Axial-Flow Fans

Fig. 9. Types of axial-flow fans where the dashed arrows denote the direction of air flow (a) propeller fans (b) tube-axial fans (c) vane-axial fans (17).

Fig. 10. Performance characteristics for axial flow fans ( ndashrule ), ptopeUet fan (—), tube-axial fan and ( ), siagle-stage and (.), two-stage...

Fig. 11. Two-truck tray dryer. A, air inlet duct B, air-exhaust duct with damper C, axial flow fan D, fan motor, 2—15 kW E, air heaters F, air-distribution plenum G, distribution slots and H, wheeled tmcks and trays. The arrows iadicate air and vapor flow pattern.

Axial-flow fans are designed to handle veiy high flow rates and low pressure. The disc-type fans are similar to those of a household fan. They are usually for general circulation or exhaust work without ducts. 

Fans Axial-flow fans are large-volume, low-pressure devices. Fan diameters are selected to give velocity pressures of approximately 2.5 mm (0.1 in) of water. Total fan efficiency (fan, driver, and transmission device) is about 75 percent, and fan drives usually have a minimum of 95 percent mechanical efficiency. 

The axial-flow fan is inherently a device for moving a consistent volume of air when blade setting and speed of rotation are constant. Variation in the amount of air flow can be obtained by adjusting the blade angle of the fan and the speed of rotation. The blade angle can be either (1) permanently fixed, (2) hand-adjustable, or (3) automatically adjusted. Air delivery and power are a direct function of blade pitch angle. 

For ease of illustration we will consider the characteristics and behaviour of a centrifugal pump which is similar in behaviour to radial/axial flow fans and centrifugal/screw compressors. Figure 63 shows the mechanical connection of a flow valve to control the output of the pump or the discharge of the fluid through the throttle of the valve. Figure 6.39 illustrates the characteristics of the pump ... 

Blowing hot air A typical arrangement is shown in Figure 31.9. The bus duct enclosure is provided with inlet and outlet valves at suitable locations. Hot air is supplied through the inlet valve until the interior of the enclosure is completely dry. The blowing equipment comprises an axial flow fan. a heater unit, an inlet air filter unit, pressure... 

Greater space requirement for a given duty than an axial flow fan. 

Axial-flow fan A fan positioned in a cylindrical casing in which the air enters and leaves the impeller in a direction parallel to the casing axis. The fan may have fixed-pitch blades or variable-pitch blades. 

Bifurcated fan An axial flow fan that directs the airstream around the motor, which is enclosed in a protective casing. It is used for handling corrosive, high-temperature, and explosive dusts, vapors and gases. 

Variable blade pitch control The impeller blade angle is varied when the fan is in operation, normally only for axial flow fans. 

Contra-rotating An axial flow fan which has two impellers arranged in series and rotating in opposite directions. 

Reversible axial flow An axial flow fan, specially designed to rotate in either direction. 

Plate-mounted An axial flow fan mounted in an orifice or spigot-... 

System efficiency is influenced by the air plenum chamber and fan housing. Industrial axial flow fans in proper system design will have efficiencies of approximately 75% based on total pressure. Poor designs can run 40%. Speed reducers are about 75% mechanically efficient. 

Centrifugal compressors usually have specific speeds of 1,500-3,000 rpm at the high efficiency point. The axial flow fans and blowers are high specific-speed wheels mixed flow units are lower and the centrifugal compressor wheels are the lowest in specific speed range because they have narrow impellers. 

The performance of the axial flow fan is represented in Figures 12-133A-B. 

Fan laws apply to blowers, exhausters, centrifugal, and axial flow fans. The relations are satisfactory for engineering... 

Fans may also be operated in series. Axial-flow fans are available having two (contra-rotating) impellers and motors in the one casing. Further staging may be arranged. If one stage fails or is switched off it will idle round with loss, but since similar losses in centrifugal fans are much... 

Air leaves the blades of an axial-flow fan with some turning motion. 

Figure 27.7 Axial flow fans, (a) Construction, (b) Typical performance curves (Reproduced, with permission, from Wood s Practical Guide to Fan Engineering [55])...

Higher pressures can he obtained hy putting two axial-flow fans in series. If they are placed close together and contrarotated, the spin imparted hy the first can he recovered hy the second, and more than twice the pressure capability can be gained. 

The best efficiency of the axial-flow fan is to the right of the trough seen in the pressure curve, and the optimum band of performance will be indicated by the manufacturer. In particular, the air flow should not be less than the given minimum figure, since the fan motor relies on air flow for cooling. 

Question s most often asked of a fan engineer about axial-flow fans for today s wet cooling towers generally cover performance efficiency corrosion resistance and noise. This article reviews such fundamentals and gives new insight for optimum tower design. 

Figures 5.4 and 5.5 show typical elevation and plot plan section sketches of horizontal air-cooled exchangers. The essential components are one or more tube sections, called tube bundles, and one or more axial flow fans, all enclosed in a structural module made to anchor footings into concrete or to stand on an offshore platform. As shown, one fan may serve more than one tube bundle. A bay may be made up of multiple tube bundles and may also be served by one or more fans. Most important is the fact that one exchanger may be composed of any number of tube bundles. Tube bundles may be arranged in series or in parallel, just like shell/tube bundles.

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