Vane-axial fans, such as the one shown in figure 17-4, are generally installed in duct systems. They have vanes at the discharge end to straighten out rotational air motion caused by the impeller. The motors for these fans are cooled by the air in the duct and will overheat if they are operated with the air supply to the fan shut off.

TUBE-AXIAL FANS

Tube-axial fans are low-pressure fans, usually installed without duct work. However, they do have sufficient pressure for a short length of duct.

CENTRIFUGAL FANS

Centrifugal fans, such as the one shown in view A of figure 17-5, are used primarily

Figure 17-4.-Vane-axial ventilating fan (cutaway view).

Figure 17-5.-Ventilating fans.

to exhaust explosive or hot gases. However, they may be used in place of axial-flow fans if they work better with the arrangement or if their pressure-volume characteristics suit the installa-tion better than an axial-flow fan. Centrifugal fans are also used in some fan-coil assemblies, which are discussed later in this chapter.

PORTABLE FANS

Portable axial fans, such as the example in view B of figure 17-5, are used with flexible air hoses aboard ship for ventilating holds and cofferdams. They are also used in unventilated spaces (voids) to clear out stale air or gases before personnel enter and for emergency cooling of machinery.

Most portable fans are the axial-flow type, driven by electric, explosion-proof motors. On ships carrying gasoline, a few air turbine-driven centrifugal fans are normally provided. You can place greater confidence in the explosion-proof characteristics of these fans.