Việc thiết kế các mũ trùm và đường ống thường là một phần rất quan trọng của kiểm soát ô nhiễm không khí. Mũ trùm và không khí hết phải đầy đủ để ngăn chặn thoát của chất gây ô nhiễm bầu khí quyển. Mặt khác, không khí hết thông qua mui xe một thường phải được xử lý để loại bỏ các chất gây ô nhiễm. Vì vậy, để giữ cho chi phí vốn và hoạt động của thiết bị điều khiển ở mức tối thiểu, không có không khí nhiều hơn nên bị cạn kiệt hơn mức cần. | 20 Hood and Ductwork Design INTRODUCTION The design of hoods and ductwork is often a very important part of air-pollution control. Hoods and the air exhausted must be adequate to prevent escape of contaminants to the atmosphere. On the other hand the air exhausted through a hood must usually be treated to remove the contaminant. Therefore to keep capital and operating costs of control equipment to a minimum no more air should be exhausted than necessary for complete capture. By designing the hood with the minimum openings necessary the air quantity to be exhausted can often be decreased. When an exhaust system must exhaust gases from a number of points it is important that the ductwork sizing and layout be carefully engineered for the proper flow and velocity in all branches both for conveying velocity and pressure drop. It is axiomatic that the pressure drop through each branch at its design flow must be the same from its intake to the point of common junction. If it is not the flow will redistribute itself in operation to create the same pressure drop in each branch. When handling abrasive dusts duct wear can be quite severe. Situations are known to have occurred where elbows have been worn through in 2 weeks time in poorly designed exhaust systems. Therefore when handling abrasive dusts the duct routing should be as simple and direct as possible. Bends where necessary should be gradual with long radii. Velocity should be kept as low as possible consistent with keeping the dust particles in suspension. Where bends in the ductwork are necessary and rapid wear is encountered designs can be developed which employ easily replaceable wear pads on the bends. These pads can be thick metal made of wear-resistant steel castings hard-surfaced with special abrasionresistant alloys such as stellite or have thick rubber-coated linings. A number of good reference sources are available for the design of hoods and ductwork and should be consulted before design of
