Environmental Engineering Reference
In-Depth Information
lated. Ventilation causes an exchange of air within building spaces and
between building interiors and the outside environment. Ventilation is used
to dilute and remove contaminants, enhance thermal comfort, remove excess
moisture, enhance air motion, improve general comfort, and in large build-
ings, maintain pressure differences between zones.
I. Natural ventilation
All buildings are subject to natural forces that result in air exchange with
the ambient (outdoor) environment. Natural ventilation depends on the
inflow of air as a result of (1) pressure differences when a building is under
closure conditions and being heated or cooled, (2) pressure-driven flows
when building windows and doors are open, or (3) the continual movement
of air through a building as it enters through some openings and exits
through others (
Figure 11.1
)
. In the last case, small buildings will experience
relatively high air exchange rates. Maximum exchange rates will depend on
wind speed, the position of open windows and doors relative to each other,
and prevailing winds. Though limited scientific data are available, air
exchange rates should be at their maximum when buildings are ventilated
by opening windows and doors when outdoor conditions are favorable.
Residential buildings, which include both single- and multi-family struc-
tures, are increasingly being provided year-round climate control. As a con-
sequence, the practice of opening windows and doors for ventilation pur-
poses is decreasing. Such residences, and those in seasonally cold or warm
climates, are maintained under closure conditions for extended periods of
time (upwards of 9 months or so).
Under closure conditions, ventilation occurs as a result of infiltration
and exfiltration processes. These processes involve pressure-driven flows
Ventilation air flow through a single-family house under open condi-
tions. (From USEPA,
Figure 11.1
Introduction to Indoor Air Quality — A Self-paced Learning Module
,
EPA/400/3-91/002, Washington, D.C., 1991.)
