ABSTRACT
Fans and blowers are based mostly on the same principles as pumps. Although they handle gases, the pressure rises are too small for compressibility to be important. The pressure rise in centrifugal fans is slightly higher than in axial fans. In the first part of the chapter, flow characteristics of axial fans are discussed, along with the effects of upstream and downstream guide vanes. The concepts of degree of reaction, pressure coefficient, flow coefficient, and blade loading coefficient are briefly introduced. The differences between the performance curves based on the shape of the blades are discussed. Fan laws as they pertain to geometrically similar machines are explained in detail. These also apply to a fan operating at different speeds. It is shown that the flow rate is proportional to speed, pressure rise is proportional to the square of speed, and power is proportional to the cubic exponent of speed. Efficiency based on static and stagnation pressure rise across fans is discussed.
