Application

Sequence of Operation
Venturi Application
15
Siemens Industry, Inc. Application Note 140-1344
Restricted 2015-11-09
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 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 from either 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 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, 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 CAL OK or a NOT CAL (EXH VLV STAT). To obtain a CAL
OK, at least 5 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 NOT CAL status for the
calibration.
This application has a table statement edit feature that allows you to view and edit the
voltage/flow values in the table. This is useful for fine tuning the air valve to meet
precise room flow setpoints and for diagnosing/editing problematic voltage/flow curves
(see Table
Venturi Air Valve Table Statement ).