Natural Ventilation

The shape of the opening is not specified but to give an idea of what three square inches looks like, Figure 15 depicts a few common geometric shapes, each of which is three square inches.

FIGURE 15 - Three Square Inches

NOTE:
Figure 15 is not drawn to scale.

The regulation requires each opening and each duct, whether a supply or an exhaust, to have more than 3.0 square inches in cross-sectional area. Therefore, openings and ducts must be larger than those shown in Figure 15.

Some of the flexible ventilation ducts that are available for marine use may be fitted with terminal fittings made to fit the duct. Some of these are designed to fit inside the flexible duct. For these fittings the reduced cross-section at the fitting is permitted to be not less than 80% of the cross-section required for the flexible duct under consideration. It is important to remember that this reduction in the cross-section is only permitted inside a terminal fitting. No reduction in cross-sectional area is permitted any place in the natural ventilation system. Also, note that the 80% figure applies to the required area of the duct. For example, if the required area is 5 sq. in., but the installed duct has a cross-sectional area of 8 sq. in., the 80% (minimum) figure applies to the 5 sq. in.

Figure 16 is a graph that is intended to aid in determining compliance. The minimum terminal fitting cross-sectional area must be at or above the 80% of required area line in order to comply with this section of the regulation. The diameters at the various areas are included on the graph to aid in evaluating terminal fittings by simply measuring the inside diameters.

The following discussion and graph has assumed that circular air ducts are used. If other shapes of flexible ducts are used, the same principles apply, but their measurements and area calculation must be appropriate for the shape under consideration

FIGURE 16 - Terminal Fitting Area

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