A certain fan system has a seasonal airflow requirement as shown in Figure 1. For four months of the year, the demand is almost double that of the other eight months. Figure 1: Monthly average air flow rate requirements. If you rearrange the months according to demand, from the highest to the lowest, you obtain what is known as a duration diagram, as shown in Figure 2. Figure 2: Air flow duration diagram. The area under a duration curve is flow rate time, which represents the total volume delivered. This diagram clearly shows that four months require a flow rate of approximately 11m3/s each, and the other eight months require a flow rate of about 7m3/s each. The power requirements for these two conditions can be approximated as shown in Figure 3. Note that the high flow rate periods require the fan to deliver 100kW to the air and the lower flow rate periods require 25kW to be delivered to the air. Figure 3: Approximated power duration diagram (power transferred from the fan to the air). At present, these two different demands are handled by a single fan that is powered by a motor controlled by a variable speed