c. Advantages of Local Exhaust Systems. Local exhaust systems are usually
preferred over general ventilation systems for these reasons:
(1) Control of the contaminant can be complete; therefore, the exposure of
the worker to the contaminant can be prevented, which results in a much more healthful
A local exhaust system can handle more highly toxic contaminants.
(3) A local exhaust system can handle higher concentrations of
(4) The velocity of the air in the system is high; as a result, performance of
the exhaust fan system is not likely to be affected by cross drafts.
d. Disadvantages of Local Exhaust Ventilation Systems.
(1) Local exhaust systems are, as a rule, mechanically complex. This fact
results in high initial costs and a greater requirement for maintenance.
(2) Installation of the enclosures and ductwork associated with local exhaust
systems results in an inflexible work area.
GUIDELINES FOR AIR-FLOW IN VENTILATION SYSTEMS
a. Basic Laws. The basic laws governing the complete motion of a fluid such
as air are complex. In the simple case of moving a layer, or layers of air (laminar flow),
the motion of the air can be computed analytically. However, in most ventilation
systems (and especially in local exhaust systems) the air-flow is usually turbulent to
some degree. As a result, the analytical solution to the motion of air in exhaust systems
depends largely on experimental data.
b. Basic Principles. A basic consideration in the principles of airflow is
conservation of mass, or put another way, the continuity equation. This equation states
that the mass rate of flow remains constant along the path taken by a fluid such as air.
Therefore, at any two points in the stream of air:
Q1 = Q2
A1V1 = A2V2
Where Q = AV is the volumetric rate of air-flow in cubic feet per minute (cfm);
A = cross sectional area in square feet (ft2);
V = velocity in feet per minute (ft/min).
There is a very definite relationship between velocity and rate of flow, which can be
expressed by the equation: