Environmental Engineering Reference
In-Depth Information
cause the re-entry of contaminants which were initially removed through
one or more exhaust systems. Buildings that are inadequately ventilated
(because of energy management concerns) are often under negative pressure
relative to the outdoor environment. As a consequence, building air may be
at significant risk of becoming contaminated due to re-entry or entrainment
(as described in the next section).
In addition to the location of exhaust and intake air systems and building
depressurization, other factors contribute to re-entry. These include the
design and height of exhaust vents, exhaust velocity, and air flow patterns
over building roof surfaces where exhaust vents are located.
For both practical and architectural (aesthetic) reasons, exhaust vents on
many large buildings are relatively short and, in many cases, flush with the
building surface. Exhaust vents are often covered with rain shields, which
significantly reduce the upward momentum of exhaust gases.
Exhaust gases released near the roof surface have limited upward
momentum. As a consequence, they become entrained in what is described
as the building wake. The building wake is a layer of air close to the roof
surface that is only weakly coupled with the free-flowing air above it. It is
produced as a result of airflow patterns over and around building roof
surfaces. This phenomenon is illustrated in
Figure 11.17
.
As contaminants
enter the wake, their dilution is limited because of the relatively stable air
that characterizes it. Contaminants within the wake move in the same
direction as air flowing above it. Commonly this results in a downwash
phenomenon on the lee side of buildings, with subsequent re-entry of
contaminants through outdoor air intakes or by infiltration through build-
ing leakage sites.
Re-entry is less likely to occur, or less likely to cause significant indoor
contamination, when exhaust vents are elevated above the roof surface and
operated to obtain significant upward momentum. In such instances, con-
taminants may be released above the building wake or break through it as
a result of upward momentum.
Figure 11.17
Building roof-top wake and re-entry phenomena.
