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fan with variable-pitch capability, the power savings can be significant, as indicated by Figure 32-12.

• Variable speed operation is usually the most efficient means of capacity control for centrifugal fans. As indicated by Figure 32-13, power savings are significant and predictable according to the fan laws, and stable

Fig. 32-15 Comparison of Power Savings Potential with Use of Multiple Parallel Fans as a Means of Control. Source: Buffalo Forge, The Howden Fan Company operation occurs at all speed settings for the given system. Lower sound levels are also achieved under partial load. The turndown ratio will be limited by the minimum speed that the variable-speed driver can achieve. The full-speed curve will be identical to the fan curve without the variable speed device if no slip occurs, although a small power loss typically occurs in the variable speed device.

Figure 32-14 compares various methods of capacity control. The potential impact of slip losses on power savings is indicated by the difference between the variable speed curves with subscripts i and o. Figure 32-15 illustrates the power savings that can be achieved using multiple fans to vary airflow capacity. Only limited control is possible when two fans are used; however, dampers, vanes, pitch, or speed control can also be used for one or both fans.

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