Application

Room Segment Functions
Device Control Functions HVAC
58
Siemens Industry, Inc.
Application Note
A6V11692055
Smart Infrastructure – Building Products
2019-04-01
⚫ 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.
Pressure Dependent Control - Low airflow
If the box cannot satisfy airflow demands due to not having enough available
airflow (or the airflow sensor object is invalid), then open loop operation begins. In
open loop operation, damper position is controlled without using the measured air
volume flow value from the sensor; instead, damper position = airflow setpoint in %
of nominal airflow.
When the air volume flow setpoint falls below 5% of nominal or the measured air
volume flow value rises above 7% of nominal, closed loop operation resumes.