Basic Calculations for Occupational Safety and Environmental Health Professionals - Mathematics and Statistics for the Environment - page 410

15.6.2 C onCept oF v entilation
The purpose of industrial ventilation is essentially to (under control) recreate what occurs in natural
ventilation. Natural ventilation results from differences in pressure. Air moves from high-pressure
areas to low pressure areas. This difference in pressure is the result of thermal conditions. We know
that hot air rises, which (for example) allows smoke to escape from the smokestack in an industrial
process, rather than disperse into areas where workers operate the process. Hot air rises because air
expands as it is heated, becoming lighter. The same principle is in effect when air in the atmosphere
becomes heated. The air rises and is replaced by air from a higher pressure area. Thus, convection
currents cause a natural ventilation effect through the resulting winds.
What does all of this have to do with industrial ventilation? Actually, quite a lot. Simply put,
industrial ventilation is installed in a workplace to circulate the air within and to provide a supply
of fresh air to replace air that has undesirable characteristics. Could this be accomplished simply by
natural workplace ventilation? That is, couldn't we just heat the air in the workplace so that it will
rise and escape through natural ports—windows, doors, cracks in walls, or mechanical ventilators
in the roof (installed wind-powered turbines, for example)? Yes, we could design a natural system
like this, but in such a system, air does not circulate fast enough to remove contaminants before a
hazardous level is reached, which defeats our purpose in providing a ventilation system in the first
place. Thus, we use fans to provide an artificial, mechanical means of moving the air.
Along with controlling or removing toxic airborne contaminants from the air, installed ventila-
tion systems perform several other functions within the workplace. These functions include
1. Ventilation is often used to maintain an adequate oxygen supply in an area. In most work-
places, this is not a problem because natural ventilation usually provides an adequate vol-
ume of oxygen; however, some work environments (deep mining and thermal processes
which use copious amounts of oxygen for combustion) the need for oxygen is the major
reason for an installed ventilation system.
2. An installed ventilation system can remove odors from a given area. This type of system
(as you might guess) has applications in such places as athletic locker rooms, rest rooms,
and kitchens. In performing this function, the noxious air may be replaced with fresh air,
or odors may be masked with a chemical masking agent.
3. One of the primary uses of installed ventilation is one that we are familiar with providing
heat, cooling, and humidity control.
4. A ventilation system can remove undesirable contaminants at their source, before they
enter the workplace air (e.g., from a chemical dipping or stripping tank). Obviously, this
technique is an effective way to ensure that certain contaminants never enter the breath-
ing zone of the worker—exactly the kind of function safety engineering is intended to
accomplish.
A mechanical fan is the heart of any ventilation system, but like the human heart, certain ancil-
laries are required to make it function as a system. Ventilation is no different. Four major compo-
nents make up a ventilation system.
1. The fan forces the air to move.
2. An inlet or some type of opening allows air to enter the system.
3. An outlet must be provided for air to leave the system.
4. A conduit or pathway (ducting) not only directs the air in the right direction, but also limits
the amount of flow to a predetermined level.
An important concept regarding ventilation systems is the difference between exhaust and sup-
ply ventilation. An exhaust ventilation system removes air and airborne contaminants from the
workplace. Such a system may be designed to exhaust an entire work area, or it may be placed at