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Engineering Bulletin: Basics

IntroductionThis technical article addresses many of the topics regarding space air diffusion as it relates to the use ofTuttle & Bailey air outlets (supply registers and diffusers) and air inlets (return grilles and louvers). Basicprinciples of human comfort, space air diffusion, selection and location of air outlets and inlets, and sound are

all discussed in sufficient detail for the purpose of educating customers.

Basics of Space Air DiffusionHuman ComfortThe primary goal in designing commercial heating,ventilating, and air conditioning (HVAC) systems isachieving human (occupant) comfort. The factorsthat determine human comfort are numerous andcomplex, but by controlling certain key parameters,the occupied space can be made quite comfortable.These parameters include thermal comfort (spacetemperature, relative humidity, thermal radiation,and local air velocity), indoor air quality (air change

rate and fresh air delivered to the breathing zone),and acoustic quality (Noise Criteria or RoomCriteria). The activity and clothing level of theoccupants are also parameters but will be ignoredbecause the design engineer has no control overthese factors.

The focus of this article will be on the parametersthat are influenced by air outlets (supply registersand diffusers) and air inlets (return grilles andlouvers), specifically space temperature, local airvelocity, and acoustic quality.

Be aware that there are instances where humancomfort may not be the primary focus of the HVACsystem, such as industrial facilities, animal rooms,or clean rooms.

Space Air DiffusionWhere human comfort is the primary concern, thepurpose of space air diffusion is to introduceconditioned air into a building space without makingthe occupants feel uncomfortable. This can beaccomplished by designing the space with thecorrect number, type, and location of air outlets andair inlets. According to ASHRAE Standard 55-92

Thermal Environmental Conditions for HumanOccupants, the comfort limits for an occupied space(relative to space air diffusion) are:

Dry bulb temperature between 73 and 77F.

Methods of Space Air DiffusionThe most common type of space air diffusion iscalled a mixing system. A mixing system is one inwhich conditioned air is discharged at velocitiesmuch greater than those allowed in the occupiedzone. The occupied zone is the region within aspace, normally occupied by people, generallyconsidered to be between the floor and six feetabove the floor and more than two feet from walls orfixed air conditioning equipment. The supply airtemperature in a mixing system can be significantlyabove the occupied zone temperature for heating(typically +15 F), below the occupied zonetemperature for cooling (typically -15 F), or equal tothe occupied zone temperature for ventilation. Thesupply air jet is delivered by the air outlet and mixeswith room air by entrainment (induction), whichhelps to reduce the jet velocity and equalize thesupply air temperature as it enters the occupiedzone. The mixing system is the predominant methodof space air diffusion in the United States and iswidely used with variable air volume (VAV) systems.Tuttle & Bailey ceiling diffusers, high and lowsidewall supply grilles, and floor registers provide

excellent mixing air patterns, which ensures that thecold or warm supply air is sufficiently temperedbefore entering the occupied zone. See Figure 1.

Displacement ventilation systems are those in

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Maximum local air velocity in the occupiedzone of 50 fpm for cooling and 30 fpm forheating. (50 fpm is a little more than 1/2mph).

A maximum vertical temperature gradientof 6 F from the floor to six feet above thefloor.

which conditioned air is delivered directly into theoccupied zone. This is possible because the supplyair temperature is slightly lower than the room airtemperature (~65 F)

and air outlet face velocities are relatively low (lessthan 100 fpm), resulting in acceptable occupiedzone conditions. Air outlets are usually located lowon the sidewall, resulting in a flow of conditioned airacross the floor. The air then naturally rises as itabsorbs heat from plumes. Heat and contaminantsfrom the occupied zone are removed from the spacethrough air inlets located in the ceiling or highsidewall. Also, a stratification zone with little or noair motion is developed just above the occupiedzone. Displacement ventilation systems are popularin Scandinavia, where cooling loads are rather low.See Figure 2.

Certain applications require an environment with aunidirectional air pattern (a.k.a. laminar flow),such as a clean room or hospital operating room.The unidirectional pattern can either be suppliedfrom the ceiling and returned through the floor orsupplied through the floor and returned through theceiling (vertical), or supplied from one wall andreturned through an opposing wall (horizontal). Theair outlets are typically distributed evenly throughoutthe supply surface (ceiling, floor, or wall) andproduce a laminar flow pattern to removecontaminants by minimizing mixing. See Figure 3.

Another method of space air diffusion is calledlocalized ventilation or task-ambientconditioning. As with displacement ventilationsystems, localized ventilation systems supplyconditioned air directly into the occupied zone attemperatures slightly below room temperature(above 63 F). the difference is that the occupantoften has local control of the airflow rate and

air distribution plenum underneath the floor.Underfloor systems are often found in computerrooms or spaces where office layotus are frequentlychanged.

The topics discussed below are primarily directedtoward mixing systems but much of the informationcan be applied to the other systems as well.

Evaluating ComfortAs mentioned previously, certain parameters can becontrolled to ensure a relatively comfortableenvironment. The question is how does one

evaluate the sytem to make sure that the actualparameters are within the allowable range?Standard methods exist to evaluate theseparameters in real or simulated spaces to determineif the conditions are being met. The first evaluationconcept is called the effective draft temperature -

, which uses the local air temperature and velocityin a single equation to determine a relative feeling ofcoolness. The equation is:

The effective draft temperature is evaluated at manylocations throughout the space and comfort studieshave shown that the effects of temperature andvelocity on comfort will be acceptable if is keptbetween -3 and +2 F. TheAir DiffusionPerformance Index (ADPI) of a space is thepercentage of locations in a space in which thelimits of the effective draft temperature are met. Inother words, an ADPI of 80 means that 80%; of thelocations in the space have an effective drafttemperature of -3 to +2 F. Obviously, an ADPI of100%; would be the ultimate condition, but theefforts required to achive this condition would oftencost much more than the added benefit. Studieshave shown that an ADPI of 80%; or more results insatisfactory comfort. However, there are somelimitations to the use of ADPI, which include:

ADPI is valid only for a cool air supply.Heating conditions should be evaluatedusing ASHRAE Standard 55.

ADPI is based on effective drafttemperature and air velocity only. Relative

humidity and the remaining comfortparameters should be evaluated inaccordance with ASHRAE Standard 55.

ADPI is not to be used as a rating methodfor a single air outlet. It is to be used only

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direction of the air jet through the use of special airoutlets (similar to those found in automobiles andcommercial airplanes). Air outlets are typicallylocated in the floor, furniture, or workstations. Airinlets (returns) are usually located in the ceilingbecause the air has a natural tendency to risesimilar to the displacement ventilation system.Localized ventilation systems are also used inraised floor applications where air is supplied

throughout an

for the evaluation of comfort in a spacethat utilizes a SYSTEM of air outlets.

Air Distribution and Diffusion Propertiesof Air Outlets and Air Inlets

Air outlets possess certain basic characteristics thatallow for the introduction of air into a space whetherit is a mixing system or any of the other systems. Asair is conveyed through an air outlet or air inlet, the

following characteristics are important factors inachieving the desired space air diffusion:

The speed of the air (face velocity) leavingthe air outlet or entering the air inlet.The direction of the air emanating from anair outlet (1-way pattern, 2-way pattern, 3-way pattern, 4-way pattern, radial pattern,etc.)The effective area of an air outlet or airinlet.The distance the air travels away from anair outlet (throw, spread, and drop).The pressure resistance of an air outlet orair inlet (static and total pressure).The quantity of the air (airflow rate).

The above characteristics of an air outlet or air inletare measured in accordance with ASHRAEStandard 70-1991 Method of Testing for Ratingthe Performance of Air Outlets and Inlets and areoften found in manufacturer's literature. Thesecharacteristics are defined as follows:

Airflow Rate (Q) - The unit volume displacement ofair per unit time. The common unit of measure iscubic feet per minute (cfm).

Throw (T) - The maximum horizontal distance fromthe center of the air outlet to a point in the airstreamwhere the air velocity has decayed to some arbitraryterminal velocity(typically 150, 100, and 50 fpm).See Figure 1. The common unit of measure is feet(ft).

Spread (S) - The overall width of a horizontallyprojected airstream envelope measured

perpendicular to the direction of airflow. Spread isusually determined for wide air patterns only, whichis typical of sidewall air outlets. The common unit ofmeasure is feet (ft).

Drop (D) - The vertical distance from an air outlet tothe lower edge of a horizontally projected airstreamenvelope. See Figure 1. The common unit ofmeasure is feet (ft).

Static Pressure (Ps) - the perpendicular force perunit area exerted on the walls of the upstream ductof an air outlet or the downstream duct of an air inlet

at a given airflow rate. The static pressure ismeasured 1.5 equivalent duct diameters upstreamof an air outlet or downstream of an air inlet via thestatic tap in a pitot tube or through small ports in theduct wall (piezometric ring). The common unit of

Face Velocity (Vk) - The average velocity of airentering an air inlet or leaving an air outlet. Facevelocity is usually measured at pre-defined locationson the face or can be calculated from the totalpressure of the device according to the followingequation. The common unit of measure is feet perminute (fpm).

Effective Area (Ak) - The actual minimum area ofan air outlet or air inlet through which air is able topass. The following two equations can be used tocalculate the effective area. The common unit ofmeasure for Ak is square feet (ft).

Effective area of an air outlet or air inlet is NOT thesame as the free area. Free area is the geometricopening of an air outlet or air inlet whereas the

effective area is essentially a percentage of the freearea based on the magnitude of the vena contracta.

Where flow hoods are unavailable or impractical,effective area and face velocity can be used by thebalancer to determine how much airflow is beingdelivered through an air outlet or air inlet bymeasuring face velocity and using it in the followingequation:

Application of Air Outlet and Air Inlet Properties(System Design Using Published EngineeringData)

The above air outlet and air inlet characteristics canbe used by design engineers to ensure aceptablespace air diffusion (ADPI of 80 or more). This isdone using a fairly common design procedure asoutlined in chapter 31 of theASHRAEFundamentals Handbook. The designer is urgedto review this procedure.

Selection and Location of Air OutletsAccording to ASHRAE, no criteria have beendeveloped for choosing among the many differenttypes of air outlets available. An acceptable ADPIcan be achieved from almost any system of air

outlets if they are sized and located properly in thebuilding space. The location and selection of airoutlets are predicated on the heating or coolingloads, the building design and size, local sources ofheat gain or loss, the specific outlet design andperformance, and the sound requirements of the

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measure is inches of water (in w.c.).

Velocity Pressure (Pv) - The force per unit areawhich would cause air to accelerate in a duct orother air conveying device. Velocity pressure istypically measured at 1.5 equivalent duct diametersupstream of an air outlet or downstream of an airinlet. The common unit of measure is inches ofwater (in w.c.).

Total Pressure (Pt) - The sum of the static andvelocity pressures. The total pressure of an air outletor air inlet will influence the size of the fan requiredto deliver a specified amount of air, as well as thesound produced by the air outlet or air inlet. Thecommon unit of measure is inches of water (in w.c.).

space. The convection effects of a concentratedload (such as a perimeter wall) should be directlycounteracted with the flow from an air outlet or theplacement of air inlet adjacent to the source.

Selection and Location of Air InletsThe location and selection of air inlets depends onthe allowable pressure loss of the device, thelocation of any concentrated heating or cooling

loads, and the permissible sound. Air inlets do notinfluence the room air motion beyond onecharacteristic length of the opening away from theinlet, so rarely do air inlets cause unwanted drafts.

SoundDue to their close proximity to the occupied zone, airoutlets and air inlets often play a critical role in theacoustical "comfort" of a building space. This is

because air outlets and air inlets not only tend togenerate sound, but sound from the fan system or aterminal unit will also pass directly through them andinto the space.

The amount of sound generated by an air outlet orair inlet depends on the entrance conditions to thedevice, the amount of restriction of the device(pressure loss), and the amount of air delivered to orfrom the device. Sound attenuation of an air outletor air inlet is not possible because they are usuallylocated near the occupied zone and have a clearpath to the occupants.

Air outlet and air inlet sound predominantly peaks inthe 500 through 4000 Hz octave bands. Becausethis frequency range is also associated with humanspeech, the devices can provide a masking soundfor reducing the amount of cross talk typically foundin an open office environment. Using these devicesfor masking sound should only be done; however,using a constant volume system. If used in variablevolume sytems, changes in airflow rate willdrastically reduce the effectiveness of the maskingsound, especially at lower air flow rates.

The sound power of air outlets and air inlets ismeasured in accordance with ASHRAE Standard70-1991. The standard requires sound power levelsto be determined in the 125 to 4000 Hz octavebands at a given operating condition. The soundpower levels can then be reduced to sound pressurelevels by subtracting the space attenuation inaccordance with ARI Standard 885-98 Procedurefor Estimating Occupied Sound Levels in theApplication of Air Terminals and Air Outlets (thisstandard can be downloaded free of charge from the

ARI homepage at www.ari.org). The resultant soundpressure levels are plotted on a noise criteria (NC)

chart to determine the actual NC level of the device.Manufacturers often use a standard deduction of 10dB per octave band for determining published NCratings. The drawback of this method is that theactual space rarely has an attenuation value of 10

Room Criteria (RC) is used to analyze the actualsound quality of a space relative to loudness as wellas spectral content. It is not to be used to rateequipment such as air outlets and air inlets.

Other factors must be accounted for in determiningthe acoustic properties of a space. Additionalsources of sound, such as the effect of multiple airoutlets and air inlets, computers, telephones, andsounds transmitted from outdoors may actuallydominate the acoustical spectrum of the space.

Recommended Sound Reduction MethodsThe following steps can be taken to reduce thesound generation of air outlets and air inlets:

Reduce the airflow delivered to eachdevice by utilizing a greater number ofdevices for the same total airflow to or fromthe space. However, the size of thedevices may need to be reduced tomaintain the proper room air motion.Increase the size of the air outlet or airinlet, which decreases the face velocityand pressure loss. However, increasingthe size of an air outlet without increasingthe airflow will result in shorter throws andmay cause conditioned air to drop into theoccupied zone.Balancing dampers should not be used inthe neck of the air outlet. This can add

anywhere from 5 to 16 NC to the soundrating, depending on the amount ofdamper closure. Balancing dampersshould be located as far upstream from theair outlet as possible.The approaching ductwork to the air outletshould be kept as straight as possible. Anyoffset or abrupt bend will add up to 15 NCto the sound rating. If flexible duct is used,sharp bends should be prevented at theneck of the air outlet.

As a general rule of thumb, the facevelocity of an air outlet should not exceed

800 fpm and the face velocity of an air inletshould not exceed 400 fpm.

References

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dB per octave band. Therefore, the standardattenuation value is to be used only for comparisonpurposes and the ARI standard should be consultedfor accurately determing actual space attenuation.Please note that ASHRAE Standard 70 does notrequire the conversion of sound power levels to NC.This is done strictly to give the end user a singlenumber sound rating for a particular device at agiven operating condition.

ASHRAE. 1997. Handbook Fundamentals, Chapter31. Atlanta: American Society of Heating,Refridgerating, and Air-Conditioning Engineers, Inc.

Ebbing, C.E. and W.E. Blazier. 1998.Application ofManufacturers' Sound Data. Atlanta: AmericanSociety of Heating, Refridgerating, and Air-Conditioning Engineers, Inc.

Copyright2005 Tuttle & Bailey.All Rights Reserved.