Boilers Part (Economizer)

Economizer:

The purpose of economizer is to preheat the boiler feed water before it is introduced into the steam drum by recovering heat from the flue gases leaving the boiler. The economizer is located in the boiler rear gas pass (second pass) below the Low Temperature Superheater. The economizer is a continuous loop of tubes (coil) and water flows in upward direction and gas in the downward direction.[1] The function of an economizer in a steam generating unit is to absorb heat from the flue gases and add this as sensible heat to the feed water before the water enters the evaporative circuit of the boiler. Earlier the economizers were introduced mainly to recover the heat available in flue gas that leaves the boiler and provision of the additional heating surface increased the efficiency of steam generation, saving the fuel consumption, thus the name is ‘Economizer’. In modern boilers power generation feed water heaters are used to increase the efficiency of the turbine unit and feed water temperature and hence relative size of economizer units are less than earlier units.[2] This is a good proposition as the heat available in the boiler exit flue gas can be economically recovered using Airheater which is essential for pulverized fuel fired boilers.

Economizers are located ahead of Airheaters and following the LTSH in the flue gas stream. Hence it is contained in the same casing as the LTSH. Counter flow arrangement is normally selected so that heating surface requirement is kept low for the same temperature drop in the flue gas. Economizer coils are designed for horizontal placements which facilitate draining of the coil and favor the arrangement in the second pass of the boiler. Water flow is from bottom to top so that steam if any formed during the heat transfer can move along with water and prevent the lock up steam which will cause overheating and failure of economizer tube

Economizer tubes are supported in such a manner that sagging, undue deflection and expansion prevention will not occur at any condition of operation. A circulation line with a stop valve may be incorporated to keep the circulation in economizer into steam-drum when there is fire in furnace but no feed flow. (e.g. During start up). Tube elements composing the unit are built into banks and these are connected to inlet and outlet headers. Manholes and adequate access and spacing between banks of tubes are provided for inspection and maintenance works. Normally the tube bank arrangement and steam soot blowers provision at appropriate locations will facilitate efficient on load cleaning. An ash hopper below the economizer is provided if the flue gas duct is taking a turn from vertical.


Ref. :

[1] www.seipub.org/ijace/Download.aspx?ID=14797

[2] International Journal of Scientific Research Engineering & Technology (IJSRET), ISSN 2278 – 0882 Volume 4, Issue 6, June 2015

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