d. Advantages of a Dilution Ventilation System.
(1) General ventilation systems have several advantages over local exhaust
systems. Perhaps the most obvious of these are simplicity of design and low initial cost.
However, it would not be wise to choose a general ventilation system solely because of
its low initial cost; because these systems invariably exhaust large volumes of heated
air from a building, their use can result in high operating costs in the form of conditioned
make-up air. Over a long period of time, this would tend to make the general ventilation
system much more expensive.
(2) Use of general ventilation systems does result in great flexibility in the
layout of the work area. In addition, it does provide a certain amount of comfort
ventilation.
e. Disadvantages of Dilution Ventilation System. Generally speaking, the
velocity of the air circulating in a general ventilation system is too low to effectively
remove high concentrations of contaminants. In addition, general ventilation is
inadequate for removing contaminants of high toxicity, that is, contaminants whose TLV
is less than 100 ppm.
4-4.
LOCAL EXHAUST VENTILATION
A local exhaust system is one in which the contaminant being controlled is
captured at or near the point at which it is created or dispersed. In contrast to general
or dilution ventilation, local exhaust ventilation places total reliance on mechanical
means of controlling airflow. A typical local exhaust system usually includes the use of
hoods or enclosures, ductwork leading to an exhaust fan, an air cleaning device for
abatement of air pollution, and finally, discharge to the outside air (see Figures 4-2 and
4-3). Local exhaust systems usually contain more mechanical components than
general ventilation systems, require more precise control of their operation, and, as a
result, require more maintenance.
a. Basic Principles. When applying local exhaust ventilation to a specific
problem, there are several basic principles that should be kept in mind.
(1)
The source of contaminants should be enclosed as completely as
practicable.
(2)
The contaminant should be captured with adequate air velocity.
(3)
The contaminant should be kept out of the worker's breathing zone.
(4)
Adequate make-up air must be provided.
(5)
The exhaust air must be discharged away from air inlet systems.
MD0165
4-5
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