Environmental Engineering Reference
In-Depth Information
G. Entrainment
Combustion-generated substances enter and contaminate buildings when
they are produced by outdoor sources. Such sources include motor vehicles,
nearby incinerators and boilers, residential heating appliances, etc.
All buildings are ventilated to some degree by mechanical systems,
natural forces (infiltration), or both. Contaminants produced outdoors
become readily entrained in air brought into buildings by either mechanical
or natural forces.
Outdoor air is commonly contaminated by motor vehicle emissions, the
dominant source of ambient air pollution in the U.S. Because of atmospheric
mixing, combustion by-products and their derivatives associated with motor
vehicles have a limited impact on IAQ, particularly in regard to health
concerns.
Problems arise when the emission source is close to a building. Entrain-
ment of motor vehicle emissions occurs near loading docks where truck
emissions (often diesel) become entrained in air drawn through open loading
dock doorways and/or outdoor air intakes located nearby. Loading docks
on the lowest level of a tall building may be a major pathway for a significant
stack effect (see Chapter 11) and entrainment of motor vehicle emissions.
Complaints of motor vehicle odors are common in school buildings where
idling motor vehicles are in close proximity to outdoor air intakes on unit
ventilators. They are also common in buildings served by diesel- and gaso-
line-powered trucks idling near loading docks or outdoor air intakes.
One of the most common entrainment phenomena occurs in the fall of
the year when millions of homeowners burn leaves (where there are no bans
on this activity). Entrainment occurs through open windows and by infiltra-
tion in residences whose occupants wish to protect themselves from neigh-
bors' leaf-burning smoke.
In residential neighborhoods, entrainment of combustion by-products
from the gas heating and wood-burning systems of nearby households is
common. Such entrainment occurs by infiltration.
The nature and significance of the entrainment of combustion-generated
contaminants in residences and other buildings have not been systematically
studied. In some field investigations there are suggestions that CO levels
may become elevated by several ppmv. For the most part, entrainment is
categorized as a nuisance. Occupants complain of “diesel odors,” “gas
odors,” etc. Such odors, however, often serve as a focus of broad IAQ con-
cerns which occur in problem buildings.
H. Re-entry of flue gases
The contamination of indoor air by combustion by-products vented to the
outdoors through flues and chimneys often occurs indirectly as a result of
re-entry. In residences it is common for fireplace wood smoke to be drawn
indoors after it has been exhausted through chimney flues. This re-entry
