| OCR Text |
Show where T , and T are the adiabatic and load temperatures, respectively. ad s In the furnace configuration used, the effective or transfer emissivity is reasonably approximated by the tube emissivity. This is rolled brass with an emissivity value in the range 0.25 to 0.3. The value required for acceptable agreement between calculation using Eq. 1 and measurement as listed in Table 4 is about 0.2. This agreement is not of a particularly high order, but it is quite adequate to substantiate the essential correctness of the structure of Eq. 3. If, then, there was a significant difference in tube absorbtivity between slurry firing and oil firing, we would expect corresponding differences in the maximum output values (H ) listed in Table 4, with values for the slurries about 2/3 of those for the oil at corresponding excess air levels. The absence of such a difference supports the view that any ash deposition had minor effects. (iii) If the oil and slurry flames had equal and low emissivity/ absorbtivities the higher relative roof temperatures for the slurry as illustrated in Fig. 4 would translate into higher thermal efficiency. The lower measured efficiency implies that the slurry flames are significantly blacker, are able to intercept a substantial fraction of the roof-to- load radiation, and the flames probably provide the major fraction of the thermal flux; conversely, with the oil flames, the flame emissivity is certainly lower than for the slurries, and the flame probably provides the minor fraction of the thermal flux to the load. Whether th. differences can then translate into the observed differences in firing rate for the same output will require detailed computation in the manner carried out previously (14) |
