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Handbook of air conditioning and refrigeration / Shan K

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Modulation <strong>of</strong> Fan-Duct Systems<br />

AIR SYSTEMS: BASICS AND CONSTANT-VOLUME SYSTEMS 20.33<br />

The performance <strong>of</strong> a fan-duct system can be modulated either by changing the fan characteristics<br />

or by varying the flow resistance <strong>of</strong> the duct system. This can be achieved by any <strong>of</strong> the following<br />

methods.<br />

Modulation Using Dampers. Use <strong>of</strong> a damper is the simplest method <strong>of</strong> modulation. A multiblade<br />

damper is usually installed inside the main duct after the centrifugal supply fan. When the<br />

damper closes, the system flow resistance increases <strong>and</strong> the system operating point moves from<br />

point A to B on a different system curve. This method is known as “riding on the curve,” as shown<br />

in Fig. 20.14a. If the fan F t curve is flatter, then the increase in �p t during reduction <strong>of</strong> the volume<br />

flow rate <strong>of</strong> the fan-duct system is small.<br />

Because <strong>of</strong> the considerable total pressure loss across the damper, power input to the fan is only<br />

slightly decreased by this method. Modulation dampers have the lowest installation cost <strong>and</strong> only<br />

small energy savings.<br />

FIGURE 20.14 Modulation <strong>of</strong> fan-duct systems: (a) using dampers; (b) using inlet vanes; (c) using ac inverter<br />

to vary fan speed.

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