Effect of scale deposits on the internal surfaces of the tubes on the superheater operation

Abstract

A mathematical model of the steam superheater exchanger with distributed parameters has been developed. Scale deposits were assumed to be present on the internal tube surfaces. It was assumed that the inner tube surfaces are covered by a thin layer of scale deposits. The finite volume method was used to solve partial differential equations describing flue gas, tube wall and steam temperature. The developed modeling technique can especially be used for modeling tube heat exchangers when detail information on the tube wall temperature distribution is needed. The numerical model of the superheater developed in the paper can be used for modeling of the superheaters with complex flow arrangement accounting scales on the internal tube surfaces. Using the model proposed the detailed steam, wall and flue gas temperature distribution over the entire superheater can be determined. The steam pressure distribution along its path flow and the total heat transfer rate can also be obtained. The calculations showed that the presence of scale on the internal surfaces of the tubes cause the steam temperature decrease and the heat flow rate transferred from the flue gas to the steam. Scale deposits on the inner surfaces of the tubes cause the tube wall temperature growth and can lead to premature wear of tubes due to overheating. Nomenclature d h -hydraulic diameter of the superheater tube (for a circular tube, this equals the inner diameter of the tube), m he -equivalent heat transfer coefficient, W/m 2 K hg -heat transfer coefficient on the flue gas side, W/m 2 K hs -heat transfer coefficient at the inner tube surface, W/m 2 K ka -ash deposit thermal conductivity, W/mK ks -iron oxides or scales thermal conductivity, W/mK kw -tube material thermal conductivity, W/mK Lr -tube length, ṁ ms -steam mass flow rate, kg/s N -number of finite volumes on the tube length p -static pressure, Pa r -radius, m ra -outer radius of deposit layer, m rin -inner radius, m ro -outer radius, m s -coordinate along the flow path in direction of flow, m s1 -longitudinal pitch perpendicular to the flue gas flow direction,m s2 -longitudinal pitch parallel to the flu gas flow direction, m Ta -deposit layer temperature o C Tg -flue gas temperature, o C Tg -mean gas temperature over the row thickness, o C Ts -steam temperature, o C Tw -tube wall temperature, o C w -steam velocity, m/s x + -dimensionless coordinate in the steam flow direction y + -dimensionless coordinate in the flue gas flow directionGreek symbols δa -thickness of ash deposits, m δs -thickness of iron oxide or scale deposits, m φ -angle between tube axis and horizontal plane, m ρ -steam density, kg/m 3 ξ -friction factor