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Show 4 DESIGN PARAMETERS With the assistance of high-speed workstation computers, parametric analyses can be costeffectively performed using the 3-D models on a number of furnace designs equipped with various burners and arrangements of overfire air ports. The results of such a study are summarized in Figure 2. The influence of various design parameters on mixing are shown for single and opposed wall-fired boilers. As expected for both boiler types, the arrangement of overfire air ports has the most significant influence on mixing. The overfire air must be injected at the correct location in the furnace for the mixing to be effective. Additionally, the type of port and design mass velocity ratio (MVR) highly influence the mixing. For most applications, more effective mixing is achieved with ports that have the capability to impart a swirl to the air, like the B&W dual zone NOx port shown in Figure 3. The MVR is the ratio of the mass flux (units of M/L2_t) through the port to the mass flux in the furnace approaching the port. This parameter is used to determine the size of the ports for a particular application. Increasing the design MVR will increase the penetration of the air from the port, but the tradeoff is a higher pressure drop across the port. Pressure drop limits the MVR that can be used for a particular design. For the single wall-fired (SWF) boiler, the modeling results indicate that the burner swirl pattern and flow control settings significantly influence the mixing through their effect on the flow field that develops in the combustion zone and approaches the overfire air ports. Consequently, the burner type and furnace shape are considerations when sizing and locating air staging ports. Failing to consider these parameters could result in considerably less effective mixing than anticipated. The sensitivities and inter-relationships between the design parameters must be considered when applying overfire air systems to SWF boilers. Parameter Single Wall-Al'8d Opposed WaU-Al'8d J3WIlflC Swirf pattern , Vane angle I I ~ Configuration , , Type I~ .. ~';J Vane angle !1i@id W il I ,*»'!.;-l Swlrf pattern 1~,~$@tt'W"M1$@ ~~~~ Mass velocity ratio I I E!.!roaQll gggmglCl Arch Shaft Height I I I I Burner-Port Spacing I N I I I --Very High - High ~ Moderate c=:J Little ~ No Cases Run Figure 2 Influence of Design Parameters |