Application

Room Segment Functions
Device Control Functions HVAC
58
Siemens Industry, Inc.
Application Note
A6V11692053
Smart Infrastructure – Building Products
2019-04-01
Dual Duct Air Volume Flow Calculation
Air Volume Flow (VavSuAirFl1, VavSuAirFl2) and Relative Air Flow
(VavSuAirFlRel1, VavSuAirFlRel2) are calculated values computed from logic that
utilizes the following:
⚫ Separate Velocity pressure from the differential pressure sensor or other flow
sensing technology
⚫ Units conversion constant to convert to air velocity
⚫ Flow coefficient value for calibration at a specific airflow setpoint
⚫ Duct area in units compatible with the units conversion constant
⚫ Nominal airflow value (user entered)
⚫ Hood air volume flow value to aid in calculating the correct flow coefficient
value
First, air velocity is a calculated value computed from logic using the following
formula:
Where:
⚫ AirVEngrgUnits is the velocity of the air in engineering units that are compatible
with the other units used in the airflow formulas above
⚫ KEngrgUnits = constant for engineering unit correction [the “4005” (English
units) or “1.29” (SI units)
⚫ CSpanCal = constant to calibrate for sensor span (also called flow coefficient)
⚫ Pdifferential = differential pressure sensor reading from duct
Next, Air Volume Flow in flow units (VavSuAirFl) is calculated by the formula:
Where:
⚫ Area = area of duct
⚫ FlCnv = conversion constant for converting between box dimensional
engineering units and desired flow engineering units
Air Volume Flow as a percentage of the nominal airflow (Relative Air Flow
[VavSuAirFlRel]) is calculated by the formula:
Where:
⚫ AirFlNom is the nominal air volume flow of the VAV terminal box.