OCR Text |
Show stock disdurge Figure 4 : Illustration of a Rapid Heating Furnace Improved Controls Air/Gas Ratio Controls:- For combustion products leaving a furnace or boiler, if either incomplete combustion occurs, or unnecessary additional excess air is present, these losses can be even higher. A striking example of this is illustrated in Figure 5, which shows the thermal input required to maintain a constant furnace output against variations in air-gas ratio. Control systems, such as the one developed by BG Technology, which is based on thermistor ratio controllers and a multiple diaphragm differential pressure controller, involve a mechanism for feedback from the flow meters in the air and gas supply lines. The feedback is analysed in an electronic ratio controller which calculates any changes required to either the air control valve or gas supply valve to ensure correct air to gas ratio is maintained. They are effective in coping with variations in air pressure, particularly for furnaces utilising air preheat systems. A s well as reducing energy consumption (up to 10%), they also result in improved product quality by better control of the furnace atmosphere plus assist in meeting plant compliance and emission regulations. Electronic ratio controllers (ERC's) can be used in conjunction with an oxygen trimming system to provide a closed-loop control system. The measured oxygen concentration in the flue gas can be directly fed back into the adjustment of the air or gas control valve. This again wjll result in improved efficiency and emissions. |