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Handbook of air conditioning and refrigeration / Shan K

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Forward-Curved Fans<br />

AIR SYSTEMS: COMPONENTS—FANS, COILS, FILTERS, AND HUMIDIFIERS 15.11<br />

FIGURE 15.6 Power-volume P-V˙<br />

performance curves for centrifugal<br />

fans with impellers <strong>of</strong> same diameters.<br />

Figure 15.8 shows the �pt -V˙ curves for centrifugal fans with impellers <strong>of</strong> the same diameter.<br />

The �pt-V˙ curve <strong>of</strong> a radial-bladed centrifugal fan is usually steeper than that <strong>of</strong> a backward-curved<br />

fan, with a higher �pt <strong>and</strong> a smaller V˙ . The fan total efficiency <strong>of</strong> a radial-bladed fan is lower than<br />

that <strong>of</strong> a backward-curved fan. Furthermore, the fan power input always increases as the volume<br />

flow becomes greater. Such an overloading P-V˙ curve indicates that the motor may be overloaded.<br />

The radial-bladed centrifugal fan is <strong>of</strong>ten used in industrial applications to transport particles or<br />

products because the spaces between the blades are not easily clogged.<br />

In a forward-curved centrifugal fan, the blade tip inclines in the direction <strong>of</strong> rotation <strong>of</strong> the impeller.<br />

At the outlet <strong>of</strong> the blade, blade angle �2 � 90°, as shown in Fig. 15.7b. Because <strong>of</strong> a greater<br />

absolute velocity v2 at the blade outlet, the blades are shorter <strong>and</strong> the speed required to produce a<br />

specific fan total pressure <strong>and</strong> volume flow rate is much lower than that for a backward-curved<br />

centrifugal fan. The impeller generally has 24 to 64 blades.<br />

If the diameters <strong>of</strong> the impellers <strong>of</strong> a forward-curved fan <strong>and</strong> a backward-curved fan are the<br />

same, then the �pt-V˙ curve <strong>of</strong> the forward-curved centrifugal fan is flatter than that <strong>of</strong> the back-<br />

ward-curved fans, as shown in Fig. 15.8. Forward-curved fans have a lower fan total efficiency than<br />

backward-curved fans, <strong>and</strong> the maximum efficiency <strong>of</strong> the forward-curved fan occurs between 40<br />

<strong>and</strong> 50 percent <strong>of</strong> the wide-open volume flow. The forward-curved fan power curve tends to bend<br />

upward more steeply than that <strong>of</strong> other centrifugal fans, as shown in Fig. 15.6. Therefore, the<br />

motor <strong>of</strong> the forward-curved fan may be overloaded when the volume flow rate is greater than the<br />

rated value.<br />

Forward-curved fans are the most compact centrifugal fans available in terms <strong>of</strong> volume flow<br />

rate delivered per unit space occupied. They are also less stable, as indicated by the saddle shape <strong>of</strong><br />

their �pt-V˙ curve. For the same size fan wheel, forward-curved fans have a greater inlet than backward-curved<br />

fans.

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