Remember to provide protection for those handling materials collected from the air cleaner. Most users install monitors on the air cleaner to warn when the air cleaner is not functioning properly. When choosing the fan, be sure to select one which will provide the necessary static pressure and volume flow rate. Remember, though, the volume flow rate is always determined by emission control requirements at the hood. The fan is to serve the hood's need, not vice versa.
Fans are divided into two main classifications: centrifugal, in which the air flows at right angles to the air axis of rotation of the rotor, and axial in which the air flow is parallel to the axis of rotation of the rotor.
Centrifugal fans are mostly used in local exhaust systems. The two important types are the radial blade fan and the backward inclined blade fan. The radial blade fan is the workhorse of the dust control industry because it is self-cleaning, rugged, and simple. Unfortunately, it is inefficient and noisy. The backward inclined blade fan is quieter, more efficient, and less likely to overload the motor at high volume flow rates. Unfortunately it tends to clog or braid when the exhaust air is laden with particulate material. The axial fan is used to move large volumes of air against small static pressures. It is often used in dilution ventilation system, and is usually noisier than equivalent centrifugal fans.
Stacks should be tall enough to assure that contaminants are adequately dispersed into the atmosphere. Stack exit velocities should be 3,000 feet per minute. One rule of thumb is that the top of the stack should be at least 10 feet above the adjacent roof line or air intake if either are within 50 feet of the stack. This will help avoid re-entrainment of exhausted contaminants into the building.
Rain protection is often used. This illustration shows a slightly larger diameter duct mounted on the exhaust deck. Rain never falls exactly straight down, so when it hits the inside of the larger duct, it drips down outside the stack. To avoid re-entrainment don't use the slanted rain cap if the stack is close to the air intake for the building.