Environmental Engineering Reference
In-Depth Information
environments, it is both desirable and practical to control them at the point
of generation rather than allow them to disperse in building air where
general dilution ventilation is less likely to be effective and economical.
1.
Applications
Local exhaust ventilation is used for specialized purposes in large office,
commercial, and institutional buildings, as well as in residential structures.
Use of local exhaust ventilation is evidenced by the often numerous exhaust
outlets or stacks on rooftops and sides of large buildings. Its use is also
evidenced by the pressure imbalances that occur when building designers
and facility operators fail to design and operate HVAC and local exhaust
systems properly.
Specific applications of local exhaust ventilation in large buildings
include the control of lavatory odors, odors and combustion by-products
associated with cafeteria and restaurant emissions, ammonia from blueprint
machines, solvent vapors from printing equipment and silk screening, paint
and varnish vapors from school wood and automobile shops, dusts from
wood-working shops, vapors from photography developing rooms, and
water vapor from swimming pools and spas. Special applications in residen-
tial environments include exhaust fans in bathrooms to control water vapor
generated in bathing, and in kitchens to control odors, cooking by-products
(such as grease), and combustion by-products associated with gas cooking.
2.
Performance
Local exhaust systems are designed to capture contaminants at or near their
source of generation before they are dispersed into the surrounding envi-
ronment. This is particularly the case with hood-based systems. It is less so
when simple fans are used to exhaust contaminants from a localized area
such as a bathroom, darkroom, etc.
Hood exhaust systems are used to capture combustion and cooking by-
products in cafeterias, restaurants, and homes; solvent vapors in laboratories
and shop rooms, etc. Such exhaust systems include one or more hoods,
blower fans, ducts, and exhaust vents. Some exhaust systems may have low-
efficiency particle filters that provide protection for the blower fan and
minimize the accumulation of potentially hazardous deposits (e.g., cooking
greases) in the blower and ducts.
The effectiveness of local exhaust ventilation systems depends on the
configuration of the hood, its proximity to the contaminant generation
source, air velocities at the face of the hood and volumetric flow rates,
availability of makeup/replacement air, and a variety of environmental fac-
tors such as temperature of contaminated air and the presence or absence
of turbulent air flows near the hood. Exhaust systems which are not properly
designed and operated are unlikely to effectively remove contaminants gen-
erated from localized sources. This is particularly true with kitchen range
hoods. Such hoods, in many cases, have insufficient fan power to capture
