OCR Text |
Show Only atomic species (H, 0, N) and homonuclear diatomic molecules (H2, N2, 02) are unable to emit and absorb radiant heat. Thus, most of the products of combustion of hydrocarbon fuels (C02, H20, CO, NO, 502, S03etc.) contribute to the radiation process. Owing to the low partial pressure of these components the emissivity of combustion products is typically 0.1 to 0.15 depending on the "beam length", which is directly related to the thickness of gas between the banks of tubes, and the gas temperature. This emissive potential is greatly enhanced by the presence of soot and particulates in the flame, as is the case with pulverized fuel and oil fired boilers. Both soot and particulates are almost black body emitters and increase the emissivity to 0.6 - 0.9 for oil and p.f. flames. Thus, a design philosophy which relies on radia tive transfer alone leads to large furnace spaces in which convective transfer is minimal. Because radiation travels in straight lines it is difficult to heat tubes evenly around their circumference. The average heat flux to boiler tubes in the radiant section is generally limited to approximately 70,000 Btu/hr ft 2 (220 kWm- J, which is about 38% of the flux between black bodies at 3250 0 R (lS00K) and l250 0 R (700K). It should be noted that these fluxes can equally well be achieved in compact heaters designed for convective heat transfer and using a separate high intensity combustor. Heat transfer rates to the wall tubes where steam is generated are extremely high, and if the heating rate is excessive the tube internal surfaces will become steam blanketed. This nonwetting of tube surfaces causes a reduction in heat transfer coefficient and an elevation in temperature. Tube integrity is then threatened, and failure of the tube can result either by creep under the influence of the high internal pressure and wall temperature or by corrosion caused by the high local concentration of dissolved salts at the position of dry out. While dry out is not unknown in coal fired plant it is a significantly more serious issue in more highly rated oil and gas fired furnaces where designers have taken advantage of the rapid combustion and clean wall conditions to achieve a reduction in furnace size relative to coal fired plant of equivalent rating. The financial penalty incurred by plant outages due to tube failures is considerable, and has prompted greater attention being given both to experimental studies and to the development of computer based models for determining the limiting thermal and hydrodynamic conditions governing the integrity of furnace tubes. |