price - air distribution guide

© Copyright E.H. Price Limited 2004. All Metric dimensions ( ) are soft conversion.Imperial dimensions are converted to metric and rounded to the nearest millimetre. EG-17

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Air DistributionEngineering Guide

Selection Procedures

Outlet SelectionThe outlet type and size must be carefullyselected so as to provide uniform airtemperatures and satisfactory velocitieswithin the occupied zone. In addition noiselevels must be acceptable. If these criteriaare met then a high level of comfort will be

1. ThrowAchieving the proper throw for a specificapplication is critical to proper outletselection. Throw data is usually presentedat terminal velocities of 150 fpm, 100 fpmand 50 fpm. Generally outlets should beselected so that the throw at 50 fpmterminal velocity equals the distance fromthe outlet to the boundary of theconditioned space. In most cases thiscriteria will produce acceptable results.

When an air stream strikes a surface ittends to spread and follow the surfaceuntil the velocity dissipates. The totalhorizontal and vertical distance travelledby the air stream is equal to the tabulatedthrow of the outlet (Figure 17). For highceiling applications it may be desirable forthe throw to exceed the space boundary(ceiling) and travel down the wall towardthe occupied zone. However penetrationof the occupied zone should usually beavoided.

In addition to physical boundaries createdby walls or partitions, boundaries can becreated by the collision of two air patterns(Figure 18). Where two patterns will meet,the outlets should be selected so that thethrow is equal to one half the distancebetween the outlets. For high ceilingapplications it may again be desirable forthe throw to travel downward toward theoccupied zone. Throw is again equal to thehorizontal and vertical distance travelledby the air stream.

It should be noted that most catalog throwdata is presented for isothermal conditions,i.e. supply air temperature equals roomtemperature. During cooling the densersupply air will shorten the horizontal throwto approximately 75% of tabulated values.

When selecting outlets for VAV application,both minimum and maximum air quantitiesmust be considered for throw. Althoughmany models of outlets provide excellenthorizontal air pattern at extremely lowflows, throws may be reduced belowacceptable limits. Slot diffusers and lighttroffer diffusers tend to maintain reasonablethrows at low air volume andare therefore a good choice for thisapplication.

SpreadSpreading the air pattern dissipates theair stream over a wider area and increasesentrainment. This reduces the mass flowper unit surface area, which in turnreduces throw. Some outlets are designedto produce a spread pattern due to theirgeometry (SDA, TBD8) while others haveadjustable vanes (510, 520, 610, 620 seriesgrilles).

In many applications it is desirable to limitthe throw due to ceiling layout, walls,partitions or other boundaries which mayobstruct the air pattern and causeunacceptable velocities in the occupiedzone. There are several methods whichmay be used to minimize throw fromoutlets.

achieved for the occupants of theconditioned space.

In order to properly select and size anoutlet, a number of performance factorsmust be taken into account.

Figure 17.

Figure 18.

EG-18 All Metric dimensions ( ) are soft conversion. © Copyright E.H. Price Limited 2004.Imperial dimensions are converted to metric and rounded to the nearest millimetre.

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Air VolumeThrow is directly related to mass flow,therefore a reduction in air volume peroutlet will reduce the throw. This can beachieved by utilizing more outlets withless air volume per outlet. For lineardiffusers or grilles the same thing can beachieved by dividing the outlet into activeand inactive sections (Figure 19). Eachactive section handles a smaller quantityof air, thereby reducing the throw. Inorder to effectively separate the airpattern, the outlet should be divided byminimum inactive length as illustrated inTable 1.

Figure 19.

Table 1. Inactive/Active Sections

Length of Active Sections, Ft. 1 5 10

Length of Inactive Sections, Ft. 1 2 3

Ceiling Height, Ft.

Outlet Type 8 9 10 12 14 16

RCDA 550 1300 2200 4000 6200 9300

RCDE 270 700 1300 2100 3300 5500

SCD, SPD 1100 2300 5000 — — —

SMD, PDF, PDN 200 350 550 900 1400 1600

CVD*

LV1* 160 250 400 600 800 1000

SDS*

LV2 320 500 800 1200 1600 2000

*cfm per side

These values are for 20°F cooling temperature differential.

Table 2. Maximum Air Quantity Guidelines2. DropSince a velocity of 50 FPM or less isdesirable in the occupied zone, dropshould be limited to a terminal velocity of50 FPM, 6 feet from the floor.

Figure 20 on the following page showsthrow and drop for sidewall grilles atseveral locations and air pattern settings.Figure 20 illustrates several methods ofreducing drop for sidewall grilles.

1. Mounting the grille close to theceiling creates a surface effectand reduces drop.

2. Deflecting the air pattern upwardreduces drop.

3. Spreading the air pattern reducesdrop.

4. Reducing the air volume reducesdrop.

5. Reducing the throw reducesdrop.

Drop for ceiling diffusers is rarelycatalogued as it is normally of no concern.However for applications with low ceilingheights or when large volumes per outletare required, the drop should be considered.Table 2 provides a guideline for maximumair quantity, allowable for given ceilingheights to satisfy 80% or higher occupantcriteria.


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Deflection: Vertical - 0°, Horizontal - 0°Mounted Within 1 Ft. of Ceiling No Ceiling


Figure 20. Throw and Drop for Sidewall Grilles


Deflection: Vertical - 20° Up, Horizontal - 0°Mounted 2 – 4 Ft. Below Ceiling No Ceiling

Deflection: Vertical - 0°, Horizontal - 45°Mounted Within 11/2 Ft. of Ceiling No Ceiling

4. Small circle in white area of each chart showscomparative performances of one size of grille at300 cfm and 600 fpm outlet velocity.

5. Shaded area to right of each chart indicates noiselevel above 30 NC.

Performance Notes:

1. Throw and drop values are based on 50 fpm terminalvelocity.

2. Data are based on tests with 20°F cooling temperaturedifferential in space with no boundary walls.

3. Data are based on Model 21 and 22 cores.

EG-20 All Metric dimensions ( ) are soft conversion. © Copyright E.H. Price Limited 2004.Imperial dimensions are converted to metric and rounded to the nearest millimetre.

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Diffuser Type Characteristic Length, L

High Sidewall Grille Distance to wall perpendicular to jetCircular Ceiling Diffuser Distance to closest wall or intersecting air jetSill Grille Length of room in the direction of the jet flowCeiling Slot Diffuser Distance to wall or midplane between outletsLight Troffer Diffusers Distance to midplane between outlets,

plus distance from ceiling to top of occupied zonePerforated, Louvered Ceiling Diffusers Distance to wall or midplane between outlets

Terminal Room Load T50/L Maximum for ADPI Range of

Device Btu/h.ft2 for Max. APDI ADPI Greater Than T50/L

High 80 1.8 68 — —

Sidewall 60 1.8 72 70 1.5-2.2Grilles 40 1.6 78 70 1.2-2.3

20 1.5 85 80 1.0-1.9Circular 80 0.8 76 70 0.7-1.3Ceiling 60 0.8 83 80 0.7-1.2Diffusers 40 0.8 88 80 0.5-1.5

20 0.8 93 90 0.7-1.3Sill Grille 80 1.7 61 60 1.5-1.7Straight 60 1.7 72 70 1.4-1.7Vanes 40 1.3 86 80 1.2-1.8

20 0.9 95 90 0.8-1.3Sill Grille 80 0.7 94 90 0.8-1.5Spread 60 0.7 94 80 0.6-1.7Vanes 40 0.7 94 — —

20 0.7 94 — —Ceiling 80 0.3* 85 80 0.3-0.7*Slot 60 0.3* 88 80 0.3-0.8*Diffusers (for T100/L) 40 0.3* 91 80 0.3-1.1*

20 0.3* 92 80 0.3-1.5*Light 60 2.5 86 80 <3.8Troffer 40 1.0 92 90 <3.0Diffusers 20 1.0 95 90 <4.5Perforated and LouveredCeiling Diffusers 11-51 2.0 96 90 1.4-2.7

80- 1.0-3.4* T100/L

Table 3. Characteristic Length for Various Diffuser Types

Table 4. Range of T/L Values

3. ADPIBy definition the ADPI is the statisticalpercentage of the points when measureduniformly within the space whose draftvelocities and temperatures fall withinthe comfort criteria established in Figure1. The higher the ADPI rating the higherthe comfort level within the space.Generally an ADPI of 80 is consideredacceptable.

Through extensive testing, relationshipshave been developed between ADPI andthe ratio of throw over characteristiclength (T / L). The throw is the isothermalthrow at a selected terminal velocity takenfrom the catalog performance charts. Thecharacteristic length is the distance fromthe outlet to the nearest boundary. Table3 provides definition of characteristiclength for various outlet types.

Table 4 illustrates the range of T/L valueswhich will result in optimum comfortconditions for various outlet types atseveral room loads. By selecting a throwfrom the catalog data which produces therequired T/L ratios, an acceptable ADPIrating can be achieved.

Example: An office 15 ft. wide by 20 ft.long is supplied with a 10" x 8" Model 520sidewall grille located on the short wall,handling 300 cfm. The supply airtemperature is 55°F and the roomtemperature is 75°F. Verify the selectionbased on ADPI.

Room load = BTU/HR

Ft2

BTU/HR = 1.085 x CFM x ∆T

= 1.085 x 300 (75-55) = 6510

Room Area = 20 x 15 = 300 ft2

Room Load = 6510 = 21.7 BTU/HR Ft2

300

Characteristic length L = 20 ft (Table 3)

From the Model 520 performance table inSection D for 10" x 8" grille:

T50 = 31 ft @ 0° DEFL

= 25 ft @ 22 1/2° DEFL

= 15 ft @ 45° DEFL

T50/L = 31/20 = 1.6 @ 0° DEFL

= 25/20 = 1.3 @ 22 1/2° DEFL

= 15/20 = 0.8 @ 45° DEFL

From Table 4 the range of T50/L for ADPIgreater than 80 is 1.0 - 1.9. For maximumADPI of 85, the T50/L is 1.5. Therefore 0°deflection will produce the optimumADPI rating.

Reference: ASHRAE Handbook

Reference: ASHRAE Handbook


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4. Noise CriteriaMost air outlets are catalogued with asingle NC (Noise Criteria) sound pressurerating based on a 10 dB room absorption.This NC value assumes an average roomand an approximate distance of 5 feet froma single sound source. These assumptionsare reliable for most applications.

Table 5 illustrates the ASHRAE recommendedspace NC values for many commercial airconditioning applications. Outlets shouldbe selected so that the tabulated NC levelsare within these design goals.

All outlet sound data is for a single source.Allowances must be made for multipleoutlets when this occurs in a space sincethe overall noise level may be the resultantof more than one outlet. Table 6 illustratesthe additive effect of multiple outlets ofequal sound.


Table 6. Allowance for Number of Outlets

No. of Outlets 1 2 3 4 8 10 20 40

dB Boost 0 3 5 6 9 10 13 16

For a large area with a large number ofoutlets, consider an area of 400 to 600 ft.2as the maximum area where more than oneoutlet will contribute to the overall noiselevel. Take area to be square or of an aspectratio of 1 to 2. By using an overlay on thediffuser layout, the maximum concentra-tion of diffusers can be determined andthus the overall noise level can becalculated.

The above procedure is a simple and fastmethod for adding multiple air outletswhich will provide reasonable accuracy.

For a more detailed sound analysis proce-dure which takes into account room size,distance from source and unequal soundsources, refer to the Terminal Unit Engi-neering Guide beginning on page EG-25.

ExampleA 25' x 40' conference room is suppliedwith 8 LTF-24-LP-6" oval inlet trofferdiffusers. Each diffuser handles 120 CFMand all 8 are evenly distributed within theroom. Determine if this selection meetsthe recommended noise criteria.

Room Area = 25 x 40 = 1000 ft2. Consider anarea of 500 ft2 with 4 diffusers (1/2 the totalarea).

From the performance tables in SectionB, the noise level of an LTF-24-LP-6" oval at120 CFM is 23 NC.

From Table 6, the boost for 4 outlets is 6 dB.Therefore overall NC = 23 + 6 = 29.

This selection maintains a noise levelwithin the recommended range (25 - 30for Table 5).

Table 5. Design Guidelines for HVAC System Noise in Unoccupied Spaces

Room Types Recommended Levelab

Private residences 25-35Hotels/Motels

Individual rooms or suites 25-35Meeting/banquet rooms 25-35Corridors, lobbies 35-45Service/support areas 35-45

Office BuildingsExecutive and private offices 25-35Conference rooms 25-35Tele-conference rooms 25 (max)Open-plan offices 30-40Corridors and lobbies 40-45

Hospitals and clinicsPrivate rooms 25-35Wards 30-40Operating rooms 25-35Corridors and public areas 30-45

Performing Arts SpacesDrama theatres 25 (max)Concert and recital halls cMusic teaching studios 25 (max)Music practice rooms 35 (max)

Laboratories (with Fume Hoods)Testing/research, minimal speech communication 45-55Research, extensive telephone use, speech communication 40-50Group teaching 35-45

Church, Mosque, SynagogueGeneral assembly 25-35With critical music programs c

a The values and ranges are based on judgment and experience, not quantitative evaluations ofhuman reactions. They represent general limits of acceptability for typical buildingoccupancies. Higher or lower values may be appropriate and should be based on a carefulanalysis of economics, space use and user needs.

b When quality of sound in the space is important, specify criteria in terms of RC(N). If thequality of the sound in the space is of secondary concern, the criteria may be specified interms of NC or NCB levels of similar magnitude.

c An experienced acoustical consultant should be retained for guidance on acoustically criticalspaces (below RC 30) and for all performing arts spaces.

d HVAC – related sound criteria for schools, such as those listed in this table, may be too highand impede learning by children in primary grades whose vocabulary is limited. Someeducators and others believe that the HVAC – related background sound should not exceedRC 25 (N).

e RC or NC criteria for these spaces need only be selected for the desired speech and hearingconditions.

Reference • 1999 ASHRAE Applications Handbook, Table 34, page 46.25• ARI Standard 885-1998, Table 15, page 28

Room Types Recommended Levelab

Schools dClassrooms up to 70 m2 40 (max)Classrooms over 70 m2 35 (max)Large lecture rooms, without speech amplification 35 (max)

Libraries 30-40Courtrooms

Un-amplified speech 25-35Amplified speech 30-40

Indoor Stadiums, GymnasiumsGymnasiums and natatoriumse 40-50Large seating-capacity spaces with speech amplificatione 45-55

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