Basic Calculations for Occupational Safety and Environmental Health Professionals - Mathematics and Statistics for the Environment - page 413

Once the system is installed, and has demonstrated that it is suitable for the task at hand, the sys-
tem must be well maintained. Careful maintenance is a must. In dealing with ventilation problems,
the industrial hygienist soon finds out that his or her worst headache in maintaining the system is
poor—or no—maintenance. A phenomenon that many environmental practitioners in the industrial
hygiene field forget (or never knew in the first place) is that ventilation, when properly designed,
installed, and maintained, can go a long way to ensure a healthy working environment. However,
ventilation does have limitations. For example, the effects of blowing air from a supply system and
removing air through an exhaust system are different. To better understand the difference and its
significance, let's take an example of air supplied through a standard exhaust duct.
When air is exhausted through an opening, it is gathered equally from all directions around the
opening. This includes the area behind the opening itself. Thus, the cross-sectional area of airflow
approximates a spherical form, rather than the conical form that is typical when air is blown out
of a supply system. To correct this problem, a flange is usually placed around the exhaust opening,
which reduces the air contour, from the large spherical contour to that of a hemisphere. As a result,
this increases the velocity of air at a given distance from the opening. This basic principle is used in
designing exhaust hoods. Remember that the closer the exhaust hood is to the source, and the less
uncontaminated air it gathers, the more efficient the hood's percentage of capture will be. Simply
put, it is easier for a ventilation system to blow air than it is for one to exhaust (or suck) it. Keep this
in mind whenever you are dealing with ventilation systems and/or problems. Moreover, pollutants
that are not captured by the hood are considered to be fugitive emissions, which includes emissions
that (1) escape capture by process equipment exhaust hoods; (2) are emitted during material trans-
fer; (3) are emitted to the atmosphere from the source area; and (4) are emitted directly from process
equipment (40 CFR Part 60, Electronic Code of Federal Regulations).
15.2.2.2 General and Dilution Ventilation
Along with local exhaust ventilation are two other major categories of ventilation systems: general
ventilation and dilution ventilation . Each of these systems has a specific purpose, and finding all
three types of systems present in a given workplace location is not uncommon. General ventilation
systems (sometimes referred to as heat control ventilation systems) are used to control indoor atmo-
spheric conditions associated with hot industrial environments (such as those found in foundries,
laundries, bakeries, and other workplaces that generate excess heat) for the purpose of prevent-
ing acute discomfort or injury. General ventilation also functions to control the comfort level of
the worker in just about any indoor working environment. Along with the removal of air that has
become process-heated beyond a desired temperature level, a general ventilation system supplies air
to the work area to condition (by heating or cooling) the air, or to make up for the air that has been
exhausted by dilution ventilation in a local exhaust ventilation system.
A dilution ventilation system dilutes contaminated air with uncontaminated air, to reduce the con-
centration below a given level (usually the threshold limit value of the contaminant) to control poten-
tial airborne health hazards, fire and explosive conditions, odors, and nuisance type contaminants.
This is accomplished by removing or supplying air, to cause the air in the workplace to move, and as
a result, mix the contaminated with incoming uncontaminated air. This mixing operation is essential.
To mix the air there must be, of course, air movement. Air movement can be accomplished by natural
draft caused by prevailing winds moving through open doors and windows of the work area. Thermal
draft can also move air. Whether the thermal draft is the result of natural causes or is generated from
process heat, the heated air rises, carrying any contaminant present upward with it. Vents in the roof
allow this air to escape into the atmosphere. Makeup air is supplied to the work area through doors
and windows. A mechanical air moving device provides the most reliable source for air movement in a
dilution ventilation system. Such a system is rather simple. It requires a source of exhaust for contami-
nated air, a source of air supply to replace the air mixture that has been removed with uncontaminated
air, and a duct system to supply or remove air throughout the workplace. Dilution ventilation systems
often are equipped with filtering systems to clean and temper the incoming air.