OCR Text |
Show As expected, NO concentrations follow the same trend as in Figure 7, all falling below 35 ppm. CO concentrations also fall as ~ decreases from stochiometry (4) = 1.0). They reach a minimum between 0.75 < ~ < 0.85, and then begins to increase sharply as ~ drops below 0.75. This upturn is due to incomplete combustion and is often considered as a deterrence to developing lean burners. However, our results show that there is an optimal operating range between 0.75 < ~ < 0.85 for 18 kW, where NOx concentrations are below 25 ppm and CO levels are about 50 ppm. At 15 kW, both NOx and CO levels at 0.75 < 4> < 0.85 are higher than at 18 kW. These results, and the earlier results showing a drop-off in efficiency at low power, suggest that the current WSB/Telstar system operates better when powers are above 15 kW. Our results also demonstrate that the use of WSB can overcome the problem associated with reduced thermal efficiencies under lean-burn. Therefore the only design criterion important for its application to commercial product is the equivalence ratio at which both NO and CO are minimized. SUMMARY AND CONCLUSIONS Our investigations have shown that the maximum and mInImum swirl requirements (corresponding to flashback and blowoff) of the WSB are independent of enclosure and power levels. The minimum swirl requirement is found to increase with exit tube length .e. A burner with.e = 7 cm was chosen for the laboratory test station. This test station consisted of a Telstar heat exchanger mounted over a WSB with all the flow rates, temperature and emissions monitored by a PC. It achieved thermal efficiencies of 80%, comparable to a current model water heater under similar power (15 - 18 k W) and water flow rate (16 liter/min.). The thermal efficiency is only weakly dependent on equivalence ratio. Most importantly, "ultra-low" NO concentrations of less than 25 ppm were observed for a broad range of equivalence ratios and power levels. The optimal operating range for 18 kW was between 0.75 < ~ < 0.85 where NO < 25 ppm and CO < 50 ppm (corrected for 3% 02)' These concentrations are well below foreseeable regulations limits. This demonstration project has shown that the WSB is suitable for use in domestic low NOx emitting water heaters. 14 |