TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Device: The two-stage spraying tower is composed of a one-stage spraying tower and a two-stage filtering spraying tower; the one-stage spraying tower is a double-cylinder series-wound structure; the upper end of the number one cylinder of the double-cylinder series-wound structure and the lower end of the number two cylinder are communicating by pipelines, and the lower parts of them are placed inside the water tank together; the upper end of the number two cylinder is communicating with the lower end of the two-stage filtering spraying tower; both the upper parts of the one-stage spraying tower and the two-stage filtering spraying tower are mounted with spray headers, each of which is connected with the submersible pump in the circulating water pond through pipeline. The upper part of the two-stage filtering spraying tower is connected with the upper end of the three-stage cyclone dust collector along tangent direction of the side; the upper end of the cyclone dust collector is columnar and the lower end of it is coniform. Moreover, air outlet of the vacuum pump is communicating with the lower part o the four-stage filter through connection pipelines; middle part of the connection pipeline is connected with a vertical air discharging pipe; a horizontally arranged reflux pipe is set between the air discharging pipe and the air inlet end of vacuum pump; and the reflux pipe is set with a vertical fire-displaying pipe. The upper part of the four-stage filter is connected to the tubular five-stage steam-water separating flame arrester; pipeline of accessed to the five-stage steam-water separating flame arrester bends downwards to the lower part and its outlet is a horn mouth; edge of the horn mouth is zigzag; the five-stage steam-water separating flame arrester is filled with water and the water level is higher than the horn mouth. The air storage tank is composed of an outer cylinder and an inner bag placed inside the outer cylinder; and there is water filling between the inner bag and the outer cylinder. The air outlet pipe of the burner is communicating with the main air inlet pipe of the heat exchanger through flanges; the heat exchanger comprises a main air inlet pipe, a main air discharging pipe and three main intermediate pipes arranged between them; the main air inlet pipe is communicating with six branched pipes through the first main intermediate pipe, and the first main intermediate pipe is connected with the next main intermediate pipe through six branched pipes; the second main intermediate pipe is also communicating with the third main intermediate pipe through six branched pipes; and the third main intermediate pipe is communicating with the main air discharging pipe through six branched pipes so as to connect orderly to form an M structure; the main air discharging pipe locating at the rear end is soldered with a wind discharging pipe connected out of the roasting rooms.