OCR Text |
Show 6 A drawback to the use of preheated combustion air is the increase in N0X emissions typically observed with conventional ribbon and multiport burners. This concern, coupled with a desire to minimize the overall dimensions of the burner system, led to the selection of a radiant burner. Such a burner cools the combustion products quickly by (largely convective) heat transfer to the radiating surface, which, in turn, radiates heat directly to the primary heat exchanger. The hot radiant surface promotes rapid ignition and burning of the incoming mixture, resulting in a short flame. The primary heat exchanger can therefore be positioned close to the burner without creating unacceptably high carbon monoxide emissions. Other components. Significant components in addition to the burner, boiler and recuperator include the igniter, gas control valve and an induced-draft fan. Two commercially available igniter types-direct spark and hot surface--are currently being tested. The gas control valve will also be of conventional design. A fan is required because of the horizontal gas flow path and the pressure drop created by ducting and heat exchangers. Induced- rather than forced-draft operation is preferred largely due to safety considerations. APPARATUS AND EXPERIMENTAL PROCEDURE The burner evaluations to date have been aimed at measurement of steady-state efficiency at the design firing rate. Thus, the primary instrumentation criteria have been repeatability, low cost, durability and ease of interpretation. Figure 2 is a s:hematic illustration of the burner test stand. Combustion air, currently supplied by compressor, is regulated and then metered with a Dwyer Rate-Master rotameter calibrated against National Bureau of Standards-traceable instruments. The air pressure at the meter (typically 10 psig) is measured with a gage and the temperature with a type-K thermocouple connected to a Doric 205 data logger. The |