Let's look at each of these components in more depth. There are three major types of hoods: those which enclose the emission source, those which capture emissions, and those that merely receive emissions, such as a canopy hood. The most effective hood usually encloses the emission source as much as possible. When using capture and receiving hoods, place the hood as close to the source as possible. Canopy hoods should only be used on hot processes. The canopy cannot be used when workers must lean over the tank or process when contaminants are rising, since they will breathe the contaminated air. Let's look at how good industrial ventilation systems are designed and installed.
At the beginning of the design process, important questions are answered. At the emission source, one question is: How fast must the air travel to capture or contain the emission source? The answer to this question tells us the hood capture velocity. Similarly: How much air must be moved the hood to control the emissions? The answer to this question becomes the volume flow rate, or Q in CFM required for effective control. Next: How fast must the air travel through the duct work in order to prevent dust particles from settling out in the duct? This air speed is caused transport velocity. Is an air cleaner required? The answer to this question must be obtained from the local air pollution authority that will issue a permit.
How large the fan be? Fans are chosen to match the requirements of the exhaust ventilation system. The fan must move the correct volume of air against the resistance to air flow caused by friction and turbulence in the system. The designer will estimate the static pressure required. How high should the stack be and were should it be located? The stack must be high enough or far enough away from intakes to avoid re-entrainment of the contaminants into the building air handling system. These questions are usually answered through carefully study of the industrial process, worker activities, the air movement in the work place, and by searching design references for appropriate standards in design information. For example, OSHA has included some design criteria in its ventilation regulations.