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Ventilation Strategies
Ventilation is achieved by either:
Its primary purpose is to dilute and remove pollutants
generated within a space to secure optimum indoor air quality for building
occupants.
Indoor pollution emissions are controlled by
'diluting' or 'displacing' indoor air with 'clean' outdoor air. For a
fixed emission rate, the steady state concentration level is reduced as
the ventilation rate is increased. If the air is 'conditioned' (i.e.
heated or cooled to maintain optimum thermal comfort, then the
conditioning energy increases with ventilation rate. The objective,
therefore is to minimise indoor sources of pollutant. Ultimately occupants
become the dominant and, hence, unavoidable source of pollutant. When this
happens, ventilation can be controlled by carbon dioxide demand control.
Natural Ventilation
Natural ventilation relies on the natural porosity of the building
and/or a combination of vents, chimneys and openable windows to provide
the primary source of ventilation. Small extractor fans may be used
to augment needs (e.g. in 'wet' rooms such as kitchens, bathrooms
etc.).
Throughout the world many buildings are naturally ventilated. While
natural ventilation may mean little more than relying on a arbitrary
combination of uncontrolled air infiltration and window opening, the
present need for energy efficiency and good indoor air quality now demands
well designed natural ventilation systems. This can be achieved by
understanding the flow mechanisms and evaluating the impact on air change
of the natural driving forces of wind and temperature (see tutorial).
Mechanical Ventilation
Mechanical systems use fans instead of natural driving forces to drive
the ventilation process. By sealing the building, complete control can be
achieved. Approaches include:
Mechanical Extract: A fan is used to 'suck' air out of a
space. A suction pressure is established which draws outdoor 'make up' air
into the space through purpose provided openings and/or leakage gaps. If
there are insufficient make up openings the fan will 'stall' and the air
flow rate will drop. If there are too many openings than the air flow rate
will be increased through air infiltration (see tutorial for design
solutions). Ideally the extracy point is placed in proxinity of
indoor pollutant sources, e.g. extracting cooker/range hoods, fume
cupboards etc.). It is possible to place a heat-pump in the extract air
stream to recover heat.
Mechanical Supply: A fan is used to 'blow' air into a
space. The resultant over pressure pushes air out of purpose provided
openings and gaps in the building fabric. Again, the sizing of openings is
critical. Since the air enters at a single location and the
'over-pressure' inhibits infiltration, the incoming air can be reliably
filtered to remove any harmful outdoor pollutant. The supply air can also
be preheated (or cooled) by placing 'heating' and 'cooling' coils in the
air-stream. Applications include cleanroom technologies and ventilation in
polluted locations. Heat recovery is not possible.
Mechanical Balanced Ventilation: Both mechanical supply
and extract networks are incorporated into the space. Advantages
include being able to filter the supply air and efficient 'air to air'
heat recovery. Disadvantages include the extra cost of the twin network,
combined with fan energy use. To work correctly, the building essentially
needs to be completely sealed. Any gaps in the fabric will allow air
infiltration to add directly to the design air change rate.
Recirculation: essentially a balanced system in which a
proportion of the indoor air is recirculated and blended with incoming
outdoor air (perhaps up to 80% of the air is recirculated). This is used
when the ventilation system is also used to provide thermal conditioning
to a space (i.e. through heating and cooling coils)
Mixing or Dilution Ventilation: This is essentially
conventional natural and mechanical ventilation. The incoming air mixes
(or is assumed to mix) uniformly with the air in a space thus 'diluting'
indoor contaminants.
Displacement Ventilation: Mechanical supply diffusers are
carefully designed to provide air at low velocity and within 2-3K above
room temperature at floor level. The incoming air ponds in the space and
pushes away rather than mixes with the existing room air. This can lead to
much improved air quality in the breathing zone than can be achieved with
mixing ventilation for the same ventilation rate.
Demand Control Ventilation: The rate of ventilation
varies in response to pollution load (typically metabolic carbon dioxide
and water vapour): This works well when the chosen pollutant is the
'dominant' pollutant and when large variations in pollutant load occurs
throughout the day. |
