topic3 ventilation part 2

8/17/2019 Topic3 Ventilation Part 2

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MECHANICALMECHANICALVENTILATIONVENTILATION


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Natural and Mechanical SystemsNatural and Mechanical

Systems

Four (4) possible combinations of natural and mechanical ventilations are asFour (4) possible combinations of natural and mechanical ventilations are asfollows;follows;

• Natural inlet and outlet (Fully Natural supply + Exhaust)Natural inlet and outlet (Fully Natural supply + Exhaust) utilizing open able windows, air bricks, louvers, door-wa andutilizing open able windows, air bricks, louvers, door-wa and

chimnes! "rovide up to about three air changes per hour butchimnes! "rovide up to about three air changes per hour butdepend upon prevailing wind direction and strength, the stack effectdepend upon prevailing wind direction and strength, the stack effectof rising warm air currents, and adventitious opening around doorsof rising warm air currents, and adventitious opening around doorsand windows!and windows!

• Natural inlet, mechanical outlet; (Natural Supply + Mechanical Exhaust)Natural inlet, mechanical outlet; (Natural Supply + Mechanical Exhaust)

#echanical e$tract fans in windows or roofs and ducted sstems#echanical e$tract fans in windows or roofs and ducted sstemswhere the air is to be discharged awa from the occupied spacewhere the air is to be discharged awa from the occupied spaceowing to its contamination with heat, fumes, smoke, water vapor andowing to its contamination with heat, fumes, smoke, water vapor andodour! % slight reduction in air static pressure is caused within theodour! % slight reduction in air static pressure is caused within thebuilding, and e$ternal airs flows inwards! &his sstem can be used inbuilding, and e$ternal airs flows inwards! &his sstem can be used indwellings, offices, factories or public buildings!dwellings, offices, factories or public buildings!

• Mechanical inlet, natural outlet; (Mechanical Supply + Natural Exhaust)Mechanical inlet, natural outlet; (Mechanical Supply + Natural Exhaust)

%ir is blown into the building through a fan convector or ducted %ir is blown into the building through a fan convector or ductedsstem to pressurize the internal atmosphere slightl with a heatedsstem to pressurize the internal atmosphere slightl with a heatedair suppl! &he air leaks out of the building through adventitiousair suppl! &he air leaks out of the building through adventitiousopenings and permanent air bricks or louvers!openings and permanent air bricks or louvers!

• Mechanical inlet and outlet; (Fully Mechanical Supply + Exhaust)Mechanical inlet and outlet; (Fully Mechanical Supply + Exhaust) 'hen natural ventilation openings unable to cope with large air flow'hen natural ventilation openings unable to cope with large air flow

rates without disturbing the architecture or causing uncontrollablerates without disturbing the architecture or causing uncontrollable

draughts! Full mechanical control of air conditioning and ventilationdraughts! Full mechanical control of air conditioning and ventilationsstem are in used!sstem are in used!


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ntroductionntroduction

• ndooroutdoor e$change of air to replenish

o$gen and to remove pollutants generated b

breathing, indoor activities and emissions from

building materials and furnishings!

• "oor indoor air *ualit causes or irritate + of

all illnesses and can lead to a host of health

problems for people of all ages!

• For ages, most buildings relied on openingsthrough the building envelope to provide

indooroutdoor air e$change! .owadas,

mechanical ventilation can make it safe to seal

unwanted air leaks and install as much

insulation as needed!

• /entilation in modern buildings (enclosed space

and without natural ventilation) re*uires

additional e*uipment to create air movement!


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&he purposesob0ectives&he purposesob0ectivesof mechanical ventilationof mechanical ventilation

sstem1sstem1

2!2! &o get enough air (*ualit&o get enough air (*ualitand *uantit)!and *uantit)!

3!3! &o control ventilation&o control ventilation

sstems (flow in and out)sstems (flow in and out)!! &o control indoor&o control indoor

environment (air *ualit, airenvironment (air *ualit, airvelocit, location, wa ofvelocit, location, wa ofuse)!use)!

4!4! &o remove heat and dirt air!&o remove heat and dirt air!

Four (4) basic tpes of

whole-house mechanical

ventilation sstems1

5$haust 6stem

6uppl 6stem

7alanced 6stem

"lenum 6stem


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(a) 5$haust 6stem(a) 5$haust 6stem

%n e$haust sstem ma consist of a single fan that is centrall located

in a hallwa or at the top of the stairs, or it can be an upgraded

bathroom fan that has a motor built to run for longer periods of timeand is sized to ventilate the whole house!

&hree () mode of operation1

2! #echanical inlet, natural e$haust (positiveover pressure)

3! .atural inlet, mechanical e$haust (negativeunder pressure)! #echanical inlet and e$haust (controlled pressure)


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Mechanical Air Supply with Natural AirMechanical Air Su

pply with Natural Air

ExhaustExhaust• %ir suppl is controlled mechanicall and

the air e$haust takes place on a naturalwa b ventilation openings, windows or

shafts! &here will be an overpressure in the

building!• &o prevent draught, the air supply in the

room has to be placed as high as possible

and the air inlet grid must have a possibility

to be regulated• 7 preheating the incoming air, draught

problems can also be decreased!• %n air filter used to clean the incoming air!

• % ventilator (or fan) controls the air suppland the outdoor air is transported into the

building b ducts!• 7 controlling the ventilator it is possible to

control the ventilation capacit for the

sstem!

5$haust5$haust

6stem6stem


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Natural air supply with mechanical air

exhaust

• popular form of ventilation in residential and offices!• &he mechanical air e$haust sstem creates an

under pressure in the building, the sstem is less

dependent on the weather! &he under pressure

creates a pressure difference over the ventilation

openings encourage the air to suck in!

• 8igh wind pressure or temperature difference canresult in draught problems!

• &o prevent draught the air supply openings have to

be placed as high as possible and the air inlet grid

must have a possibility to be regulated• /entilation capacit (air out) controlled b an

e$haust ventilator!• n residential buildings suction takes place from at

least the kitchen, the bathroom and the toilet! (need

for suction ducts)• n non-residential buildings suction mostl takes

place from the corridor!• For locations where the sound load is high

sometimes baffle filters are used!

5$haust 6stem5$haust 6stem


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#echanical 6uppl and 5$haust#echanical 6uppl and 5$haust• % mechanical ventilation sstem can be combined

with all sorts of heating and cooling sstems!

• 9ften the heating, cooling and ventilation of abuilding are combined in the air-conditioning

sstem (8/%:)!• n a mechanical ventilation sstem the suppl air

and the e$haust air are transported mechanicall!

%dvantages of an 5$haust /entilation are %dvantages of an 5$haust /entilation are1• ood control of the ventilation capacit; nodependence of the outdoor weather conditions and

despite possible nois environment!• &he possibilit of e$tracting heat from the e$haust

air and use it to preheat the fresh air suppl (heat

recover)!• &he possibilit of preheating and pre-cooling of the

air suppl!• &he possibilit of humidif and dehumidif of the air

suppl!• &he possibilit of cleaning the air b an air filter or

suppling the air from a relative clean site of the

building!

5$haust 6stem5$haust 6stem


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Exhaust FreshupExhaust FreshupRaising exhaust air decreases room pressure to prevent the

leaking of odors or floating bacteria into other rooms


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(b) 6uppl 6stem(b) 6uppl 6stem

% suppl sstem slightl pressurizes the house b drawing outdoor air into the

house! &his pressurization pushes indoor air out through small openings in the

building, which helps to prevent outside moisture and pollutants from getting in

through these passages! "ositive indoor pressure also protects against back

drafting!


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n a suppl sstem, a fan pullsoutside air into the house,

creating positive pressure!


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F$ESH%& 'perati'n( 7oth the e$cessive suppl mode and the e$cessive

e$haust mode are selectable! &his function allows for a more comfortable

air environment whether the occupied space is re*uired to be pressurized

or de-pressurized!

Supply rich m'de( Raising the air supply maintains proper room

pressure to prevent back-flow to toilet or kitchen odours or moisture inflow!


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(c) 7alance 6stem (benefits)(c) 7alance 6stem (benefits)

7alanced ventilation7alanced ventilationsstems can providesstems can provideman benefitsman benefitsincluding1including1

Impr')ed ind''rImpr')ed ind''rair *ualityair *uality Impr')ed c'm+'rtImpr')ed c'm+'rt Impr')ed healthImpr')ed health L'wer utility "illsL'wer utility "ills Impr')ed resaleImpr')ed resale

p'siti'np'siti'n


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Impr')ed ind''r airImpr')ed ind''r air*uality,*uality,

Impr')ed c'm+'rt,Impr')ed c'm+'rt,

Impr')ed health,Impr')ed health,

L'wer utility "ills,L'wer utility "ills,

Impr')ed resaleImpr')ed resalep'siti'n,p'siti'n,

-alance System !"ene+its#-alance System !"ene+its#

7alanced ventilation sstems suppl fresh air to the living7alanced ventilation sstems suppl fresh air to the livingand sleeping areas of homes while e$hausting stale air at anand sleeping areas of homes while e$hausting stale air at ane*ual rate from the bathrooms! &his proactive approach toe*ual rate from the bathrooms! &his proactive approach to

ventilation can result in improved indoor air *ualit!ventilation can result in improved indoor air *ualit!

5.5=> 6&%= labeled homes with tight construction and5.5=> 6&%= labeled homes with tight construction andbalanced ventilation sstems can have fewer drafts and abalanced ventilation sstems can have fewer drafts and aconstant suppl of outdoor air resulting in improved comfort!constant suppl of outdoor air resulting in improved comfort!

6tale air can cause health problems! t can be responsible6tale air can cause health problems! t can be responsible

for smptoms such as headaches, drowsiness, andfor smptoms such as headaches, drowsiness, andrespirator problems! &hese smptoms are more common inrespirator problems! &hese smptoms are more common inhomes with poor ventilation and moisture control!homes with poor ventilation and moisture control!:ontinuousl providing fresh air can result in the improved:ontinuousl providing fresh air can result in the improvedhealth and well being of the occupants!health and well being of the occupants!

?ess energ is consumed to operate ventilation sstems?ess energ is consumed to operate ventilation sstemsthan to heat and cool e$cessive amounts of outdoor air thatthan to heat and cool e$cessive amounts of outdoor air that

infiltrates leak homes! %dditional savings are capturedinfiltrates leak homes! %dditional savings are capturedwhen these sstems are e*uipped with either a sensible orwhen these sstems are e*uipped with either a sensible ortotal heat e$changer! &his can result in lower utilit bills,total heat e$changer! &his can result in lower utilit bills,making homes less e$pensive to operate!making homes less e$pensive to operate!

5.5=> 6&%= labeled homes with balanced ventilation5.5=> 6&%= labeled homes with balanced ventilationsstems can provide the man impressive benefits listedsstems can provide the man impressive benefits listed

including more comfortable homes with better indoor airincluding more comfortable homes with better indoor air*ualit and lower utilit bills! &hese benefits can translate*ualit and lower utilit bills! &hese benefits can translateinto higher resale value!into higher resale value!


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(d) "lenum 6stem(d) "lenum 6stem• "lenum is the space that can

facilitate air circulation for heatingand air conditioning sstems, b

providing pathwas for either

heated conditioned or return

airflows!• 6pace between the structural

ceiling and the dropped ceiling orunder a raised floor is tpicall

considered plenum!• %ir pressure in plenum is higher

than outside!• For buildings, plenum is an

enclosed space that are not forhuman occupanc, but are often

used for heating, ventilating

andor air conditioning e*uipment

and airflow, and are for e*uipment

such as cables, piping, and

luminaries


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5*uipment for #echanical 6stem5*uipment for #echanical 6stem

&he motive power for the distribution of air

in ventilation sstems is invariabl provided

b electricall driven e*uipement!

• Fan 7lower • 5$tractor

• %ir :onditioning


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5*uipment for #echanical 6stem5*uipment for #echanical 6stem

&here are three main tpes of fan in use

for blowing or e$tracting wind in or out of

the building 1

2! "ropeller fans

3! :entrifugal fans

! %$ial flow fans


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"ropeller fans are suitable for situationswhere no great resistance to airflow has to

be overcome!

&he free intake and discharge condition of

ventilation fans situated in wall openings

giving direct inout and outin movementare eminentl suited to this tpe of fan,

which under these conditions can move

large volumes of air economicall and with

ver low installation costs!

5fficienc A-A+

6hort duct sstems can also be served,

provided the resistance of the sstem is

low!

(i) "ropeller fans(i) "ropeller fans

% tpical fan


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(ii) :entrifugal fans(ii) :entrifugal fans

:entrifugal fans can generate high-pressure

rises in the gas stream BA"a! %ccordingl,

the are well-suited for industrial processes

and air pollution control sstems! E++iciency

/012 hi.h c'st2 silence2 "i. si3e and need

m're space,

% centrifugal fan has a fan wheel composed of

a number of fan blades mounted around a

hub! &he hub turns on a shaft that passes

through the fan housing! &he gas enters from

the side of the fan wheel, turns C° and

accelerates as it passes over the fan blades!

&he term, centri!ugal , refers to the tra0ector of

the gas stream as it passes out of the fan

housing!

&he centrifugal

tpe generall

supplies more

cfm perhorsepower at

static pressures

over 4!-4!+

inches of water

and is often

used where low

noise level is

desired


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(iii) %$ial flow fans(iii) %$ial flow fans

&his tpe of fan is becoming popular!

Dse in the ductwork sstem

5fficienc is high to B+, simpleinstallation and appearance neat,

particularl in a line of ducting!

&he vane-a$ial tpe has a propeller and vanes, and is best for static pressures under 4!-4!+ inches of water


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Ad)anta.es '+ .''d )entilati'nAd)anta.es '+ .''d )entilati'n

"# $E%&'*N N &M% -E.E-S"# $E%&'*N N &M% -E.E-S

-


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Example ( T%$-INE $OOF VENTILATIONExample ( T%$-INE $OOF VENTILATION

&ornado &urbine =oof

/entilation

&he 'hirling &urbine %ction1

=emoves :onvicted 8eat

"ollution 7uild-Dp

- mproves %ir ualit

- 'ater and


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SI %NIT FO$ VENTILATIONSI %NIT FO$ VENTILATION

%nit used to determine a /entilation in abuilding1

2!%ir :hange per 8our (%:8)

3!/olumetric flow rate (msec)!#ass flow rate (kgs)


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5$ercise5$ercise

A room 15m x 7m x 2.8m high has a ventilation rate of 11 airchanges per hour. Air enters from a duct at a velocity of 8.5m/s. Calculate the air volume flo rate to the room and thedimensions of the s!uare duct.


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5$ercise5$ercise Anser "he air flo rate is given #y$

% & '( air changes/hour) * '+m,/air change) * '1h/,-s) here room volume +m, & 1 air change. ence % & '(+/,-) m/s & '11 * 15 * 7 * 2.8) / ',-) m,/s & .0 m,/s Also % m,/s & duct crosssectional area A m2 * air velocity + m/s "herefore A & %/+ & '.0/8.5) m2 & .1- m2 3f the duct side is l m then A & l2m2. "herefore l & + A m & 4.1- m & .,25 m


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5$ercise5$ercise

% lecture theatre has dimensions of 3+m G 33m G Am

height with 2 occupants; Hls of fresh air and 3+ ls of

re-circulated air are supplied to the theatre for each

person! % single-duct ventilation sstem is used! f 2

of the suppl volume leaks out of the theatre, calculate

the room air change rate and the air volume Iow rate in

each duct


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%nswer1

6uppl air *uantit @ (3+ J H) ?s $ 22 m?

@ ! ms

8ence

s @ ! $ (2 J 2)

@ !A ms

%lso @ ./A

!A ms @ . (3+ $ 33 $ A)A

. @ (!A $ A) (3+ $ 33 $ A)

@ !CA airchanges per hour

?eakage fron the theatre is l @ 2 $

@ 22 $ !A ms

@ !A ms

5$ercise5$ercise


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5$ercise5$ercise :ontinue!!!

&he *uantit of air e$tracted from the theatre e @ (!A K !A) m

s @ !3B ms

uantit of fresh air f @ (H $ 22)2 ms

@ !HH ms

&he *uantit of recirculated air is r @ - f

@ !A ms - !HH ms

@ 3!B+ ms

uantit of e$aust air e$ @ e K r

@ !3B ms - 3!B+ ms

@ !+3 ms


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2!2! 'h proper ventilation is re*uired inside'h proper ventilation is re*uired insidea buildingLa buildingL

3!3! 5$plain factors to consider in5$plain factors to consider inma$imizing natural ventilation in ama$imizing natural ventilation in abuilding!building!

!! 6ketch and briefl describe 4 basic6ketch and briefl describe 4 basicsstem in mechanical ventilation!sstem in mechanical ventilation!

topic3 ventilation part 2

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