Ventilation is a primarily an control engineered to reduce or extract the concentration of gases, dusts, vapours, smoke, and fumes in the air.
There are two types of ventilation systems: general and local exhaust:
General (dilution) ventilation systems supply clean air that mixes with the air in the workplace, diluting the concentration of the contaminant. General ventilation is not suitable to control exposure to toxic substances because these systems actually spread the contaminant throughout the workplace before exhausting it. Also, they require large amounts of air and may be costly to operate during the winter because of additional heating. General ventilation systems are used primarily to control temperature and humidity, to remove odours, and sometimes to remove traces of volatile organic compounds (VOCs) and microorganisms emitted from carpeting, paneling, furniture, and people.
Local exhaust ventilation systems remove the contaminant before it spreads through the workplace. The systems are designed to take advantage of the motion of the contaminant in order to capture it without drawing in large amounts of air as well. They are most useful for controlling toxic materials when their airborne concentrations could exceed legislated standards. There are four elements that make up a local exhaust ventilation system: the hood, the duct work, the air cleaning device, and the fan.
The hood captures the contaminant by overcoming its momentum and then drawing it into the system. Factors affecting the design and location of the hood include the form of the contaminant (dust, fume, vapours, or gas), and the speed at and direction in which the contaminant is released. For large, heavy dust particles released at high speeds (e.g., grinding), the hood must be positioned in the path of the particles.
Typical capture velocities required are:
100 feet per minute (fpm) for vapours and gases
200 fpm or more for dusts
The duct work provides a pathway to carry the contaminant to the air cleaning device. The velocity of air in the duct must be high enough to prevent heavy particles from settling in the ducts. The heavier the particle, the greater the velocity needed.
Typical velocities required are:
1000 fpm for vapours and gases
4000 fpm for heavy dusts
Also, there should be no obstructions or unnecessary bends and constrictions. These can cause excessive pressure drops. The air cleaning device removes contaminants from the air stream before it is passed to the fan and expelled to the atmosphere or recycled to the work area.
Air filters are designed to remove low dust concentrations of the magnitude found in atmospheric air. Dust collectors are designed for the heavier concentrations that are generated by industrial processes.
The exhaust fan is the device that draws the fumes through the entire system. The extractor fans must be capable of generating enough of pressure drop to draw the required volume of air through the hood, ducts, and collecting devices at the correct velocity, and of overcoming the resistance to air flow from hoods, ducts, and collecting devices
Each of these elements must be designed specifically for the type of contaminant to be removed. Special attention should be given to ignitable dust, such as aluminum and magnesium (I.E.R. s.65). The dust collector must be capable of meeting the environmental standards set by the Ministry of the Environment, such as the application of a Certificate of Approval for air emissions.
The elements must be maintained in optimum condition through regular inspection and maintenance.
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