Economizer Function An economizer is a device in which the waste heat of the flue gases is utilized for heating the feed water. Construction and Working It consists of large number of vertical cast iron or steel water tubes connected with two horizontal header pipes A and B, one at the top and other at the bottom as shown in Figure 1. Header pipe A is the bottom pipe through which the feed water is pumped into the economizer. The water comes into the top header pipe B from the bottom pipe and finally flows into the boiler. The flue gases flows around the pipes in the direction opposite to the flow of water. Consequently, heat transfer to the surface of the pipes takes place and water is thereby heated. A blow off cock is provided at the back end of vertical pipes to remove sediments deposited in the bottom boxes. The safety valve is provided at the end of the pipe A for the safety of pipes. The soot of flue gases deposited on the pipes reduces the efficiency of economizer. To prevent the soot deposit, the scrapers move up and down to keep the external surface of pipe clean. By-pass arrangement of flue gases enables to isolate or include the economizer in the path of flue gases. Super Heater Function Super heater increases the temperature of the steam above its saturation temperature by utilizing exhaust flue gases. Construction and Working Figure 2 shows Sugden’s superheater installed in a Lancashire boiler. It is located in the path of furnace gases so that heat is recovered from hot gases. It consists of two steel headers to which are attached solid drawn ‘U shaped’ tubes of steel. These tubes are arranged in groups of four and one pair of the headers generally carries ten of these groups or total of forty tubes. The U-tubes are placed in the path of flue gases. The amount of hot gases passed over the superheater tubes should be propertion to the steam passing through tubes and degree of superheat required. The degree of superheat is control by damper which is operated by hand wheel. Some amount of hot gases may be dirverted with the help of the damper to avoid overheating. When damper is in vertical position then flue gases not passes over the superheater but passing directly to bottom flue passage, no superheating of steam takes place. When the stop valve A is closed, stop valves B and C are in open position, the wet steam from boiler flows into right hand header via stop valve C. After superheating of steam in the tubes, it flows into the left hand header, from where it is withdrawn through the stop valve B. If the superheated steam is not needed, the stop valves B and C are closed and the wet steam is directly taken out from the boiler through stop valve A.
