Basic Calculations for Occupational Safety and Environmental Health Professionals - Mathematics and Statistics for the Environment - page 412

The total pressure of a ventilation system can be either positive or negative (i.e., above or below
atmospheric pressure). Generally, the total pressure is positive for a supply system, and negative for
an exhaust system.
For the environmental professional to evaluate the performance of any installed ventilation sys-
tem, he or she must make measurements of pressures in the ventilation system. Measurements are
normally made using instruments such as a manometer or a Pitot tube. The manometer is often used
to measure the static pressure in the ventilation system. The manometer is a simple, U-shaped tube,
open at both ends, and usually constructed of clear glass or plastic so that the fluid level within can
be observed. To facilitate measurement, a graduated scale is usually present on the surface of the
manometer. The manometer is filled with a liquid (water, oil, or mercury). When pressure is exerted
on the liquid within the manometer, the pressure causes the level of liquid to change as it relates
to the atmospheric pressure external to the ventilation system. The pressure measured, therefore,
is relative to atmospheric pressure as the zero point. When manometer measurements are used to
obtain positive pressure readings in a ventilation system, the leg of the manometer that opens to the
atmosphere will contain the higher level of fluid. When a negative pressure is being read, the leg
of the tube open to the atmosphere will be lower, thus indicating the difference between the atmo-
spheric pressure and the pressure within the system.
The Pitot tube is another device used to measure static pressure in ventilation systems. The Pitot
tube is constructed of two concentric tubes. The inner tube forms the impact portion, while the outer
tube is closed at the end and has static pressure holes normal to the surface of the tube. When the
inner and outer tubes are connected to opposite legs of a single manometer, the velocity pressure is
obtained directly. If the engineer wishes to measure static pressure separately, two manometers can
be used. Positive and negative pressure measurements are indicated on the manometer as above.
15.6.2.1 Local Exhaust Ventilation
Local exhaust ventilation (the most predominant method of controlling workplace air) is used to
control air contaminants by trapping and removing them near the source. In contrast to dilution
ventilation (which lets the contamination spread throughout the workplace, later to be diluted by
exhausting quantities of air from the workspace), local exhaust ventilation surrounds the point of
emission with an enclosure, and attempts to capture and remove the emissions before they are
released into the worker's breathing zone. The contaminated air is usually drawn through a system
of ducting to a collector, where it is cleaned and delivered to the outside through the discharge end
of the exhauster. A typical local exhaust system consists of a hood, ducting, an air-cleaning device,
fan, and a stack. A local exhaust system is usually the proper method of contaminant control if
• The contaminant in the workplace atmosphere constitutes a health, ire, or explosion
hazard.
• National or local codes require local exhaust ventilation at a particular process.
• Maintenance of production machinery would otherwise be dificult.
• Housekeeping or employee comfort will be improved.
• Emission sources are large, few, ixed and/or widely dispersed.
• Emission rates vary widely by time.
• Emission sources are near the worker-breathing zone.
The environmental professional must remember that determining beforehand precisely the effec-
tiveness of a particular system is often difficult. Thus, measuring exposures and evaluating how
much control has been achieved after a system is installed is essential. A good system may collect
80 to 90% or more, but a poor system may capture only 50% or less. Without total enclosure of the
contaminant sources (where capture is obviously very much greater), the environmental profes-
sional must be aware of the limitations and must be familiar with handling problems like these.