chapter 3 the air distribution system

8/12/2019 Chapter 3 the Air Distribution System

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The Air Distribution System

MEC 351

DR HAMID YUSOFF


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AHU


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The purpose of air distribution system

is to create the proper combination oftemperature, humidity and air velocity

in the occupied zone of the

conditioned room. If not properly designed, the air

distribution system has the potential to

compromise the comfort conditions it

was designed to maintain.


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Terminology

Diffusiontemperature

Supply air to space airtemperature difference is

termed diffusiontemperature. In theexample, the room designtemperature, Tr is 25 deg C.and the supply temperatureTla, is 13 deg.C, hence thediffusion temperature is (2513) = 12 deg C.


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Terminology

Induction,entrainment, oraspiration

Involve mixing of roomair with the supply air

outside the occupiedzone. Entrainment isdefined as themovement of room airinto the jet caused bythe air stream

discharged from theoutlet (secondary airmotion).

aspiration


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Terminology

Throw (blow) andradius of diffusion

The throw or blow andthe radius of diffusionrefer to the horizontaldistance that the supply

air travels before reachinga specified low velocityknown as the terminalvelocity. Generally, throwor blow refers to dischargefrom the sidewall outlet,and the radius of diffusionrelates to the ceilingoutlets.


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TERMINOLOGY

AHU

An air handler, or air handling unit (often abbreviatedto AHU), is a device used to condition and circulate airas part of a heating, ventilating, and air-conditioning(HVAC) system. Usually, an air handler is a large metal

box containing a blower, heating and/or coolingelements, filter racks or chambers, sound attenuators,and dampers. Air handlers usually connect to ductworkthat distributes the conditioned air through the

building, and returns it to the AHU. Sometimes AHUsdischarge (supply) and admit (return) air directly to andfrom the space served, without ductwork.


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Terminology

Occupied Zone 3 inches to 72 inches above the floor level.

Two feet in from the walls


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Evaluating Comfort

Comfort criteriaGood room air distribution requires the following characteristics forcomfort:Comfort is maintained through the change in seasons when thefollowing conditions are maintained in the space occupied zone.

Air temp maintained between 73-77F

RH maintained between 25-60%

Maximum air motion in the occupied zone1. Air velocities throughout the occupied zone (called residual

velocities ) between 2535 FPM for applications where peopleare seated. (FPM feet perminute)

2. Moving peopleare higher air velocities are acceptable ( 50 -70FPM)

Maximum temperature gradient (temperature fluctuation)1-2 cooling

4 heating

Note: Occupied zone of most spaces is considered to be from the floor to an

elevation of 6 ft. Above this height, greater temperature fluctuations are

permissible


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effective draft temperature - Kelvin

As mentioned previously, certain parameters can be controlled to ensure a

relatively comfortable environment. The question is how does one

evaluate the system to make sure that the actual parameters are within

the allowable range? Standard methods exist to evaluate these

parameters in real or simulated spaces to determine if the conditions are

being met. The first evaluation concept is called the effective draft

temperature - Kelvin , which uses the local air temperature and velocity in

a single equation to determine a relative feeling of coolness. The equation

is:

The effective draft temperature is evaluated at many locations throughout

the space and comfort studies have shown that the effects of temperature

and velocity on comfort will be acceptable if is kept between -3 and +2F


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TheAir Diffusion Performance Index

(ADPI)

The percentage of locations in a space in whichthe limits of the effective draft temperature aremet. In other words, an ADPI of 80 means that80%; of the locations in the space have an

effective draft temperature of -3 to +2 F.Obviously, an ADPI of 100%; would be theultimate condition, but the efforts required toachieve this condition would often cost much

more than the added benefit. Studies haveshown that an ADPI of 80%; or more results insatisfactory comfort.


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Successful Design Achieves

Good ventilation effectiveness

Avoids dumping (lambakan)

Avoids draft


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Description of terms used in air distribution

Throw (T)- The maximum horizontal distance from the center of the air

outlet to a point in the airstream where the air velocity has decayed

(menyusut) to some arbitrary terminal velocity(typically 150, 100, and

50 fpm). The common unit of measure is feet (ft)

Drop (D)- The vertical distance from an air outlet to the lower edge of

a horizontally projected airstream envelope. The common unit of

measure is feet (ft).


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Air patterns

The are a few facts about how facts about how an air supply to aroom will behave which are important to understand in order toselect and locate air supply devices properly and to balance andadjust the devices.

1. When air lower temperature than room air is supplied (as in

summer), it will drop.2. When air higher in temperature than room air is supplied (as in

winter), it will rise.

3. When air is supplied parallel to and near a ceiling, it will tend tohug the ceiling for some distance. This is called the ceiling orsurface effect.

4. The supply air to the room ( called the primary air) whendistributed from air supply device, will induce room air (calledsecondary air) into air stream, thus rapidly mixing the supply airand the room air.


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Outlet location

High wall. This is a good location for coolingbecause the cold air will drop naturally andadequate air circulation throughout the occupiedzone will occur.

Ceiling. This is excellent location for coolingbecause the cold air will drop naturally.

Low wall. This is a good location for heatingbecause the warm air will rise naturally.

Floor or sill. This is an excellent location forheating if located under windows, because itcounteracts the cold air downdraft.


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location

Coolinggood air distribution

Floor or sill location under window forheating provides good air distribution

Heatingpoor air distribution

Ceiling outlet location for cooling providesgood distribution


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Supply air outlets and air distribution

patterns

Grilles and registers These consist of a frame and

fixed or adjustable parallel bars.

Ceiling diffusers These consists of a series of

separated concentric rings or louvres with a collar or

neck to connect to the duct.

Slot diffusers A slot diffuser is a long strip-shaped

outlet with one or more narrow openings.

Plenum ceiling panels The confined space above

the ceiling is used as a large plenum from which the

supply air is delivered through slots or holes,

particularly to large zones of uniform temperature.


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Outlet Classification

Types of air supply devices

Group A. Outlets mounted in or near the ceiling that discharge air

horizontally.

Group B. Outlets mounted in or near the floor that discharge air

vertically in a non-spreading jet.

Group C. Outlets mounted in or near the floor that discharge air

vertically in a spreading jet.

Group D. Outlets mounted in or near the floor that discharge airhorizontally.

Group E. Outlets mounted in or near the ceiling that project

primary air vertically.


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Group A

High Sidewall Grilles


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Group A Ceiling Diffusers


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Neck, face, core & free area


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Neck Area & Neck Velocity


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Outlet Discharge Patterns


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Circular

Diffuser

The downward-jet

pattern may be of

benefit when the

outlet is installed in a

tall space and HVAC

(heating, ventilation air

conditioning) is in

heating Mode Thevisualization

Shows consequences for

cooling mode


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Square/Rectangular Diffusers


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Square

Diffuserprovides flexibility

for distributing air

in a variety of

patterns


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Slot Diffuser


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Slot Diffuser

allows directingthe air downward


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Air distribution system design

Room air distribution

Outlet selection and location procedure

Return air system


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Room air distribution

Conditioned air should be supplied so that natural convection,radiation and conduction effects are counteracted.

Return grilles should be so located that direct by passing of air fromthe supply outlet to the return grill will not occur.

Adequate return grilles should be provided and the same should be

sized for low friction loss. Supply outlets should be so selected that their throw puts the

conditioned air where it is needed, but excessive throw should notbe allowed in which case the air would bounce off the walls andcreate draft. The desirable length of throw is up to three fourth ofthe distance to the opposite side of the wall.

Noise level of supply outlets must kept low.

Supply outlets should have adjusting features so that good room airdistribution can be obtained.


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Outlet Selection and location

procedure1.Determine Air Volume Requirements & Room Size. In case more than one air outletis used, the total air flow rate needs to be divided by the number of outlets. =

1.2

= (3 /)

= (

)

= ()

2.Select diffuser type and location within room. Considering factors as air quantity, distanceavailable for throw, structural characteristics and architectural constraints. For example, if circularceiling diffusers are to be used, each would be centred in a ceiling area that is roughly square inshape as the air is supplied in all directions from the diffuser.

3.Determine room characteristic length from table, L


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ASHRAE Outlet Selection Steps

4.Select recommended throw to length ratio from the ADPI selection guide


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ASHRAE

Outlet Selection Steps

5.Calculate throw .


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Return air system

Several factors should be considered in locating return airgrilles

Location of grilles should avoid causing drafts in theoccupied zone.

The return air duct system should be sized for low pressuredrop to reduce noise generation.

It is not required to locate a return air grille in each space.Excessive air collected at a single return should be avoided,because of the possibility of creating drafts or noise.

Return air grilles should be so located that short circuiting isavoided prior to complete mixing and heat exchange ofsupply air.


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Recommended Sound Reduction

Methods

The following steps can be taken to reduce the sound generation of air outlets and airinlets:

Reduce the airflow delivered to each device by utilizing a greater number of devicesfor the same total airflow to or from the space. However, the size of the devices mayneed to be reduced to maintain the proper room air motion.

Increase the size of the air outlet or air inlet, which decreases the face velocity and

pressure loss.However, increasing the size of an air outlet without increasing theairflow will result in shorter throws and may cause conditioned air to drop into theoccupied zone.

Balancing dampers should not be used in the neck of the air outlet. This can addanywhere from 5 to 16 NC to the sound rating, depending on the amount of damperclosure. Balancing dampers should be located as far upstream from the air outlet aspossible.

The approaching ductwork to the air outlet should be kept as straight as possible. Anyoffset or abrupt bend will add up to 15 NC to the sound rating. If flexible duct is used,sharp bends should be prevented at the neck of the air outlet.

As a general rule of thumb, the face velocity of an air outlet should not exceed 800fpm and the face velocity of an air inlet should not exceed 400 fpm.


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Example 1

Select a round ceiling diffuser for a room in an office building, as shown in below.

The room, 4.2 m wide by 4.8 m long and with ceiling 2.4 m above the floor, is to be

supplied with 120 litres/s of conditioned air. The room load was estimated to be

125 W/m2. A noise criteria (NC) of 20 -35 would be acceptable.

4.2 m

4.8 m

diffuser


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Example 2

Specify circular ceiling diffusers for cooling a school room to

be maintained at 25 deg. C. The cooling load for the room is

13.2 kW and the supply air temperature is 13 deg.C.

10 m

12 m

chapter 3 the air distribution system

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