Building Services Engineering 5th Edition Handbook

Ventilation and air conditioning 133
around sedentary occupants will always be the main challenge. Low-level radiant heating from
warmed floors or overhead radiant panels can offset high rates of air movement. Anyone who
has sat within cathedral ceiling spaces will recognize the potential discomfort problems in cold
weather within intermittently used high thermal-mass buildings, for example, stone churches and
sports halls. The use of natural ventilation in the UK is accompanied by the problem of allowing
the uncontrolled ingress of the moisture that is present in the outside air. Any surface within the
building that is below the dew-point temperature of the space will accumulate damp and create
long-term mould growth. This becomes a comfort, health and maintenance cost if damage to the
building is to be avoided. In climates where the outdoor air temperature rises above 20 ◦ C d.b.
on most days of the year, the use of outdoor air natural ventilation is often out of the question.
This is due to the combination of intense solar radiation heat gains in these regions and the lack
of a natural cooling function by the outdoor air.
Hot dry climates within the Middle East and southern continents make use of evaporative
low-cost cooling. Figure 5.2 shows an Australian type where outdoor air of above 25 ◦ C d.b.
and around 10–30% saturation is drawn inwards through wetted vertical panels and blown
directly into occupied rooms as quickly as possible. The only operational cost being that for
a 400 W axial fan and a small water consumption in this case. Larger units serve commercial
premises and may have an indirect gas-fired heat exchanger to provide winter heating. The
supply air has a high humidity but is at reduced dry-bulb air temperature. All cooling is created
from evaporation of water, latent heat transfer, but provides sensible cooling of the conditioned
space through high air flow. Air is released from the building through window and screened door
openings.
Within the tropics, the constant high moisture content of the outdoor air makes it unsuitable
for natural ventilation practice in commercial buildings. Buildings in the UK that have an internal
atrium have restored a historical precedent in turning the building inside-out. The exterior surfaces
need less glazing as the occupants’ view is directed inwards towards a planted open space
that has natural daylight. The atrium can be used to return conditioned air back to the airhandling
plant room without the need for a return air fan, collect exhaust air and expel it through
roof openable vents and facilitate the removal of smoke during an emergency. Heat produced
from the occupants, fluorescent lighting, computers and electrical equipment assists the upward
flow of air away from the occupied zones. Atria are used in all climates from sub-zero through
40 ◦ C d.b. environments and fully provide indoor office, retail shopping malls, casino, hotel and
entertainment spaces throughout the world.
5.2 Ducted evaporative cooler used in hot climates.