Building Design and Construction Handbook - Merritt - Ventech!

13.4 VENTILATION
HEATING, VENTILATION, AND AIR CONDITIONING 13.27
Ventilation is utilized for many different purposes, the most common being control
of humidity and condensation. Other well-known uses include exhaust hoods in
restaurants, heat removal in industrial plants, fresh air in buildings, odor removal,
and chemical and fume hood exhausts. In commercial buildings, ventilation air is
used for replacement of stale, vitiated air, odor control, and smoke removal. Ventilation
air contributes greatly to the comfort of the building’s occupants. It is considered
to be of such importance that many building codes contain specific requirements
for minimum quantities of fresh, or outside, air that must be supplied to
occupied areas.
Ventilation is also the prime method for reducing employee exposure to excessive
airborne contaminants that result from industrial operations. Ventilation is used
to dilute contaminants to safe levels or to capture them at their point of origin
before they pollute the employees’ working environment. The Occupational Safety
and Health Act (OSHA) standards set the legal limits for employee exposures to
many types of toxic substances.
13.4.1 Methods of Ventilation
Ventilation is generally accomplished by two methods: natural and mechanical. In
either case, ventilation air must be air taken from the outdoors. It is brought into
the building through screened and louvered or other types of openings, with or
without ductwork. In many mechanical ventilation systems, the outside air is
brought in through ductwork to an appropriate air-moving device, such as a centrifugal
fan. With a network of ductwork, the supply air is distributed to areas where
it is needed. Also, mechanical ventilation systems are usually designed to exhaust
air from the building with exhaust fans or gravity-type ventilators in the roof or a
combination-type system.
Many mechanical ventilation systems are installed for fire protection in buildings
to remove smoke, heat, and fire. The design must be capable of satisfying the
provisions of the National Fire Protection Association ‘‘Standard for Installation of
Air-Conditioning systems,’’ NFPA 90-A. The standard also covers installation provisions
of air intakes and outlets.
Natural ventilation in buildings is caused by the temperature difference between
the air in the building and the outside air and by openings in the outside walls or
by a combination of both. With natural ventilation, there should be some means
for removing the ventilation air from the building, such as roof-mounted gravity
vents or exhaust fans.
13.4.2 Minimum Ventilation Requirements
There are many codes and rules governing minimum standards of ventilation. All
gravity or natural-ventilation requirements involving window areas in a room as a
given percentage of the floor area or volume are at best approximations. The amount
of air movement or replacement by gravity depends on prevailing winds, temperature
difference between interior and exterior, height of structure, window-crack
area, etc. For controlled ventilation, a mechanical method of air change is recommended.