment used in the HVAC system. A part-load factor (a.k.a. part-load ratio) is first calculated for the fan as the ratio of the actual operating mass flow rate to the maximum fan mass flow rate. The run time fraction is then calculated as the part-load factor divided by the part-load fraction. The part-load fraction is determined by other HVAC equipment in the simulation.

f_flow = m / m_max

RTF = f_flow / PLF

The total fan power is then calculated as the maximum fan power multiplied by the run time fraction.

Q_tot = RTF.[m.δP/(e_tot.ρ_air)]

Q_shaft = e_motor.Q_tot

Q_toair = Q_shaft + (Q_tot - Q_shaft).f_motortoair

h_out = h_in + Q_toair / m

w_out = w_in

T_out = PsyTdbFnHW(h_out,w_out)

Nomenclature:

f_flow is the flow fraction or part load ratio

f_motortoair is the fraction of motor heat generated passed into the air stream

Q_tot is fan power in W

m is mass flow in kg/s

m_max is the maximum or design mass flow in kg/s

RTF is runtime fraction

PLF is part load factor

δP is the design pressure increase

e_tot is fan total efficiency

e_motor is the motor efficiency

ρ_air is air density at standard conditions (kg/m3)

Q_shaft is the fan shaft power in W

Q_toair is the power entering the air in W

h_out,h_in are the outlet and inlet specific enthalpies in J/kg

w_out,w_in are the inlet and outlet air stream humidity ratios

PsyTdbFnHW is the EnergyPlus psychrometric routine relating enthalpy and humidity ratio to temperature.

Fan - On/Off Outputs

HVAC,Average,Fan Electric Power[W]

HVAC,Average,Fan Delta Temp[C]

HVAC,Sum,Fan Electric Consumption[J]

HVAC,Average,On/Off Fan Runtime Fraction

Fan Electric Power [W]

This output field contains the average electricity consumption rate for the fan in Watts for the timestep being reported.

Fan Delta Temp [C]

This output field contains the average rise in air temperature across the fan (outlet air temperature minus inlet air temperature) (in °C or °F)elsius for the timestep being reported.

Fan Electric Consumption [J]

This output field contains the electricity consumption of the fan in Joules for the timestep being reported. This output is also added to a report meter with Resource Type = Electricity, End Use Key= Fans, Group Key= System (ref. Report Meter).

On/Off Fan Runtime Fraction

This output field contains the fraction of time that this fan operated for the timestep being reported