Study of Improvement in Boiler Efficiency through Incorporation of Additional Bank of Tubes in the Economiser

Abstract

The major efficiency loss of a boiler is caused by the hot stack gases discharging to the atmosphere which is polluting the atmosphere and on other side Pollution Control Board is forcing the norms of Pollution levels in atmosphere. One of the most cost-effective ways of improving the efficiency of a high pressure steam boiler is to install an economizer on the boiler. An economizer is a heat exchanger, which transfers heat from the stack gases to the incoming feedwater. Typically, on a high pressure water tube boiler, the efficiency improvement with an economizer is 2 to 4%, depending on firing rate. On a high pressure fire tube boiler, the improvement is 2 to 3.5%, depending on boiler size and firing rate. The economizers are located in the boiler stack close to the stack gas outlet of the boiler. They may be supported from overhead or from the floor. A feedwater line, which serves the boiler, is piped to the unit. No additional feedwater control valves or stack gas dampers are required. Presently in NTPC stage II units there are banks of tubes in economizer. There was a proposal from management to add another bank of tubes in the economizer so that there will be control of pollutants coming out from boiler. We took that proposal as a task and made a detailed investigation of it. An investigation is conducted on the effect of performance of the boiler by incorporating the additional bank of tubes in the space below the lower bank of tubes. The main idea is to extract maximum amount of heat from the flue gases and increase the heat pick up rate of the feed water outlet into the boiler. By assuming the additional bank of tubes, various heat transfer calculations are done and the reduction in the flue gas out let temperature and increase in the feedwater outlet temperature are found out. Finally a comparison of efficiencies of the boiler is made between the existing unit and the unit when additional bank of tubes are assumed. At last, various pros and cons have been found out by conducting above investigation i.e. by incorporating additional bank of tubes in the economizer.