OCR Text |
Show indirect heating furnace, however, the heat energy generated by combustion of fuel gas is used once for heating the radiant tube, and the heat energy is then transferred to other elements in the furnace by radiant heat transfer. A heating time lag thus can not be avoided early in the heating process. Figure 14(a) (b) shows the heat loss rate due to the exhaust gas. For both types of furnaces, the heat loss rate is increased with atmospheric temperature. In addition, heat loss rate in the directly heating furnace is very weakly affected by the circulating rate. Figure 15(a)(b) shows thermal storage rates into the thermal insulating wall for both types of furnaces. The thermal storage rates were estimated subtracting the sum of the work input, the heat loss by exhaust and the heat input into the atmosphere from total heat input generated by combustion. For both types of furnaces, thermal storage rates into the thermal insulating wall decreased monotonically. However, for the directly heating furnace, as circulating rate increased, the thermal storage rate increased as well. 3.3 Thermal energy budget From the fuel gas supply rate and heat flow rates into the furnace elements, the heat energy budget can be estimated in order to explain transient furnace performance. Figure 16(a) shows the energy budget of the directly heating furnace at a fun rotating speed of 1060r.p.m.. The horizontal axis indicates the atmospheric temperature in the furnace and vertical axis indicates heat inputs into furnace elements, and is shown using logarithmic scale for convenience. It is clear that the rate of heat storage into the insulating wall plays a dominant role in the thermal energy budget. However, as the atmospheric temperature increases, the heat loss by exhaust comes to be of the same order as the heat storage in the insulating wall. This means that the heat storage in the thermal insulating wall cannot be ignore when the furnace is in transient state due to an expected or an unexpected interruption such as batch operation or an intermission. It is also evident from the experimental result discussed in the previous section that heat storage into the thermal insulating wall causes an unexpected increase in fuel gas consumption which depends on the rare of circulation of the atmosphere. Figure 16(b) shows the energy budget for the indirectly heating furnace. The behavior of the energy budget is almost the same as for the directly heating furnace. 3.4 Convection and Radiation contribution to a work input Figure 17 shows the contribution of the radiant heat transfer rate to total heat input into the work for each of the four rates of circulation of the atmosphere. Filled dots corresponding to the radiant heat transfer rate and white dots shows the total heat input into the work. Within the temperature range up to 500/C, heat transfer to the work is due mainly to convective heat transfer. For temperatures higher than bOOK, the radiative |