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Operating point = EXAMPLE 14-1 Operating Point of a Fan in a Ventilation System A local ventilation system (hood and exhaust duct) is used to remove air and contaminants produced by a dry-cleaning operation (Fig. 14-12). The duct is round and is constructed of galvanized steel with longitudinal seams and with joints every 30 in (0.76 m). The inner diameter (ID) of the duct is D = 9.06 in (0.230 m), and its total length is L = 44.0 ft (13.4 m). There are five CD3-9 elbows along the duct. The equivalent roughness height of this duct is 0.15 mm, and each elbow has a minor (local) loss coefficient of K = Co = 0.21. Note the notation Co for minor loss coefficient, commonly used in the ventilation industry (ASHRAE, 2001). To ensure adequate venti- lation, the minimum required volume flow rate through the duct is ű = 600 cfm (cubic feet per minute), or 0.283 m/s at 25°C. Literature from the hood manufacturer lists the hood entry loss coefficient as 1.3 based on duct velocity. When the damper is fully open, its loss coefficient is 1.8. A centrifugal fan with 9.0-in inlet and outlet diameters is available. Its perfor- mance data are shown in Table 14-1, as listed by the manufacturer. Predict the operating point of this local ventilation system, and draw a plot of required and available fan pressure rise as functions of volume flow rate. Is the chosen fan adequate? 22 1 0.9 Havailable 0.8 Operating point 0.7 0.6 H, inches H20 0.5 Fan 0.4 TABLE 14-1 Manufacturer's performance data for the fan of Example 14-1 0.3 Hrequired 0.2 Damper V.com 0.1 250 0 0 Inches HO 0 0.90 0.95 500 0.90 750 0.75 1000 0.40 1200 0.0 *Note that the head data are listed inches of water, even though air is the fund. This is common practice in the ventilation industry 200 800 1000 Hood 400 600 ů, cfm 23 FIGURE 14-13 Net head as a function of volume flow rate for the ventilation system of Example 14-1. The point where the available and required values of H intersect is the operating point. FIGURE 14-12 The local ventilation system for Example 14-1, showing the fan and all minor losses