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Handbook of air conditioning and refrigeration / Shan K

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Rectangular Ducts<br />

AIR SYSTEMS: AIR DUCT DESIGN 17.13<br />

FIGURE 17.4 Various types <strong>of</strong> <strong>air</strong> duct: (a) rectangular duct; (b) round duct with spiral seam; (c) flat oval<br />

duct; (d) flexible duct.<br />

Class 0. Zero flame spread, zero smoke developed.<br />

Class 1. A flame spread rating <strong>of</strong> not more than 25 without evidence <strong>of</strong> continued progressive<br />

combustion <strong>and</strong> a smoke developed rating <strong>of</strong> not more than 50.<br />

Class 2. A flame speed <strong>of</strong> 50 <strong>and</strong> a smoke developed rating <strong>of</strong> 100.<br />

National Fire Protection Association (NFPA) St<strong>and</strong>ard 90A specifies that the material <strong>of</strong> the ducts<br />

be iron; steel including galvanized sheets, aluminum, concrete, masonary; or clay tile. Ducts fabricated<br />

by these materials are listed as class 0. UL St<strong>and</strong>ard 181 allows class 1 material to be used for<br />

ducts when they do not serve as risers for more than two stories or are not used in temperatures<br />

higher than 250°F (121°C). Fibrous glass <strong>and</strong> many flexible ducts that are factory-fabricated are<br />

approved by UL as class 1.<br />

Ducts can be classified according to their shapes into rectangular, round, flat oval, <strong>and</strong> flexible,<br />

as shown in Fig. 17.4.<br />

For the space available between the structural beam <strong>and</strong> the ceiling in a building, rectangular ducts<br />

have the greatest cross-sectional area. They are less rigid than round ducts <strong>and</strong> are more easily

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