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Natural Natural Ventilation: TheoryVentilation: 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

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Natural and Mixed Mode Ventilation Mechanisms

Courtesy of Martin Liddament via Yuguo Li

Mixed Mode Ventilation

Sketch of school systemSketch of B&O Building

Natural Ventilation

Cross Flow Wind

Mixed Mode Ventilation

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

BogotáCold dry

Cold humid

San Francisco

Mt. Fuji

Temperate seasonal – RH

Montreal, Canada

LimaBostonTemperate seasonal -- Temp

Quito, EcuadorHigh desertTemperate dry

Milan, ItalyLondonTemperate humid

Low desertHot dry

SingaporeHot humid

No seasonal variation

Seasonal variation

Steady daily cycle

Diurnal swing

Climate type

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What is ventilation?

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

• Purpose provided (intentional) ventilation: Ventilation is the process by which ‘clean’ air (normally outdoor air) is intentionally provided to a space and stale air is removed. This may be accomplished by either natural or mechanical means.

• Air infiltration and exfiltration: In addition to intentional ventilation, air inevitably enters a building by the process of ‘air infiltration’. This is the uncontrolled flow of air into a space through adventitious or unintentional gaps and cracks in the building envelope. The corresponding loss of air from an enclosed space is termed ‘exfiltration’.

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

Element Description Requirements/ Guideline

Design or Operation

Buildings

Primary External air flow rate

Minimum ACHMinimum L/s

Fan, duct, openings or streets

ASHRAE 62

1-12 ACH

Secondary Overall flow direction between zones

Flow clean to “dirty” spaces

Pressure control through airflow imbalancePrevailing winds

Positive/negative 2.5-15 PaIsolation/smoke control

Tertiary Air distribution within a space

Ventilation effectiveness, no short-circuiting

Use of CFDSmoke visualization

Ventilation strategies

Cities

?

Dirty industrydownwind

Buy upwind

Urban planning

9Courtesy of Yuguo Li

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

23oC 23oC 23oC

The purpose is Not to have a 2.5 Pa negative pressure, but no air leaks to the corridor!

23oC

Cor

ridor

Ant

eroo

m

Toile

t/bat

hroo

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 differences between 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 pressure regimes

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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 / m2-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 air exchange rate

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1

2

3

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0 1 2 3 4 5Ventilation air changes per hour (ach)

Con

cent

ratio

n (µ

g/m

3 )

EF = 1 µg/m2•hr

EF = 5 µg/m2•hr

EF = 10 µg/m2•hr

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Applications: Convective cooling

• convection /con·vec·tion/ (kon-vek´shun) the act of conveying or transmission, specifically transmission of heat in a liquid or gas by bulk movement of heated particles to a cooler area.

• Air flow person can be caused by the higher temperature of the person’s skin relative to the air around it, giving rise to an air flowknown as the “thermal plume,” air movement predominantly in an upward direction.

• Or, it may be caused by forced air movement, as from a fan or wind.

Temperature variation in an object cooled by a flowing liquid

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Convective cooling

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Physiological cooling

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Applications: 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 box

effect).– Expand folds of skin.– Expose skin surfaces.

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Convective + Physiological cooling

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Physiological cooling

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Impact of wind and temperature difference on natural ventilation

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Concept of the neutral level

External pressure gradient

Internal pressure gradient

h1

h2

TiT0

Neutral plane

External pressure gradient

Internal pressure gradient

h1

h2

TiT0

Neutral plane

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Single-sided ventilation

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Influence of wind and temperature (stack effect) 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 driving flows in a building

Neutral pressure plane

+

+

_

_

+

+

_

_

_

_Indoor air

temperature is greater

than outdoor

Neutral pressure plane

+

+

_

_

+

+

_

_

_

_Indoor air

temperature is greater

than outdoor

Neutral pressure plane

+

+

+

_

_

+

+

+_

_Indoor air

temperature is less than

outdoor

Neutral pressure plane

+

+

+

_

_

+

+

_

_

+

+

+_

_+

+_

_Indoor air

temperature is less than

outdoor

(A)

Indoor air warmer Indoor air cooler than outdoor than outdoor

(B)

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Wind floor 18F

Memorial Hall

Entrance Hall

Lecture Rooms

Graduate School

Offices

Library Car Parking

Roof Garden

119.5m

Heat Storage TankRain Water Tank

Canteen

Roof Garden

Stack effect in a high rise buildingLiberty Tower, Meiji University, Tokyo

Stack effect

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Wind Floorfor Hybrid Ventilation

Gross Floor Area: 59000 m2

completed in 1998

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

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Natural ventilation in buildingsFrancis Allard, Mat Santamouris, Servando Alvarez, European

Commission. Directorate-General for Energy, ALTENER Program

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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 buildingsFrancis Allard, Mat Santamouris, Servando Alvarez, European

Commission. Directorate-General for Energy, ALTENER Program

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

apertures and locations

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

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

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Effect of oblique wind direction with openings on opposite walls

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

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

Wind perpendicular Wind oblique

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

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Simple formulation for Vent Calculation

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Single-sided ventilation

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Single-sided ventilation

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Natural and Mixed Mode Ventilation Mechanisms

Courtesy of Martin Liddament via Yuguo Li

Mixed Mode Ventilation

Sketch of school systemSketch of B&O Building

Natural Ventilation

Cross Flow Wind

Mixed Mode Ventilation

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 air quality at high ventilation rates

• The higher the outdoor air ventilation rate, the higher the indoor/outdoor pollutant concentration

• The effect of the building on reducing outdoor pollutants varies by pollutant and by building ventilation pathways

• Where outdoor air pollution is high, natural ventilation must be considered not only as a means for reducing concentrations from indoor sources (infectious airborne agents as well as chemicals emitted indoors), but also as a means of delivering un-cleaned outdoor air.

• With highly susceptible health care facility occupant populations, consideration must be given to the effects of outdoor pollutants on the occupants’ health.

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Chapter 2.

Global ambient air pollution concentrations and trends

Bjarne Sivertsen

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

WHO, 2005. Air Quality Guidelines: Global Update

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Ranges of annual average concentrations of outdoor air pollutants by continent based on

selected urban data

120–300100–250120–310150–380150–350200–600

10–1006–653–179–358–3640–70

35–6520–7511–2835–7018–5730–82

40–15035–22028–12720–6020–7030–129

AfricaAsiaAustralia/New ZealandCanada/United States EuropeLatin America

Ozone(1-hour maximum

concentration)Sulfurdioxide

NitrogendioxidePM10Region

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

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

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Pollutant concentrations by national level of development

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

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Average annual PM10 concentrations in selected cities world wide (part 1)

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Average annual PM10 concentrations in selected cities world wide (part 2)

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Average annual PM10 concentrations in selected cities world wide (part 3)

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Average annual PM10 concentrations in selected cities world wide (part 4)

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NORTH AMERICA

Average annual PM10 concentrations in selected cities world wide (part 5)

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Average annual PM10 concentrations in selected cities world wide (part 6)

Europe

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EUROPE

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Ultrafine particle number concentrations measured 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 ozone concentrations measured in selected cities

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Modeled surface ozone concentrations (ppb) over Europe during July for the years 2000– 2009

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Indoor O3 concentration as a function of outdoor 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 neither stable 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 an example.

• 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

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90

180

270

Test 4: Wind speed = 3.0 m/s

Test 17: Wind speed =4.5m/s

(A)

10 15 200

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

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100

150

200

250

300

350

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

Win

d di

rect

ion(

o )

Spee

d (m

/s)

Tem

pera

ture

(o C)

Time (hours)

Temperature Wind speed

Nov 06,2005

Wind direction

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Natural Ventilation: Theory Summary - 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 one zone with uniform temperature” Building and Environment 38: 1281–1289.

Yuguo Li 2000. “Buoyancy-driven natural ventilation in a thermally stratifed one-zone building.” 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 be read on-line at http://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/