Basic Documentation

Airflow Control Loop Performance
Performance of an airflow control system can be
evaluated without regard to the type of components.
Figure 7 displays dynamic airflow data from two
chemi
cal fume hoods, one with a Venturi air valve
and the other with a single-blade damper. Each plot
shows the airflow as a function of time when the
fume hood sash is opened. Both hoods exhibit the
quick, stable flow control needed for safe laboratory
work.
0
100
200
300
400
500
600
8 101214
time (seconds)
Air Flow Rate (cfm)
0
100
200
300
400
500
600
8101214
time (seconds)
Figure 7. Dynamic Airflow Control by a Single-Blade Damper (L) and a Venturi Air Valve (R).
Applications for Mechanical
Pressure Independence
Siemens Industry, Inc. Page 7 of 8
Document No. 149-985
A common application of mechanical pressure
independence is in constant air volume (CAV)
ventilation systems. Constant airflow can be
maintained in critical parts of the CAV system by
applying Venturi air valves with pressure
independent capability. This eliminates the need for
separate airflow controllers and valve actuators and,
thus, reduces the overall control system cost. CAV
pressure independent applications include CAV
fume hood exhaust, biological safety cabinet
exhaust, laboratory room specialty equipment
exhaust, lab bench exhausts (snorkels), and even
the overall room ventilation supply and general
exhaust airflow.
In pressure independent applications, the Venturi air
valve is used without an actuator and with the lever
arm in a fixed position. (See Figure 1 for a
descri
ption of the Venturi air valve components.) To
obtain the required airflow, the lever arm must be
manually positioned until the required airflow is
attained and then the lever arm can be locked into
position by a wing nut or a similar means.
7
Conclusion
This report explains the issues surrounding use of
the Venturi air valve and the single-blade damper as
an airflow control device. The Venturi air valve is a
more intricate mechanical device compared to the
simple damper. Both are successfully applied as
components high performance ventilation systems.
Table 2 compares the major attributes of a Venturi
air valve and
a single-blade damper r.
The performance achieved in a particular application
is primarily a function of the overall airflow control
system that includes, and depends on, the controller
capability. Performance being equal, owners and
engineers should base the decision of Venturi air
valve versus single-blade damper on specific
application, economical, and safety needs.
Regardless of the end device, knowledge of actual
airflow is important for informed operation. A
thoroughly effective design includes airflow sensors.
7. The airflow through the Venturi air valve must be measured
during the setup process in order to establish the proper
lever arm position.