OCR Text |
Show the regenerator being heated is sufficiently charged, the reversing system operates, cold air flows to the newly heated regenerator and is preheated, the previously cooled regenerator being reheated by the exhaust gas generated by the other burner firing ( Figure 1). --- FIRING COLLECTING ~ W.l ~ MODE 1 \l.!J ~~--~--~ /------~~~ - -- COLLECTING FIRING @ MODE2 @ ~--------~ ~--------~~ F,GURE 1 - Diagram of Firing Modes The direct connection of a burner to each regenerator and the cycling of the burners eliminates hot valves working at full exhaust gas temperature. The reversing valve is a purpose built unit, requiring a supply of compressed air for actuation and an electrical supply for the logic circuit to program reversal based on a combination of time and exhaust temperature. This reversing system is completely independent of other furnace controls with the exception of flame protection (if fitted) to which it requires to be interlocked for burner start sequencing. The fuel supply is switched to the firing burner by an electrical solenoid valve timed from the reversing system. The pilot runs, under flame supervision (if fitted), during the period its burner is in exhaust mode to negate the need for spark re-ignition at reversal to firing. The pilot may be run as interrupt i b Ie by i ncorporating time delay relays into the flame control circuit, such that it is initiated for reignition by the main flame immediately prior to reversal to exhaust mode and extinguished immediatel y after mai n flame establ ishment on reversal to firing. Fuel/air ratio is typically controlled using a differential signal from an orifice plate in the combustion air line and a multiplying air regulator to supply a flow based loading signal to a conventional air/gas ratio regulator. Air and exhaust flow for the unit are con trolled by tandem linked valves driven fro~ the process temperature controller. A fine trim on exhaust flow is provided by a motorized valve, mounted on the outlet of the suction fan, controlled from furnace pressure. The exhaust fan is protected against overheating caused by system malfunction. CONTROL OF REVERSAL Reversal is controlled by a combination of time and temperature of the exhaust gas from the exhausting regenerator. In order to 'maintain balance of the heat content of the two regenerators and to keep operation at the optimum level, the primary. control on reversal is the temperature, reversal being initiated when a predetermined temperature is reached. If the furnace temperature is so low that this predetermined exhaust temperature is not reached within a preset maximum time from the previous reversal, the reversal is initiated at this maximum time. An additional minimum time parameter exists to assist in nudging the heat storage of the regenerators toward equality should imbalance have arisen for any reason. The chart section shown under the Performance heading shows reversal being initiated by temperature of the exhaust gas. PERFORMANCE The chart below (Figure 2) is taken from a record of a laboratory trial of a pair of Regenerative Burners operating on an 980°C (1800°F) test furnace. In this case thermocouples were installed to measure the fluid temperatures at the top and bottom of each regenerator, and it is these traces which the chart shows. TEMPERATURE ·c 300 _ L 400 500 600 700 800 I I I I I .. : Right· Hand .... ::. ........ _ Waste Gas Exit ...... Temperature Left · Hand Waste Gas Exit Temperature . Right · Hand :~ ................. ........... Air Entry '. Temperature t----'7"'-- Left · Hand Air Entry Temperature Right· Hand . Waste Gas Entry -_._.... .-·_ i Temperature .... Right · Hand . Air Preheat ... _.. . ..;.. .. Temperature .... Left · Hand _ Air Preheat Temperature FURNACE TEMPERATURE 980"C (1800"F) AVERAGE AIR PREHEAT TEMP. 92O"C (1690'F) 1600 1800 FIGURE 2 - Chart Record from Furnace Trial 78 |