Economiser

Economiser is a type of heat exchanger which exchanges some parts of the waste heat of flue gas to the feed water. It is installed between exit of the furnace and entry into the chimney. Generally, economiser is placed after feed pump to avoid the problem of priming in feed pump. If economiser is installed before feed pump, some amount of water may be transformed into vapour which can create a priming problem in feed pump. In this case, limit of temperature rise of water is fixed so that water cannot be transformed into steam. The constructional details of economiser are shown in Figure 4.24. It consists of vertical cast iron tubes attached with scrapers. The function of scraper is to remove the soot deposited on the tube. Water flows through the tube to the boiler drum. These tubes are arranged in the path of waste flue gas entering into the chimney. The flow of water is controlled by two valves attached with down header and up header of the tubes. Waste heat of flue gas is transferred to the tube material and then tube material to water.

Figure 4.24 Economiser

Advantages

• It increases the power output of the plant. For 6°C increase in temperature of water efficiency of boiler increases by 1%.
• It increases the evaporation capacity.
• It increases the life of boilers due to less thermal stress.