Control fan pitch

Air-Flow Control Process operating reqmrements and weather conditions are considered in determining the method of controlling air flow. The most common methods include simple on-off control, on-off step control (in the case of multiple-driver units), two-speed-motor control, variable-speed drivers, controllable fan pitch, manually or automatically adjustable louvers, and air recirculation. 

Figure 17.6 Pressure control by condensing medium flow variations, (a) Liquid coolant (5) refrigeration coolant, level control (c) refrigeration coolant, outlet flow control (d) air coolant, fan motor speed or fan pitch variation (e) turboexpander inlet guide vanes variation.

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... 

Fan speed or fan pitch control generally works well with induced-draft fans, but is less satisfactory with forced-draft fans. In the latter case, precipitation tends to affect condensation to a far greater degree than variations in fan speed or pitch. Where precipitation severely interferes with control action, a roof some distance above the air cooler may be desirable. 

If a battery of condensers is used, often only few of the condensers need to have louvers, fan pitch, or motor speed controls. The others are controlled by switching fans on or off when the modulating unit reaches the limits of its control range. The switching can be done automatically (362). 

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. 

When the mean annual temperature is 16.7°C (30°F) lower than the design dry-bulb temperature and when both fans in a bay have automatically controllable pitch of fan blades, annual power required has been found to be 22, 36, and 54 percent respectively of that needed at the design condition for three process services [Frank L. Rubin, Tower Requirements Are Lower for Air-Cooled Heat Exchangers with AV Fans, Oil Gas J., 165-167 (Oct. 11, 1982)]. Alternatively, when fans have two-speed motors, these dehver one-half of the design flow of air at half speed and use only one-eighth of the power of the full-speed condition. 

The rate at which each fan circulates air can be varied by changing the pitch of the fan blades. In final drying stages, in which diffusion controls or the product is light and powdery, the circulation rate is considerably lower than in the initial stage, in which high evaporation rates prevail. In the majority of applications, air flows through the... 

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... 

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. 

Adjustable pitch fan A fan in which the pitch angle can be set to provide the required airflow rate. The pitch angle may be preset or controlled with the fan running. 

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

Manual (for seasonal changes only) or automatic pitch control operated by air-motor on fan blades. 

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.)...

TCV = Temperature Control Valve TE = Temperature Element VAV = Variable Air Volume XP = Positioner for Fan Volume Control, such as a Blade Pitch Positioner... 

Tube-axial fans are provided with adjustable pitch blades that permit the balancing of the fan to match the varying process load either automatically or by infrequent manual adjustments. Vane-axial fans are also available with controllable pitch blades (that is, pitch that can be varied while the fan is in operation) for use when frequent or continuous flow adjustment is needed. 

PIC-10 is the load-following controller. It compares the optimized set point with the actual header pressure and adjusts the fan speeds or the blade pitch angles of the fans. When its output signal reaches the maximum set on PSH-07, it starts another fan. 

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