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Handbook of air conditioning and refrigeration / Shan K

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20.26 CHAPTER TWENTY<br />

The total pressure loss <strong>of</strong> the combined fan-duct system �p t, in in. WC (Pa), is<br />

�p t ��p 1 ��p 2<br />

(20.26)<br />

Graphically, the combination <strong>of</strong> the curves <strong>of</strong> two fans connected in series on a �pt-V˙ diagram can<br />

be performed by drawing several constant-volume flow rate lines. For each constant-volume line,<br />

the fan total pressure <strong>of</strong> the combined fan curve Ft1 � Ft2 always equals the sum <strong>of</strong> the sections <strong>of</strong><br />

fan total pressure <strong>of</strong> fan curve 1, Ft1 <strong>and</strong> fan curve 2, Ft2, represented by �p1 <strong>and</strong> �p2, as shown in<br />

Fig. 20.9. The purpose <strong>of</strong> connecting two fan-duct systems in series is to increase the fan total<br />

pressure that can be provided by the combined system.<br />

Figure 20.10 shows the system pressure characteristics <strong>of</strong> a combined fan-duct system with<br />

fans F1 <strong>and</strong> F2 <strong>and</strong> a single fan-duct system that uses only one fan F. It can be seen that the static<br />

pressure across the duct wall is considerably higher in a fan-duct system with a single fan than in<br />

the combined fan-duct system with two fans. Of course, a single-fan system has a lower initial cost.<br />

For two fan-duct systems connected in series, the volume flow rates <strong>of</strong> the two fans should be<br />

similar. If a large fan is connected in series with a small fan, at large volume flow rates, the fan total<br />

pressure <strong>of</strong> the combined system may be less than if the large fan were operated alone. This loss <strong>of</strong><br />

efficiency results from the influence <strong>of</strong> negative total pressure <strong>of</strong> the small fan when it is operated at<br />

a volume flow rate greater than its wide-open volume flow.<br />

Fans Combined in Parallel <strong>and</strong> Connected in Series with a Duct System<br />

When two fans F1 <strong>and</strong> F2 combined in parallel are connected to a duct system represented by flow<br />

resistance R, as shown in Fig. 20.11, the volume flow rate <strong>of</strong> the parallel combined fans V˙ P is the<br />

sum <strong>of</strong> the volume flow rates <strong>of</strong> individual fans V˙ 1 <strong>and</strong> V˙ 2.<br />

The purpose <strong>of</strong> such a combination is to increase the volume flow rate <strong>of</strong> the system. Usually,<br />

the fans in parallel combination are identical fans, i.e., fans <strong>of</strong> the same model with the same rated<br />

FIGURE 20.11 Two fans combined in parallel <strong>and</strong> connected in series with a duct system.

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