Venturi Air Valve or Blade Damper

7/29/2019 Venturi Air Valve or Blade Damper

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Technology Report

September, 2008

Siemens Industry, Inc. Page 1 of 8

Venturi Air Valve or Single-Blade Damper

Whats Right for You?

This report examines and compares the attributes ofa Venturi air valve and a single-blade damper toassist a ventilation system designer.

Single-blade dampers have been on the scene forgenerations and are probably one of the earliestHVAC control devices. They were initially used to

manually adjust the chimney draft of wood and coalburning stoves. This made it possible to regulate thestove's heat output. Blade dampers were latercoupled with automatic operating mechanisms forvarious HVAC control applications and continue tobe widely used, especially to control ventilationsystems.

Single-blade dampers function by simple rotationand perhaps provide the least obstruction to airflowwhen in the maximum open position. Round bladedampers are sometimes called butterfly dampers.

This term may be better reserved for dampers thatfunction by a folding and unfolding action (much likea butterfly's wings).

The Venturi air valve is a newer airflow controldevice with a more elaborate mechanical design.

Mechanical Operation

Figure 1 shows the basic mechanical operation of

each device. The single-blade damper consists of adisk mounted on a shaft. As the shaft rotates, thedisk blocks more or less of the air path. This ruggedarrangement allows gradual adjustment of the flowarea from nearly blocked to almost fully open. AVenturi air valve has a curved body that functions asa valve seat, and a cone that moves in and out ofthe throat of the Venturi to restrict airflow.

When used with a suitable airflow controller, either aVenturi air valve or single-blade damper caneffectively modulate airflow.

THE ACTUATOR SHAFT IS CONNECTED TOTHE CONE SHAFT BY A LEVER ARM.

AS THE ACTUATOR SHAFT EXTENDS OR RETRACTSIT CAUSES THE LEVER ARM TO MOVE THE HORIZINTALCONE SHAFT. MOVEMENT OF THE CONE SHAFT VARIESTHE AIRFLOW AREA BETWEEN THE CONE AND THEVENTURI BODY.

ACTUATOR

ACTUATOR SHAFT

DAMPER SHAFTCRANK ARM

THE ACTUATOR SHAFT IS CONNECTED TOTHE DAMPER SHAFT CRANK ARM.

AS THE DAMPER SHAFT EXTENDS ORRETRACTS IT ROTATES THE BLADEDAMPER WHICH VARIES THE AIRFLOW

AREA BETWEEN THE DAMPER BLADEAND 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.

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Mechanical PressureIndependence

The brief description of the damper operationskipped over an interesting characteristic of theVenturi air valve.

In many valves, the actuator shaft does not directlymove the cone. Instead, they are connected by aspecial spring. This gives the cone some freedom tomove along the shaft. The spring exerts a force onthe cone, but so does the air that flows through thevalve. The cone slides along the shaft to the positionwhere the air pressure balances the spring.

Through this mechanical force balancing process,the Venturi air valve can be made pressure

independent. That is, as pressures change in theduct system, the cone moves on the shaft, alteringthe airflow path, counteracting the pressure change,and tending to keep that airflow rate constant. Thisbehavior depends on a careful mechanical designthat matches the characteristics of the speciallydesigned, variable-stiffness spring to the shapes ofthe cone and the Venturi body.

EXPANDED SP RING --INCREASED AIRFLOW AREA

LOWERSTATIC

PRESSURE

COMPRESSED SPRI NG --DECREASED AIRFLOW AREA

HIGHERSTATIC

PRESSURE

AIRFLOWREMAINS

CONSTANT

Figure 2. Pressure Independence by the VenturiAir Valve Pressure Compensat ion Spring.

Figure 2 illustrates action of the cone and spring.The upper diagram shows, that when a lower staticpressure acts on the upstream side of the cone, thespring is only slightly compressed, and the cone sitsrelatively far out of the throat of the valve. The

resulting airflow area between the cone and Venturibody allows the required airflow rate.

The lower diagram shows what happens when the

duct pressure increases. Pressure on the upstreamside of the cone pushes the cone along the shaft(towards the throat) and compresses the spring. Thismovement of the cone restricts the airflow,countering the effect of the pressure increase. Theresult is that the airflow rate stays nearly constant.When the system static pressure decreases, thecone spring expands and slides the cone back(away from the throat). This increases the airflowarea and maintains the required airflow rate.

Clearly, this is a sophisticated mechanical device.Performance depends on carefully selected andmaintained mechanical parameters. Pressure

independent operation is effective over a range ofoperating pressures specified for the valve (typically0.6 in. WC to 3.0 in. WC, 150 Pa to 750 Pa).

Stability of this non-linear, spring-mass systemdepends on the shock absorbing effect of the dashtube (the hollow core of the cone) and the conebushing (a spacer that supports the small end of thecone on the shaft). As the cone moves along theshaft, it squeezes air through the precise openingbetween the bushing and the dash tube wall. Thisshock absorber dissipates energy and keeps thecone from bouncing continually on its spring. Criticalmechanical tolerances allow the cone sufficientfreedom for motion with sufficient damping.

Air flow Control Concepts

Closed Loop Control: The most common approachto airflow control in a ventilation system is the closedloop, also called feedback control. By definition,each adjustment by a closed loop controller dependson the measured results of previous movements.

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As the flow controller adjusts the damper (single-blade, Venturi, or other) it also reads an airflowsensor to measure the flow rate and compare it to

the desired value (called the setpoint).

If the duct pressure changes, and affects the airflow,the controller measures that change and quicklyadjusts the damper opening, continuing to senseairflow until the required rate is restored. This is theusual way to accomplish pressure independent flowcontrol.

1. 2005 ASHRAE Handbook - Fundamentals of Control,Chapter 15, Page 15.1.

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Open Loop Control: Another approach uses aVenturi air valve as a simple metering device withoutmeasuring airflow. In this scenario, the Venturioperates as an open loop flow controller. This

concept is valid as long as the system keeps thevalve within the operating range of the spring andcone mechanism.

In some installations, the Venturi assembly includesa sensor, which measures position of the actuatorshaft. This can be useful. That position signal issometimes mislabeled as flow feedback, but its notactually sensitive to airflow. In the open loop design,events in the duct system that change the airflow donot move the actuator, and are not reflected by theposition sensor. Operators and designers havemistaken the speed and stability of the positionsignal as a true indication of airflow, overlooking the

dynamic events that occur inside the valve, as thecone and spring continually seek the changingbalance point.

Most ventilation system designers recognize thevalue of an airflow measurement. Nearly allspecifications call for a flow feedback signal.Position of the actuator should not be construed tosatisfy that need.

Linearity of Control Components

Control engineers characterize components in termsof input/output responses. They consider dynamiccharacteristics such as speed and overshoot, as wellas the steady-state responses. If the steady-stateresponse of a component (for example, airflowversus damper position) can be described with astraight line, the component is called linear.

In the days before digital control, linearity wasgreatly beneficial to engineers piecing systemstogether from signal processing components.Product developers went to great lengths to linearizeresponses because it simplified system design.

Today, non-linearity in a component is easier toaddress in software.

The physics of airflow is full of non-linearity. Forexample, the relationship between the position of thecone in the valve, and the corresponding airflow at aparticular pressure depends on the complexgeometry of the cone and valve body, and is highlynon-linear. Similarly, the relationship between theposition of the actuator and the resulting position ofthe cone depends on the non-linear spring in thecone and the pressure forces in the duct.

Some manufacturers still linearize their valves byadding a non-linear electronic circuit to distort therelationship between the command voltage and theposition of the actuator. These days, that step is

usually unnecessary.

Air flow Control Accuracy

In a closed loop flow control application, accuracydepends mainly on the airflow sensor; thecharacteristics of the damper (blade or Venturi) havelittle effect. In such a case, the expression accuracyof the flow control damperdoesnt actually meananything.

Sensors are available for a wide range of accuracyrequirements. Its important to select sensorsappropriate to the application. That means relating

the sensing range and accuracy to the ventilationobjectives. There is no single accuracy specificationthat makes sense for all applications. It also meansconsidering the geometry and vulnerability of thecomponents in the air stream. In exhaust systemsits crucial to prevent fouling; a rugged geometry withminimum obstruction of the air path is preferred. Alow-profile orifice ring with large pressure taps isvery reliable.

AIR FL OW

VENTURI AIR VALVE

SINGLE BLADE DAMPER

AIR FLO W

AIRFL OWMEASUREMENT

SENSOR

Figure 3. Venturi Air Valve and Damper AirTerminals with Integrated Airflow Sensors.

In an open loop application, (a Venturi air valvewithout an airflow sensor) flow control dependsentirely on the reliability of the flow versus positionrelationship of the valve. This depends, in turn, onthe precisely coordinated mechanical parametersdescribed earlier. Deviations over time in themechanical parameters degrade open-loop

Siemens Industry, Inc. Page 3 of 8Document No. 149-985


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accuracy. The following issues are known to causemechanical parameters to degrade:

1. With continued use, springs exhibit somedeparture from their original spring rate curve

due to material aging and fatigue. Therefore,mechanical pressure independence is not asprecise, nor will it provide the long-term stabilitythat is achieved with closed loop flow control.


2. Reliable pressure independent operationdepends on precise mechanical clearance insidethe cone, where the dash tube slides back andforth over the cone bushing. Material from the airstream is normally deposited on exposedsurfaces in the valve, and over time, can workinto this critical area. Figure 4 shows thecondition of several air valves removed fromservice. The center left image shows the cone

assembly covered in dust. The bottom left imageshows the cone bushing has been damaged bydeposited material. Because performance is sosensitive to the condition of these frictionsurfaces, a clean air stream improves reliability.

Figure 4. Venturi Air Valves Fouled During Use.

3. Proper valve orientation (horizontal or vertical) isvery important for mechanical pressureindependence. In the vertical position, theweight of the cone has an impact on thepressure compensation spring. Thus, the properspring must be installed in the factory

corresponding to the position (horizontal orvertical) in which the valve will be installed.

Therefore, it is mandatory that a Venturi airvalve is installed in the proper horizontal or

vertical position and not in a slanted (angular)position.

4. The air valve is not only susceptible to fouling ofthe precise friction surfaces. Grosscontamination can also upset the open looprelationship of flow versus position. The imagesin the top and bottom right ofFigure 4 illustratethe sort of debris that an air valve can catch inan exhaust system (in this case, large debris).

These valves were removed from service after itwas observed that the airflow did not seem right.

The actuator position signal did not indicate anyproblem.

Flow and Pressure Characteristics

The fundamental selection criteria for a flow controldevice are the range of flow rates, andcorresponding pressure drops across the device.

These numbers (flow range and pressure range) area good starting point for a designer selecting adevice.

Its important to remember that they are not exactlycomparable. For the Venturi air valve, these valuesare the hard limits of the pressure independent

operation. Because these values are hard, physicallimits, prudent designers leave a cushion, and do notapply the valves right at the limit.

For a single-blade damper, the ranges are based ona more flexible set of engineering rating criteria. Itsalways possible to push more or less air through thedamper if the effects on the system are acceptable.

Table 1 defines each term and contrasts themeaning between a single-blade damper and aVenturi air valve.

Air Capaci tyBecause of the greater airflow area of a single-bladedamper type of air terminal, its airflow capacity issignificantly more than a Venturi air valve of thesame diameter. Since the maximum Venturi air valvesize is typically 12 inches in diameter, larger airflowsrequire having multiple units arranged in parallel(ganged together). Figure 5 indicates airflow rangesof Venturi air valves and single-blade air terminals ofvarious sizes.


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Table 1. Definition of Sizing Parameters.

Single-blade Damper Venturi Air ValveSizingParameter

definition typical value definition typical value

MaximumAirflow

A selected rating value. Abovethis point, system may beconsidered too loud or lose toomuch pressure.

Often selectedbetween 2,000 and3,000 fpm.

The flow rate approximatelymaintained by the springwhen the actuator is at theend of the stroke.

Typically occursat 1,700 to 1,900fpm.

MinimumAirflow

A selected rating value. Belowthis point, the flow sensor maybe inaccurate. Depends on thesensor and the requiredaccuracy.

Usually between 0and 500 fpm.

The flow rate approximatelymaintained by the springwhen the actuator is at theother end of the stroke.

Typically occursat 100 to 200fpm.

MaximumPressureDrop

Above this pressure, controlmaybecome difficult.

Dampers have beenapplied successfullyat 6 in. WC of drop.

At this pressure drop, thespring is fully compressed andcan no longer regulate airflow.

3 in. WC for allavailable Venturiair valves.

MinimumPressureDrop

Pressure measured across thefully open damper at a ratedflow.

Usually less than 0.1in. WC.

At this pressure drop, thespring is fully extended andcan no longer regulate airflow.

Usually 0.6 in.WC or 0.3 in. WCfor low pressurevalves.

FLOW RANGES OF TERMINAL UNITS & VENTURI AIR VALVES

0 1000 2000 3000 4000 5000 6000 7000 8000

TYPE

LGS_04

AV_106

LGS_06

AV_108

LGS_08

AV_110

LGS_10

AV_112

LGS_12

LGS_14

AV_210

LGS_16

AV_212

AV_312

LGS_18

FLOW (CFM)

STARTING FROMTU SHUT-OFF

to FLOW SIGNAL0.02" (~350FPM)

to 1000 FPM

Figure 5. Airf low Ranges for Venturi Air Valvesand Single-blade Dampers of Various Sizes.

Minimum Pressure DropMinimum pressure drop is an important parameter of

an airflow control device. It is the pressure dropacross a fully open device at a given airflow rate.

The minimum pressure drop (along with thepressure drop of the other system components)determines the static pressure that the supply andexhaust fans must provide to achieve the desiredsystem airflow.

The system pressure drop is an important factor infan energy consumption. Designers motivated by theGreen Building movement and sustainability strive toselect low-pressure components, so they favorsingle-blade dampers.

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Like the air capacity, the minimum pressure dropdepends on the size of the airflow area. Figure 6

compares of the airflow open area for a Venturi airvalve and a single-blade damper.

2. U.S. Department of Energy, Low-Pressure-Drop HVACDesign for Laboratories, DOE/GO-102005-2042 (February2005).

3. Siemens Building Technologies, Inc., Green Lab Facilities:Steps Toward Sustainability Technology Report, 149-488,(April 2008).



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SINGLE BLADE

DAMPER

VENTURI AIR

VALVE

AIRFLOWAREA

AIRFLOWAREA

(END VIEW) (END VIEW)

Figure 6. Maximum Airfl ow Areas for the WideOpen Position .

Figure 6 shows that a blade damper's fully open

airflow area is much greater in comparison to a fullyopen Venturi air valve of the same diameter. A fullyopen single-blade dampers airflow area equals theinternal duct area less the area occupied by thedamper shaft. For a fully open Venturi air valve, theairflow area is limited by the diameter of the cone.

The airflow through a Venturi air valve also changesdirection as it flows around the cone. For thesereasons, a Venturi air valve has a significantly higherminimum (wide open) pressure drop than a single-blade damper air terminal of the same diameter.

Typically, a Venturi air valve has a non-recoverableminimum static pressure drop of 0.6 in. WC (150Pa). Some manufacturers also offer low pressureVenturis with a drop of about 0.3 in. WC (75 Pa) Incontrast, the larger airflow area of a fully opensingle-blade damper results in a non-recoverableminimum static pressure drop of about 0.01 to 0.05in. WC (2.5 Pa to 12.5 Pa).

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Cost of Static Pressure LossThe effect of pressure loss on energy consumptioncan be complicated, but it doesnt have to be. Somelab control publicationshave confused the issue,perhaps unintentionally.

5With the right perspective,

it can be simple.

There are many mathematical ways to express thepower that a fan consumes in a ventilation system.For this purpose (calculating the effect of pressure


4. For single duct supply air terminals the pressure drop of thereheat coil must also be considered when selecting andsizing air terminals for a given application.

5. For more information, contact Systems Applications inBuffalo Grove.

losses for a given airflow rate) the following equationapplies:

EfficiencyFan

ssurePreFanAirflowPowerFan

This means fan power is directly proportional topressure loss: twice as much pressure consumestwice as much power.

If a fan system runs at 5 in. WC of pressure, and wecan save 0.5 in. WC by selecting more efficientterminals, that saves 10% of the fan power. If thesystem is more efficient (for example, 3.0 in. WC)the percentage savings achievable at the terminalsis even greater.

Some lab control publications mistake the valvepressure drop for the:

1. Static pressure measured at the terminal. Ifa system runs with 0.5 in. WC (125 Pa) atthe terminal, that includes the drop acrossother components, not just the valve.

2. Signal pressure generated by the airflowsensing element. The sensing pressure isnot a loss in the system, and is often manytimes greater than the drop across thevalve.

Air flow Sound

When airflow through a device causes a pressure

drop, that energy is dissipated as heat and sound.The heat component of this energy transformationcauses a slight rise in the air temperature flowingthrough the device, but this is usually small and isdisregarded for practical purposes. However, thesound component can be significant and annoying.

Therefore, sound ratings are an importantconsideration when choosing air terminals. A Venturiair valve terminal will typically create somewhatmore discharge and radiated sound power for agiven airflow than a single-blade damper type of anair terminal due to its greater pressure drop for agiven airflow.

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6. Determining the resulting room sound level caused by HVACcomponent sound can be a very complex matter. Aside fromthe air terminal sound, many other factors affect the room'sambient sound level. For additional insight into ventilationrelated sound, see the Technology Report No. 5Attaining

Acceptable Ventilation Related Sound in Laboratory Rooms(149-979) and to Siemens comprehensive Application GuideMinimizing Excessive Sound in Ventilation System Designs(125-1929).


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Airflow Control Loop Performance

Performance of an airflow control system can beevaluated without regard to the type of components.

Figure 7 displays dynamic airflow data from two

chemical fume hoods, one with a Venturi air valveand the other with a single-blade damper. Each plotshows the airflow as a function of time when thefume hood sash is opened. Both hoods exhibit the

quick, stable flow control needed for safe laboratorywork.

0

100

200

300

400

500

600

8 10 12 14

time (seconds)

AirFlowR

ate(cfm)

0

100

200

300

400

500

600

8 10 12 14

time (seconds)

Figure 7. Dynamic Ai rflow Contro l by a Single-Blade Damper (L) and a Ventur i Air Valve (R).

Applications for MechanicalPressure Independence


A common application of mechanical pressureindependence is in constant air volume (CAV)ventilation systems. Constant airflow can bemaintained in critical parts of the CAV system byapplying Venturi air valves with pressureindependent capability. This eliminates the need forseparate airflow controllers and valve actuators and,thus, reduces the overall control system cost. CAVpressure independent applications include CAVfume hood exhaust, biological safety cabinetexhaust, laboratory room specialty equipmentexhaust, lab bench exhausts (snorkels), and eventhe overall room ventilation supply and general

exhaust airflow.

In pressure independent applications, the Venturi airvalve is used without an actuator and with the leverarm in a fixed position. (See Figure 1 for adescription of the Venturi air valve components.) Toobtain the required airflow, the lever arm must bemanually positioned until the required airflow is

attained and then the lever arm can be locked intoposition by a wing nut or a similar means.

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ConclusionThis report explains the issues surrounding use ofthe Venturi air valve and the single-blade damper asan airflow control device. The Venturi air valve is amore intricate mechanical device compared to thesimple damper. Both are successfully applied ascomponents high performance ventilation systems.

Table 2 compares the major attributes of a Venturiair valve and a single-blade damper r.

The performance achieved in a particular applicationis primarily a function of the overall airflow controlsystem that includes, and depends on, the controllercapability. Performance being equal, owners andengineers should base the decision of Venturi airvalve versus single-blade damper on specificapplication, economical, and safety needs.Regardless of the end device, knowledge of actualairflow is important for informed operation. Athoroughly effective design includes airflow sensors.

7. The airflow through the Venturi air valve must be measuredduring the setup process in order to establish the properlever arm position.


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Product or company names mentioned herein may be the trademarks of their respective owners. 2009 Siemens Industry, Inc.

Siemens Industry, Inc. Printed in the USABuilding Technologies Division Document No. 149-9851000 Deerfield Parkway Page 8 of 8Buffalo Grove, IL 60089-4513USA

Table 2. Comparison of Major Air Terminal Characteristics.

Att ribute Ventur i Ai r Valve Single-Blade Damper

Physical Configuration More complex physical configuration with moreoperating components. Higher airflows requiremultiple ganged units, which takes up more space,increases the unit cost, and requires more ducttransitions.

Very simple physical configuration with minimaloperating components only two damper shaftpivot points. Duct transitions and the installationare easier especially when higher airflows arerequired.

Airflow Capacity Lower maximum airflow due to less airflow area fora given size (diameter) of air terminal. Higherairflow capacities require multiple (parallel ganged)units.

Higher maximum airflow (70% to 100% greaterthan the Venturi air valve) for the same size(diameter) air terminal due to the large airflow area.

Cost Higher cost per unit. Lower cost per unit.

Mechanical PressureIndependence

Yes (within a specific static pressure range) No

Control Curve Non-Linear (equal percentage) airflow controlaction. (Linear control is accomplished by applyinga controller with closed loop control capability or

the unit must be factory calibrated for use with anopen loop controller.)

Non-linear (quick opening) airflow control action.(Linear control output is accomplished by applyinga controller with closed loop control capability.)

Control Accuracy Nominally +/- 5% as an open loop device Depends on the airflow sensor; which should beselected according to the application

Control Turndown 8-to-1 with closed loop control.From 10-to-1 through 16-to-1 without closed loopcontrol.

Limited by leakage around damper seal.Sometimes reaches 20-to-1.

Minimum Pressure Drop 0.30 in. WC (75 Pa) for low pressure model.

0.60 in. WC (750 Pa) for medium pressure model.

(This pressure drop is required to enable themechanical pressure independent function tooperate.)

Pressure drop is very low (0.01 to 0.05 in. WC or2.5 to 12.5 Pa) with the damper wide open.

(No minimum pressure drop is required foroperation.)

Sound Generation Somewhat higher than a round blade damper for

the same airflow and pressure drop.

Somewhat lower than a Venturi air valve for the

same airflow and pressure drop.Materials available to resistcorrosion

Spun aluminum outer shell with Hersite

andTeflon

protective coatings are available for the

shell, cone and shaft.

Multiple materials available for the entire unit:Galvanized steel, Type 316L Stainless Steel and

Teflon

coating.

Susceptibility to the effects ofchemical or airborne particulate

Particulate accumulation on the internal surfaceswill degrade the self-contained (non-closed loop)pressure independent function. External closedloop control is required to prevent degradedperformance.

Particulate accumulation will not degrade theclosed loop control action.

Installation Requirements Each unit must be installed in either the horizontalor vertical orientation in which it was factorycalibrated. Higher airflow (ganged) units arecumbersome, heavy, and more difficult to install.

Units can be installed in any orientationvertical,horizontal, or angular. Higher airflow units are lesscumbersome.

Maintenance No routine maintenance required.* No routine maintenance required.*

* All laboratory ventilation safety standards require periodic (typically on an annual basis) inspections of a laboratory ventilation system'sperformance.

Documents

Venturi Air Valve or Blade Damper