AN INFORMATIVE BULLETIN PROVIDED BY
Warren Environment, Inc.
FLOW DESIGN
Steel stack heights are set during the process of obtaining stationary source permits by dispersion modeling in accordance with EPA regulations found in Part 40 of the Code of federal regulations. Other parameters are usually left with discretion of the stack designer such as diameter, inlet arrangement and thermal hold capabilities. Good engineering practice and experience are necessary to provide appropriate draft characteristics and corrosion resistance.
Any column of hot air or gas such as that conveyed by a steel stack, provides a natural draft proportional to the height of the column and the temperature difference above ambient. When the natural draft of a stack is large enough to overcome the inherent losses in the stack, the stack is said to be self-drafting. Since the stack height and stack inlet temperature are normally predetermined, the stack designer has control only over the stack losses to design a self-drafting stack. These losses in decreasing order are the stack exit loss, the stack inlet loss and friction losses. The stack diameter is a major variable in controlling exit loss and friction loss. The inlet arrangement and size primarily control the inlet loss. Normal flue velocities are in the range of 50 ft. - 60 ft. per second for low and medium temperature conditions and approximately 100 ft per second for high temperature applications.
