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Show ) INCREASED TERTIARY AIR VELOCITY, DOUBLE PORTS: In this run, the single port (per burner) design was replaced by twin ports located along the quarl periphery. This concept was run in an attempt to have the tertiary air bypass the area of highest throat gas momentum and more evenly distribute the air into the combustion stream. The resultant flowfield is shown in Figures 7a and 7b. This solution shows a significant improvement over the previous two cases, with further penetration of the tertiary air stream into the combusting throat gases. By observation of Figure 7a, we see the tertiary air enveloping the flame body, resulting in improved tertiary air mixing. As shown in Figure 7b, the upper portion of the high fuel residual recirculation zone has narrowed, and the fuel residual level at the furnace exit is significantly reduced. INCREASED VELOCITY, DOUBLE PORTS, BIASED Oil MASS FLOW: In addition to floor port design optimization, a further improvement in system performance was simulated by substituting the symmetrical oil drilling with an oriented drilling. The oriented drilling biased an additional 20 percent of the total fuel towards the bottom of the furnace, in the direction of the floor ports. The resultant flowfield for this case is shown in Figures 8a through 8c. Although hardly noticable from observation of Figure 8a fuel residual contours, a summation of individual nodes at the fur- TABLE 3 CASE ALTERNATIVE SUMMARY NO. DESCRIPTION CHANGED BOUNDARY CONDITIONS 1 BASE N/A 2 HIGH VELOCITY PORT VEL = 150 fps 3 TWIN PORTS, PORT VEL = 100 fps, 2 HIGH VELOCITY PORTS/BURNER 4 ORIENTED CAP, BIASED OIL FLOW, TWIN PORTS, HI PORT VEL = 100 VEL fps, 2 PORTS/BURNER nace exit revealed a further reduction in fuel residual from the previous case. In addition to the further depleted high fuel residual zone at the top front end of the furnace, it is interesting to note the increased size of the high fuel residual zone at the bottom of the furnace, due to the 20 percent increase in fuel mass flow entering this region. SYSTEM DESIGN The design of the staged air system was based on the above described modelling results. As shown in Figure 9, four (one per burner) 12" diameter duct headers were routed from the common wind box to the individual floor-mounted port assemblies. Located upstream of each port assembly is a manual damper for individual control of tertiary air to each of the four bu~ners. Twin tertiary air sleeves are routed from each port assembly to the port nozzles in the furnace interior. The exterior dimensions of the sleeves and nozzles are 5" X 12.4", designed to minimize relocation of existing floor water-tubes, and to maximize available windbox velocity head. The fuel atomizer nozzles were oriented to bias the flow towards the tertiary air mixing region, and away from the previously modelled reducing region above the quart. In addition, the nozzles were oriented to be compatible with the shaped throats. CONCLUSIONS Quantity, velocity, and location of tertiary air ports can have a Significant effect on system performance; Flame length, unburned hydrocarbons, and NOx reduction (as a function of staging level) will all be effected by the overall retrofit design. External air staging of an existing furnace installation requires careful consideration of the existing system layout: The number of burners, the burner arrangement, and the burner spacing will all have an impact on fuel/air mixing and residual |