Blade pitch increase

The fan blades themselves may be adjusted to obtain more airflow. This is done by increasing the fan blade pitch. The pitch can usually be adjusted between 12° (for low airflow) to 24° (for high airflow). Any... 

Cooler weather always increases the airflow produced by a fan. This always increases the amp load on the fan s motor driver. To prevent the motor from tripping off, or simply to save electricity during the winter, you might reduce the fan blade pitch. 

The effect of changing Sj, the clearance, depends on the design of the turbine. Increasing Sj increases i fp for a disk turbine of the type shown in Fig. 9.7. For a pitched-blade turbine increasing Sj lowers Np considerably, as shown in Table 9.2 for an open straight-blade turbine it lowers Np slightly. 

Setting the blade pitch cannot be done with great precision, and it s not too critical. I once increased the blade pitch from 15° to about 22°. Air flow increased by only 5 percent measured by the increased amperage load on the motor driver. 

A basic stirred tank design is shown in Fig. 23-30. Height to diameter ratio is H/D = 2 to 3. Heat transfer may be provided through a jacket or internal coils. Baffles prevent movement of the mass as a whole. A draft tube enhances vertical circulation. The vapor space is about 20 percent of the total volume. A hollow shaft and impeller increase gas circulation (as in Fig. 23-31). A splasher can be attached to the shaft at the hquid surface to improve entrainment of gas. A variety of impellers is in use. The pitched propeller moves the liquid axially, the flat blade moves it radially, and inclined blades move it both axially and radially. The anchor and some other designs are suited to viscous hquids. 

The 45° axial-flow turbine is more efficient at all Reynold s numbers than the flat-blade radial turbine. However, as the pitch angle decreases below 45°, the impeller becomes increasingly sensitive to high viscosity. 

The difference in processing behavior between rotors provided with straight blades and those with pitched blades is shown in Figure 6 by an application referring to solvent recovery from a polymer solution of low initial viscosity. The evaporation capacity of a thin-film evaporator equipped with straight blades decreases considerably as soon as the concentrated polymer reaches a viscosity between 1000 and 2000 P. The greatly increased mean film thickness that characterizes this vis-... 

Pitch-blade turbine (paddle stirrer with pitched blades) and propeller stirrers provide high mixing with an axial flow pattern. Both of these stirrers are normally used for low-viscosity liquids and in vessels with baffles. They are well suited for providing liquid homogenization and suspension of solids in slurry reactors. The stirrers can also be used in viscous fluids and for vessels with H/dT > 1, which are generally encountered in fermentation processes. For these situations, axial flow is increased with the use of multistage stirrers with pitched stirring surfaces. 

Air coolers 87-94 Aerodynamic fan blades, decrease rev./min and increase pitch, tip and hub seals, decrease pressure drop. 

Pavlushenko et al. (P2) studied the suspension of screened fractions of sand and iron ore in a variety of liquids, with a 1-ft. diameter unbaffled vessel filled to a depth of one foot. Square-pitch three-blade propellers of 3-, 4-, and 5-in. diameter were used, and most of the observations were made with a 1 to 4 weight ratio of solids to liquid. Thief samples were taken at various levels in the vessel. In some cases, the contents did not become uniform at any impeller speed in other cases the contents became uniform at some impeller speed and remained so at higher speeds in a third type of behavior, the upper part of the vessel reached the over-all vessel average and then exceeded it as impeller speed was increased. Using the observations from the second and third types of behavior, a critical speed was defined as the lowest impeller speed at which the solids concentration at each level, or in the upper layers of the liquid, was equal to the over-all average solids concentration. This critical speed Nc in revolutions per second had the following relation to the operating variables ... 

Turbine designs are intermediate between paddles and propellers. Turbines are effective mixers over a wide viscosity range and provide a very versatile mixing tool. The ratio of radial to tangential flow, the predominating parameters with this impeller, increases as the operating speed increases. Pitched-blade turbines are sometimes used to increase axial flow. Baffles must be used to limit swirling unless the turbine is shrouded. 

The proportionality constant fe depends upon the stirrer type. For propeller stirrers fe = 10 ]68], for turbine stirrers fe = 11.5 and pitched-blade stirrers fe = 13 [104]. For blade stirrers k = 2.5, for cross-beam stirrer fe = 4.1 and for helical ribbon stirrer fe = 6.0 [411]. Calderbank [66] found that when turbine stirrers were used with Bingham and pseudoplastic fluids fe = 10 and when used with dilatant liquids fe = 12.8 (d/D) 5. Lower fe values were found as the viscoelasticity of fluids increased [104]. In the case of close-clearance anchor stirrer fe depended upon the wall clearance [24]. 

H. Judat has comprehensively extended this work. In [246] Ne(Q) relationships are given for different stirrer types, namely for perforated disk stirrers with 12 holes (after Brauer [48]), for 3-wane propeller stirrers with a pitch a = 26°, for pitched-blade stirrer with a pitch a = 45° and for conventional turbine stirrers with different numbers of blades i = 6 12 18 24 (Fig. 2.6). The measuring range of Q was extended for turbine stirrers to Q 1.0. It was shown that in the Q range 0.2 to 1.0 expression (2.23) did not apply. With increasing Q, Ne decreases ffirther. 

With a straight-blade open turbine the effect of changing S4, the ratio of blade width to impeller diameter, depends on the number of blades. For a six-blade turbine Np increases directly with S4 for a four-bladed turbine Np increases with With pitched-blade turbines the effect of blade width on power consumption is much smaller than with straight-blade turbines (see Table 9.2). 

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