Complex sentence: "Flash boiler" can mean a type of water-tube boiler, whose tubes are strong and close together with water pumped through the tubes, the tubes are kept very hot so the water feed is quickly flashed into steam and superheated.
Split into simpler sentences: Flash boiler can mean a type of water-tube boiler, whose tubes are strong and close together with water pumped through the tubes. The tubes are kept very hot so the water feed is quickly flashed into steam and superheated.