Fan - Constant Volume

This object models a constant air volume fan that is intended to operate continuously based on a time schedule. This fan will not cycle on and off based on cooling/heating load or other control signals.

General

Name

A unique auto-assigned name for the Fan. Any reference to this fan by another object will use this name.

Type

The type of the constant volume fan is fixed as:

• 2-Constant volume

Note: To change from constant volume to variable volume you must open the parent AHU dialog and change the AHU Fan type to 2-Variable volume. To allow the AHU Fan type to be edited you must first disconnect the demand side of the air loop from all zone ADUs by deleting connecting branches.

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.

Flow Rates

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

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.

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 way that fan and motor efficiency values affect the condition of the air at the fan outlet is given by the following equations:

Q_tot = 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_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

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 - Constant Volume Outputs

HVAC,Average,Fan Electric Power[W]

HVAC,Average,Fan Delta Temp[C]

HVAC,Sum,Fan Electric Consumption[J]

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).