b. Dynamic Losses. Dynamic losses are those energy losses encountered in
airflow that result from turbulence caused by a change in direction or velocity within a
duct. The pressure drop in a duct system due to dynamic losses increases with the
number of elbows or angles and the number of velocity changes within the system.
a. Rate of Air-Flow. The purpose of a local exhaust system is to capture and
transport airborne contaminants from the source through an air cleaner to the
atmosphere. Precise measurements of capture velocities as well as estimates of
exhaust or supply volumes can be made at the point where the airflow system interacts
with the work environment.
b. Anemometers (Air-Flow Measuring Instruments). In using any of the
various airflow instruments, which are known as anemometers, the need to take
multiple measurements of a given slot, hood, or diffuser must be kept in mind. Only by
making a uniform traverse of the opening being evaluated will you be able to arrive at a
satisfactory average velocity to use in the calculation of airflow.
(1) Rotating vane anemometer. The rotating vane anemometer is
comprised of a vane or propeller on a shaft connected to gears. The air movement
causes the vane to rotate, turning gears that register the revolutions on the dial of the
instrument as linear feet. Readings are usually taken for one-minute periods, thus
giving air velocity in linear fpm. These instruments are best suited for determining air
velocities and estimating air flow through large openings such as mine shafts and air
supply and discharge grilles. It will be necessary to consult the instrument operating
instructions for correction factors to be used in calculating actual airflow rates. In
addition, these instruments must be handled with extreme care; they require the use of
a timing device and must be frequently calibrated. They should be used in relatively
(2) Swinging vane anemometer. The swinging vane anemometer indicates
air velocity as a function of the pressure exerted by the air stream on a spring-loaded
swinging vane. They are quite portable and used extensively by industrial hygienists
and ventilation engineers in the field. They are used primarily for measuring velocities
of exhaust or supply openings. By using the fittings available for some brands of
swinging anemometers, they can be used to measure static pressures. It is important to
follow the instructions carefully, using the recommended correction factors and
calibrating the instrument periodically. Figure 4-5 illustrates several possible
applications of a swinging vane anemometer.