87-12: Anomalies in Fan Efficiency Calculations

Fan total efficiencies as calculated by the CTI Field Test Handbook (Chapter 5.5) can, in certain situations, exceed 100%. This has caused confusion among specifiers and users. Therefore, efficiencies calculated from experimental data are sometimes adjusted downward to maintain credibility. The net result is that fan motor horsepower, as specified, are sometimes higher than actually needed. The reason for this anomaly is the incorrect assumption that the static pressure at the fan outlet is zero. In actuality, it is a negative number due to rotational flow and turbulence effects. This paper will discuss the problem and propose corrective action. In addition, the influence of inlet shape and obstacles will be discussed. Supporting data will be presented.