Boiler economizer is a heat exchanger located in convection flue pass for flue gas heat recovery. It receives feedwater from deaerator and gives water to steam drum at an elevated temperature. Economizer can absorb heat from high temperature flue gas to reduce exhaust temperature. With it, boiler efficiency is improved to a higher level. That’s why it gets the name.
By feedwater condition: steaming type and non-steaming type. For a steaming type economizer, feedwater entering steam drum reaches its saturation temperature, and part of feedwater (about 10-20%) turns into steam. For a non-steaming type economizer, feedwater entering steam drum should be 30℃ or more than saturation temperature.
By tube material: cast iron tube economizer and steel tube economizer. Cast iron has a good wearing & corrosion resistance ability, but it is brittle and not as good as steel to bear stress. So, cast iron tube economizer is widely used for small boilers without deaerator. For a boiler with working pressure higher than 2.4MPa, steel tube economizer must be used.
By arrangement: vertical or horizontal.
By tube structure: bare tube economizer, finned tube (spiral finned tube, H finned tube or gilled tube) economizer, heat pipe economizer. Finned tube could greatly enlarge heating surface of economizer to make a compact design.
By flue gas temperature: condensing economizer and non-condensing economizer. Non-condensing type is mostly used in coal fired power plant. Due to sulfur content in fuel, flue gas should be kept above dew point temperature to avoid corrosion. Condensing type is mostly used for natural gas fired power plant. High quality natural gas contains barely sulfur content. This make it possible to bring flue gas below condensation temperature.
By flow direction of water/flue gas: co-current flow, countercurrent flow, hybrid flow.
During boiler initial startup or low load, feedwater supplied to economizer is at a low level and intermittent. This heat transfer medium not sufficient to take enough heat away from flue gas. Economizer tubes may get overheated. To avoid this problem, a line is connected between steam and economizer. When necessary, economizer recirculation valve is opened to supply excess water to cool it down.
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