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Handbook of air conditioning and refrigeration / Shan K

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15.14 CHAPTER FIFTEEN<br />

plug/plenum fans are <strong>of</strong>ten used as return fans in many ro<strong>of</strong>top packaged units <strong>and</strong> as exhaust fans<br />

in industrial applications.<br />

Centrifugal Ro<strong>of</strong> Ventilators<br />

15.3 AXIAL FANS<br />

Types <strong>of</strong> Axial Fans<br />

FIGURE 15.10 Centrifugal ro<strong>of</strong> ventilator.<br />

A centrifugal ro<strong>of</strong> ventilator is <strong>of</strong>ten mounted on a ro<strong>of</strong> to exhaust <strong>air</strong> from a ventilated space, as<br />

shown in Fig. 15.10. Ro<strong>of</strong> ventilators can also be used as intake devices to draw outdoor <strong>air</strong> through<br />

the ro<strong>of</strong>. In Fig. 15.10, <strong>air</strong> is extracted from ventilated space <strong>and</strong> enters the inlet cone directly or<br />

through a connecting duct having a total pressure loss usually less than 0.2 in. WC. After flowing<br />

through a backward-curved centrifugal impeller, <strong>air</strong> is discharged radially against the inner surface<br />

<strong>of</strong> the hood. The sheet-metal or <strong>air</strong>foil blades <strong>of</strong> the centrifugal ro<strong>of</strong> ventilator are made <strong>of</strong> aluminum<br />

alloy, stainless steel, or well-protected structural steel to provide good corrosive resistance<br />

above the ro<strong>of</strong>.<br />

The centrifugal ro<strong>of</strong> ventilator usually operates at a low total pressure <strong>and</strong> a large volume flow<br />

rate, as illustrated by a downward inclined �pt-V˙ curve, as shown in Fig. 15.8. The power P-V˙<br />

curve is a nonoverloading curve.<br />

For an axial fan, a parameter called the hub ratio is closely related to its characteristics. Hub<br />

ratio R hub is defined as the ratio <strong>of</strong> hub diameter D hub, in ft (m), to the tip-to-tip blade diameter or<br />

diameter <strong>of</strong> impeller D bt, in ft (m),<br />

R hub � D hub<br />

D bt<br />

(15.19)

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