Abstract :
In selecting a controller for application to various types of mill machinery all the conditions must be considered, such as the class of operator, conditions of location, operation and maintenance, relative importance of reliability, simplicity, cost and repairs, etc., and initial cost. These points are illustrated by three different types of control as applied to a blast furnace skip hoist, to a high-speed coal hoist, and to a slow-speed ash hoist. In the first case reliability and continuity of operation are imperative, and other requirements are of secondary importance; in the second case, where an ordinary delay does not affect plant operation, questions of simplicity, repairs, maintenance and inital cost can be given more consideration, as continuity of operation is not absolutely essential; in the third case where the operations are performed by unskilled labor the most important requirements are that the controller perform its functions positively, accurately and automatically, and it should be provided with all the necessary protective features to avoid damage to the apparatus controlled.
