Bridge walls

The approaeh deseribed above introduees the eoneept of a giant partiele (wall)-Q partiele direet eorrelation funetion. As far as the relation between hga and cga is not speeified, Eq. (32) is exaet. However, in eontrast to Eq. (14), (32) eontains the direet eorrelation funetion for a uniform fluid. The general relation between hg and Cg involves the bridge wall-a particle function, Bg r)... 

For steam boilers, fuels are natural gas and blast furnace gas. A large number of small burners (about 30 hp. each) is used for the former. Burners are thoroughly distributed. As with fuel oil, they may be directed either slightly downward toward the rear or may face from the bridge wall toward the front. The combustion space should be large. The air supply is drawn in around the gas jet. High eflSciency is easily possible. For blast furnace gas, use a furnace space of 2 cu. ft. per rated horsepower, 0.8 sq. in. of gas passage area per horsepower and 2 in. blast (based on 6 to 8 in. pressure in main). Provision must be made for removal of dust. With all gas burners, the gas and air supply should be separately controllable. 

S.2.3.5 Process Related Parameters Ultralow NO and the latest generation burners have longer flames that change the heat flux profile. This is especially important on thermal cracking heaters such as cokers and visbreakers in oil refineries. The longer flames may increase the bridge wall temperature (BWT) and change the duty split between the radiant section and convection section. 

BWT bridge wall temperature TMT tube metal temperature TWT tube wall temperature... 

Should be 0.1 to 0.2 in. water, and according to Reed the draft at the bridge wall or highest point at which gases are turned down in the still should not be less than 0.02 in. The draft at various elevations in the still can be computed from the following tabulation ... 

Flame Emissivity (Pp). Ibis is related to PL, the atmosphere-feet of radiating gas cloud, in Fig. 18-9 by means of assumed values of the gas temperature at the bridge wall (O and the tube skin temperature (f,). 

Reading across from 3.84 on Fig. 18-9 to a tube skin temperature of lOOO F, then upward to an assumed flue gas temperature at the bridge wall (tg) of 1800, and then to the right, Pf, the flame emissivity, is found to be 0.495. 

The flue gas that enters the convection section still contains 100 — 44 — 3, or 53 per cent of the net heating value of the fuel. Figure 14-2 shows that the temperature of the gas at the bridge wall will be approximately 1800°F. The furnace temperature, that is recorded in plant operation is lower than this, but it is usually measured at another point. 

Bridge walls of brick running down the center of the furnace may be used to achieve better control of the heat distribution inside the unit as seen in Figure 4. Care must be taken during shutdowns to clean out and repack the expansion joints to accommodate the thermal expansion that will occur when the furnace is brought on-line. Figure 5 shows the effect of inadequate expansion allowance in a division wall. It is imperative that all the expansion joints be fully functional to avoid buckling of the wall. 

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