More Questions and Answers
Q: What are the benefits of a screw pump feeder over a rotary feeder in a cement plant?
A: Les Bartholomew and Lou DiBuo, FLSmidth, says:
Let's start by discussing the rotary feeder and its uses and then consider the benefits of a screw pump for use in a cement plant.
A pneumatic conveying system fed by a rotary feeder is designed to operate at low line pressures and high velocities and is seldom used to transfer cement products. In a cement plant a rotary feeder is generally restricted to nuisance dust transfer systems. Cement-related materials are abrasive, and the close tolerances between the feeder's rotor and the housing wear extremely fast under pressure-differential conditions where the conveying line pressure is greater than the pressure above the feeder. Additionally, the maximum design pressure differentials for rotary feeders are limited to about 10 psig. Generally, requirements for cement transfer systems are too high for economical rotary feeder designs.
A pneumatic conveying system fed by a screw pump (see Figure 1 for an example configuration) is the most prevalent type of pneumatic conveying in a cement plant because of its reliability, easy maintenance, capacity, flexible equipment layout, simplicity, and ability to adapt to future capacity changes.
The screw pump is actually a line-charging device. A properly designed feed hopper brings the material to the screw's inlet, and the screw advances and compacts the material to provide a material seal. This seal prevents the conveying air from blowing back up into the feed hopper and pushes the material through a mixing chamber where the material and conveying air combine, accelerating into the conveying line.
Additional advantages of a screw pump over a rotary feeder include longer operational life and relatively low conveying volumes because of the denser material stream. The screw pump can also operate at a constant speed while the conveying capacity can vary, operate at much higher pressure than a rotary feeder, and seal against back pressure more effectively than a rotary feeder.
FLSmidth, Bethlehem, Pa., is a global engineering company and supplier of one-source plants, systems, and services to the cement and minerals industries.