Transcript
Natural ventilation
Objectives
1 Discuss the different types of natural ventilation application in
building design
2. Discern some techniques to reduce solar and internal gains in a
building
Key Words
Stack Effect Positive Air pressure Inlet Openings Solar Gain
Airtight Building Envelope Windward Glazing Exhaust Openings
Introduction
A building is a man-made environment (a machine in Le Corbusier
terms) within the bigger natural surrounding. The whole endeavor of
architecture, apart from space definition, is to moderate the level
of interaction with the environment.
In doing so, openings are provided on the envelope of the
building for ventilation and space cooling purposes.
However, providing the “holes” on the “walls” is not a simple task
of necessity. Rather it is a multi variable function of the immediate
surrounding (wind direction, nearby elements like buildings, trees,
shrubs) and the opening parameters (size, placement and
orientation).
Taking into consideration all above the elements
reduces energy consumption for air conditioning
and mechanical cooling.
Description
Natural ventilation is the use of
outdoor airflow into the building
to provide ventilation and space
cooling.
It can be achieved with operable
windows when the spaces to ventilate
are small and the architecture permits.
In more complex systems warm air in
the building can be allowed to rise and
flow out upper openings to the outside
(stack effect) thus forcing cool outside
air to be drawn into the building
naturally though openings in the lower
areas.
These systems use very little energy but
care must be taken to ensure the
occupants‘ comfort. In warm or humid
months, in many climates, maintaining
thermal comfort via solely natural
ventilation may not be possible so
conventional air conditioning systems are
used as backups
Concepts
Natural ventilation is a whole building design concept. the design
utilizes the stack effect and wind pressure to supply
outdoor air to building interiors for ventilation or space cooling
purposes.
The aim is to have an airtight building envelope while
controlling outdoor air supply to provide the required ventilation.
Features of naturally ventilated buildings include operable
windows, exhaust units located high in the building with
intakes located low in the building, and open building plans
to facilitate air movement. Designs can incorporate lobbies,
internal stairwells, vent chimneys and small fans to move
vent air.
Natural ventilation reduces energy
consumption for fans and mechanical cooling
and in most cases gives occupants control
over their office space. Further benefits
include no fan noise and in some cases
elimination of the mechanical cooling
Benefits
provides ventilation (outdoor air) to ensure safe,
healthy and comfortable conditions for building
occupants without the use of fans, provides free
cooling without the use of mechanical
system, reduces building construction cost
and operation cost,
when carefully designed, reduces energy
consumption for air conditioning and circulating fans
and eliminates fan noise
Limitations
Greater temperature swings than normal with
mechanically cooled spaces will occur and have to
be acceptable to the occupants, flow rate varies
with the outdoor conditions, Cool air drawn
into room, warm air extracted occupants must be
willing to open or close vents and windows
to regulate thermal comfort, not as effective
during warm, humid, summer months, difficult to
retrofit in buildings and fire code may restrict
design options.
Application
1.The long façade of the
building and majority of the
openings should be oriented with
respect to the prevailing
summer breezes,
2. exhaust vents or outlets should
be on the leeward side as high as
possible in the building,
3. vegetation and site objects should
not obstruct inlet openings,
4. rooms should have an inlet and
outlet openings located in the
opposing pressure zones,
e.g. windward and leeward walls,
windward wall and roof
5.inlets should supply air low in the
room, outlets Should Be Located
Possibly Across the room and at
high level,
6. vertical distance between
the inlet and exhaust openings should take
advantage of the stack,
7. all occupied spaces should
have an inlet opening and outlet
opening, one or both of which may be
an operable window,
8. the total area of outlet
openings should be operable and
accessible by the occupants,
9. inlet openings should not be
obstructed by furniture and interior
partitions, to ceiling height
should not be less than 3m
Maximize reflectivity of ground and
building surfaces outside facing winter
sun Provide reflective panels outside
of glazing to increase winter irradiation
Some techniques to reduce solar and internal gains
1.window areas should not be excessive and be
protected with exterior shading devices
Window size > 0.3 A x B
Window sizes for industrial buildings: glass
area-1/20 of room area; window width-
1/10 (M+N-O+P)
2. design for high thermal capacity
and exposed ceilings for night cooling
3. minimize warming of walls by the sun through the use of light
colored exteriors, trees and shrubs to provide shading and
evaporative cooling,, grass and other ground cover to keep ground
temperature low, and ponds and fountains to enhance evaporative
cooling
4. internal loads such as lighting controls, high efficiency
lighting,, high efficiency mechanical equipment and pipe and
duct installation should be low
5. narrow floor plan
Basic Principles
•As the wind approaches the face of the building, the
airflow is slowed, creating positive
pressure. Separated air increases suction (negative
pressure) along wall surfaces.
•If windows are placed in both windward and leeward,
the building would be cross ventilated and eddies will
develop
•Windows placed in sidewalls will enhance ventilation
sue to increased suction and greater
air circulation Ventilation is best for room with windows
on 3 adjacent walls than 2 opposite walls
• If wind is from windowless side, there is poor
ventilation
• A building with one window wall negligible ventilation
will occur, but architectural solutions are available
• As airflow passes through a well-ventilated room it
forms an air-jet
• If openings are near to wall, ceiling, floor, the air
stream attaches itself to the surface
• It is best to offset the inlet and outlet
• The window size for the inlet and outlet should be the
same
Attic and crawl space ventilation
1.when there is attic space under roof, venting of attic will
reduce heat build up, provide escape route for moisture and
in cold climates, help prevent ice dams from forming
2. Even when there is no attic, the venting effect
can still be achieved with at least a 2.54cm air
space above the insulation
3. In some cases, the argument can be made for having no venting
at all. This can be done in dry climates and building types where
vapor is less of a problem or if the “wet” side of
the roof is sealed against vapor migration.
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