Abstract :
In a recent paper the author dealt with the design of propeller fans, and in the present paper he discusses the advantages of propeller fans as applied to the cooling of electrical machines. Illustrations are given of various types of electrical machines fitted with propeller fans, and the author points out the flexibility of design as compared with that of the centrifugal fan. Special tests show the advantages of a fan designed according to modern aerodynamics over the simple ¿paddle wheel¿ type of fan. Reference is made to fans designed to give maximum slip-stream rotation. The author points out that the windage efficiency of a machine with fan is far more important than that of the fan alone, and the most efficient fan does not necessarily give the most efficient machine. Great care must be taken in computing from friction and windage tests the actual power taken by the fan. The fan can be designed to rotate the air more efficiently than can the machine itself, so that the increased fan power may be more than balanced by the reduced machine windage. The author stresses the need for good inlet conditions for the satisfactory operation of propeller fans. Tests made upon a turbo-alternator illustrate the effect of guide vanes in the air intake to propeller fans, and the author points out that both circumferential and radial guide vanes are essential. Discussing the problem of external fans for cooling large turbo-alternators, the author shows that two fans in series on a variable load permit greater efficiencies, and therefore greater flexibility, than two fans in parallel. Tests on two propeller fans in series bear out the author´s statements. High efficiencies are claimed for a special scheme of series propeller fans which simplifies the dampers in a turbo-alternator ventilation scheme.
