STEAM PLANT GROUP HEAT TRANSFER IN THE FURNACES OF SHELL TYPE BOILERS

Peak heat fluxes which occur near the oil burner can be predicted by a simple mathematical model to within 12 per cent and thus help to estimate peak metal temperatures. Although peaks of 150 000 Btu/ft2 h can occur in current practice, the metal of the furnace is not at risk provided the waterside is clean. Conversely, the risk is great when even a small amount of scale has built up. 1 MODERN SHELL BOILERS WITHIN the last 20 years shell boilers have changed from large Lancashire and Economic type plant, mostly coal fired and manually controlled, to fully automatic units of much smaller dimensions, fired with oil or gas. Sur-prisingly, this has been achieved with no increase in capital cost per unit of heat produced and the thermal efficiency has been raised from around 70 to over 80 per cent of the gross calorific value. A typical shell boiler of British design is shown in Fig. 1. The reversal chamber at the rear end is entirely surrounded by water in this 'wet-back ' type; in the 'dry-back ' the rear part is lined with refractory. The essential purposes of a boiler furnace are to accom-modate the combustion process and to reduce the temper-ature of the combustion products before they reach the tube plate in the reversal chamber to an acceptable level, say 1750°F. In the larger snell boilers furnaces are commonly about 4 ft in diameter and 12 ft long, the metal being about $ in thick. For up to about 35 000 lb/h evaporation one furnace is used, for larger outputs two may be required. 1.1 Notation A Heat receiving area, ft2. c, D Furnace internal diameter, ft. d Radiation beam length, ft. F Heat flux, Btu/ft2 h.