Heat transfer analysis of the CFFF superheater by the effectiveness-NTU method

The Superheater Test Module (SHTH) at the Coal Fired Flow Facility (CFFF) is a 124 foot long heat exchanger assembled from flanged sections. The purpose of the SHTM is to study fouling and tube metal corrosion in flue gas from a coal fired magnetohydrodynamic (MHD) power cycle, while cooling the flow to temperatures that the exhaust gas cleanup equipment can handle. The SHTM is composed of three sections with steam cooled tubes, six with water cooled tubes, and one section with air cooled tubes. The other sections are geometric transitions without cooling surface. Arrangement of the steam, water and air cooled tube banks to achieve specific temperature regimes along the flue gas flow path is critical to the test program.

The original sizing and arrangement of the SHTM was based on hand calculations checked by the Babcock and Wilcox Company with a proprietary boiler sizing computer code. After three years of testing with the SHTM, much more is known about fouling factors, flow distributions and the accuracy of measured data. Based on this operational history it appears that a major redesign of the SHTM will be necessary. The heat transfer analysis and subsequent development of a computer model were undertaken in order to accurately and conveniently evaluate options for redesign of the SHTM. The computer model is based on the "effectiveness-NTU" method of heat exchanger analysis and allows for a great deal of freedom in modeling different tube bundle arrangements.