Proper circulation of air is necessary for all ventilating and air-conditioning systems and related processes. Therefore, we will first consider methods used aboard ship to circulate air. In the following sections, you will find general information on shipboard equipment used to supply, circulate, and distribute fresh air, and to exhaust used, contaminated, and over-heated air from shipboard working and living spaces.

Shipboard ventilation systems normally use two-speed electric motors to drive intake fans and exhaust fans. For proper operation, the speed settings must be the same. This is because the exhaust fans have a higher air-handling capacity in cubic feet per minute (cfm) than the intake fans. When the fans are operated at different speeds, problems occur. For example, when the exhaust is on high and the intake is on low, a partial vacuum is caused and it will be difficult to open watertight doors to gain entry to the space. When the opposite condition occurs (high intake, low exhaust), the watertight doors will be difficult to close.

Fans used in Navy ships in conjunction with supply and exhaust systems are divided into two general classes-axial flow and centrifugal.

Most fans in duct systems are of the axial-flow type because they generally require less space for installation.

Centrifugal fans are generally preferred for exhaust systems that handle explosive or hot gases. The motors of these fans, being outside the air stream, cannot ignite the explosive gases. The drive motors for centrifugal fans are subject to overheating to a lesser degree than are motors of vane-axial fans.