OCR Text |
Show disk-shaped flame that delivers even heat to a typical fin-and-tube heat exchanger found in pool heaters, a blockage of 65% seems to be optimum. As stated earlier, commercial interest in the WSB, either air-swirled or vane-swirled, stems from its capability to s~abilize lean ($ < 1.0), premixed flames so that NOx emissions can be lowered. Regulations are already in place within California restricting NOx emissions to 40 nanograms of NO x per Joule (ng/J) of useful heat output for water heaters « 75,000 BtuIhr)8 and self-contained space heaters « 175,000 Btu/hr)9. It is not unlikely that these regulations will be expanded to other firing rates and appliances in the future. As NOx generation is strongly dependent on temperature, lean flames are desirable as they have lower flame temperatures than the partially-premixed flames generated by conventional rack burners. To evaluate the performance of the vane-swirler in a practical situation, we used the laboratory test station (described in our earlier work2 ) that simulates the operation of a 60,000 BtuIhr Telstar spa heater. Figure 7 compares NO and CO emissions, as well as thermal efficiency of the heat exchanger, for both the air and vane-swirled WSB. Note that the CO readings are in ppm while NO readings are in ng/J. This was done as NOx and CO regulations are written in those different units. NO emissions are essentially the same for the air and the vane-swirled WSBs, ranging from 1 ng/J at $ = 0.70 to about 17.5 ng/J at $ = 0.95. Of particular significance is that these emission levels are well below the 40 ng/J regulation limit. When operated with its conventional partially premixed burner, the Telstar unit produces approximately 70 ng/J of NO, an order of magnitude higher than the WSB emissions. As current regulations for CO are set at 400 ppm, it is seen that both the airswirled and the vane-swirled WSB fall below these limits. A difference is found between the CO emissions for the two WSBs, with the air-swirler consistently producing at least double the amount of CO as the vane-swirler generates. The likely cause is incomplete combustion at the mixing region between the premixture and the swirling air where dilution may reduce the local stoichiometry below the flammability limit. The vane swirler does not encounter such a problem as both the core flow and the swirling annular 10 |