| OCR Text |
Show The heat flux calculation indicates higher heat loads in the lower wall sections of the boiler when oil firing compared to the gas firing. This is caused by the increased emissivity of the oil flames. The maximum predicted tube fluxes when oil firing are in the region of 110,000-115,000 Btu/hr ft 2 (350-360 kW/m2) of projected surface area, whilst for gas firing these are reduced to 89,000-92,000 Btu/hr ft 2 (280-290 kW/m2). The general 'smoothing-out' of the flux distribution when gas firing should improve the water-side conditions. The calculations for horizontal gas firing have been included to give a direct comparison of the effect of a fuel change alone. As can be seen, this change reduces the heat fluxes in the bottom of the boiler by approximately 30%. The effect of angling the gas burners downwards is to recover this loss of flux and increase the heat transfer relative to oil by approximately 9%. The variations in heat flux between surface zones in the same horizontal plane are caused by local variations in the convective heat transfer component. These variations have been determined by the mass flow mapping variations as defined by the physical modelling studies, which in turn defines the amount of local convective heat transfer. It is important to observe that under gas firing conditions the upper region of the boiler carries a higher hea t loading, although the predicted fluxes are not excessive. Some experimental measurements have been reported by Bagley (16), investigating the relationship between tube wall temperatures, heat flux and water mass flux for both vertical and horizontal tubes. These indicate dramatically the importance of waterside mass flow on the potential for tube failure due to excessive temperature, and when considered in combination with the heat flux data predicted, can be used to indicate areas of concern in the radiant section of the boiler. |
