In recent years there has been increased emphasis placed upon the rating, cataloging, and testing of fans and systems to assure their proper operation and to minimize the consumption of energya As a result, safety factors in system design have been reduced so that there is very little allowance for error either in the calculated system resistance or the fan performance. It is, therefore, essential that those items which can affect fan performance and the magnitude of that, effect become an important part of our general knowledge.

It has been known for many years that a fan tested in the laboratory may not perform the same once it is installed in the field. Several articles have been written which describe these phenomena. This project was initiated to address the problem, in much greater detail, of variations between laboratory fan ratings and the actual performance obtained under field conditions. Field installation conditions, including the specific location and design of ductwork components, can affect a fan's performance. These elements influence not only the flow rate and fan static pressure, "but also the fan brake horsepower.