grd guidelines

7/30/2019 Grd Guidelines

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Engineering Guidelines - GRD

ENGINEERINGGUIDELINES

Grilles & Diffusers ..........................................................................................................................................................................B5

Basic Principles of Air Distribution ................................................................................................................................................B5

The Goal of an Air Diffusion System: Maintaining Comfort ...................................................................................................B5

Comfort Limits Set by ASHRAE Handbook, ASHRAE Standard 55 and ISO Standard 7730 ..................................................B5

Comfort: A Function of Room Air Velocity .............................................................................................................................B6

Fangers Comfort Index ..........................................................................................................................................................B7

General Comfort Guidelines ...................................................................................................................................................B7

Outlet Location and Selection .......................................................................................................................................................B7

Three Methods .......................................................................................................................................................................B7

Method I. Selection by Noise Criteria (NC) ............................................................................................................................B7Method II. Selection by Supply Jets Mapping.......................................................................................................................B8

Method III. Selection by Comfort Criteria - ADPI...................................................................................................................B8

Isothermal Air Jets ......................................................................................................................................................................B10

Jet Characteristics: Four Zones of Expansion ......................................................................................................................B10

Room Air Induction Rate for an Outlet.................................................................................................................................B11

Jet Characteristics Surface Effect (Coanda Effect) ..............................................................................................................B12

Procedure to Obtain Catalog Throw Data ............................................................................................................................B13

Isothermal Jet Theory for All Outlets ...................................................................................................................................B13

Nonisothermal Jets ..............................................................................................................................................................B13

Exhaust and Return Grille Pressure .....................................................................................................................................B13

Throw and Drop from Side Wall Outlets in Free Space ........................................................................................................B14

Other Diffusers With Cooling ...............................................................................................................................................B17

Supply Outlet Classifications .......................................................................................................................................................B17

Typical Air Distribution Characteristics................................................................................................................................B17

Classification of Supply Outlets ...........................................................................................................................................B19

Horizontal, Circular and Cross Flow Patterns ......................................................................................................................B21

Vertical Downward Projection From Ceiling ........................................................................................................................B23

Estimating Downward Vertical Projection ...........................................................................................................................B24

Diffuser Applications on Exposed Ducts ..............................................................................................................................B24

Horizontal Projection at Floor Level: Displacement Ventilation ...........................................................................................B25

Vertical Upward Projection from Floor, Low Side Wall and Sill ...........................................................................................B26

Perimeter Applications ................................................................................................................................................................B27Outlet Located Back from Perimeter Wall ............................................................................................................................B28

Outlet Vertical Projection at a Window................................................................................................................................B28

Rolling a Room .....................................................................................................................................................................B28

Some Perimeter Considerations ...........................................................................................................................................B28

ADPI-Air Diffusion Performance Index ........................................................................................................................................B29

ADPI Can be Obtained by Measurement or Through Prediction ..........................................................................................B30

Jet Calculations ...................................................................................................................................................................B31

Industrial Applications .................................................................................................................................................................B32

Thermal Standards for Industrial Work Areas ......................................................................................................................B32

Control of Heat Exposures by Isolating the Source ..............................................................................................................B32

General Ventilation ...............................................................................................................................................................B33

overview

grilles & diffusers

Engineering Guidelines Overview ..................................................................................................................................................B4

Table o Contents


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Engineering Guidelines - GRDTable of Contents (continued)

Acoustical Applications & Factors ...............................................................................................................................................B39

Noise Criteria (NC) ...............................................................................................................................................................B39Room Criteria (RC) ...............................................................................................................................................................B41

Air Terminal Sound Issues ...................................................................................................................................................B43

AHRI Standard 885 ..............................................................................................................................................................B44

Environmental Adjustment Factor .......................................................................................................................................B44

Discharge Sound Power Levels ............................................................................................................................................B45

Acceptable Total Sound in a Space ......................................................................................................................................B46

Maximum Sound Power Levels for Manufacturers Data ....................................................................................................B48

Desired Room Sound Pressure Levels ..................................................................................................................................B48

Radiated Sound Power Level Specifications ........................................................................................................................B49

Discharge Sound Power Level Specifications ......................................................................................................................B49

Diffuser Specifications .........................................................................................................................................................B50

Determining Compliance to a Specification .........................................................................................................................B51

acoustical applications & factors

Local Relief ..........................................................................................................................................................................B33

Guidelines for Local Area or Spot Cooling Ventilation: ........................................................................................................B33

Outlet Selections ..................................................................................................................................................................B33

Other Grille and Diffuser Application Factors ..............................................................................................................................B36

Pressure Measurements ......................................................................................................................................................B36

Airflow Measurements ........................................................................................................................................................B36

Velocity Distribution from Linear Diffusers ..........................................................................................................................B36

Ducts for Linear Diffusers ....................................................................................................................................................B36

Expansion and Contraction of Aluminum Linear Grilles ......................................................................................................B36

Installation Acoustics ..........................................................................................................................................................B37

references

glossary

References ...................................................................................................................................................................................B52

Glossary .......................................................................................................................................................................................B54


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ENGINEERINGGUIDELINES

The selection and performance data contained in thiscatalog are the result of extensive studies conducted inthe Titus engineering laboratories under professional

engineering guidance, with adherence to soundengineering applications. They are intended to be aidsto heating and air conditioning engineers and designerswith skill and knowledge in the art of air distribution.The data has been obtained in accordance with theprinciples outlined within the American Society ofHeating, Refrigerating and Air Conditioning Engineers

(ASHRAE) Standard 70 and Standard 113. Although Titushas no control over the system, design and applicationof these products, a function which rightfully belongs to

the designer, this data accurately represents the productperformance based on the results of laboratory tests.Furthermore, the recommended methods of applyingthis information have been shown by eld experienceto result in optimum space air distribution.

Overview


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BASIC PRINCIPLES OF AIR DISTRIBUTION

THE GOAL OF AN AIR DIFFUSIONSYSTEM: MAINTAINING COMFORTAn understanding of the principles of room air distributionhelps in the selection, design, control and operation of airsystems. The real evaluation of air distribution in aspace, however, must answer the question: Are theoccupants comfortable?

In general, a person is thermally comfortable when bodyheat loss equals heat production without being consciousof any changes in the bodys temperature regulatingmechanisms. The human body heat loss to the environmentcan occur through the following:

Radiation Convection Conduction Evaporation

The comfort of an occupant is determined by both occupantvariables and the conditions of the space. Occupant factorsinclude activity level and metabolic rate (reported in Metunits), as well as occupant clothing levels (reported in Clounits). The factors that inuence space comfortconditions include:

Dry bulb and radiant temperatures Relative humidity Air velocity

The design of the air distribution system should address the

above factors so that the occupants heat loss is maintainedat a comfortable rate.

COMFORT LIMITS SET BY ASHRAEHANDBOOK, ASHRAE STANDARD55 AND ISO STANDARD 7730For many years, it has been shown that individual comfortis maintained through the change in seasons when thefollowing conditions are maintained in the occupied zone ofa space:

1. Air temperature maintained between 73 - 77F

2. Relative humidity maintained less than 60%

3. Maximum air motion in the occupied zone

4. (6 to 6 vertical, within 1 of walls):50 fpm cooling30 fpm heating

5. Ankle to head level, 5.4F standing & 3.6F seatedmaximum temperature gradient

Note: The comfort standards state that no minimum airmovement is necessary to maintain thermal comfort,provided the temperature is acceptable. To maximize energyconservation, maintain proper temperatures at the lowestpossible air speed.

The previous conditions assume occupants are sedentary

or slightly active individuals and appropriately dressed.Variations in clothing can have a strong effect on desired

temperature levels, often creating circumstances where asingle setpoint will not satisfy all individuals in a space.

In meeting the above criteria for comfort, the temperatureof the space and the relative humidity is largely controlledby the mechanical equipment including chillers or packageunits, air handlers, room thermostat, and air terminal unit.The air motion in the occupied zone is a function of thedischarge velocity, discharge temperature (and room load)and the pattern of the air diffusion device into the space. Attodays relatively low (< 1 cfm / sq.ft.) air delivery rates, andwith properly selected diffusers, room load (and resultantDt) is often the strongest variable in setting room air motion.

Figure 1. Comfort Chart - Neck Region

Figure 2. Comfort Chart - Ankle Region

Grilles and Difusers

-2

0

-6 -4 0 2 4

Feeling of

coolness

80

20

40

60

10%

20%

30%

100

40%

of warmth

Feeling

Ankle region

LocalAirVelocity,

FPM

Local Air Temp. minus Ambient Temp.(TX - TA)

10%

-2

0

-6 -4 0 2 4

40%

Neck region

20

40

60

80

30%

20%

100

of warmth

coolness

Feeling of

FeelingLocalAirVelocity,

FPM

Local Air Temp. minus Ambient Temp.

(TX - TA)


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COMFORT: A FUNCTION OFROOM AIR VELOCITYSome interesting relationships exist between room air

motion and the feeling of occupant comfort. Figure 1shows the effect of air motion on comfort. The charts showthat the feeling of comfort is a function of the local room airvelocity, local temperature and ambient temperature.

Local temperature (Tx) (Figures 1 and 2) is the

temperature at a given point in a space. Ambienttemperature (T

A) is the desired room temperature and can

be considered the thermostat setpoint.

The basic criteria for room air distribution can be obtainedfrom the curves shown in (Figure 1). The chart shows theequivalent feeling of comfort for varying room temperaturesand velocities at the neck. The % curves indicate thenumber of people who would object to the temperature

and velocity conditions. The same comfort perceptions areshown in (Figure 2) for the ankle region.

If 20% objections or 80% acceptance at the same velocitiesare allowed between (Figure 1 and Figure 2), thetemperature deviation allowed between the ankle and necklevels would be about 4F (less than ASHRAE values of5.4F).

Table 1 shows the relationship between local velocities andtemperatures on occupant comfort. As an example,at a local velocity of 80 fpm, the local temperature can bemaintained at 75F to reach an 80% comfort level in thespace. The same 80% comfort level can be maintained withlocal air velocity of 15 fpm and a local temperature reduced

to 71F.

The lower portion ofTable 1 shows the effect on comfort ofroom air velocity with local temperature remaining constantat 75F. For example, with a local velocity of 30 fpm and alocal temperature at 75F, the comfort reaction is neutral.Increasing the velocity to 60 fpm results in the objectiveincreasing to 10%. This phenomenon of feeling can beillustrated by using a ceiling fan. A person can be cooledwithout decreasing the actual temperature by turning on aceiling fan. The fan, in effect, increases the local air velocityand increases the feeling of coolness. It is shown that avelocity change of 15 fpm produces approximately the sameeffect on comfort as a 1F temperature change. The dottedlines in (Figures 1, 2 and 3) show the division between

the feeling or perception of heating and cooling.

Generally, the acceptable level of comfort for a space isconsidered to be at the point where 20% or less of the roomoccupants may object to the room conditions. This wouldindicate that the given condition is acceptable to 80% of

the occupants.

We all have perceived this above change and thesesubjective responses to drafts (temperature differenceand air velocity). In 1938 Houghten et. al. developed thecurves shown in (Figures 1 and 2). Utilizing this data, theequation for effective draft temperature was generated.

Equation 1: Effective draft temperature = ( t

X- t

C) - 0.07 ( V

x- 30 )

where: = effective temperaturetX

= local air temperatures, FtC

= ambient temperature (average roomtemperature or control temperature, F)

Vx = local air velocity, fpm

ADPI (Air Diffusion Performance Index) was derived byNevins and Ward. The percentage of all local points in anoccupied space where - 3<


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FANGERS COMFORT INDEXA third comfort index which is used frequently in reportsto compare research results is Fangers procedure which

is the basis for ISO Standard 7730. Fangers methoddetermines the Predicted Mean Vote (PMV) and thePredicted Percentage of Dissatised (PPD) at eachof a number of measuring points uniformly distributedthroughout the occupied zone.

The Fanger equation includes thermal parameters notconsidered in the ADPI (relative humidity, mean radianttemperature, clothing insulation, and activity levels).The PMV approach has the advantage of providing asingle number rating combining all comfort elements.The ASHRAE 55 and ISO 7730 Standards yieldessentially the same space conditions for acceptability.ASHRAE 55 incorporates PMV in its current revision. Fromboth ASHRAE and ISO standards, the estimated comfort

of 80% of the individuals in a space can be plotted. Froma program developed as part of the ASHRAE 55 reviewprocess, a consensus computer program was developedand published. This program has been used to plot dataon a psychrometric chart for two sets of typically occurringconditions:

Condition 1 (Executive):Met Rate = 1.1 (Typical for ofce)

Clothing Rate = 1.0 (Shirt, tie, long pants, socks)Air Speed = 20 fpm (Typical interior ofce)

Condition 1 (Clerical):Met Rate = 0.9 (Sedentary)Clothing Rate = 0.5 (Skirt, blouse, no socks)Air Speed = 20 fpm (Typical interior ofce)

It can be seen from this graph that a single setpoint, suchas 75F, 50% RH is not likely to satisfy even 80% of all

individuals in a space.

GENERAL COMFORT GUIDELINESMost published guidelines for comfort suggest the below

conditions are maintained, adjusted for seasonal andoccupational clothing and activity levels:

HEATING

Generally, during heating, local air velocities are low, often

below 30 fpm. If the 80% comfort factor is to be met, themaximum temperature gradient from ankle to the neckshould be no more than 5.4F.

COOLING

During cooling, which is the predominant mode in mostoccupied spaces, local air temperature differentialsgenerally are not more than 1 to 2F from ankle to neckregion with properly designed air distribution systems.Therefore, to maintain the 80% comfort level, the airdistribution system should be selected to limit the local airvelocities to not exceed 50 fpm.

OUTLET LOCATION AND SELECTION

THREE METHODSOutlets are located in the side wall, ceiling, sill, etc., by thedesigners preference or by necessity due to the buildingconstruction. The type and size are selected to mosteffectively overcome stratication zones created by thenatural convection and internal loading. At the same timeselection should result in acceptable noise levels and roomvelocities and temperatures to satisfy as high a percentageof occupants as possible per (Figure 1, page B6). (At least80% of space occupants.)

METHOD I. SELECTION BY NOISE CRITERIA (NC)The most frequently used procedure to select an outlet sizeis by using the tabulated Outlet NC level (which typicallyassumes a 10 dB room absorption at the observers location)equal to the desired space NC. See page B39. (While RC isreplacing NC in the ASHRAE Handbook, most specicationswill continue to reference NC for some time, and withdiffusers, there is seldom any difference between NC andRC.)

To take into account the number of outlets, distance, roomsize, etc., see the discussion on the Outlet NC Level andNoise Criteria/Room Criteria in the Acoustical Designsection, page B39. Multiple outlets in a space at the samecataloged NC rating will result in an increase in the actual

sound levels heard. A second outlet within 10 ft. will add nomore than 3 dB to the sound pressure level.

Guidelines for Selection by NC Generally within a 10 ft. module the catalog NC

rating will apply for diffusers and continuous linears

We hear only 10 ft. of a continuous diffuser A wide open balancing damper in the neck may

add 4 - 5 NC

Signicantly closed balancing dampers can addmore than 10 NC, depending on the duct pressureand supply fan characteristics

The effect of inlet dampers can be determined by calculating

the ratio of the Velocity to Total Pressure.

Figure 4. PMV Chart

Basic Principles of Air Distribution (continued)

Executive & Clerical

50

0.0

0.3

0.5

0.8

55 60 65 70 75 80 85

10%

20%

40%

50%

60%80%

Temperature, F.

PartialPressure,

Millibars

%R

H

Executive Clerical


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Flow restrictions increase the space sound levels. Whenmaking a selection at a given cfm and adding a damper, adevice with a low total pressure will have a higher actualincrease in NC as compared to a device with a higher total

pressure. The sound level increases above the cataloged NCrating due to the pressure increases from dampers or othercontrol devices, can be approximated by the use ofTable2. An inlet balancing damper can be expected to add about3 dB when fully open, and as much as 10 dB or more ifsignicantly closed.

METHOD II. SELECTION BYSUPPLY JETS MAPPING

This selection procedure uses the throw values to terminalvelocities of 150, 100 and 50 fpm from the performancetables. Examples of how terminal velocities are used areshown in (Figures 7 and 8, pages B11 and B12).Temperature differences at these terminal velocities areadded to the map by using the following equation:

Equation 2: Temperature change of supply jet D t

x= 0.8 D t

o

where: D tx= t

x- t

c

D to

= to

- tc

tx= local air temperature, F

tc= ambient temperature (average room

temperature for control temperature, F)to= outlet air temperature, F

Note: Temperature Differential between total air and roomair for various terminal velocities. Calculated with Equation 2,with Dt

o= 20F and V

o= 1000 fpm.

Selection by supply jet mapping identies the most probableportion of the space to be uncomfortable. Portions of aspace away from the supply jet will have velocities and

temperatures that are nearly equal to the spaceambient conditions.

Not all applications result in overblow in a connedspace as shown in (Figure 8). In some cases the throwterminates with the airstream dropping into the occupiedspace. This is due to the buoyancy effect between theairstream and space air and/or external forces. Drop mustthen be considered as shown in (Figures 10 through 15,pages B13 - B16) for side wall outlets and Table 5, pageB17 for ceiling diffusers. Many examples of jet performancemapping are shown throughout this engineering section.

MAPPING PROCEDURES

1.Select type of diffuser (Reviewing the Classicationof Supply Outlets section on page B17 will help

determine the best device).2. For diffusers, checkTable 5 on page B17 to

determine if the air quantity is less than the maximum.

3.When selecting a side wall grille, check congurationin (Figures 10 through 15), in this section for dropduring cooling (use cfm and jet velocity).

4. Plot isothermal T150

, T100

, T50

from performance datain catalog for a selected size and cfm at the throwdistances.

5. If the outlet provides a horizontal pattern below theceiling, the pattern will tend to leave the ceiling nearthe 100 fpm terminal velocity.

6. Repeat steps 1 through 5 as necessary to meetjob requirements.

METHOD III. SELECTION BYCOMFORT CRITERIA - ADPIADPI (Air Diffusion Performance Index) statistically relatesthe space conditions of local or traversed temperatures andvelocities to occupants thermal comfort. This is similar tothe way NC relates local conditions of sound to occupantsnoise level comfort. High ADPI values are desirable asthey represent a high comfort level, and also increasedprobability of ventilation air mixing. Acceptable ADPIconditions for different diffuser types are shown in (Figure5) for velocities less than 70 fpm and velocity-temperaturecombinations that will provide better than the 80%

occupant acceptance.

The curves in (Figure 5) summarize some of the testswhich established ADPI and the relationships from whichthis selection procedure originates.

Table 2. Effect of Dampers on Outlet NC

Total Pressure Ratio 100% 150% 200% 400%

dB Increase 0 4.5 8 16

Table 3. Jet Velocity vs. Temperature Rise

VX, fpm 500 400 300 200 100 50

tX, F 8 6.4 4.8 3.2 1.6 0.8

Figure 5. Throw vs. Characteristic Room Length

o Vx pVo

Outlet Location and Selection (continued)

0 3.0 4.0

20

40

60

80

100

T50/L

ADPI

4 Light Troffer Diffusers

12 Two Slot

Diffuser

RoundCeilingDiffusers

HighSidewall

= 20 BTUH/Sq.Ft.

= 40 BTUH/Sq.Ft.

3.52.01.00.5 1.5 2.5


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The curves show relative comfort for: Four different outlet types Catalog throw and space characteristics

Loading (one cfm/sq. ft. with a 20F differential isa load of about 20 Btuh/sq. ft.) Flow rate (variable volume)

L is the space characteristic length in feet. This is usually thedistance from the outlet to the wall or mid-plane betweenoutlets. This can also be considered the module line whenoutlets serve equal modules through a space, and allconsideration can then be based on the module parameters.

T50

is a catalog throw value to a terminal velocity of50 fpm. A throw value can be selected using a catalogperformance table by multiplying the throw ratio (T

50/L) by

the characteristic length (L). The throw ratio is based on a 9ft. ceiling height. The throw can be increased or decreased

by the same amount that the ceiling height exceeds or isless than 9 ft. To obtain optimum comfort in the space, ADPItests indicate selecting the outlet from the throw ratios inTable 4 as follows.

For more details on how ADPI is obtained from basic tests oreld tests see ADPI topic, page B29.

ADPI SELECTION PROCEDURE

1. Select type of diffuser.

2. If a ceiling diffuser is being used, checkTable 5, page

B18 to determine if the air quantity is less than themaximum for the ceiling height of the room. Whenselecting a side wall gril le, check conguration in(Figures 10 through 15, pages B15-B17) for dropduring cooling (use cfm and duct velocity).

3. Select the characteristic length from the plans - thedistance from a diffuser to a wall or the distance tocenter line between two diffusers.

4. Select the range of acceptable throw values from Table4 below at the corresponding characteristic length.

5. From Performance Table for diffuser at required cfm,select a size with a T

50within range. With a VAV

system, this must be done for both maximum ow rate(maximum load) and at the lowest ow rate expectedwhen the space is occupied. (This may be higher thanthe minimum ow shown in the building plans.)

6. Check sound levels for NC compatibility.

This selection will result in maximum comfort and ventilationmixing for the application. If this selection cannot be madeas outlined, supply jet mapping can determine areas ofdiscomfort in the space.

Recommended T50/L range for PAS: 0.9-1.8

T50

Isothermal throw to terminal velocity of 50 fpm.Select diffuser size within these ranges.

L Characteristic length from diffuser to module line.L* Distance between units plus 2 ft. down for overlapping

airstream.L** Distance to ceiling and to far wall.

Table 4. Recommended ADPI Ranges for Outlets

Outlet T50/L Range Calculated T

50& L Data

Sidewall Grilles L 10 15 20 25 30

1.3-2.0 T50

13-20 20-30 26-40 33-50 39-60

Ceiling Diffusers Round Pattern L 5 10 15 20 25

TMR, TMRA, TMS, PAS 0.6-1.2 T50

3-6 6-12 9-18 12-24 15-30

Ceiling Diffusers Cross Pattern L 5 10 15 20 25

PSS, TDC, 250 1.0-2.0 T50

5-10 10-20 15-30 20-40 25-50

Slot Diffusers L 5 10 15 20 25

ML, TBD, LL1, LL2 0.5-3.3 T50

8-18 15-33 23-50 30-66 38-83

Light Troffer Diffusers L* 4 6 8 10 12

LTT, LPT 1.0-5.0 T50

4-40 6-30 8-40 10-50 12-60

Sill and Floor Grilles L** 5 10 15 20 25

All types 0.7-1.7 T50

4-9 7-17 11-26 14-34 18-43

Outlet Location and Selection (continued)


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The vast majority of air diffusion systems in the UnitedStates are overhead forced air systems. A grille or diffusergenerates a high velocity jet and comfort conditions aremaintained through mixing. The primary airstream from the

air diffuser device draws the room air into the supply air jetas shown in (Figure 6, page B10). The primary airstreaminduces room air (or secondary air) to provide completeroom air mixing and maintain thermal comfort conditionsby creating uniform room temperatures. The room airmotion is largely a function of the discharge velocity andinduction rate of the air diffusion device at higher ows,but is increasingly dependent on room load and dischargetemperature as the ow rate decreases.

Isothermal jets (Isothermal refers to the supply air beingat the same temperature as the room air) are studied

because the data is repeatable and predictable withouthaving to correct for the buoyancy effect associated withheated or cooled air. Because of predictability, test methodsused to obtain throw data for grilles and diffusers are based

on isothermal air (see ASHRAE Standard 70). Extensivestudies of isothermal jets have shown that the air projectionis related to the average velocity at the face of the air supplyoutlet or opening. The distance an airstream will travel isbased on the relationship between the discharge velocity,cfm, discharge area and velocity prole. Isovel testing hasshown that as an isothermal jet leaves a free opening, it canbe described by its predictable characteristics and knownequations. A free jet has four distinct zones of expansionwith the centerline velocity of each zone related to the initialvelocity as shown in (Figures 6 & 7).

As air leaves an outlet, four distinct zones of expansiondene the jet. These zones are shown in the dimensionlessgraph (Figure 7, page B11). This graph can be generatedfor any diffuser or grille by experimentation, and is used tocalculate the throw for a diffuser at any ow condition. (TheK value for each zone is the value of the X axis where theY axis is = 1). A description of each zone and an equation todene the characteristics of each zone is shown below.In the rst zone, the jet maintains a constant velocity withminimal mixing of supply and room air. This zone extendsapproximately one and a half duct diameters from the face.

Equation 3: First zone velocity

Vx

Vo

Where Vx= air speed at a point

Vo= outlet air velocity

In the second zone, the jet begins to mix with room air.Induction of room air also causes the jet to expand. Linear

outlets typically have a long second zone and long throws.

Equation 4: Second zone velocityWhere Q = outlet ow rate

Where K2

= the second zone throw constantA

o= outlet effective area (This value may be

less than the actual opening or the outlet.)X = throw distance

Equation: 4A: Second zone throw

Figure 6. Expansion of primary air jet

Vx

= o K2

Ao p

Vo

X

X = o Q p 2 K2V

x A

o

3

ISOTHERMAL AIR JETS

JET CHARACTERISTICS: FOUR ZONES OF EXPANSION

= constant

PRIMARYAIR

Induced room airgentle movement

High velocity 22Totalair

II III

Zone

IV

Greatest possiblesource of drafts

I


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The fourth zone is the zone with relatively low velocities.The performance in zone four is a function of the cfm. Theperformance is independent of the outlet size.

Equation 6: Fourth zone velocity

Equation 6A: Fourth zone throw

Figure 7 above shows a non-dimensional plot of thevariables in the zones of an expanding jet for a grille(shaded area). The mathematical relationship betweenthe centerline velocity and the distance to this velocityis indicated for each zone. These relationships for alloutlets are obtained by the methods outlined in ASHRAEStandard 70. Average constants are included in the ASHRAEHandbook [2] for Zone III for a number of different typesof outlets. Throw for each outlet type can be predicted byequations 3 through 6. Catalog throw data is generated byobtaining the constants for an outlet type and calculatingthe throw using the zone equations. The area for A

oand V

o

must be the same for the relationships to be valid.

ROOM AIR INDUCTION RATE FOR AN OUTLETThe amount of room air induced into a primary air jet canbe approximated using the equation shown below:

Equation 7: Room air induction equation

Induction ratio =

Qo

= supply cfm

Qx = cfm at distance x distance from outletV

o= discharge velocity

Vx

= velocity at distance x from outletC = entrainment coefcient, 1.4 for innite

slots and 2.0 for round free axial jets

Vx

= o K4

Ao p 2

Vo

X

X = oQ

p

K4V

x

Qx

= C o Vx pQ

oV

o

Isothermal Jets (continued)

Figure 7. Four Zones of Expansion of Primary Air Jet

1.0

Decre

asing

ZoneI

ZoneII

ZoneIII

ZoneIV

Increasing

TYPICALLINEAR

Vx

/Vo

150 fpm

100 fpm

50 fpm

K3

X/ A

o

Vx

= o K3

Ao p

Vo

X

The third zone is the zone where most of the inductionoccurs. This is the most important zone because it hasthe most effect on room air velocities and room induction.The relationship between initial velocity and jet center line

velocity for the third zone is given by the equation below.Equation 5: Third zone velocity

where K3

= third zone constant


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The previous section discussed the relationships betweenvelocity and throw in free space applications. If thejet is projected parallel to and within a few inches of asurface, the jets performance will be affected by thesurface. This is called surface effect or Coanda Effect. Thesurface effect creates a low pressure region and tends toattach the jet ow to the ceiling or surface. (The higherpressure in the room holds the airstream to the ceiling.)

As the jet ows along a surface, secondary room air can nolonger mix with the part of the jet adjacent to the surface,

which causes the amount of induction to decrease.

This surface effect will occur if: The angle of discharge between the jet and

the surface is less than 40 for circular patterndiffusers, somewhat less for jets.

A side wall outlet is within 1-foot of the ceiling. Floor or sill outlet is near (within 10) to a wall. A ceiling outlet discharges along the ceiling.


Figure 8. Mapping of Isothermal Throw

Figure 9. Mapping of Cooled Air Throw

JET CHARACTERISTICS SURFACE EFFECT (COANDA EFFECT)

Isothermal

Feet

50 60 80 100 120 150 200 300

25

30T

50fpm

20 15 10 5 0

24'

150 cfm Velocity Measurements

300 cfm Throw T150, T100, T50

T100

fpm

T150

fpm T150

fpm

50 fpm Isovel 14" x 4"

150 cfm

500 fpm

9'

Isothermal and Cooling

20 15 10 5 0

24'

150 cfm Velocity Measurements

300 cfm Throw T150

, T100

, T50

T100

fpm 9'

Isothermal &20 F Cooling

T50

fpm

T100

fpmT

50fpm T

150 fpm

T150

fpm


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PROCEDURE TO OBTAINCATALOG THROW DATATest data in (Figure 7, page B11) shows typical velocity

measurements obtained during isothermal testing of atypical 12 x 6 grille with 155 cfm. At each measuringdistance (x), velocities are obtained below the ceiling at1, 2, 3 inches, etc. The highest sustained velocities (V

x) at

each distance are used in(Figure 7). For a complete seriesof different size outlets, (Figure 7) is a dimensionless plotof V

x/V

ovs. x /A

o.

When the curves of (Figure 7) are established for acomplete series, the throw values are included in theTitus performance tables for terminal velocities of 150,100, and 50 fpm. These catalog data can then be used tomap velocities. A uniform distribution of the three catalogvelocities across the ceiling are shown in (Figures 8 and9, page B12) for 150 cfm. In general, the T

50location for a

free jet with 20BDT cooling will be approximately the samedistance from the outlet as the T

100isothermal jet.

Figures 8 and 9 also show that for 300 cfm, the threevalues overblow to the opposite wall. These values areapproximately the total distance to the wall and down thewall. In both the 150 and 300 cfm conditions the 50 fpmisothermal envelope (isovel) is near the ceiling and wall.Higher velocities occur only near the wall with the 300 cfm.Velocities in the rest of the room are below 50 fpm.

Two outlets handling the same opposing airow valueswould result in airow in the space like that shown for onejet at the wall. The 150 fpm throw would exist at the sameposition whether heating or cooling. With overblow, as with

300 cfm, drop would not be the same as the 150 cfm. Thehigher airow is also an example of Rolling the Room,(Figure 35, page B28).

ISOTHERMAL JET THEORY FOR ALL OUTLETSAlthough the information so far has dealt with grille or walloutlets, the same principles included in (Figure 7) applyequally well for all types of outlets: ceiling diffusers withair patterns on the ceiling, ceiling slots, and upward anddownward projection of air. Slots and linear outlets usuallyshow a large second zone characteristic, which results inlonger throws as indicated by the dotted line in (Figure 7).Testing using isothermal air is repeatable. Therefore,isothermal diffuser testing has been the basis for throw

determination in all standards since the original ADC TestCode 1062. These include 1062 thru 1062 R4, ISO, and thelatest ASHRAE Standard 70.

In addition, Titus has used the three throw values at 150,100 and 50 fpm as aids in applications by mapping andapplication of the Air Diffusion Performance Index (ADPI).All results from different diffusers and diffuser locations areshown in the section on Classication of Supply Outlets,Table 4, page B9.

NONISOTHERMAL JETSYear round applications often require heating, cooling andisothermal conditions through the same device.

Titus originally tested all outlets using the method ofanalysis in (Figure 7). During cooling with a 20Fdifferential, a modication of a similar method proposedby Koestel, taking into account the buoyancy effect, wasused. As a result, Titus built an elaborate database on jetcharacteristics from all diffusers and grilles as shown in(Figures 10 through 15, pages B15 - B17).

The most signicant results are shown in these gures withside wall grilles in many applications, locations, and bladecongurations. These charts show the relationship of cfm,jet velocity, throw and drop with a 20F differential betweenthe supply air and the space air temperatures. Each circle onthese curves represents the 50 fpm terminal velocity from

the same size outlet under these congurations handling300 cfm with a jet velocity of 600 fpm. Each intercept ofcfm and velocity indicates the same grille size which can bedetermined from the tables in Sections K, L and M. From(Figure 10), the 50 fpm cooling throw and drop for a 300fpm jet at 150 cfm are 12 ft. throw and 4.5 ft. drop belowthe ceiling. For 300 cfm and 300 fpm, the throw is 13 ft. andthe drop is 6 ft. below the ceiling. The characteristics with300 cfm in this room size in (Figures 12 and 13) are verysimilar during cooling and isothermal conditions. The dropduring cooling with 150 cfm results in low temperatures andvelocities 50 fpm and above in the middle of the room.

In (Figures 14 and 15), the throw is reduced due to the45 spread of the jet and drop is reduced.

EXHAUST AND RETURN GRILLE PRESSUREFor all exhaust and return grilles, the Negative StaticPressure shown in the return performance data charts is aconservative value which can be used for design purposes.For actual pressure drop, subtract the velocity pressure fromthe reported pressure value.



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Figure 10. Throw and Drop for Outlet 2-4 feet Below Ceiling, 20 Degrees Vertical Deection, 0 Degrees Spread

Figure 11. Throw and Drop for Outlets Mounted Without Ceiling, 20 Degree Vertical Deection, 0 Degree Spread

THROW AND DROP FROM SIDEWALL OUTLETS IN FREE SPACEThe following gures show the effect of buoyancy on dropand throw for cooling jets. The charts are based on sidewall outlets.

The relationship of cfm, velocity, drop, and throw is basedon deection angle and ceiling effect. The gures belowrepresent 50 fpm terminal velocities and can be used toestimate throw and drop for ceiling diffusers. The dots

on the gures (or the same intercept of cfm and velocity)represent a single outlet using varying congurations.

1. Throw and drop values are based on 50 fpm

terminal velocity.

2. Data are based on tests with 20F cooling temperature

differential in space with no boundary wall.

3. Data are based on Models 271 and 272 (AeroBlade

Grille).

4. The small circle in the white area of each chart shows

comparative performances of one size grille at 300 cfm

and 600 fpm outlet velocity.

5.Deection settings and resulting patterns are shown onPage B21.

6. Shaded areas to right of each chart indicate noise level

above 30 NC.

7. Velocities shown are outlet discharge Jet Velocity. To

calculate Vo, divide the owrate by the effective area.


0

5

10

15

2050403020100

Throw, Ft.

Drop,

Ft.

100cfm

200

400

600

800

1000

1500

2000

cfm

TYPICAL 100 fpm ENVELOPE

700

500

1000fpmJET VELOCITY

300fpmJET

VELOCITY

0

5

10

15

20 50403020100

Throw, Ft.

Drop,

Ft.

100cfm

200

400

600

800

1000

1500

2000

cfm


500

10001500fpmJET VELOCITY

300fpm JET

VELOCITY

700


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Engineering Guidelines - GRDIsothermal Jets (continued)

Figure 12. Throw and Drop for Outlet 1 Foot Below Ceiling 0 Degree Deection, 0 Degree Spread

Figure 13. Throw and Drop for Outlet Without Ceiling, 0 Degree Deection, 0 Degree Spread

1. Throw and drop values are based on 50 fpmterminal velocity.

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

3. Data are based on Models 271 and 272 (AerobladeGrille).

4. The small circle in the white area of each chart showscomparative performances of one size grille at 300 cfmand 600 fpm outlet velocity.

5.Deection settings and resulting patterns are shown onPage B21.

6. Shaded areas to right of each chart indicate noise levelabove 30 NC.

7. Velocities shown are outlet discharge Jet Velocity. Tocalculate V

o, divide the owrate by the effective area.

0

5

10

15

2050403020100

Throw, Ft.

Drop,

Ft.

200

400

600

800

1500

2000cfm


5001000

100 cfm1500fpm JETVELOCITY

300fpm JE

TVELO

CITY

700

1000

0

5

10

15

2050403020100

Throw, Ft.

Drop,

Ft.

100cfm

200

400

600

800

1000

1500

2000cfm


700

500

1000

300fpm JE

TVELO

CITY

1500fpm JETVELOCITY


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Figure 14. Throw and Drop for Outlet 1-1/2 Feet Below Ceiling, 0 Degree Vertical, 45 Degree Spread

Figure 15. Throw and Drop for Outlet 2 to 4 Feet Below Ceiling, 0 Degree Vertical, 45 Degree Spread

1. Throw and drop values are based on 50 fpmterminal velocity.

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

3. Data are based on Models 271 and 272 (AerobladeGrille).

4. The small circle in the white area of each chart showscomparative performances of one size grille at 300 cfmand 600 fpm outlet velocity.

5.Deection settings and resulting patternsare shown on Page B21.

6. Shaded areas to right of each chart indicate noise levelabove 30 NC.

7. Velocities shown are outlet discharge Jet Velocity. Tocalculate V

o, divide the owrate by the effective area.

0

5

10

15

50403020100Throw, Ft.

Drop,

Ft.

200

400600

800

2000cfm


500

1000

1001500fpm JETVELOCITY300fpm

JETVELOCITY

1000700

0

5

10

15504020100

Throw, Ft.

Drop,

Ft.

100

200400

600

800

1000

2000cfm


700500

1000

30

1500fpm JETVELOCITY

300fpm JET

VELOCITY


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OTHER DIFFUSERS WITH COOLINGIn (Figures 10 through 15, pages B14-B16) the drop isprimarily a function of the cfm. Therefore, to obtain a small

drop, a small air quantity is necessary. For all outlets withhorizontal airow along the ceiling, the drop can be relatedto the air quantity and the ceiling height as shown in Table5. In these cases airow less than maximum shown resultsin drop above the occupied zone.

* cfm per side

SUPPLY OUTLET CLASSIFICATIONS

Independent comfort reactions have been covered and showthe effect of local velocities and temperatures at a personsneck or ankle sections. The basic jet characteristics thatcan be obtained by testing have been diagrammed. Manydifferent outlets can be selected and can be placed in manylocations for conditioning a space. This section covers theprinciple of analyzing the airow into the space from sometypical outlets during comparative heating and coolingconditions. From early air distribution tests conducted atthe University of Illinois from 1950-1955 and continuingon through today, the principles of stratication, naturalconvection currents, stagnation layers and air motion fortypes of outlet locations have been studied.

From these results, some general outlet classications

appear. These classications may be used in the rstapproach to outlet selection.

TYPICAL AIR DISTRIBUTION CHARACTERISTICSThe characteristics shown in (Figure 16, page B18), areprinciples obtained during heating and cooling in any typeof system. The following gures will classify the outlets bylocation and air pattern.

The magnitude of the stratication zones and gradientsis representative of each type of outlet in a specic spaceand can be compared directly. During cooling, the slope ofthe gradient curve in the stratication zone for all outlets isnearly the same due to constant loading and construction.

The same characteristic is shown for heating. In general,only the size of the stratication zone is changed by the typeof outlet and application.

Results from air distribution studies show that thetemperature gradient and size of the stratication zone weredecreased by a decreased temperature differential and anincrease in airow rate or supply velocity. These conditionstend to reduce the buoyancy effects, and also result in anincreased and more uniform room air motion.

In the example shown, the heating tests were conductedwith about 0F on two exposures. The cooling tests wereconducted with about 100F on two exposures. The loadingduring these tests was excessive compared to the wall andwindow R values of today. Thus the temperature gradients,stratication zones, and temperature differentials weregreater than would be expected. However, these conditionspermitted the general characteristics to be observed and

compared between different congurations.

FIVE STEPS TO ANALYSIS

The following are very important considerations toremember when selecting outlets and applications. They arebest illustrated by following the step-by-step procedure inFigure 16, page B17.

STEP 1 - PRIMARY AIR

Primary air is the starting point when laying out orinvestigating the space room air motion. Primary air isdened as the mixture of air supplied to the outlet andinduced room air within an envelope of velocities greater

than 150 fpm. The primary pattern can be completelydened by high velocity isovels taken through twoperpendicular planes. These show the number and angles ofthe jets in the primary airstream.

Maximum velocities in the primary air can be obtainedanalytically as shown in (Figure 7, page B11). Dataobtained isothermally (no temperature difference betweenthe supply and room air) down to a velocity of 150 fpmapply equally well for heating and cooling for most diffusers.

STEP 2 - TOTAL AIR

Total air is dened as the mixture of primary and roomair (that portion of the air in a space not included in theprimary and total air envelopes) which is under the inuence

of the outlet conditions. Normally, the total air has arelatively high velocity but it has no sharply dened lowerlimit. Even though the total air follows the general patternindicated by the primary air, its spread and travel may notbe in proportion to that of the primary air. Other factorssuch as ceiling height, obstructions, internal and externalloads, may disturb the orderly course of the airstream.

The temperature difference between the total and room airproduces a buoyancy effect. This causes cool total air todrop and warm total air to rise.The most complete mixingof total and room air occurs during isothermal conditions.Consequently, the location and type of outlet reduces thebuoyancy effects and increases the travel of the total air

Table 5. Maximum cfm for Diffusers Based on Drop

Outlet TypeCeiling Height, Ft.

8 9 10 12 14 16

TMRA 550 1300 2200 4000 6200 9300

TMR 270 700 1300 2100 3300 5500

TMS 1100 1500 2000

PAS 650 1000 1500

TDC250 400 650 900 1400 1600

PSS

250*

160 250 400 600 800 1000LL2*

Modulinear*

LL1 320 500 800 1200 1600 2000



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Supply Outlet Classications (continued)

Figure 16. Characteristic of Non-isothermal Supply Jets in a Space

Exposedwall

COOLING

STEP 1Primary air

Outlet

STEP 2Total air

Insidewall

HEATING

STEP 3Natural convection

currents andstratification zone

STEP 4Return intake

IntakeIntake

6'

4'

9' Ceiling

STEP 5Room air

Stratification

Stratification

Stratification

Stratification

Stratification

Stratification

Stratification

Stratification

Intake Intake

Temperature, F+- 0

Temperature, F+- 0

6'

4'

9' Ceiling

Height

Temperature Setpoint

Average Room Vertical Temperatures

during heating when cool air is induced and mixed rapidlywith primary air. This also will occur during cooling whenwarm air is induced and mixed with primary air.

In addition to the outlet type and location, the action due tobuoyancy effects is greatly dependent on the temperaturedifferential between the supply air and the room air.


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Air has a tendency to scrub surfaces. A near perfectsituation can be envisioned where the total air covers allof the walls and ceiling in a thin lm. The occupied space

would then be completely enclosed within an envelope ofconditioned air.

Since the total air within a conned space is affected byfactors other than outlet conditions, it is not subject to acomplete analytical treatment. However, air characteristicsfor cooling and heating within a free space can beestimated.

STEP 3 - NATURAL CONVECTION CURRENTS

Natural convection currents are created by a buoyancyeffect caused by the difference in temperature between theroom air and the air in contact with a warm or cold surface.The air in contact with a warm surface will rise and the air

in contact with a cold surface will fall. Convection currentsare caused not only by the windows and walls, but also byinternal loads such as people, lights, machines, etc. In mostcases, natural convection currents will not only affect roomair motion, but also play a major role in the comfort of aspace. At todays lower supply air rates, natural convectionmay be the predominant variable in determining actual roomair motion levels in the occupied zone.

Results of tests have shown ankle level temperatures duringheating may be 5F below room temperature and thatvelocities ranged from 15 to 30 fpm. (Figure 2) indicatedthat about 10% of the occupants would object to theseconditions.

Stratication layers as shown in (Figure 16) actually existin many tests. A similar situation often occurs in practice asidentied by a region where a layer of smoke will hang forsome time. Whether a stratication layer actually exists isnot important, but the concept of a stratication layer andstratication zone leads to a better understanding of airdistribution.

It should be noted that natural convection currents form amixing zone between the stratication layer and the ceilingduring cooling and between the stratication layer and theoor during heating.

STEP 4 - RETURN INTAKEThe return intake affects only the air motion within itsimmediate vicinity. Even natural convection currents possessenough energy to overcome the draw of the intake. Thisdoes not mean that the return location is not important, butonly that it has little effect on the room air motion. Otherreturn intake considerations will be discussed later.

STEP 5 - ROOM AIR

The room air diagram is completed when the remainingroom air drifts back toward the primary and total air. The

highest air motion in the space is in and near the primaryand total air. The most uniform air motion is between thetotal air and stratication layer. The lowest air motion is in

the stratication zone.The temperature gradient curves emphasize how some ofthe factors discussed are interrelated and how they affectthe space temperature distribution. Where the air motion isuniform (between the total air and the stratication layer),the temperatures are approximately equal and uniform (asindicated by the almost vertical portion of the gradient). Asthe stratication layer is crossed, the temperatures in theneutral zone vary considerably. Gradients in the straticationzones show that the air is stratied in layers of increasingtemperatures with an increase in space height.

Since the stratication zone depends primarily on natural

currents, it also must depend on the magnitude of theheating or cooling load, space construction and volume,the area of exposure, or the load (in the case of internalloading). The complete relationship is not fully understood,but many tests conducted in residences indicate thatthe gradient changes with indoor-outdoor temperaturedifference and from house to house. Consequently, themagnitudes of the temperature variations between levelswill be smaller in mild climates than in severe climates, inspaces having exposed walls with greater resistance to heatow, and with minimum internal loads. With no loading,temperature gradient curves would be vertical, indicatingthat all of the air temperatures in the conditioned spacewere equal to the control temperature.

CLASSIFICATION OF SUPPLY OUTLETS

HORIZONTAL AIR PROJECTION BELOW CEILING

Figures 17 and 18 show two outlets in this group thatprovide the most important characteristics.

Figure 17 shows a spreading jet from a high side wall grilleprojecting air horizontally near the ceiling in arestricted space.

Previous cooling curves in (Figures 10 through 15,pages B14-B16), showed different congurations andoutlet locations with and without a ceiling and no far wall

restriction.

Additional data on grille patterns are shown in GrillePerformance Tables.



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Figure 17. Spreading Horizontal Air Projection at the Ceiling Level

Total air will follow the wall down to oor level if itsprojection is sufciently long. During heating, this willresult in a smaller stratication zone and lower temperaturedifferentials in the occupied zone. Room air motion will not

be excessive unless total air extends part way into the roomat the oor. The two airstreams project into and down theexposed corners.

Figure 18. Circular Horizontal Air Projection at the Ceiling Level

Greater spread of this pattern results in more uniform spacetemperatures during cooling than with those obtained withthe two-jet pattern shown previously.

The uniform temperatures from a circular pattern indicatethat this pattern is most efcient for cooling in spaces suchas open ofce areas.

The slight overlap of the total air in the space helpsgenerate very uniform temperatures. The inuence ofthe natural currents on the total air reduces horizontaland overlap projection at the window during cooling andincreases overlap projection during heating.


A

A

Section A-A

Stratification

COOLING

HEATING

Section A-ATemperature, F

+- 0

Temperature, F

+- 0

High side wallStratification S

etpoint

Setpoint

PlanView

SideView

Stratification

A

A

Section A-A

COOLING

HEATING

Temperature, F+- 0

Temperature, F

+- 0

CeilingPlan

Outlet

Stratification StratificationSetpoint

Setpoint

PlanView

SideView

Section A-A


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Engineering Guidelines - GRDSupply Outlet Classications (continued)

Figure 19. Some Specic Airow Characteristics at High and Low Airow Rates

HORIZONTAL, CIRCULAR ANDCROSS FLOW PATTERNSWith ceiling diffusers, we must consider the different airowpatterns than with side wall outlets. Ceiling diffusers, exceptfor linear diffusers, typically exhibit ow in one of twopatterns: circular or cross ow. The diagrams in (Figure19) show the main differences between circular andcrossow patterns.

In general, a circular pattern has a shorter throw than across ow pattern. The vertical diagram shows that duringcooling the circular pattern has a tendency to curl back fromthe end of the throw toward the diffuser.

This action reduces dumping and ensures that cool airremains near the ceiling, resulting in uniform temperaturesthroughout the space. A circular pattern has greaterinduction than a crossow pattern.

Cross ow patterns with longer throw and individual sidejets react in a manner similar to side wall jets. Near theend of the throw, cross ow patterns will continue in thesame direction away from the diffuser. The drop can also bedependent only on the airow from each diffuser side. Bothpatterns are usually centered in equal spaces in large openareas. The longer throw of cross patterns can be used withgreater change in airow during VAV.

CIRCULAR FLOW CROSSFLOW

PLAN VIEWS

PLAN VIEWS

SIDE WALLGRILLES

45 DeflectionSee Fig. 17

0 DeflectionSee Fig. 13

PLAN VIEWS

ELEVATION VIEWS

LINEAR TBD

WITH SPREAD

Low Flow RateShort Throw

High Flow RateLong Throw

CEILING

DIFFUSERS

ELEVATION VIEWS

ELEVATION VIEWS


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Comparing the plan views of the circular and cross owceiling diffusers, the circular pattern has a much shorterthrow than the cross ow for a given ow rate. Theelevation views indicate that the circular supply jet at theend of its trajectory has a tendency to return to the diffuserbeing reinduced into the primary air. On the other hand,cross ow jet projection continues after the low terminalvelocity just like the airow from side wall registers at 0deection. During cooling, the circular recirculating airowresults in less drop than with cross ow jets.

The wide spreading 45 deection from the side wallgrilles is very similar to one-half of the circular pattern.

The 0 deection grille is a pattern followed by all singlenonspreading patterns.

The horizontal projection from diffusers and grilles hasbeen used extensively in commercial applications. Perimeterheating may need special treatment over the conventionalhorizontal air discharge at the ceiling level, see page B23for more information.

The linear diffuser air pattern has a tendency to fold backlike the circular pattern. This reaction results in less dropthan expected during cooling from the linear diffusers asairow is reduced.

Figure 20. Horizontal Discharge from Outlets or Near the Ceiling Applications Guidelines


For constant volume applications, select diffusers so that the T50/ L values are close to the

midpoint of the recommended range for the specic outlet. This will result in maximum

comfort for the occupants in the space. Mapping the throw for the space will indicate theconstant airow characteristics.

Guidelines for selecting outlets with variable air volume systems are included in theASHRAE Fundamentals Handbook (2009), Chapter 20, and Applications Handbook (2011),Chapter 57, and include:

1. Diffusers should be selected on the basis of both maximum and minimum(occupied) ow rates. VAV systems can vary the air delivered to the space, anddiffusers need to be selected to provide an acceptable air distribution over thatrange. Minimum ows listed on building plans are often below that expectedto be experienced when a space is occupied, so higher and more realistic owsshould be used in these cases.

2. Outlets should be chosen for relatively small quantities of air. This has the effect

of limiting the variation in throw to a minimum with the variation in ow ratefrom the outlet. In modular arrangements, this requirement may not increasethe number of outlets required.

3. Consistent with satisfactory sound levels, the minimum outlet velocity whichshould occur with the minimum airow should be approximately 500 fpmto maintain mixing at the diffuser. At maximum ow, the diffuser should beselected as loud as possible. This maximizes mixing at all points, and may helpsatisfy occupants where at high load (and high ows) when the space will be atits warmest, there is air coming into the space.

4. The maximum T50/ L or slightly higher can be used for VAV outlet selection for

maximum airow. Except for maximum load conditions, the VAV airow willbe less than the maximum. The optimum ADPI comfort can then generally bemaintained with airow rates from maximum to less than 70% of maximum.

Test results indicate that long throw outlets will result in higher air movement in a space.Slight adjustments in the temperature control would affect the occupants more than theairow change in the variable air volume system. The results indicate that problems aremore likely to exist at high airow rates. A simple rule to follow: Select the combination ofoutlets and maximum airow rate to avoid high air velocities in the occupied space. Lowerairow rates will be acceptable. High air velocities occur when the total air enters theoccupied zone. ASHRAE Research has shown that properly selected ceiling diffusers canprovide an ADPI of 100% in open ofce areas over a broad range of loads and airows,and provide Air Change Effectiveness rates of 100% at the same time.


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Figure 22 indicates a laminar ow outlet. This type of anoutlet is the opposite of the turbulent ow derived from aconventional mixing diffuser. Laminar ow implies lines ofequal velocity moving in a single direction; in this case, avertical direction. A laminar ow outlet must have relativelyuniform velocities over the whole face of the outlet, usuallyat relatively low velocity. When this is accomplished, the

airow below the outlet produces uniform velocities tothe outlet. Air is only induced at the ceiling level, andconsequently the pattern is uniform for some distance belowthe ceiling with almost the same velocity. Usual conceptsof throw do not apply in this case. Laminar ow does notproduce the normal mixing noticed with standard diffusers.The piston-like delivery of primary air is usually directedover a work area, hospital operating room, parts assembly,or clean rooms. (See the TLF Critical Environment Diffusers.)

VERTICAL DOWNWARDPROJECTION FROM CEILINGFigure 21 shows downward projection characteristics from

ceiling diffusers with circular patterns (See TMRA diffusers).With a ceiling these diffusers project either a full horizontalcircular pattern, as in (Figure 18, on page B20), or a fullyformed vertical projection. Intermediate diffuser positionsare not recommended as the airow becomes unstableduring normal operation.

During cooling the air will often be projected to the oor.A stratication zone can be formed near the ceiling,which may result in nonuniform temperatures below thestratication layer. The size of the stratication zone willvary depending on the primary air and the natural heatsources in the space. With a constant heat source a VAVsystem that reduces the ow will allow a larger straticationzone to form. This type of distribution allows high levelstratication where air must be introduced from the ceiling.Return intakes can be located between supply sources. Thistype of cooling can maintain constant temperatures aroundmachinery by projection to the oor near the machines withthe return over the machines. Projection may be obtained atlow jet velocities with the machines located in a near equaltemperature. Controlled heat loss over the machines can becontrolled by projecting more or less air to the oor level,providing more air to the oor if the main heat source is atthe oor level.

During heating, the warm supply air demonstrates lessprojection, spread and buoyancy. The oor level becomescooler and a stratication layer can form. Heat sources nearthe oor or the air projected all the way to the oor helps toreduce the neutral zone. Return intakes located at the oorin stratication zones will help.

Figure 23 indicates a hemispherical vertical ow from theoutlet. When mounted at the ceiling as indicated, a greatamount of induction occurs at the ends of the diffuser nearthe ceiling. Special arrangements in the outlet must then bemade so that the end pattern is maintained over the lengthof the room. Normally, the velocity pattern from this type ofunit will produce shorter vertical projection than the laminarow unit. Vertical projections from these diffusers will notbe as stable as those obtained from a laminar ow diffusersince the air is spreading to a position outside the diffuseroutline. This permits an obstruction directly below thediffuser to cause the airow to increase toward the outside.An obstruction near the outside of the air jet will cause theairstream velocity to increase directly below the diffuser.(See the TriTec, VersaTec or RadiaTec CriticalEnvironment Diffusers.)

Figure 21. Downward Projection Characteristics from CeilingDiffusers

Figure 22. Laminar FlowDiffusion Pattern

Figure 23. Hemispherical FlowDiffusion Pattern


Setpoint

COOLING

Setpoint

HEATING

Stratification

Temperature, F+- 0

Temperature, F

+- 0

Stratification

Stratification

Stratification

SideView

SideView

LAMINAR FLOW HEMISPHERICAL FLOW


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ESTIMATING DOWNWARDVERTICAL PROJECTIONVariations in vertical projection of a

free jet (not entrained along a surface)can be estimated by utilizing (Figure24). The chart on the right handside of this diagram shows a verticalprojection with varying temperaturedifferentials during heating, atemperature differential of 20 duringcooling and isothermal. The chart isused to estimate different projectionswith different temperature differentials.When air is entrained along a surface,as at a perimeter, the convection fromthe hot or cold surface will have acounter effect to the buoyancy of the

jet, reducing the effect shown here.

When an isothermal jet throw isknown, throws corresponding todifferent temperature differentialscan be obtained horizontally from theisothermal in the right column. Thethrow values are for a terminal velocityof 50 fpm. The chart is based on A

D=

1 sq. ft.

DIFFUSER APPLICATIONS

ON EXPOSED DUCTSA good horizontal circular pattern, when located below theceiling, will rst project upward toward the ceiling. Thisaction is caused by a low pressure region being formedabove the diffuser, allowing the higher pressure in the roomto push the air up. It is also possible for these diffusersto adjust the air pattern so that air can come down in anintermediate vertical projection, rather than the full verticalprojection as obtained from the ceiling location.

Cross ow pattern diffusers are not capable of causingthe airow to go upward when set for a horizontal patternbecause the air pressure between the jets is equal to thatin the room and, therefore, no low pressure is developedabove the diffusers.

On the other hand, a cross ow pattern diffuser generallyproduces a better intermediate vertical projection than thatof a circular pattern diffuser. Vertical projections from thecross ow jet pattern and with the velocities can be spreadout almost uniformly from the center to the outside edgesof these jets.

A good diffuser for this application is the TDCA, with a4-way pattern.

Figure 24. Downward Vertical Projection

Figure 25. TMRA Mounted in Exposed Duct

Figure 26. TDC, PSS and 250 Mounted in Exposed Duct


2400

2000

1600

1400

1200

1000

800

600

400

300

250

200

150100 200

Flow Rate, cfm

300 400 800600 1000

JETVE

LOCITY

48 5 10

030002000

fpm

1500 4000 5000

COOLING

TEMP. DIFFERENTIAL

ISOTHERMAL

HEATING

2010

7

30

5

4

40 50

3

20

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8

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WARDVERTICALPROJECTION,FT.

25

10 9

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76

8

9

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8

15

2012

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80 50

fpm

fpm

fpm

fpm

fpm

fpm

fpm

fpm

fpmfpm

fpm

50 fpm

30"

6"12"

30"

3"Ceiling

1630

TDC with 9x9 neck,4-way patternand 240 cfm


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HORIZONTAL PROJECTION AT FLOORLEVEL: DISPLACEMENT VENTILATIONFigure 27 matches the actual denition given for

displacement ventilation. Dened as a low velocity, lowtemperature differential air distribution across the oor level,displacement ventilation utilizes natural convection currentswithin the space to cause air to rise and form a neutral zoneabove a stratication level. The stratication level usuallyoccurs at a level where the room load and air loading match.

Figure 28 shows heating and cooling from a spreadinghorizontal projection at oor level. This performancecompares to previous outlet types and locations withheating and cooling loads. Statements accompanying Figure28 were made in the 1950s and considered the heatingand cooling conditions of the time. Today, a displacement

system will use larger air quantities at low temperaturedifferentials and low discharge velocities. These applicationscan be used under seats in theaters, around walls toform a ring of air around a space with people, or internal

loads forming the rising plume up to the ceiling returns. Itshould be remembered that temperatures and velocitieswill be relatively low and uniform in occupied zones. Thisdistribution system has been used extensively in Europeand is being promoted on the basis of the reduced energyconsumption realized by the availability of economizer inthat climate. With displacement ventilation, the percentageof outside air may be reduced while meeting ASHRAEStandard 62s requirements.

Note: Vertical jet oor diffusers, discussed later, which producewell mixed air in the occupied zone rather than displacementow, do not qualify for this credit.

Figure 28. Horizontal Projection Spread at Floor Level

Spreading horizontal projection at oor level permitsconsiderable stratication at high levels during cooling whilemaintaining uniform temperatures in the occupied zone

as indicated by characteristics given here. Although high

velocities in the occupied zone preclude use of this type ofdistribution for comfort cooling, application to situations withhigh internal load where occupant comfort is not of primary

concern should be considered.

Figure 27. Schematic of Displacement Ventilation


Section A-A

Setpoint

COOLING

Setpoint

HEATING

Section A-A+- 0

Temperature, F

Temperature, F

+- 0

BaseboardPlan near floor

A A

StratificationStratification


Side ViewPlan View

Stratificationlevel

Upper zone

Convection plume

Return

Lower zone

Heat/contaminant source Temperature,F

StratificationSetpoint

Supply

0


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Figure 29. Spreading Upward Projection of Primary Air in the Stagnant Zone, Relatively Large Temperature Variations Occur

VERTICAL UPWARD PROJECTION FROMFLOOR, LOW SIDE WALL AND SILLFigure 29 shows conditions where the air is projected from

the oor with a spreading jet. Natural convection currentsand the buoyancy effect of the low velocity region of thejet causes the jet to drop into the room during cooling.A stratication zone forms above the natural convectioncurrents. Temperatures are relatively uniform, exceptwarmer in the neutral zone and cooler in the drop zone ofthe jet.

During heating the warm primary air goes up and across theceiling to join the descent of the natural currents on the cold

opposite wall. The room temperature above the straticationlayer is relatively constant, but higher than desired. (Figure30) is similar, except the natural convection currents fromthe work stations are causing a similar condition duringcooling. During heating the natural currents oppose andmay stop the projection and satisfy the space reducing theprimary air until cooling is required.

Figure 29 with spreading vertical jets located on anexposed wall shows that, during cooling, the primary airdid not project as high as the nonspreading jet in (Figure31). The spreading jet did a better job of reducing thestratication layer. This was also the case in (Figure 29)during heating.

Figure 30. Upward Projection in a Work Station Figure 31. Nonspreading Upward Projection


A

Floor diffuser

Side View

Section A-A

COOLING

Setpoint

HEATING

Section A-ATemperature, F

+- 0

Temperature, F

+- 0

Side View

Setpoint



A

Stratification

COOLING

Side View

Temperature, F

+- 0

Stratification

Temperature, F

+- 0

Setpoint

Setpoint

COOLING

HEATING

Stratification

Stratification Str atification

SideView


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When using oor or low level air distribution at theperimeter, stratication from the oor can be controlledby correctly applying the 150 fpm isothermal throw data.Complete stratication across the room can occur as shownin (Figure 29, page B26), where the spreading jets do notproject to the ceiling. The stratication can be minimized bycontrolling the jet velocities and jet patterns. The verticaljet should be projected upwards to a terminal velocity ofover 150 fpm above the occupied zone. For example, if avertical throw is only 6 ft. to a terminal velocity of 150 fpm,stratication will occur above that level.

The examples discussed above apply equally as wellfor outlets located in window sills, low side wall outletsprojecting air up a wall, oor outlets near a wall or in theopen, baseboard outlets, etc.

Underoor diffuser applications are being introduced intoaccess oor facilities. These are primarily cooling andventilation applications, and the jets are seldom attached toa surface as shown in (Figures 29 and 30, page B26).Instead, the diffuser is designed to have rapid inductionand have throws which seldom exceed 5 or 6 ft. Thetemperature differential is usually less than 20F betweenthe occupied zone and discharge used with a ceilingapplication. The return air temperature near the ceilingis 5F or more, warmer than the room, and the resultantsupply-exhaust Dt is again 20F. When selected properly,oor diffusers directed upward as a free jet can result inwell-mixed, uniform temperature zones from the oor to 6ft. high, with a stratied region above.

Underoor air distribution is being proposed as a betterway to satisfy indoor air quality and comfort. Work stations

and task distribution can be considered also in thiscategory. Clean ventilation air is distributed to the occupiedspaces and contaminated air can be exhausted from thestratication zone overhead. The rules for proper ventilationand air diffusion are being investigated at this time todetermine optimum congurations.

ASHRAE has sponsored several tests with oor baseddiffusers used in this manner, and again, high levels of airdistribution, comfort and Air Change Effectiveness (ACE)were observed.

Note: The observed ACE from these tests was the same asproperly designed ceiling systems, near a level of 1.0, or 100%mixing.PERIMETER APPLICATIONS

In perimeter applications, concerns focus mainly on heating.However, it is also well understood that the air conditioningsystem must handle both heating and cooling. The heatingconcern is shown dramatically as the previous diagrams ofthe various classications of outlets are reviewed. The oorlevel and low level distribution classication diagrams showthat both heating and cooling the perimeter can be handledbest from these locations. Economic reasons, however, favorheating and cooling from overhead.

Each heating diagram shows low oor level temperatureswith neutral zones of varying sizes from the oor up to somehigher level.

Especially bad conditions are shown for overhead heating.Because this is a major concern, emphasis is placed oncei

grd guidelines

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