pressurized water and air mixture supply source 57 such that they are operative to inject a fine spray of a mixture of water and air under pressure in a horizontal direction below the first collection tray towards the side wall at which the gap 38 is located a plurality of horizontally directing jetting nozzles 58 are mounted on a jetting pipe 59 located in a horizontal manner just below the second collection tray the jetting pipe 59 may be connected to a second pressurized water and air mixture supply source 60 or alternatively to the same pressurized water and air mixture supply source 57 the jetting nozzles 58 are operative to inject a fine spray of the mixture of water and air under pressure in the horizontal direction below the second collection tray towards the side wall at which the gap 39 is located similarly a plurality of horizontally directing jetting nozzles 61 are mounted on a jetting pipe 62 located just below the third collection tray in a horizontal manner and are operative to inject a fine spray of the mixture of water and air toward the side wall at which the gap 40 is located finally a plurality of upwardly directing steam nozzles 63 are mounted on a steam pipe 64 located just above the upper third collection tray the second scrubber unit 65 has the same construction as the first scrubber unit 36 except it is oriented such that the lower collection tray therein is located in the opposite position as the upper collection tray of the first scrubber unit 35 so that the combination forms a continuous tortuous path for the rising exhaust gas 13 in operation when the polluted exhaust gas 13.