VENTILATION & AIR

MOVEMENT
CLIMATOLOGY
INTRODUCTION

• Natural ventilation and air movement could-be considered under the heading of 'structural
controls’ as it does not rely on any form of energy supply or mechanical installation, but due
to its importance for human comfort, it deserves a separate section.
FUNCTION

• 1 . SUPPLY OF FRESH AIR

• 2 . CONVECTIVE COOLING
• 3. PHYSIOLOGICAL COOLING
SUPPLY OF FRESH AIR

• The requirements of fresh air supply are governed by the type of occupancy, number and activity of the
occupants and by the nature of any processes carried out in the space . Requirements may be stipulated
by building regulations and advisory codes in terms of m 3/h person, or in number of air changes per
hour, but these are only applicable to mechanical installations. Nevertheless, they can be taken as
useful guides for natural ventilation
 SUPPLY OF FRESH AIR

• For natural ventilation usually certain limited

solutions are prescribed and not the expected
performance. •The provision of 'permanent
ventilators', i.e. of openings which cannot be
closed, may be compulsory. •These may be
grilles or 'air bricks‘ built into a wall, or may be
incorporated with windows
 CONVECTIVE COOLING

• The exchange of indoor air with fresh out-door air can provide cooling, if the
latter is at a lower temperature than the indoor air. The moving air acts as a
heat carrying medium.
PROVISION FOR VENTILATION: STACK EFFECT

• Ventilation, i.e. both the supply of fresh air and convective cooling, involves the movement of air at a
relatively slow rate. The motive force can be either thermal or dynamic (wind).
•
• • The stack effect relies on thermal forces, set up by density difference (caused by temperature
differences) between the indoor and out-door air. •It can occur through an open window (when the air is
still): the warmer and lighter indoor air will flow out at the top and the cooler, denser outdoor air will Flow
in at the bottom.
•
• •The principle is the same as in Wind generation.
PROVISION FOR VENTILATION: STACK EFFECT

• Special provision can be made

for it in the form of
VENTILATING SHAFTS. The
higher the shaft, the larger the
cross-sectional area and the
greater the temperature
difference: the greater the
motive force therefore, the
more air will be moved. Such
shafts are often used for the
ventilation of internal,
windowless rooms (bathrooms
and toilets) in Europe
PROVISION FOR VENTILATION: STACK EFFECT
PROVISION FOR VENTILATION: STACK EFFECT

• WIND CATCHER
PROVISION FOR VENTILATION: STACK EFFECT

The motive force is the 'stack pressure' multiplied by the

crosssectional area (force in Newtons– area in m²). The stack
pressure can be calculated from the equation:
PROVISION FOR VENTILATION: STACK EFFECT

This Graph gives a quick guide

for establishing the size of
ventilating shafts. These
systems operate satisfactorily
under winter conditions when
the temperature difference is
enough to generate an
adequate air flow.

• DUCT DESIGN GRAPH

PHYSIOLOGICAL COOLING

The movement of air past the

skin surface accelerates heat
dissipation in two ways: 1.
Increasing convective heat loss
2. Accelerating evaporation

Cooling by air movement is

most needed where there are
no other forms of heat
dissipation available, when the
air is as warm as the skin and
the surrounding surfaces are
also at a similar temperature.
PROVISION FOR AIR MOVEMENT: WIND EFFECTS

Thermal forces will rarely be

sufficient to create appreciable air
movements. The only 'natural’ force
that can be relied on is the dynamic
effect of winds. When the creation
of air movements indoors is the
aim, the designer should try to
capture as much of the available
wind as possible.
Provision for air movement: WIND
EFFECTS
Negative control – when the wind is
too much – is easy, if windows and
• AIR FLOW AROUND A BUILDING
openings can be shut.
AIR FLOW THROUGH BUILDINGS

As no satisfactory and complete Wind simulators may be of

theory is available, air flow patterns
can only be predicted on the basis 1. The Open-jet Type
of empirical rules derived from
measurements in actual buildings
or in wind tunnel studies. Such
empirical rules can give a useful
or
guide to the designer but in critical
cases it is advisable to prepare a
model of the design and test it on a 2. The Wind Tunnel Type .
‘Wind Simulator’.
OPEN JET WIND SIMULATOR
WIND TUNNEL WIND SIMULATOR
AIR FLOW THROUGH BUILDINGS
On the basis of such experimental observations the following factors can be isolated which affect the
indoor air flow (both patterns and velocities):

1. ORIENTATION
2. EXTERNAL FEATURES
3. CROSS-VENTILATION
4. POSITION OF OPENINGS
5. SIZE OF OPENINGS
6. CONTROLS OF OPENINGS

Each of these will be examined in the following paragraphs

ORIENTATION
The greatest pressure on the windward side of a building is generated when
the elevation is at right angles to the wind direction, so it seems to be
obvious that the greatest indoor air velocity will be achieved in this case.

•A wind incidence of 45° would reduce the pressure by 50%.

•Thus the designer must ascertain the prevailing wind direction from wind
frequency charts of wind roses and must orientate his building in such a way
that the largest openings are facing the wind direction.

•It has, however, been found by Givoni that a wind incidence at 45° would
increase the average indoor air velocity and would provide a better
distribution of indoor air movement.
ORIENTATION
 ORIENTATION
Figure a shows the outline of
air flow at 90 ° and Figure b
at 45 °, to a building square
in plan. In the second case a
greater velocity is created
along the windward faces,
therefore the wind shadow
will be much broader, the
negative pressure (the
suction effect) will be
increased and an increased
indoor air flow will result.
 ORIENTATION
If often happens, that the optimum solar orientation and the optimum orientation for wind do not
coincide. In equatorial regions a north-south orientation would be preferable for sun exclusion but most
often the wind is predominantly easterly. The usefulness of the above findings is obvious for such a
situation – it may resolve the contradictory requirements

Massing & Orientation for Cooling Massing and orientation are important design factors to consider for
passive cooling, specifically, natural ventilation. As a general rule, thin tall buildings will encourage
natural ventilation and utilize prevailing winds, cross ventilation, and stack effect.

Massing Strategies for Passive Cooling Thinner buildings increase the ratio of surface area to volume.
This will make utilizing natural ventilation for passive cooling easy. Conversely, a deep floor plan will make
natural ventilation difficult-especially getting air into the core of the building and may require mechanical
ventilation. Tall buildings also increase the effectiveness of natural ventilation, because wind speeds are
faster at greater heights. This improves not only cross ventilation but also stack effect ventilation.
 ORIENTATION

While thin and tall buildings can improve the effectiveness of natural ventilation to cool buildings, they also increase the
exposed area for heat transfer through the building envelope. When planning urban centres, specifically in heating
dominated climates,. The height difference between neighbouring buildings should not exceed 100%.
 EXTERNAL FEATURES
External features of the building itself can
strongly influence the pressure build-up.

•For example, if the air flow is at 45◦ to an

elevation, a Wing Wall at the downwind end
or a projecting wing of an L-shaped building
can more than double the positive pressure
created.
Building structures can redirect prevailing winds to cross-
ventilation
•
Wing Walls

Wing walls project outward next to a window, so that even a slight breeze against the wall creates a high
pressure zone on one side and low on the other. The pressure differential draws outdoor air in through one
open window and out the adjacent one. Wing walls are especially effective on sites with low outdoor air
velocity and variable wind directions.
EXTERNAL FEATURES
 CROSS VENTILATION

When placing ventilation

openings, inlets and
outlets are placed to
optimize the path air
follows through the
building. Windows or
vents placed on opposite
sides of the building give
natural breezes a pathway
through the structure.
This is called cross-
ventilation. Cross-
ventilation is generally the
most effective form of
wind ventilation.
CROSS VENTILATION

It is generally best not to place openings exactly across from each other in a space. While this does give effective
ventilation, it can cause some parts of the room to be well-cooled and ventilated while other parts are not.
Placing openings across from, but not directly opposite, each other causes the room's air to mix, better
distributing the cooling and fresh air. Also, cross ventilation can be increased by having larger openings on the
leeward faces of the building that the windward faces and placing inlets at higher pressure zones and outlets at
lower pressure zones.
CROSS VENTILATION

Placing inlets low in the room and outlets high in the room can cool spaces more effectively, because they
leverage the natural convection of air. Cooler air sinks lower, while hot air rises; therefore, locating the opening
down low helps push cooler air through the space, while locating the exhaust up high helps pull warmer air out
of the space. This strategy is covered more on the stack ventilation.
 CROSS VENTILATION
The following figure in the absence of an outlet opening or with a full partition there can be no effective air
movement through a building even in a case of strong winds. With a windward opening and no outlet, a pressure
similar to that in front of the building will be built up indoors, which can make conditions even worse, increasing
discomfort. In some cases oscillating pressure changes, known as 'buffeting' can also occur. The latter may also be
produced by an opening on the leeward side only, with no inlet.
 CROSS VENTILATION
To be effective, the air movement must be directed at the body surface. In building terms this means that air
movement must be ensured through the space mostly used by the occupants: through the 'living zone' (up to 2 m
high). As Figure shows, if the opening at the inlet side is at a high level, regardless of the outlet opening position,
the air flow will take place near the ceiling and not in the living zone.
 SIZE OF OPENINGS
Window or louver size can affect both the amount of air and its speed.

•For an adequate amount of air, one rule of thumb states that the area of operable windows or louvers should be
20% or more of the floor area, with the area of inlet openings roughly matching the area of outlets.

•However, to increase cooling effectiveness, a smaller inlet can be paired with a larger outlet opening.

•With this configuration, inlet air can have a higher velocity.

•Because the same amount of air must pass through both the bigger and smaller openings in the same period of
time, it must pass through the smaller opening more quickly.
SIZE OF OPENINGS

With a given elevational area – a given total wind force (pressure x area) – the largest air velocity will be obtained
through a small inlet opening with a large outlet. This is partly due to the total force acting on a small area, forcing air
through the opening at a high pressure and partly due to the ‘Venturi Effect’: in the broadening funnel (the imaginary
funnel connecting the small inlet to the large outlet) the sideways expansion of the air jet further accelerates the
particles. Such an arrangement may be useful if the air stream is to be directed (as it were focused) at a given part of
the room.
SIZE OF OPENINGS

With a given elevational area – a given total wind force (pressure x area) – the largest air velocity will be obtained
through a small inlet opening with a large outlet. This is partly due to the total force acting on a small area, forcing air
through the opening at a high pressure and partly due to the ‘Venturi Effect’: in the broadening funnel (the imaginary
funnel connecting the small inlet to the large outlet) the sideways expansion of the air jet further accelerates the
particles. Such an arrangement may be useful if the air stream is to be directed (as it were focused) at a given part of
the room.
 SIZE OF OPENINGS
When the inlet opening is large, the air velocity through it will be less, but the total rate of air flow (volume of air
passing in unit time) will be higher. When the wind direction is not constant, or when air flow through the whole space
is required, a large inlet opening will be preferable.

The Venturi Effect is the

reduction in fluid pressure that
results when a fluid flows
through a constricted section of
pipe. The Venturi effect is
named after Giovanni Battista
Venturi (1746– 1822), an Italian
physicist
SIZE OF OPENINGS

The Venturi Effect is a phenomenon of

the flow of fluids. Fluids in this case are
all gases & liquids. The experience of
this effect happens in many places in
our world. You may have experienced
this dynamic effect when trying to
open a door on a windy day that does
not want to open, or when walking
through a windy urban canyon or
narrow passage. The phenomenon of
high wind areas and difficult doors is
created by Venturi effect. The Venturi
Effect is created by a fluids natural
tendency to equalize pressure across
two or more zones.
SIZE OF OPENINGS
CONTROLS OF OPENINGS

Sashes, canopies, louvres and

other elements controlling the
openings, also influence the
indoor air flow pattern.

Sashes can divert the air flow

upwards. Only a casement or
reversible pivot sash will channel
it downwards into the living zone
.
CONTROLS OF OPENINGS

Louvres and shading

devices may also present a
problem. The position of
blades in a slightly upward
position would still channel
the flow into the living
zone (up to 20° upwards
from the horizontal) .
CONTROLS OF OPENINGS

Louvres and shading

devices may also present a
problem. The position of
blades in a slightly upward
position would still channel
the flow into the living
zone (up to 20° upwards
from the horizontal) .
AIR FLOW AROUND BUILDINGS
The effect of tall blocks in mixed developments has been examined in experiments conducted by
the Building Research Station at Garston. Figure shows how the air stream separates on the face
of a tall block, part of it moving up and over the roof part of it down, to form a large vortex leading
to a very high pressure build-up. An increased velocity is found at ground level at the sides of the
tall block. This could serve a useful purpose in hot climates, although if the tall block is not fully
closed but is permeable to wind, these effects may be reduced.

Air stream separation

at the face of
buildings
AIR FLOW AROUND BUILDINGS
If a low building is located in the wind shadow of a Tall block , the increase in height of the
obstructing block will increase the air flow Through the low building in a direction opposite to that
of the wind. The lower (return-) wing of a Large vortex would pass through the building.

Reverse flow behind a

tall block
AIR FLOW AROUND BUILDINGS
If in a rural setting in open country, single storey buildings are placed in rows in a grid-iron pattern,
stagnant air zones leeward from the first row will overlap the second row (Figure 83). A spacing of
six times the building height is necessary to 129 ensure adequate air movement for the second
row. Thus the 'five times height' rule for spacing is not quite satisfactory

Air flow: grid-iron lay-

out
AIR FLOW AROUND BUILDINGS
in a similar setting, if the buildings are staggered in a checker-board pattern, the flow field is
much more uniform, stagnant air zones are almost eliminated.

Air flow: checkerboard

lay-out
HUMIDITY CONTROL
Controlled air supply , Filtering out sand and dust, Evaporative cooling & Humidification are served
by a device used in some parts of Egypt – the Wind Scoop.

The large intake opening captures air movement above the roofs in densely built up areas. The
water seeping through the porous pottery jars evaporates, some drips down onto the charcoal
placed on a grating, through which the air is filtered. The cooled air assists the downward
movement – a reversed stack effect

WIND SCOOP
• Thank you for the patience 