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Technology Report

September, 2008

Siemens Industry, Inc. Page 1 of 8

Venturi Air Valve or Single-Blade Damper

What’s Right for You?

This report examines and compares the attributes of

a Venturi air valve and a single-blade damper to

assist a ventilation system designer.

Single-blade dampers have been on the scene for

generations and are probably one of the earliest

HVAC control devices. They were initially used to

manually adjust the chimney draft of wood and coal

burning stoves. This made it possible to regulate the

stove's heat output. Blade dampers were later

coupled with automatic operating mechanisms for

various HVAC control applications and continue to

be widely used, especially to control ventilation

systems.

Single-blade dampers function by simple rotation

and perhaps provide the least obstruction to airflow

when in the maximum open position. Round blade

dampers are sometimes called butterfly dampers.

This term may be better reserved for dampers that

function by a folding and unfolding action (much like

a butterfly's wings).

The Venturi air valve is a newer airflow control

device with a more elaborate mechanical design.

Mechanical Operation

Figure 1 shows the basic mechanical operation of

each device. The single-blade damper consists of a

disk mounted on a shaft. As the shaft rotates, the

disk blocks more or less of the air path. This rugged

arrangement allows gradual adjustment of the flow

area from nearly blocked to almost fully open. A

Venturi air valve has a curved body that functions as

a valve seat, and a cone that moves in and out of

the throat of the Venturi to restrict airflow.

When used with a suitable airflow controller, either a

Venturi air valve or single-blade damper can

effectively modulate airflow.

THE ACTUATOR SHAFT IS CONNECTED TO

THE CONE SHAFT BY A LEVER ARM.

AS THE ACTUATOR SHAFT EXTENDS OR RETRACTS

IT CAUSES THE LEVER ARM TO MOVE THE HORIZINTAL

CONE SHAFT. MOVEMENT OF THE CONE SHAFT VARIES

THE AIRFLOW AREA BETWEEN THE CONE AND THE

VENTURI BODY.

ACTUATOR

ACTUATOR SHAFT

DAMPER SHAFT

CRANK ARM

THE ACTUATOR SHAFT IS CONNECTED TO

THE DAMPER SHAFT CRANK ARM.

AS THE DAMPER SHAFT EXTENDS OR

RETRACTS IT ROTATES THE BLADE

DAMPER WHICH VARIES THE AIRFLOW

AREA BETWEEN THE DAMPER BLADE

AND THE DUCT HOUSING.

ACTUATOR SHAFT

LEVER ARM

CONE SHAFT

VENTURI BODY

CONE

ACTUATOR

AIRFLOW AREA

VENTURI AIR VALVE

SINGLE BLADE DAMPER

DUCT HOUSING

AIRFLOW AIRFLOW

Figure 1

. Basic Operation of a Venturi Air Valve and Single-Blade Damper Air Terminal.

Document No. 1

49-985

Summary of content (8 pages)

PAGE 1
Technology Report September, 2008 Venturi Air Valve or Single-Blade Damper What’s Right for You? This report examines and compares the attributes of a Venturi air valve and a single-blade damper to assist a ventilation system designer. The Venturi air valve is a newer airflow control device with a more elaborate mechanical design. Single-blade dampers have been on the scene for generations and are probably one of the earliest HVAC control devices.
PAGE 2
resulting airflow area between the cone and Venturi body allows the required airflow rate. Mechanical Pressure Independence The brief description of the damper operation skipped over an interesting characteristic of the Venturi air valve. In many valves, the actuator shaft does not directly move the cone. Instead, they are connected by a special spring. This gives the cone some freedom to move along the shaft. The spring exerts a force on the cone, but so does the air that flows through the valve.
PAGE 3
Open Loop Control: Another approach uses a Venturi air valve as a simple metering device without measuring airflow. In this scenario, the Venturi operates as an open loop flow controller. This concept is valid as long as the system keeps the valve within the operating range of the spring and cone mechanism. In some installations, the Venturi assembly includes a sensor, which measures position of the actuator shaft. This can be useful.
PAGE 4
accuracy. The following issues are known to cause mechanical parameters to degrade: 1. With continued use, springs exhibit some departure from their original spring rate curve due to material aging and fatigue. Therefore, mechanical pressure independence is not as precise, nor will it provide the long-term stability that is achieved with closed loop flow control. 2.
PAGE 5
Table 1. Definition of Sizing Parameters. Sizing Parameter Single-blade Damper definition Venturi Air Valve typical value definition typical value Maximum Airflow A selected rating value. Above this point, system may be considered too loud or lose too much pressure. Often selected between 2,000 and 3,000 fpm. The flow rate approximately maintained by the spring when the actuator is at the end of the stroke. Typically occurs at 1,700 to 1,900 fpm. Minimum Airflow A selected rating value.
PAGE 6
VENTURI AIR VALVE SINGLE BLADE DAMPER losses for a given airflow rate) the following equation applies: Airflow  Fan Pressure Fan Efficiency This means fan power is directly proportional to pressure loss: twice as much pressure consumes twice as much power. Fan Power  AIRFLOW AREA AIRFLOW AREA (END VIEW) (END VIEW) Figure 6. Maximum Airflow Areas for the Wide Open Position.
PAGE 7
Airflow Control Loop Performance Air Flow Rate (cfm) Performance of an airflow control system can be evaluated without regard to the type of components. Figure 7 displays dynamic airflow data from two chemical 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.
PAGE 8
Table 2. Comparison of Major Air Terminal Characteristics. Attribute Venturi Air Valve Single-Blade Damper Physical Configuration More complex physical configuration with more operating components. Higher airflows require multiple ganged units, which takes up more space, increases the unit cost, and requires more duct transitions. Very simple physical configuration with minimal operating components – only two damper shaft pivot points.