Levin NatVentTheory

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NaturalNatural Ventilation: Theory Ventilation: Theory

Hal LevinHal Levin

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Natural Ventilation: Theory

DefinitionsPurpose of ventilation What is ventilation?Types of natural ventilation (Driving forces): Buoyancy (stack effect; thermal) Pressure driven (wind driven; differential pressure)

Applications Supply of outdoor air Convective cooling Physiological coolingIssues Weather-dependence: wind, temperature, humidity Outdoor air quality Immune compromised patients Building configuration (plan, section) Management of openings

44

Natural and Mixed Mode Ventilation Mechanisms

Courtesy of Martin Liddament via Yuguo Li

Mixed Mode Ventilation

Sketch of school system

Sketch of B&O Building

Natural Ventilati on

Cross Flow Wind


Wind Tower Stack (Flue) Stack (Atrium)

Fan Assisted Stack

heated/cooledpipes

heated/cooledceiling void

Top Down Ventilation

chilled pipes

Buried Pipes


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Climate Typology (oversimplified!)Not in handout materials to be completed by class

BogotCold dry

Cold humid

San

FranciscoMt. Fuji

Temperate

seasonal RH

Montreal,Canada

LimaBostonTemperateseasonal -- Temp

Quito, Ecuador High desertTemperate dr y

Milan, ItalyLondonTemperate humi d

Low desertHot dry

SingaporeHot humid

No seasonalvariation

Seasonalvariation

Steady d ailycycle

Diurnalswing

Climate type

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What is ventilation?

Definitions covering ventilation and the flow of air into andout of a space include:

Purpose provided (intentional) ventilation: Ventilationis the process by which clean air (normally outdoor air)is intentionally provided to a space and stale air isremoved. This may be accomplished by either natural ormechanical means.

Air infil tration and exfil tration: In addition to intentional

ventilation, air inevitably enters a building by the processof air infiltration. This is the uncontrolled flow of air intoa space through adventitious or unintentional gaps andcracks in the building envelope. The corresponding lossof air from an enclosed space is termed exfiltration.


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Three elements of ventilation(source: Yuguo Li, personal communication)

Element Des cr ip ti on Req ui rem en ts /Guideline

Design orOperation

Buildings

Primary External airflow rate

Minimum ACHMinimum L/s

Fan, duct,openings orstreets

ASHRAE62

1-12 ACH

Secondary Overall flowdirectionbetween zones

Flow clean todirty spaces

PressurecontrolthroughairflowimbalancePrevailingwinds

Positive/negative2.5-15 PaIsolation/smokecontrol

Tertiary Airdistributionwithin a space

Ventilationeffectiveness,no short-circuiting

Use of CFDSmokevisualization

Ventilationstrategies

Cities

?

Dirty industrydownwind

Buy upwind

Urbanplanning

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Courtesy of Yuguo Li

Isolation room ventilationGoal: ~12 ach or 160 l/s-p (?)

23oC 23 oC 23 oC

The purpose is Not to h ave a 2.5 Pa negative pressure, but no air leaks to the cor ridor !

23 oC

C o r r i d o r

A

n t e r o o m

T o i l e t / b a t h r o o m

Cubicle

Suspended ceilings

Recommended negative pressure is 10 Pa with wind, -2.5 Pa without wind


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Types of natural ventilation

Stack effect (buoyancy) Warm air is lighter (less dense)

than cold air Warm air rises, cold air falls Intentional chimneys (stacks)

can create larger differencesbetween top and bottom,increasing the air flow rate

Wind-driven (pressure) Pressure differences result in

air mass movement Packets of air flow from

higher to lower air pressureregimes

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Wind driven vs. Stack effect


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Natural Driving Mechanisms Pressure:Wind-driven air flow

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Natural Driving Mechanisms Pressure:Wind-driven air flow


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Natural driving mechanisms -- BuoyancyStack effect

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Hot air = buoyancy


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Natural driving mechanisms -- BuoyancyStack effect

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Applications: Supply of outdoor air

Supply of outdoor air removal of pollutants In air changes per hour (AER or h -1) or liters per person per

second (l/s-p) What happens if you have a very tall space?

Pollutant concentration = source strength/removal rate Removal rate includes dilution/exhaust plus deposition on

surfaces or chemical interactions/transformation Chemicals: source strength expressed as mg of pollutant / m 2-h

or mg/h Dilution/exhaust rate expressed as dilution ventilation (air

changes per hour) Deposition: gcm -1s -1


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Pollutant concentration as a function of outdoor airexchange rate

0

1

2

3

4

5

6

7

8

0 1 2 3 4 5

Ventilation air changes per hour (ach)

C o n c e n t r a t i o n

( g

/ m 3 )

EF = 1g/m2 hr

EF = 5 g/m2 hr

EF = 10 g/m2 hr

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Applications: Convect ive cooling convection /convection/ (kon-vekshun) the act of conveying or

transmission, specifically transmission of heat in a liquid or gas bybulk movement of heated particles to a cooler area.

Air flow person can be caused by the higher temperature of thepersons skin relative to the air around it, giving rise to an air flowknown as the thermal plume, air movement predominantly in anupward direction.

Or, it may be caused by forced air movement, as from a fan orwind.

Temperature variation in an objectcooled by a flowing liquid


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Convective cooling

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Physiological cool ing


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Appl ications: Physiological cooling

Ectothermic cooling Vaporization:

Getting wet in a river, lake or sea. Convection:

Entering a cold water or air current. Building a structure that allows natural or generated air flow for cooling.

Conduction: Lie on cold ground. Staying wet in a river, lake or sea. Covering in cool mud.

Radiation: Find shade.

Enter a cave or hole in the ground shaped for radiating heat (Black boxeffect). Expand folds of skin. Expose skin surfaces.

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Convective + Physiological cooling


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Physiological cool ing

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Impact of wind and temperaturedifference on natural ventilation


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Concept of the neutral level

Externalpressuregradient

Internalpressure

gradienth

1

h 2

T iT 0

Neutralplane

Externalpressuregradient

Internalpressure

gradienth

1

h 2

T iT 0

Neutralplane

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Single-sided ventilation


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Influence of wind and temperature (stackeffect) on ventilation and air flow pattern

(source: AIVC, 2009)

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Cross flow ventilation(source: AIVC, 2009)


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Stack ventilation (dwellings)(source: AIVC 2009)

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Stack ventilation (atrium)(source: AIVC 2009)


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Stack driv ing flows in a building

Neutralpressureplane

+

+

_

_

+

+

_

_

_

_Indoor air

temperatureis greater

thanoutdoor


+

+

_

_

+

+

_

_

_

_Indoor air

temperatureis greater

thanoutdoor


+

+

+

_

_

+

+

+_

_Indoor air

temperatureis less than

outdoor


+

+

+

_

_

+

+

_

_

+

+

+_

_+

+_

_Indoor air

temperatureis less than

outdoor

(A)

I n d o o r a i r w a r m e r I n d o o r a i r c o o l e r

t h a n o u t d o o r t h a n o u t d o o r

(B)

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Wind floor 18F

Memorial Hall

Entrance Hall

LectureRooms

Graduate School

Offices

Library Car Parking

Roof Garden

119.5m

Heat Storage TankRain Water Tank

Canteen

Roof Garden

Stack effect in ahigh ri sebuildingLiberty Tower,Meiji University,

Tokyo

Stack effect


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Wind Floor for HybridVentilation

Gross Floor Area: 59000 m 2

completed in 1998

Meiji University L iberty Tower, Tokyo, Japan(source: Professor Toshihara Ikaga, Keio University)

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Natural ventilation in bui ldingsFrancis Allard, Mat Santamour is, Servando Alvarez, European

Commissio n. Directorate-General for Energy, ALTENER Progr am


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Natural ventilation in buildingsBy Francis Allard, Mat Santamouris, Servando Alvarez, European Commission.

Directorate-General for Energy, ALTENER Program

http://books.google.com/books?hl=en&lr=&id=1tdQMyhPA2gC&oi=fnd&pg=PR9&dq=Natural+ventilation+theory&ots=mFzmfd4mct&sig=XA3zksH_OBkkS8tILbxmwJqbWyo

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Natural ventilation in bui ldingsFrancis Allard, Mat Santamour is, Servando Alvarez, European

Commissio n. Directorate-General for Energy, ALTENER Progr am


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Indoor air velocities for naturally ventilated spacesunder different wind directions and different number of

apertures and locations

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Effects of inlet / outlet sizes in cross-ventilatedspaces with openings on opposite walls


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Velocity as percent of wind velocity: openingson opposite walls, wind perpendicular to inlet

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Effect of oblique wind direction withopenings on oppos ite walls


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Velocity as percent of wind velocity: Openingson opposite walls, wind oblique to inlet

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Effects of inlet / outlet sizes in cross-ventilatedspaces, openings on adjacent walls

Wind perpendicular Wind oblique


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Effect of inlet and outlet sizes, openings onadjacent walls, wind perpendicular to inlet

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Simple formulation for Vent Calculation


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4949

Natural and Mixed Mode Ventilation Mechanisms

Courtesy of Martin Liddament via Yuguo Li


Sketch of school system

Sketch of B&O Building

Natural Ventilati on

Cross Flow Wind


Wind Tower Stack (Flue) Stack (Atrium)

Fan Assisted Stack

heated/cooledpipes

heated/cooledceiling void

Top Down Ventilation

chilled pipes

Buried Pipes


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Natural Ventilation Issues

Weather-dependence: wind, temperature,humidity

Outdoor air quality Immune compromised patients Building configuration (plan, section) Management of openings Measurement of ventilation rate(s)

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Outdoor air quality becomes indoor airquality at high ventilation rates

The higher the outdoor air ventilation rate, the higher theindoor/outdoor pollutant concentration

The effect of the building on reducing outdoor pollutantsvaries by pollutant and by building ventilation pathways

Where outdoor air pollution is high, natural ventilationmust be considered not only as a means for reducingconcentrations from indoor sources (infectious airborneagents as well as chemicals emitted indoors), but also asa means of delivering un-cleaned outdoor air.

With highly susceptible health care facility occupantpopulations, consideration must be given to the effects ofoutdoor pollutants on the occupants health.


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Chapter 2.

Global ambient airpollutionconcentrations andtrends

Bjarne Sivertsen

http://www.who.int/phe/health_topics/outdoorair_aqg/en/

WHO, 2005. Air Quali ty Guidelines:Global Update

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Ranges of annual average concentrations of

outdoor air pollutants by continent based onselected urban data

120300100250

120310150380150350200600

10100665

317935836

4070

35652075

1128357018573082

4015035220

2812720602070

30129

Africa Asia

Australia/New ZealandCanada/United StatesEuropeLatin America

Ozone(1-hour maximum

concentration)Sulfur

dioxideNitrogendioxidePM 10Region

(source: World Health Organization, 2005. Air Quality Guidelines: Global Update)


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Where are the people who will arrive innaturally ventilated health care facilities?

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Pollutant concentrations by national levelof development


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U.S. EPA National Ambient Air QualityStandards (NAAQS) http://www.epa.gov/air/criteria.html

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Average annual PM10

concentrations inselected cities world wide (part 1)


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Average annual PM 10 concentrations inselected cities world wide (part 2)

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NORTH AMERICA

Average annual PM10

concentrations inselected cities world wide (part 5)


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Europe

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EUROPE


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Ultrafine particle number concentrationsmeasured in urban and roadside environments

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Mean afternoon (13:00 to 16:00) surface ozone

concentrations calculated for the month of July(comment: where are people living?)


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Highest (1-hour average) ground-level ozoneconcentrations measured in selected cities

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Modeled surface ozone concentrations (ppb) overEurope during July for the years 2000 2009


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Indoor O 3 concentration as a function ofoutdoor concentration and ventilation rate

180162144126108907254361820

16014412811296806448321612

132119106927966534026136

1109988776655443322114

75676052453730221572

50454035302520151051

AER(h -1)

20018016014012010080604020

Outdoor Air Ozone Concentration (parts per billion)

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Wind: direction and velocity are neitherstable nor consistent Selected data from almost any city will show daily cycles

and variations in wind direction and velocity Seasonal variations are more reliable, but daily

variations are still the rule rather than the exception Even with many predictable situations, wind direction will

change over the diurnal cycle California coast is anexample.

Relying on wind alone can result in both under and over-ventilation relative to a design objective.


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Lima, Peru: May 1, 2008

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Lima, Peru: September 1, 2008


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Lima, Peru: January 1, 2009

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Lima, Peru: March 1, 2009


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Boston, MA: July 24, 2009

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Boston, MA: March 1, 2009


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Boston, MA: January 1, 2009

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Boston, MA: October 1, 2008


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Grantham Hospital Study, Hong KongYuguo Li

0

90

180

270

Test 4:Wind speed= 3.0 m/s

Test 17:Wind speed=4.5m/s

(A)

10 15 200

5

10

15

20

25

30

35

10 15 20 10 15 20 10 15 20 10 15 200

50

100

150

200

250

300

350

Aug 28,2006Nov 10,2005Nov 09,2005Nov 07,2005

W i n d d i r e c t

i o n (

o )

S p e e d

( m / s )

T e m p e r a

t u r e

( o C )

Time (hours)

TemperatureWind speed

Nov 06,2005

Wind direction

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Natural Ventilation: TheorySummary - Review

Purpose of ventilation What is ventilation?Types of natural ventilation (Driving forces): Buoyancy (stack effect; thermal) Pressure driven (wind driven; differential pressure)

Applications Supply of outdoor air Convective cooling Physiological coolingIssues Weather-dependence: wind, temperature, humidity Outdoor air quality Immune compromised patients Building configuration (plan, section) Management of openings


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Natural Ventilation: TheoryReferences

Karl Terpager Andersen, 2003. Theory for natural ventilation by thermal buoyancy in onezone with uniform temperature Building and Environment 38: 12811289.

Yuguo Li 2000. Buoyancy-driven natural ventilation in a thermally stratifed one-zonebuilding. Building and Environment 35: 207-214.

Francis Allard, Mat Santamouris, Servando Alvarez, Natural ventilation in buildings.European Commission. Directorate-General for Energy, ALTENER Program. Can beread on-line athttp://books.google.com/books?hl=en&lr=&id=1tdQMyhPA2gC&oi=fnd&pg=PR9&dq=Natural+ventilation+theory&ots=mFzmfd4mct&sig=XA3zksH_OBkkS8tILbxmwJqbWyo

WHO, 2005. Air Quality Guidelines, 2005 Update . Geneva: World Health Organization.http://www.who.int/phe/health_topics/outdoorair_aqg/en/

Heiselberg, P., Ed., 2002. Principles of Hybrid Ventilation . IEA-ECBS Annex 35 report.Downloadable from http://hybvent.civil.auc.dk/

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