If you have ever driven through a tunnel you not doubt will have noticed devices suspended from the ceiling that almost look as if they are engines from the most recent fighter aircraft. They are not aircraft engines, they are inline exhaust fans and without them it would be impossible to get from one end of the tunnel to the other without suffocating.
As one can imagine the air quality in an enclosed space like a tunnel is terrible, laced with fumes from automobiles and trucks. Although tunnels are creating a natural draft, it is far from enough to exhaust a heavy concentration of fumes and other harmful particulates. The only way to keep the tunnel bearable is to help the air move using inline exhaust fans.
Rarely will you see the same type of fan used to provide ventilation to a metro subway system, in these cases vertical shafts to street level are used in conjunction with either axial or centrifugal fans that pull the foul air up to street level where it is exhausted. This is possible in a subway as they are not all that far below the surface, this is not necessarily the case with tunnels which can be hundreds of feet below grade.
This is but one application of inline exhaust fans albeit a major one. These types of fans are also employed in more mundane applications in the home or industry. Whenever there is a need for a ventilation fan and there is no appropriate way to install it in a ceiling or wall, then the fan can be installed in the ductwork of the building. The contaminated air is drawn into the duct through a series of vents; this air is then either pushed or pulled through the duct and then exhausted outside.
Inline exhaust fans consist of a propeller type fan blade which is housed in a tube. The tube is designed to actually fit into the duct as an active component and the purpose is to increase the effectiveness of the duct system that it is part of. The inline fan propels air from one place to another, replacing stale air with fresh. These fans are used in situations where the natural draw of the duct or tunnel is insufficient to exhaust the air or in many cases; heat it or cool it.
Many inline fans are located at the entry point of the duct although they can also be placed at intervals along the duct or tunnel depending on the length and geometry.
