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Show ventilation so that the air carries away any heat generated. In the case of a ground stock pile, and also for smalls coal in a hopper, the usual technique is to prevent access of air into the coal, e.g. by compacting the surface of the stockpile or even coating the pile with a layer of bitumen. 3.3 Conveying systems Conventional mechanical coal conveying systems such as belt elevators, bucket elevators and drag link conveyors are fairly expensive to install, require regular maintenance to ensure reliability and are prone to blockage when conveying difficult-to-handle coals such as wet (9) smalls with a high content of fines . Screw conveyors provide an inexpensive means of handling singles coal in small boilerhouses but can cause a certain amount of degradation. All these systems have the disadvantage that they cannot accommodate a horizontal bend, so multiple units must be used if the site layout is difficult. A modern alternative to these mechanical systems is pneumatic conveying, which has competitive capital cost and low maintenance (9) . .. requirements . Two types of pneumatic conveying are available, dense and dilute phase. Dense phase systems operate at solids to air mass ratios in excess of 40:1, require compressed air and are particularly suitable for smalls. By contrast dilute phase systems, which can use positive pressure or suction, operate at ratios below 20:1, require relatively low pressure air as supplied by a fan, and are at present limited to conveying singles. In the UK dense phase conveying equipment suitable for coal transport is supplied by four commercial companies, Macawber, Send-air, Sturtevant and Neu Engineering and a typical example is shown in Fig. 7. Coal is gravity fed from the bunker into a pressure vessel below and a valve shuts off the flow when it is full. Inflatable rubber seals around the valve are activated to make it airtight before compressed air is supplied to the top of the vessel forcing the coal out into the conveying pipe as a plug several metres long. The plug of coal is blown through the pipe to the boiler feed hopper before the cycle is repeated. 20-23 |