Large blade low speed

Radial blade blowers as seen in Fig. 10-68 are usually used in large-diameter or high-temperature apphcations. The blades being radial in direction have veiy low stresses as compared to the backward or forward cui ve blades. The rotors have anywhere between 4 to 12 blades and usually operate at low speeds. These fans are used in exhaust work especially for gases at high temperature and with suspensions in the flow stream. 

Impellers with a large blade area, whieh rotate at low speeds. 

These fens usually operate with no piping or duct work on either side and move air or gas from one large open area to another. Pressures are usually very low, and volumes depend on size, blade pitch, the number of blades, and speed. 

The horizontal mixers normally used consist of a w shaped mixing bowl with two Z blades (Figure 2). These mixers do not have a removable bowl so the dough has to be tipped out. The mixers can deliver large amounts of energy at low speeds. 

Four blades are ordinarily considered a desirable minimum for mediumsized fans, with eight blades a minimum for large diameters. Twelve blades are a practical maximum. Fan diameters are basically determined by the volume of air to be handled. Air velocities through the fan range from 1200 to 2500 fpm. An economic design value is about 1800 fpm. Acceptable fan blade tip speeds, with low noise as the criterion, vary with the type tower, the type of fan cylinder and the blade tip clearance. 

The mixer as a solid liquid reactor is generally most elTicient, i.e. Rq/cj is a maximum) at the just fully suspended condition for all impellers and for any particular solid-liquid pair, constant. Thus the overall optimum is obtained when (eT)ts lowest and an impeller-tank configuration that is eflicient for suspension is also efficient for reaction. In many cases, this corresponds to a large low-speed hydrofoil impeller, or pitched blade turbines of D/T 0.4. In this respect, the recent development of cheap and reliable gearboxes capable of handling the larger torques that low speeds imply is noteworthy. ... 

Precoat slurries are prepared batchwise in a dedicated tank. This must have a working volume greater than that of the filter(s) to be treated along with the associated piping. An agitator is essential to keep the filter aid in suspension. Large-bladed turbines or propellers rotating at low speed are best. 

Ap = pd —Pc. The magnitude of the constants C20 and as, given in the Table 12-5, are a measure of the ease of suspension formation. Low C20 values indicate that dispersions are formed at low speeds. Large C20 values (single impellers) suggest that higher speeds are required for minimum suspension. Turbines at the o/w interface require lower speed than in other locations. Radial flat-blade turbines placed in the light phase appear to be inefficient. 

In 1933, the first roU coater was installed as an integral part of a paper machine. These on-machine coaters produce a low cost coated paper used largely for magazines. Coating of paper off-machine at speeds greater than paper machine manufacturing speeds is possible with the use of the various available blade coaters (17). 

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