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Handbook of air conditioning and refrigeration / Shan K

Handbook of air conditioning and refrigeration / Shan K

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AIR SYSTEMS: COMPONENTS—FANS, COILS, FILTERS, AND HUMIDIFIERS 15.13<br />

FIGURE 15.8 Pressure-volume �pt-V˙ performance curves for centrifugal<br />

fans with same impeller diameter.<br />

Coward (1997) noted the influence <strong>of</strong> the gap between the plenum wall <strong>and</strong> the impeller on the<br />

fan volume flow rate <strong>and</strong> fan power input as well as various types <strong>of</strong> duct entrance losses. The performance<br />

<strong>of</strong> an unhoused plug fan should be predictable <strong>and</strong> should be carefully taken into consideration<br />

during design.<br />

Unhoused plug/plenum fans with backward-curved impellers may require more power than<br />

centrifugal fans with outer casings (fan total efficiency is usually between 58 <strong>and</strong> 63 percent). They<br />

are lower in sound power level if the plenum is properly sized <strong>and</strong> acoustically treated. Unhoused<br />

FIGURE 15.9 Tubular centrifugal fan.

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