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Educational Package Ventilation
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Lecture 7: Sizing natural ventilation
systems
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IEE/09/631/SI2.558225
28.10.2011
Summary
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Introduction and Typology of natural ventilation systems
Prediction methods
Network models
Methodologies for sizing openings
Critical Barriers
Building design
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Natural
ventilation
is
ventilation
without
the
assistance of fans or other
mechanical
air
moving
equipment.
Airflow is supplied by
natural means using wind and
temperature difference to
naturally drive air through
buildings.
In this lecture we will focus
on
the
sizing
natural
Wind-Driven Natural Ventilation Systems
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ventilation systems.
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Introduction and typology of NV systems
Introduction and typology of NV systems
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Natural ventilation systems
rely on pressure
differences to move fresh air through buildings.
Pressure differences
can be caused by
wind
the buoyancy effect
created by temperature differences
or differences in humidity
In either case, the amount of ventilation will depend critically on
the size and placement of openings in the building.
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Introduction and typology of NV systems
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Natural ventilation : no specific systems
Ducts
(Shafts)
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Free Openings on façades
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Fig. 1 a,b
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Description of the airflow in the case of the natural
ventilation of buildings
¾ 4 different approaches:
ƒ Empirical models;
ƒ Network models;
ƒ Zonal models
ƒCFD models;
Source: Francis Allard, Natural ventilation in buildings, (a design handbook)
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Empirical models
¾2 categories of empirical methods are presented:
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Simplified empirical methods for the prediction of
the airflow rates
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• Simplified methodologies for the predictions of the
aire velocity inside a
building
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Source: Francis Allard, Natural ventilation in buildings, (a design handbook)
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Simplified empirical methods for the prediction of
the airflow rates within naturally ventilated buidings
The British Standards method
¾ propose formulas for the calculation of the air infiltration &
ventilation in single-sided and cross-ventilation configurations.
The method assumes two-dirrectional flow through a
building and ignores all internal partitions.
Source: Francis Allard, Natural ventilation in buildings, (a design handbook)
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Formulae for single-sided ventilation
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Source: Francis Allard, Natural ventilation in buildings, (a design handbook)
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Formulae for cross ventilation
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Source: Francis Allard, Natural ventilation in buildings, (a design handbook)
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The ASHRAE method
¾This method requires knowledge of the total effective leakage area of the Building,
witch can either be determinated using pressurization/depressurization techniques .
According to the method, the bulk airflow rate Q, in a single-zone building is:
The coefficient
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Where:
A is the total effective leakage area of the building,
a is a stack coefficient,
b is the wind coefficient,
∆T is the average indoor-outdoor temperature difference, (K)
U met is the meteorological wind speed,
a has 3 different values according to the number of storeys of the buiding.
b takes different values according to the number of storeys of the building, but also according
to the local shielding class to witch the building belongs.
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Source: Francis Allard, Natural ventilation in buildings, (a design handbook)
The Aynsley method
¾ simple method for global airflow prediction in the case of cross ventilation.
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™ two main openings on two opposite facades of a building, the
method uses the definition of the pressure coefficients Cp1 and Cp2
on each facade to calculate the flow rate of aire through the
building.
¾ the following expression is derived for the global airflow rate:
Where:
ƒCd1 & Cd2 - discharge coefficients given as functions of the openings configuration;
ƒA1 & A2 - areas of openings
ƒVz - the reference wind velocity.
The main interest of the method is its simplicity and efficiency in giving a rough
estimate of the order of magnitude of the global airflow rate in a crossventilated building!
Source: Francis Allard, Natural ventilation in buildings, (a design handbook)
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The De Gidds and Phaff method
¾ a general expression is given for the ventilation rate, Q, through an open window as
a fonction of temperature difference, wind velocity and fluctuating terms.
™ For the case of single-sided ventilation, an effective velocity, Ueff is defined as:
U
™leading to the form:
Where :
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Umet - the meteorological wind velocity;
H - the vertical size of the opening;
C1 - a dimensionless coefficient depending on the wind;
C2 – a buanduary constant;
C3 - aturbulence constant.
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Source: Francis Allard, Natural ventilation in buildings, (a design handbook)
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Simplified methods for the estimation of the air
velocity inside naturally ventilated buildings
THE PROPOSED TECHNIQUES ARE CLASSIFIED INTO 5 MAIN
GROUPS:
¾ research based on full-scale invzstigations;
¾ research based on computerized numerical simulations;
¾ methods based on tabulated data obtained from parametric wind-tunnel
studies;
¾ methods making use of wind discharge coefficients;
¾ methods based on direct measurements of the indoor air velocities in a scale
model of the investigated building placed in a buandary-layer wind tunnel.
Source: Francis Allard, Natural ventilation in buildings, (a design handbook)
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Givonni’s method
¾ a general correlation method, based on experimental data, to calculate the
average indoor air velocity in rooms with a square floor plan and with identical
upwind and downwind openings located in opposite walls.
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According to the method the average velocity inside the room is given by the
following expression:
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Where:
Vi - the average indoor velocity;
X - the ratio of the opening area to wall area where the opening is located;
Vr - the reference external wind speed.
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Source: Francis Allard, Natural ventilation in buildings, (a design handbook)
Methods based on tabulated data
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Different values of the mean indoor air speed for cross-ventilation configurations
without internal partitions, as a fonction of the inlets and the outlets. For aligned
inlets and the outlests and for perpendicular winds.
Effect on inlet and outlet sizes in cross-ventilated
spaces with openings on opposite walls
Openings on opposite walls; wind oblique to inlety
Effect of inlet and outlet sizes in cross-ventilated spaces; openiongs on adjacent
walls, wind perpendicular and wind oblique to inlets
Source: Francis Allard, Natural ventilation in buildings, (a design handbook)
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The CSTB methodology
¾ based on data obtained from architectural scale models in a wind
tunnel for the prediction of the wind-induced indoor air motion;
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¾ based on the evaluation of a Global Ventilation Coefficient CG.
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The method therethore propose the evaluation of four corresponding
coefficients, Csite, Corientation, CArch.Exter., Caero.Inter. Then, global ventilation ceofficient
CG of a given space is equal to the minimum of the four previously defined
coefficients.
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Source: Francis Allard, Natural ventilation in buildings, (a design handbook)
Zonal models
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9 Zonal models have been developed in the 80th for
coupling heating systems with Indoor Environment.
9 Zonal models provide an estimate of air flow
distribution or temperature gradients.
9 They are quick, flexible and can be used for long
time analysis.
9 Their weakness is due to the empirical knowledge
they need about the main driving flow in the room to
be modeled.
Source: Francis Allard - Champs Seminar Nanjing 20-22/03/2011
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Multizone models
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Francis Allard - Champs Seminar Nanjing 20-22/03/2011
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Interpretation of the physical mechanisms involved in
natural ventilation
¾The effect of the wind;
¾The stack effect;
¾Combined action of wind & temperature difference.
Source: Francis Allard, Natural ventilation in buildings, (a design handbook)
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Fig. 2 a
Where:
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The effect of the
wind
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∆Pw – the wind-induces pressure (Pa);
CP – the pressure coefficient;
ρ – the air density (
);
U – the wind speed at a reference height(
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Source: Francis Allard - Champs Seminar Nanjing 20-22/03/2011
)
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Air movement by the stack effect occurs when temperature differences between a zone and
the environment adjacent to it, be it another zone or the exterior, cause light warm air to rise
and flow out of the warm zone.
The stack effect
Fig. 2.b
Stack pressure drivin natural ventilation
Pressure stack variation as a function of
temperature difference and buiding height
Source: Francis Allard - Champs Seminar Nanjing 20-22/03/2011
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Combined action of wind & temperature difference
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Fig. 2 c
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Source: Francis Allard - Champs Seminar Nanjing 20-22/03/2011
Methodologies for sizing openings
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¾2 sizing methods to calculate the surface areas of the
openings, especially for cross-ventilation conficurations:
• simplified empirical methodologies
-based on simple analytical expressions for calculating the inlet and
outlet areas for cross-ventilation confiogurations in a room or a
monozone building;
• computerized iterative methods
- based on network models who combine the effect of the wind and
temperature difference and do not have the limitations of the
simplified empirical methods.
Source: Francis Allard, Natural ventilation in buildings, (a design handbook)
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Barriers during bulding design
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Source: Francis Allard, Natural ventilation in buildings, (a design handbook)
Barriers during bulding operations
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¾Safety concerns: preventing unuathorized entry of other people, of animals, including
bugs and insects or, simply, preventing rain from damaging the
furnishings;
¾Noise from outdoors: witch may interfere with normal activities and sleep or simply be
unpleasant;
¾Air pollution: kept out of the building, from urban pollution to dust in the coutryside;
¾Shading: for solar controlor for privacy considerations, that may require partial or total
covering of the openings provided in the outer envelope for natural ventilation;
¾Draught prevention: stemming from confort or from work requirements;
¾Ignorance on the part of the occupants: about the correct strategies that should be
adopted to take the best advantage of natural
ventilation.
Source: Francis Allard, Natural ventilation in buildings, (a design handbook)
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Barriers during bulding design
¾Regulations : fire regulations,witch may prevent the free flow of air to prevent smoke or odour
propagation; acoustic regulations may also pose some restrictions;
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¾Type of building use: the designer may have about the ability of the occupants, to choose suitable
strategies for each operating mode;
¾The need to provide: shading, privacy & daylighting => witch may require devices or solutions that
seriosly hamper the free flow of air
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¾The adoptations of the sothisticated automatic controls => that could to optimize the operation of
the building at each moment
¾Tha lack of suitable, reliable design tools, => witch also introduce and added degree of
difficulty to the implementtionsof a control strategy incorporating natural ventilation.
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Source: Francis Allard, Natural ventilation in buildings, (a design handbook)
Other barriers
Architectural impact
Lack of suitable standards
Fee structure for design
Increased risk for designers
Source: Francis Allard, Natural ventilation in buildings, (a design handbook)
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Natural ventilation influence on the design & on the
architectural expression
THE ASPECTS OF BUILDING DESIGN RELATED CAN BE GROUPED:
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¾The form of the building envelope;
¾The internal distribution of spaces and functions;
¾The dimensions and location of openings;
¾The characteristics and dimensions of the exposed thermal mass;
¾The interactions with HVAC system
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Source: Francis Allard, Natural ventilation in buildings, (a design handbook)
Designing NaturalVentilation Systems
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Fig. 3 a,b
Ventilation system : a) house
b) apartment building
¾ Designed natural ventilation can provide ventilation even when infiltration does not occur (zero air
permeability of building envelope);
¾ Ventilation openings are usually manually controlled
for most time of the year;
ventilating rates are within acceptable limits
¾ In the case of cross ventilation the air volume flows mainly result from wind effects (wind pressures),
the vertical lifting forces (buoyancy) within the building being of less importance.
¾ In the case of stack systems, the air volume flow result from the vertical lifting forces (buoyancy) within
the building
Source: HealthVent WP5 - Draft Report 2011_09_04
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Design Recommendations
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The specific approach and design of natural ventilation systems will vary
based on building type and local climate. However, the amount of
ventilation depends critically on the careful design of internal spaces, and
the size and placement of openings in the building.
Maximize wind-induced ventilation by siting the ridge of a building
perpendicular to the summer winds.
Naturally ventilated buildings should be narrow..
Each room should have two separate supply and exhaust openings. Locate
exhaust high above inlet to maximize stack effect. Orient windows across
the room and offset from each other to maximize mixing within the room
while minimizing the obstructions to airflow within the room.
Window openings should be operable by the occupants.
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Source: Francis Allard, Natural ventilation in buildings, (a design handbook)
OTHER CONSIDERATIONS IN SYSTEM DESIGN
Roof slope is important to good ventilation.
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Obstructions protruding from the underside of the roof, such as deep purlins, can
trap moist air and increase metal corrosion and wood deterioration.
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Sidewall height can also affect natural ventilation. For instance, if walls are not
high enough, mechanical bunks can disrupt proper air flow through the building in
summer. Also, winter sun cannot penetrate open-front buildings adequately if wall
height is insufficient.
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Wall height becomes more important as building width increases, if enough
sidewall vent area is to be available for summer air flow.
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Source: Francis Allard, Natural ventilation in buildings, (a design handbook)
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References
¾Natural ventilation in buildings, (a design handbook) Francis Allard
¾HealthVent WP5 - Draft Report 2011_09_04
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¾Francis Allard - Champs Seminar Nanjing 20-22/03/2011
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