| OCR Text |
Show the actual installation at C-E's Kreisinger Development Laboratory. A screw pre-feeder is located at the bin outlet to insure uniform removal of material over the entire length of the bin outlet. Figure 7 details the design of the screw used in this pre-feeder. The screw volumetric capacity increases in the direction of the discharge. This is done to prevent complete filling of the screw flights, which could result in stagnation of parts of the mass flow bin. The screw has a sealing section, placed just past the outlet of the bin. This sealing section length must be at least equal to the pitch diameter of the screw at this point. The end of the screw contains a triple flight to improve the consistancy of feed leaving the pre-feeder. The pre-feeder is powered by a D.C. motor, whose speed is controlled by the demand of the gravimetric feeder. The normal maximum speed of this screw at the design rate of 9 tons/hr is 5 rpra. Coal Metering: A gravimetric feeder was chosen to control the feed rate to as many as eight individual burners. This is a belt type feeder with an electronic load cell, which continuously monitors the weight of material on the belt. The belt speed is controlled by a feedback control system, which calculates the actual feed rate based on material weight on the belt and belt speed and compares this rate to the desired feed rate. The belt speed is then increased or decreased as required to minimize the error between the set point and the actual rate of feed. The speed of the screw is also a function of gravimetric feeder set point. The gravimetric feeder set point is controlled by the master burner control and ranges in increments of 12.5% from 50% load to 100% load depending upon the number of burners in service. The system turndown is, therefore, 2 to 1. Coal Transport: When the pulverized coal leaves the gravimetric feeder, it falls down a feed pipe into a C-E pneumatic screw pump. This pump provides the pressure seal required to transport the pulverized coal to the various burners. 11-13 |
