Quantcast Chapter 4. Industrial Ventilation Systems

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4.1 GENERAL.  Industrial ventilation is defined as "the use of supply and
exhaust ventilation to control emissions, exposures, and hazards in the
workplace."  General ventilation systems such as heating, ventilation, and
air conditioning systems (HVAC) are used for comfort control (e.g.,
temperature, humidity, and odor).
Design and installation of the system is usually done in accordance with
the guidelines found in MIL-HDBK-1003/17. Industrial Ventilation Systems;
and Industrial Ventilation, published by the American Conference of
Governmental Industrial Hygienists.
The objectives of the industrial ventilation system are to:  (a) con-
trol airborne concentrations, dust, and vapor to keep below explosive or
flammable levels, and (b) control toxic particulates, gases, and vapors to
keep below exposure levels called threshold limit values (or TLVs) harmful
to workers.
There are five basic components to an industrial ventilation system.
Each component (hood, duct, air cleaner, fan, and stack) is critical to the
operation and success of the system.
4.2 HOOD.  The hood or capture device is any device whose purpose is to
receive and contain the emitted contaminant at the source of generation.
The three basic types of hoods are:
a. The enclosing hood.  Examples are glove box, lab hoods, bench
hoods, grinder hoods, and others with four of more sides.
b. The capture hood.  These are hoods with one to three sides. A
welding snorkel type hood is typical. Others include side draft, down draft,
and push pull hoods.
c. The receiving hood.  These hoods are designed to receive the
emission source (which has some initial velocity imparted to it by the
emitting source).  A canopy hood is a receiving hood because it receives hot
rising air and gases.  A small hand held tool hood may be a receiving hood.
Figure 4-1 shows the three different types of hoods.
4.2.1 Capture and Face Velocities.  The capture or face velocity is the
velocity of the air into the hood that is required to remove the contamina-
tion.  The velocity that is required should be obtained from the design
information.  However, if the design information is not available, then
Table 4-l can be used.
The velocity should be measured across the face of the hood. The
measurement can be made using various flow measuring devices (velometers,
discussed in Chapter 2) or smoke generators.  A minimum of 16 measurements
should be taken for rectangular hood openings.  If the measurement is taken
over a round opening, then 20 measurements should be taken and averaged.


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