| CHARGING
A LOSS-IN-WEIGHT SYSTEM AT HIGH REFILL RATES BACKGROUND
A Loss-In-Weight (LIW) system consists of a hopper and feeder mounted on load cells. Such systems are commonly used for precise metering of powders and other bulk solids. When operated in a continuous discharge mode, accurate gravimetric operation is achieved by controlling the speed of the feeder in order to provide a constant decrease in the weight of the feed hopper. When this hopper is refilled with material, most LIW systems switch to a less accurate volumetric mode. In order to minimize the time that the system must operate in a volumetric mode, the upstream surge bin must be designed to quickly refill the LIW feed hopper.
THE PROBLEM
Control & Metering, Inc. of Illinois, a manufacturer of LIW systems, asked us to design a surge bin to reliably handle pulverized lignite and coal. The end user, Satna Cement Company in India, required that the surge bin have a capacity of 60 metric tons and a maximum diameter of 5m. System requirements made it necessary for the LIW feed hoppers to be filled with 88 cubic feet of material in 10 seconds. This corresponds to an instantaneous discharge rate from the surge bin of approximately 600 tph at a bulk density of 38 pcf.
THE SOLUTION
The first step in designing this surge bin was to determine the flow characteristics of the pulverized lignite and coal by testing in our laboratory. By simulating the worst case conditions of moisture and temperature in our laboratory tests, we could provide a design that would reliably handle the materials under those conditions.
We found that both the lignite and coal had little cohesive strength. This meant that both materials would flow through a relatively small outlet in a mass flow bin without forming a stable arch. We also determined the hopper wall angles required for the materials to flow in a mass flow pattern, where all of the material in a bin is moving when any is discharged, which is important for coal that can spontaneously combust.
In addition to calculating the parameters for mass flow, we determined the design requirements to achieve an instantaneous discharge rate of 600 tph.
We calculated limiting flow rates for various outlet dimensions based on the powders' compressibility and permeability (i.e., how readily air passes through the voids). Our computer analysis indicated that both powders would aerate readily and deaerate rapidly.
Based on the test results and computer analysis, we designed the surge bin that provided mass flow and the required capacity.
In order to achieve the required discharge rate with a reasonably sized outlet, the material must be aerated. We recommended that several air pads be positioned on the lower hopper walls to aerate only the material near the outlet.
Another important aspect of the design was to ensure that the heel of material in the LIW hopper did not become fluidized during rapid refill. If this were to occur, it could lead to flooding and loss of control of the feed rate. To prevent this, we designed a deflector plate to be placed inside the hopper.
THE RESULT
The system was installed and has worked as intended for years.
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