Variable pitch fan

Variable Air Flow Fans. Variable air flow fans are needed ia the process iadustry for steam or vapor condensing or other temperature critical duties. These also produce significant power saviags. Variable air flow is accompHshed by (/) variable speed motors (most commonly variable frequency drives (VFDs) (2) variable pitch fan hubs (J) two-speed motors (4) selectively turning off fans ia multiple fan iastaHations or (5) variable exit louvers or dampers. Of these methods, VFDs and variable pitch fans are the most efficient. Variable louvers, which throttle the airflow, are the least efficient. The various means of controlling air flow are summarized ia Table 3. 

In petrochemical plants, fans are most commonly used ia air-cooled heat exchangers that can be described as overgrown automobile radiators (see HeaT-EXCHANGEtechnology). Process fluid ia the finned tubes is cooled usually by two fans, either forced draft (fans below the bundle) or iaduced draft (fans above the bundles). Normally, one fan is a fixed pitch and one is variable pitch to control the process outlet temperature within a closely controlled set poiat. A temperature iadicating controller (TIC) measures the outlet fluid temperature and controls the variable pitch fan to maintain the set poiat temperature to within a few degrees. 

Temperature control of the process fluid may be accomplished easily through the use of shutters, variable pitch fan blades, variable speed drives, or, in multiple fan installations, by shutting off fans as required. 

The effect of cold weather on the freezing of tube-side fluids and increasing horsepower due to increased air density can not be overlooked. Usual practice is to reduce fan output by using a two-speed motor, louvers on variable-pitch fans, or drivers. ... 

When only one fan and/or exchanger exists per process service, it may be advisable to control with an automatic variable pitch fan, unless a single- or two-speed drive is considered adequate. If the process service consists of several exchanger sections or tube bundles per cell (groups of bundles) and multiple fans are used, see Figure 10-193. If single fans are used per cell, see Figure 10-194. If several... 

Figure 10-191. Temperature control and horsepower savings with automatic variable pitch fans. (Used by permission Hudson Products Corporation.)...

Rubin, F. L., Power Requirements Are Lower for Air-Cooled Heat Exchangers with Variable-Pitch Fan Blades, Oil and Gas Jour, Oct. 11, (1982). 

Figure 17.6d shows a coolant flow variation arrangement for air condensers. The controller varies fan speed or fan pitch to control pressure. This arrangement is energy-efficient and minimizes fan power consumption, but it also entails high maintenance and requires the use of a variable-pitch fan or a variable-speed motor. Fan pitch... 

Monroe, R. C., "Consider Variable Pitch Fans", Hydrocarbon Processing, Dec. 1980. 

Air cooler is designed to operate at warm summer air temperature. When air temperature is cooler than the design air temperature, process fluid will be overcooled, which may be undesirable. One way to solve this problem is to reduce the air flow through the air cooler. Thi.s can be done by using multiple Ians at the unit, 2-speed or variable. speed motor, louver, or variable pitch fans. Air flow can be reduced by turn off some ns, manual or automatic control of motor speed, change louver air inlet shutter position, or n ntch. 

For internal air recirculation system, the manual fan moves the air through the half portion of the tube bundle. The auto-variable pitch fan operating in reverse mode will draw the hot exit air fiom the upper recirculatton... 

FIG. F-3 Automatic, variable-pitch fan used to adjust airflow and fan horsepower. (Source The Marley Cooling Tower Company.)... 

Recent developments in cooling-tower fan energy management also include automatic variable-pitch propeller-type fans and inverter-type devices to permit variable fan speeds. These schemes involve tracking the loaa at a constant leaving-water temperature. 

The variable-pitch arrangement at constant motor speed changes the pitch of the olades through a pneumatic signal from the leaving-water temperature. As the thermal load and/or the ambient wet-bulb temperature decreases, the blade pitch reduces air flow and less fan energy is required. 

The controllable pitch fan provides an infinitely variable air delivery across the K-Fin Sections through automatic changes in the fan blade angle. Temperature can be closely controlled to meet the varying demands of operating conditions and fluctuating atmospheric temperatures with appreciable power savings under low load conditions. For certain control applications, the ability of... 

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. 

Fan motors should be sized at 25% above the brake horsepower. Fans should be belt driven with variable pitch drives up to and including 25 FIP and fixed pitch drives at about 25 FIP. Fans must be radial and they must be balanced (static and dynamic) and mounted on anti-vibration supports. The connection of the fan outlet to the inside panels must be done by means of an internal flexible connector. 

Fans are belt drive with variable-pitch drives for 25 FIP or less and hxed-pitch drives for drives above 25 HP. 

The fans may be driven by electric motors or by turbines. In either case, it is almost always necessary to use either a gear reducer or a belt drive to achieve the proper fan speed maximum fan tip speed is about 1500 ft/min (7.7 m/s). Where feasible, two fans (or more) should be used with each bay of the exchanger. This allows one fan to be on constant speed, while the other is controlled to achieve the desired process outlet temperature for existing ambient conditions. Both variable and variable-pitch control systems are used. API [65] provides the standards for air-cooled heat exchangers in the process industries. 

Air is circulated by multiblade propeller-type ns that provide forced or induced drafts. Fans can be supplied with either adjustable-speed or variable-pitch blades. The fen blade pitch can be changed to vary the air-flow rate to compensate for rising or falling air temperatures. Air coolers supplied with multiple fens may be operated with some of the fens shut down. Dampers, baffles, and bypasses can also be tised to further control liquid outlet temperature. For elevated installations, platforms are generally furnished for access to header boxes and motors. 

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