Fusible Plug Function The main function of the fusible plug is to extinguish fire when water level in the boiler falls below an unsafe level. Construction & Working It consists of three plugs. The hollow plug A having hexagonal flanges is screwed to the fire box crown plate. The plug B gunmetal plug is screwed to the body A. The third plug C is made up of copper is locked with metal like tin or lead which has a low melting point. In normal working condition, water covers the fusible plug remains cool. In case the water level falls below the danger levels, the fusible plug gets exposed to ste am, this overheats the plug and fusible metal having low melting point melts quickly. Due to this plug S falls, the opening so made allows the steam to rush on to the furnaces and extinguishes the fire or it gives warning to the boiler attendant that the crown of furnace is in danger of being overheated. Air Preheater Function It increases the temperature of air before it supply to the furnace using heat from flue gases passing through the chimney. Construction and Working It is installed between economizer and the chimney. It consists of large numbers of tubes which arranged in path of flue gases. A hot flue gas enters into tubes from top of shell and leave from the bottom to the chimney. The inlet air at room temperature is admitted into shell at lower end with the help of the fan. The air passes upward around the tubes in the opposite direction to the flow of hot flue gases. The soot hopper provided at the bottom is used to collect soot during cleaning operation of the tubes.
