The different methods of pressurizing powdered solid fuels which have been developed are reviewed briefly, including proposals as well as those in operation. The work has been devoted particularly to a study of the characteristics of these pressurizers, in relation to their common problems of: (a) Power loss due to venting the pressurizing air, (b) wear of the surfaces in contact with the solids, and (c) effective sealing. The most practical methods have been compared on power loss, rather than power required in their cycles. It has been found that power loss due to air venting cannot be totally eliminated and that the simple lock hopper is, so far, the best available device which has the lower capital cost, maintenance requirements, power loss, and better reliability. Where space is of major importance, however, the lock hopper with a moving wall can be considered as an alternative, though it may suffer from an increased wear rate and a reduced volumetric efficiency.

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