Machinery's Handbook, 31st Edition
FAN AND BLOWERS 2811 (squirrel cage fans) are the most common type, but with an efficiency of approximately 75 percent they are the least efficient of all the centrifugal designs. Backward-curved centrifugal fans are less likely to overload than forward-curved fans, and their increased efficiency over forward-curved centrifugals makes up for their increased cost. Radial blade fans are commonly found in industrial cooling applications where a large volume of air is required at moderate pressure. Centrifugal blowers look more like centrifugal pumps than fans. The impeller is typically gear-driven. The two most common types of centrifugal blowers are pressure blowers and volume blowers. Pressure blowers are de- signed to draw or push air at high pressures, rated in static pressure water gauge (SPWG). Volume blowers are designed to draw or push larger volumes of air, rated in cubic feet of air per minute (CFM), at lower static pressures than pressure blowers. Cross-flow fans and blowers have air enter and exit the impeller tangentially. The pres sure produced by these impellers is low. Mixed-flow fans and blowers use impellers that impart both axial and radial momentum to a gas. These impellers can be housed in either axial or scroll-type casings to output either axial or radial flow. Positive displacement blowers are essentially positive displacement pumps used for moving air. Positive dis placement pumps are described earlier in this text. Fan and Blower Calculations: When selecting a fan or blower, one must consider the cleanliness and type of gas being moved, the pressure differential or velocity needed, the volumetric flow rate required, the energy loss in the ductwork or system, the noise pro duced, and the cost. Fan capacity is a measure of the maximum volumetric flow rate of the fan or blower. Total pressure of a fan or blower is the difference between the inlet and outlet pressures, and it is also the sum of static pressure and velocity pressure. Static and velocity pressures are often measured in inches of water (in. w.g.). Fan static, velocity, and total pressures at standard conditions can be calculated using the following equations: p s in. w. g. ( ) p psig 0.0361 = --------
2
v fpm 4005 ------
=
p v in. w.g. ( )
p γ -- ρ
v 2
p t = where p s is static pressure, and the specific weight of water is assumed to be 0.0361 lbf/in 3 ; p v is velocity pressure; v fpm is velocity in fpm; and p t is total pressure. p s p v + 2 + ----- = When a fan is connected to ductwork, the pressure drop through the duct and fittings should be taken into account using the following equations, by using the Moody diagram given in Fig. 1, or by using friction loss graphs like those published by ASHRAE. The Darcy-Weisbach equation (see Darcy-Weisbach Equation and the Moody Diagram on page 2779 ) should be used for “non-standard” duct type such as flex duct. Q CFM 0.61 = ----------------------- where p f is friction pressure of air moving through ducts. This assumes clean, galvanized, circular ducts with a specific roughness of 0.0005 ft. p f in. w.g. ( ) K v fpm p f in. w.g. ( ) 3.9 –9 × 10 ( ) v fpm 2.43 L ft 2 = where p f is friction pressure of air moving through fittings. The loss coefficient K can be found in Table 34. 4005 ------
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