Application

Room Segment Functions
Device Control Functions HVAC
57
Siemens Industry, Inc.
Application Note
A6V11692042
Smart Infrastructure – Building Products
2019-04-01
Nominal Air Volume Flow
Nominal air volume flow refers to the "nominal" flow rating of the terminal box, the
maximum flow for which it is rated.
Default value of AirFlNom
⚫ Standard preloaded VAV / FPB templates: AirFlNom default = 1200 ft3/min
⚫ Raw factory value in unconfigured VAV or FPB type: AirFlNom default = 100
m3/h (59 ft3/min)
⚫ Lab (CET) type: see Lab documentation
(Dual Duct standard preloaded templates)
AirFlNom is used to define the scaling value that converts physical flow values to
their relative (percent) flow values. When AirFlNom is used, all of the application’s
air volume flow percent calculations are based on the value of AirFlNom. This
means that the value of AirFlNom - when used - is crucial for the proper
functioning of the automation station’s air volume flow control algorithms.
Dual Duct applications have the option of setting AirFlNom to zero, in which case it
is not used. See the following guidelines for specific information.
CAUTION
Duct size must be considered when setting AirFlNom
For all application version numbers, when AirFlNom is used (i.e., set to a non zero
value), duct size must be considered.
Example: a 6-inch round duct (approx 0.2 sq ft) with a typical maximum velocity
of 3000 ft/min suggests a nominal air volume flow of 600 cfm (3000 ft/min * 0.2
sq ft = 600 cfm (approximately 300 lps)).
In this example, AirFlNom must not be left at the preloaded template default
value of 1200 cfm! It requires a different value following the guidelines above.
(AirFlNom is not a BACnet object. It is a configuration parameter that can only be
set using ABT Site or ABT-SSA.)
Air Volume Flow Setpoint Calculation
In control mode the airflow setpoints (VavSuSpAirFl1 and VavSuSpAirFl2) are
configured to provide cooling or heating demand and the configured ventilation
demand.
Although request values are in percent, the comparison to select the max is made
in physical airflow units. This is because the airflow limits (minimum flow and
maximum flow) are different for each request type.
The mapping from percent to airflow units for cooling and ventilation includes a
step-up at the low end, with a switch on point of 4% and a hysteresis of 2%. Thus,
the request percent must rise above 4% before its corresponding airflow value rises
from 0 to the request’s minimum airflow.