Large Boilers Screen Area Partition Model And Application Of Radiation Heat Transfer

The relations between the character of steam temperature and the position that the reheater is disposed were investigated deeply, basing on the operating data and literature in this paper. The phenomena that the water spray quantity is excessive can be found out from the above data, especially for the 1025T/H boiler of 300MW unit. The quantity of reheater spray water influences the boiler efficiency badly. The quantity of reheater spray water is also affected by the operation and the kind of coal. Meanwhile, the difference between the using coal and the designed coal exists. However, if there is a proper proportion of the radiation part and the convection part, the reheater steam temperature is stable comparatively with the load changing (that is the steam temperature stability character). Therefore, the quantity of reheater spray water is decreased fundamentally.Fist of all, in order to discover the proper arrangement and proportion of reheater, the rule and quantitative relationship of radiation or convection of different parts should be found out.Tube bursting of screen superheater is one of the main problems in boiler accident. But as far as the asymmetrical factor of heat absorption on the screen zone height direction along furnace is concerned, the reasonable academic calculation value is also deficient. Hence, the material choice of screen superheater, the bursting analysis and precaution are influenced directly.On the basis of "The Standard Method of Boiler Heating Calculation" developed by original Soviet Union, the problem in "two great regions" calculation was researched in this paper. Aimed to the study emphasis, the mature zero dimension method was adopted for the calculating of lower hearth under the screen type superheater. In order to study the heat transfer of the screen area, the computation model of the sectorization heat transfer by radiation was established.Though Monte Carlo method and discrete coordinate was adopted mainly to study the radiant heat transfer, the influence of flue-gas has not been considered when the method was applied in screen area. The two-dimension sectorization method wascarried out on the account of the engineering actual demand and calculation reduction for the establishment of the computation model. Based on the fundamental principle of' radiation, the model of radiation heat transferring among different faces was established. Moreover, the heat brought into by the flue gas convection was included. It can not only simplify the computation of large size engineering design computation but also satisfy the computational accuracy.Under different operating conditions of load, the computation and analysis was carried out with the computation model. Based on the above computation and analysis, the conclusions are as following.1. That the steam temperature character of wall reheater is radiating type temperature character has been proved by this computation model. Meanwhile, the two-dimension distribution of temperature field in the screen zone and the quantitative relation between the character of stream temperature and the load swing was found out. Moreover, this model can be applied for the computation of the screen reheater.2. Compared with the flue-gas tempe/ature computed by the model took into account of the radiation heat transfer barely and the result computed by the model adding the convection factor of the flue gas, the latter flue-gas temperature is accordant with the measured flue-gas temperature during the practical operation on the whole. However, the former average flue-gas temperature is less about 300°C than the normal flue-gas temperature.3. The heat-absorbing unbalanced coefficient of the screen superheater along the , altitude of boiler was found out. Under the condition of designed load, the coefficient is 1.26 at the lowest part, and it is about 0.7 on the upper part. For the choice of tube material and the accurate analysis and precaution of bursting accidents in the screen zone it has the important significance.Finally, the site proof test was done to validate the computing result. The calculating results are in accordance with running practice very well. Meanwhile, the relation between steam temperature character and arrangement locations was also analyzed.