Application

Sequence of Operation
Table Access Feature (Mode 1, 3)
18
Siemens Industry, Inc.
Application Note, App 2926
140-1308
2015-07-07
a)
These voltage/flow values constitute the “low flow”
element (or “point”) for the Supply and the General
Exhaust Venturi Air Valves. They are shown here
with factory default values. They are not altered
during calibration—they must be set manually.
However, they only need to be set if the Venturi Air
Valve is going go be operating at low flow settings of
less than 350 fpm. Otherwise, they can be left at
default and ignored.
NOTE:
The first pair (or “point”) of flow/voltage values in the table statement is the low flow
point. It is provided for low flow situations where airflow through the Venturi Air Valve
must be controlled at velocities less than 350 fpm. Otherwise, this point can be left at
factory default of 0 cfm and 0V (10V if exhaust) and ignored, as is the case in the
example table statement in the Table
Venturi Table Statement Example . See the
Table
Venturi Air Valve Table Statement for how to manually set this point.
If there is a low flow cfm value, it is taken either from the room schedule or the Venturi
Air Valve housing. Cubic feet per minute (cfm) flows in this range (where velocity
equals less than 350 fpm) and related voltages must be determined and/or confirmed
with help from a balancer. See the Table
Venturi Airflow @ 350 fpm for cfm flows
equal to 350 fpm. The following equation associates airflow to air velocity:
Airflow (cfm) = Velocity (fpm) × Duct Area (sq ft) × Flow Coefficient.
Venturi Airflow @ 350 fpm.
Valve Size in
Inches
cfm
5
48
6
69
8
122
10
191
12
275
Dual 10
380
Dual 12
550
Triple 12
825
During calibration, up to 15 voltage/flow values are automatically generated (the first
pair of voltage/flow values—the low flow point—is not generated; it must be set
manually). The Venturi Valve actuator is then fed the voltages and the application
reads the resulting airflows. At the end of calibration, the airflow readings are analyzed
and the calibration is either given a PASS or a FAIL (GEX VLV STAT, or the SUP VLV
STAT is set to PASS or FAIL). To obtain a PASS, at least 12 of the airflow readings
must increment correctly (the points in the table increase as the voltage increases). For
example, if one point on the voltage/flow curve shows 5V and 500 cfm and the next
point shows 6V and 450 cfm, the second point (6V, 450 cfm) would fail. But 6V at 550
cfm would pass. (This example assumes the actuator is direct acting, where more volts
equal more flow. Exhaust devices are usually reverse acting and have an inverse
voltage/flow relationship.) Too many failed airflow points along the voltage/flow curve
will result in a FAIL status for the calibration.