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, since they are unable to differentiate between the weight being gained due to refill versus the weight being lost due to discharge. 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.

Control & Metering, Inc. of Batavia, Illinois (a manufacturer of LIW systems) asked us to design such 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 first step in designing this surge bin, as it should be with any bulk solid storage bin, was to determine the flow characteristics of the material to be handled. Satna Cement sent representative samples of the pulverized lignite and coal to our laboratory for testing. We ran flow tests at the maximum expected moisture content (3% for the lignite and 5% for the coal) and at both 72° and 150° F (the maximum expected temperature of the materials in the surge bin). By simulating the worst case conditions 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.

It is important to have a mass flow pattern when handling materials such as coal, which can spontaneously combust in a storage bin. In mass flow, all of the material in a bin is moving when any is discharged. The first material placed in the bin is the first material out, resulting in a uniform residence time. The alternative is a funnel flow pattern—where an active flow channel develops over the outlet, but material around the periphery of the bin remains stagnant. This flow pattern is undesirable because stagnant material will remain in the bin for an extended period if the bin is not completely emptied. It is this stagnant coal which is most likely to 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. Pulverized lignite and coal are typical of most fine powders in that they will deaerate if given sufficient time in a storage bin. As the powder reaches the outlet of a mass flow bin, the solids pressure decreases, causing the voids to expand and a vacuum to develop just above the outlet. This tends to create a counter-current air flow that holds up the material and severely limits its discharge rate from the bin.

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 tests indicated that both materials to be handled in this system were relatively impermeable; hence the desired rates would be difficult to achieve without special design considerations. 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 shown in the figure. It consists of a 5m diameter cylinder above a carbon steel cone with walls sloped at 15° from vertical. The cone outlet diameter is 305mm. This design provides mass flow and the required capacity.

In order to achieve the required discharge rate with a reasonably sized outlet, we decided that the material must be aerated. We recommended that several air pads be positioned on the lower hopper walls (below the inverted cone shown in the figure) to aerate only the material near the outlet. We calculated that air at 250 cfm, turned on for 5 seconds prior to discharge, would be sufficient to aerate the material below the inverted cone, and that this volume of material would be enough to fill the LIW feed hopper. The inverted cone would aid in aerating the material by relieving the pressure at the bottom of the bin and providing space (under the cone) for the aerated material to expand.

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 [524K QuickTime video] and loss of control of the feed rate. To prevent this, we designed a deflector plate to be placed inside the hopper. While such a plate might create dust, this would be easier to deal with than flooding.

The system was installed and has worked as intended for several years. This design, however, was only half of the project. Control & Metering required another surge bin with two outlets to feed two LIW hoppers. But that is a discussion for another time...