Environmental Engineering Reference
In-Depth Information
chapter eleven
Ventilation
Ventilation has historically been applied and viewed as both a desirable and
effective technique in improving thermal comfort and general air quality and
comfort in buildings. It has been used to dilute and exhaust unwanted con-
taminants such as combustion by-products, lavatory and cooking odors, heat
and moisture, etc. from residential and nonresidential buildings. Exhaust
ventilation is widely used in industry to remove contaminants at their source
to reduce worker exposure and health risks. General dilution ventilation
induced and delivered by mechanical fans is used in office, commercial and
retail, and institutional buildings to maintain acceptable air quality.
Ventilation is a physical process that involves the movement of air
through spaces. It has two dimensions. When air flows into a space, venti-
lation is characterized by mixing, dilution, and partial replacement. When
air flows (is removed) from a space (because it is under negative pressure),
it is replaced by air from a nearby area. General dilution ventilation describes
the first case; exhaust ventilation, the second.
Ventilation, because it involves the flow of air due to pressure differ-
ences, is a natural phenomenon. However, natural ventilation is often too
variable or inadequate. As a consequence, it is necessary to use systems that
deliver controlled, mechanically induced ventilation to provide for the needs
of a variety of building spaces.
Ventilation in its dilution, displacement, and replacement aspects causes
changes in chemical composition and environmental factors in the air envi-
ronment of ventilated spaces. These changes may result in reduced overall
contaminant levels or a decrease in concentrations of some substances, with
an increase in others. The outcome depends on the nature of air in ventilated
spaces as well as air used for ventilation. The same is true for environmental
factors such as temperature and relative humidity.
The desired effect of ventilation, whether it is natural or mechanically
induced, is to enhance or protect the quality of air in the space being venti-
