Developments from the 1930s to the 1960s
In the late 1930s, Stork Brothers of Hengelo, in the Netherlands, introduced its aerofoil backward bladed centrifugal fan (Figure 1.40) which enabled efficiencies in the high 80s% to be achieved over a considerable portion of the characteristic. It coincidentally produced a reduction in noise levels.
In 1955 tests by Professor Sorensen had shown that the Schicht fan (Figure 1.41), produced by KKK of FrankenthalPfalz, Germany, could produce efficiencies in excess of 80%. Static pressure through the impeller remained constant and was only increased by retardation in the diffuser section. Due to the accelerated flow velocity, shaped blades were unnecessary, and the fan capacity was, therefore, unchanged by deposits, rust or erosion. In consequence the fan has been widely used for induced draught applications, control being by means of a radial vane inlet damper.
Aerex Ltd evolved a series of axial flow fans for mine ventilation using an impeller having patented blades of fabricated stainless steel, hollow formed to true aerofoil section. The blades could have their pitch angle changed at the periphery without entering the hub. Both up and downstream guide vanes were used. Fans were often arranged for horizontal drive through vee-belts from a side mounted motor and an integral outlet bend/diffuser was fitted. Many such fans were supplied to South Africa for use in gold and coal mines. Atypical example for Wankie Colliery is illustrated in Figure 1.42.
The Axcent mixed flow fan was originally patented by Keith Blackman Ltd in 1958 (Figure 1.43) and was claimed to combine the advantages of both the axial and centrifugal types. With its steep pressure/flowrate characteristic and non-overloading power curve, its performance was more akin to a two stage axial fan. Subsequently improved versions have been produced with fan static efficiencies in excess of 70% and noise
Figure 1.43 An Axcent mixed flow fan
Levels comparable with centrifugal fans. Such fans are widely used offshore for the ventilation of oil rig platforms in the North Sea. Their ability to maintain almost constant airflow under strong contrary winds has been as much valued as their low mass and compact dimensions.
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