This paper presents alternative methods for evaluating how field tested power compares to a fan vendor’s rated power when the tested operating point is marginally off the specified reduced load operating point. ASME PTC 11-2008 Mandatory Appendix I requires four separate field test points be obtained to verify a single reduced load operating point. Each of the four test points must fall within two to four separate and relatively precise target windows that adjoin and surround the specified operating point. This approach will involve numerous system adjustments, fan control adjustments and accompanying field performance tests just to verify a single specified operating point. A methodology is proposed in this paper for which it is postulated that only a single field test point needs to be obtained within a flow-pressure target window of ±3% of the specified flow and ±2% of the specified pressure rise to accurately evaluate reduced load power. Three different evaluation methods are presented that compare the field tested power to the fan vendor’s rated reduced load power; i) power evaluation – 4-point distance weighted interpolation ii) power evaluation – 2-point linear interpolation and iii) efficiency evaluation. The proposed power evaluation methods require that in addition to identifying the rated power at the specified operating point, the fan vendor will be asked to identify what the rated power is at each of the four corners of the flow-pressure target window. These rated powers are used to interpolate the rated power that corresponds with the single field test point which can then be compared directly to the field tested power. This approach will result in significant reductions in field testing time and expenses. In addition, working with a single field tested power value for a single test point should reduce the potential combined uncertainty associated with interpolating test power from multiple field tested powers. It is opined that applying an interpolation method to the fan vendor’s rated reduced load power predictions rather than field test data may be more agreeable to the fan vendor. In effect, the fan vendor is being asked to guarantee the reduced load power for a relatively narrow flow-pressure target window.

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