9 ac air_outlets

7/23/2019 9 AC Air_outlets

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ME 538Dr. Ahmed F. Elsafty

ec an ca ar ne ng neer ng ep .

College of Engineering and Technology

Arab Academy for Science, Technology &


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Low Velocity Ducts.


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The object of air distribution is to create an

acce table condition of tem eraturehumidity and air motion in the occupied

.

applying of these principals should result

user.


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Grilles

Linear grilles

Diffusers

Disk valves

Louvres


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Floor Baseboard Low sidewall High sidewall Ceiling

performance

used with

perimeter

systems

designed to

discharge

upward

Heatingperformance

Excellent Excellent ifused with

perimeter

Excellent ifused with

perimeter

Fair shouldnot be used to

heat slab

Good shouldnot be used to

heat slab

systems systems ouse n

Northern

climates

ouse n

Northern

climates

Air Registers and diffusers, from Long, Principals of Air Conditioning, 1979 by

Delmar Publishers Inc.


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Lack of uniform conditions within the s ace or

excessive fluctuations of conditions in the same

part of the space also produces discomfort.Discomfort can arise due to any of the following:

Excessive air motion (draft).

Excessive room air temperature variations(horizontal, vertical or both).

Failure to deliver or distribute air according to load

requirements at different locations Overlay rapid fluctuations of room temperature.


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Recommended velocities


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Effective Draft Tem erature: oC

= Tx - Tr a(vx - vrm)Where:

: Effective temperature difference between any point in

e occup e zone an e con ro con ons.Tx: Space air temperature in a specific location, (oC).

x , .

Tr: Mean space air temperature or set point, (oC).

, .

Vrm: Mean space air velocity (0.15 m/s).

ASHRAE Fundamentals 1997


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Air Diffusion Performance Index ADPI:

N

NADPI

100=

ere:N

: Number of points measure in occupied zone

in which (-1.7oC < < 1.1oC)

N: Total number of points measure in occupied zone

ASHRAE Fundamentals 1997


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e e ect veness o a space a r

diffusion system can also be

assessed by using a spacediffusion effectiveness factor for

air temperature T or for air

.

are dimensionless. T temperature, F (C)C concentration of air contamination, g/m3

subscript re represents the re-circulating air, ex

Effectiveness factorT compares

temperature differentials and C

the exhaust air, r the space air or air at the

measuring point, and s the supply air.

differentials .en , e space a r us on s

considered effective.

If 1 , a portion of supply air has failed to

u y u z x u

through the return or exhaust inlets directly.


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Isothermal jet

Supply temperature is the same

as the room temperature

Shortly after the high velocityprimary air leaves the outlets it

induces surrounding air into the

stream

an expanding cone. Throw: is the distance from the

outlet to a oint at which the

velocity of the air stream has

reached a definite minimum

value.


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Non-Isothermal jet

ere s a erence e ween e room

and supply air temperatures then there

will be a drop or rise due to densitydifference.

As the jet progresses into the room

primary air mixes with room air

absorbing the room load.

If the velocity is higher than 150 fpm themomentum of the jet will overcome the

buoyant force and keep the flow

.

Drop (rise): is the vertical distance the

air moves between the time it leaves the

its throw


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Ceiling effect

e e s pro ec e para e o an w n a ew nc es o a wa or ce ng e e w

take on the form of a half cone.

Essentially the same flow must now be distributed thru one half of a larger cone.Maximum velocities will remain close to the surface, creating a low pressure region

between the jet and the surface. Thus the jet hugs the surface while induction is

limited to the free side of the jet.

The net result: the throw of the

jet will be increased, and the

drop for horizontal projections

toward the surface by the rush

of induced air.


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Outlets in parallel en wo e s oca e a a s ance are sc arg ng n para e , eac as a

maximum velocity core, and each behaves independently until it reaches a distance

L where the two interfere.

jet. Past the point of interference the maximum velocity then occurs on a line midway

between the outlets.

At this oint a secondar et conical rofile is formed which behaves as if it were

emanating from an outlet twice the size of either of the actual outlets.

The net result of the mixing under

these conditions is that the throw of

the two parallel jets exceeds that of

one individually and likewise the.


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Induction

room air by the air ejected fromthe outlet and is a result of thevelocity of the outlet air.

The air coming directly from theoutlet is called primary air. Theroom air which is picked up andcarried alon b the rimar air iscalled secondary air.

The entire stream, composed of amixture of primary and secondary

, .

Induction is expressed by themomentum equation

3212211 )( VMMVMVM +=+M1 = mass of primary air

M2 = mass of secondary air

=

V2 = velocity of secondary airV3 = Velocity of total air


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IMPORTANCE OF INDUCTION: Induction

Since throw is a function of velocity and sincethe rate of decrease of velocity is dependent onthe rate of induction, the length of throw is

dependent on the amount of induction that

Induction ratioInduction ratioInduction ratio

iiioccurs.

The amount of induction for an outlet is a directfunction of the perimeter of the primary air stream cross-section.

Is defined as the ratio of the

total air to the primary air.

For two outlets having the same area, the outletwith the larger perimeter has the greatestinduction and, therefore, the shortest throw.

airTotal us, or a g ven a r quan y sc arge n o a

room with a given pressure, the minimuminduction and maximum throw is obtained by a

single outlet with a round cross-section.

airPrimary=

,shortest blow occur with a single outlet in theform of a long narrow slot.

airPrimary

a recon aryr maryi =


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Blades setting SPREAD: The angle of divergence of the air .

Spread is the result of the momentum law.

Straight Vanes: Outlets with vanes set at a

approximately 19-20 in both the horizontal

and vertical plane.

Diverging Vanes: Outlets with vanes set to give

an angular spread to the discharge air have a

marked effect on direction and distance of

travel. Vertical vanes with the end vanes set at

,

intermediate angles to give a fanning effect,

produce an air stream with a horizontal

included angle of approximately 60. Under this condition throw is reduced about 50%.


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Return grilles Even though relatively high velocities

are use ru e ace o e re urn

grille, the approach velocity drops

markedly just a few inches in front of .

This means that the location of a

return grille is much less critical than a

.

Also a relatively large air quantity canbe handled thru a return grille without

causin drafts.

General drift toward the return grille

must be within acceptable limits of

less than 50 f m; otherwisecomplaints resulting from drafts may

result. Return gril le500 cfm at a face velocit of 500 f m


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r ou e s an us ng equ pmen n ro uce a r n o a con one space o

obtain a desired indoor atmospheric environment. Return and exhaust air is

removed from a space through return and exhaust inlets. Various types ofu u v u u u

Slot diffusers are deployed primarily in applications in which suspended

panel ceilings leave a narrow gap of only 16-120 mm in width.

They are suitable for installation in rooms ranging in height from approx. 2.60

m to 4.00 m. They offer high induction, resulting in swift reduction of the

supply air temperature differential and the air discharge velocity.

The recommended volume flow range is 25 to l/s m, while the permissible

supply air temperature differential stands at 10 K. The stable discharge

behavior of slot diffusers makes them suitable for use in systems withconstant or variable volume flow. The air discharge direction can be adapted

as necessary to the desired room conditions.

Slot diffusers are generally supplied with rear-mounted plenum box, whereby

the air duct system is connected via the side-mounted round spigot.


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Slot Diffuser

A slot diffuser consists of a plenum box with single

or multiple slots and air deflecting vanes.

Air discharged from a slot diffuser can be projected

horizontally or vertically.

With multiple slots, air can be horizontally

discharged either left or right, or a combination of

both, or one slot can discharge vertically while

another discharges horizontally.

e unc on o e p enum ox s o s r u e e

air more evenly at the slot.

.


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Ceiling Diffusers

A ceiling diffuser consists of a series of concentric rings or inner cones made

up of vanes arranged in fixed directions and an outer shell or frame

Ceiling diffusers can be round, square, or rectangular.

Square diffusers are most widely used.

Supply air is discharged through the concentric air passages or directional

passages in one, two, three, or in all directions by using different types of inner

cone an vanes.


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w r users cons s o a user ace w

fixed, radially arranged air control elements.

They are available in square or round design.Connection to the air duct system is effected

via the plenum box, in either vertical or

horizontal configuration.

Swirling, horizontal discharge of the supply air

at a high induction rate guarantees swift

temperature equalization and fast reduction of

t e ow ve oc ty.

Up to 30 room air changes per hour are

attainable at supply air temperatures between+10K and -10K.

In order to stabilize the supply air flow, all sizes

must be installed flush with the ceiling.


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supply and exhaust air.

-available in a width of 27 mm

or 23 mm, with horizontal,

individually adjustable dripblades and concealed screw

.

Optionally available with screw

screw attachment (warted

holes .


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e ect on o t e supp y out et epen s on t e o ow ng:

Requirements of indoor environmental control.

, , .

Surface effect.

Volume flow per ft2 of floor area.

Appearance.


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Select the Specific Supply Outlet from

Manufacturers Catalogue

oun eve . e com ne soun eve o erm na an ou e s ou

be at least 3 dB lower than the recommended NC criteria in the

conditioned space. For optimum noise control, the recommended airv u y u w : Residences, apartments, churches, hotel guest rooms, theaters, private offices, 500 to 750

fpm (2.5 to 3.75 m / s)

General offices 500 to 1250 f m 2.5 to 6.25 m / s

The outlet velocity for the ceiling diffuser can be calculated by dividing the volume flow byarea factorAk, given in the manufacturers catalog.

Drop of Cold Air Jet. Drop of a cold air jet should be checked if the

cold jet enters the occupied zone directly.

Total Pressure Loss of the Su l Outlet.


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Introduction to sound SOUND POWER L :

is a measure of the total acousticenergy being emitted at the source in

all directions WHAT IS A DECIBEL (dB)WHAT IS A DECIBEL (dB)WHAT IS A DECIBEL (dB)room surroundings.

Sound power data is derived fromtesting methods as outlined by ARI.

ec e s a un o

measurement used to express

the relative difference in If the equipment sound power data

and the acoustic characteristics(attenuating effects) of the space areknown, it is ossible to estimate or

power between acoustic

signals.

calculate the sound pressure level inthe space.

Because sound power levels cannot

(watts)levelSound=,calculated from sound pressuremeasurements, in dB, conducted in asound laboratory.

watts)(10levelReference 12-


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Introduction to sound SOUND PRESSURE L :

measured in dB, is what the ear

(receiver) detects, and it varies in

direct relationship to the distance to WHAT IS A DECIBEL (dB)WHAT IS A DECIBEL (dB)WHAT IS A DECIBEL (dB)the source , the room volume, the

sound absorption of the room, and the

background noise.

ec e s a un o

measurement used to express

the relative difference oun pressure va ues w ou

reference to both the distance from thesource and the frequency range are

essentiall meanin less.

between acoustic signals.

A sound pressure level in the space

may be calculated from known point

source sound power level data by levelSoundknowing the acoustic characteristics ofthe environment surrounding the

source.Reference is 0.0002 microbars

levelReference


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Introduction to sound The A-weighted sound level dB(A): - ,

low to moderate levels, and is often used to rate HVAC noise in the areas of conversation and community noise exposure limits as well as to establish acoustical

design goals. The weighting is achieved by subtracting (filtering) decibels from thesensitivity in those frequency ranges. A-weighted sound level measurements arerelatively easy to perform and may be measured with an inexpensive sound meter.

e -we g e soun eve s

most useful when comparingthe relative loudness of one

acoustic environment to

another similar environment.

Measuring the A-weighted

sound level at various CFMdeliveries could help determine

if reducing the air quantity to

the diffuser is an effective

.


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Introduction to sound NOISE CRITERIA (NC):evaluate acceptable noise criteria limits for occupied spaces. These curves do notadequately predict acoustic quality other than attempting to achieve reasonable

loudness levels and avoid speech interference from HVAC noise. The NC curves work

The NC curve establishes the

maximum acce table u er

- .

limit for each octave band.

Experience has shown that

noise roblems are not

avoided unless the shape of

the actual noise spectrum

approximates that of the

c osen curve over ree

to four contiguous octave

bands.


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Recommended Criteria for

Range

o e: ese are or unoccup e

spaces, with all systems operating .

*Design goals can be increased by 5

constraints or when intrusion from

other sources represents a limiting

condition.

An acoustical expert should be

consulted for guidance on these critical

spaces.

Source: ASHRAE Handbook 1987,

HVAC Systems and Applications.


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Room effect LpNC, LwNC:The difference between Lw Lp is

called room effect and reflects the

decay in the sound power caused by:

1. The distance between the outlet and

the occupant.

2. Sound absorptivity characteristics of

the room.

The softer the room the higher the

room effect.

The figure reflects a 10dB room effect

which is normal for most general office

spaces.

The performance of the outlet with a

10 dB room effect results in a room

NC of 44.


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IMPACT OF DIRECTIVITY Sound tends to radiate from its source in a s herical attern.

When an outlet is placed in a only 1/2 a sphere is available. So, the intensity of sound

doubles as it leaves a ceiling outlet, its location is said to have a directivity factor of 2.

10 and >5-92-40-1Difference between two sources dB

.

Should an outlet be placed near the

0123Decibels to be added to the higher

unc on o wo sur aces, e rec v y

factor increases to 4. This doubles the

sound power of the outlet and results in a

3dB increase in its sound ratin . Placing an outlet in a corner would

increase the directivity factor to 8 and

result in a 6dB increase in the outlet

sound rating.


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TYPICAL EXAMPLE Two Krueger 14 inch round ceiling outlets were picked whose rated resultant room NC

was 35 dB. The following paragraphs will take you through the steps required to arrive at

the final room NC level.. : rev ew o e rueger ra ngs revea s a room e ec o was

assumed to produce the ratings. Normally a 10 dB room effect is typical for office

spaces. The difference 18 10 = 8 dB must be added back to the 35 dB to reflect more

accuratel the outlet erformance. The 14 inch outlet will therefore roduce a room NC

of 35 + 8 = 43 db

2. CHECK DIRECTIVITY: Each outlet is located well toward the center of the ceiling and is

not close to any walls or columns. Therefore, there is no directivity adjustment.

3. CHECK ACCESSORIES: Balancing dampers will be needed for each outlet. Assume

they are located in the ductwork 5 Feet upstream of the outlet and a review of the

manufacturers data shows an extra 3 dB of sound will be generated. Each outlet now

.

4. MULTIPLE OUTLETS: The room is equipped with two 14 inch outlets each producing

NC 46 dB. The difference between the two Sound sources is zero. Combining the two,

+ =


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TYPICAL EXAMPLE

NC 49


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Problems: Selection

Architectural problems:

Cornice

Duct layout problems

Structure problems: Beams

Variable air volume

Sound level


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Air outlets Nomenclature


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ir outlets Nomenclature


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ir outlets Tabular selection

i tl t


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ir outlets Selection of double deflection grilleGrilles with ceiling effect

i tl t


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ir outlets Effective outlet area

i tl t


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ir outlets Drop and rise

i tl t


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ir outlets Linear Grille

i tl t


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ir outlets Slot diffuser


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Introduction to sound

Equal loudness curve:

wo eren ec e soun s w no n genera ave e same ou nessmeasured in dB, is what the ear (receiver) detects, and it varies in direct relationshipto the distance to the source , the room volume, the sound absorption of the room,

and the background noise. Saying that two sounds have

equal intensity is not the

same thing as saying that

ey ave equa ou ness.

Since the human hearingsensitivity varies with

,

curves was plotted which

show that variation for the

average human ear.

If 1000 Hz is chosen as a

standard frequency, then

each equal loudness curve

decibel level at 1000 Hz.


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Room air movement

The performance of the outlet and the room air movement are related to

ratioInductinxCFMOutletncirculatioinairTotal1) =

ncirculatioinCFMtotalx1.4velocityroomAverage2) =

outletoppositewallofareaclearratioinductionx1.4

==

9 ac air_outlets

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