Packed towers allowable velocity

Flood point Packed towers are usually designed to 70 to 80 percent of the Good point velocity (17,55,56,96). This practice provides sufficient margin to allow for uncertainties associated with the flood-point concept (Sec. 8.2.3) and prediction (Sec. 8.2.6) and to keep the design point away from the region at which efficiency rapidly diminishes (just below the flood point). 

It will be noticed that the two-stage spray tower (installation 4) gives slighdy betto performance than the six-stage spray tower. The two major reasons for this appear to be (a) the lower gas velocity which allows the mist formed to setde out and (b) the preciably higher water-circulation rate. Silica-dqiosition problems generally favor the use of a spray tower for this service over the more compact packed tower. 

In a horizontal flow concurrent scrubber, the gas velocity carries scrubbing liquid into the packed bed and the device actually operates as a wetted entrainment separator. Normally, superficial gas velocity is limited to a maximum of 9.6 ft/s due to liquid reentrainment at higher velocities. Packed bed thickness is restricted because the depth of liquid penetration into the tower packing is limited by the allowable gas velocity. 

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