ECONOMIZER

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ECONOMIZER
Economiser is a pressurized back end equipment because of it is installed at the end of boiler. Where after no heat is added to the water or steam.
  1. Heat transfer surface used to increase the temperature of the feed water before it enters to the boiler drum.
  2. When flue gases travel across water-walls then latent heat is absorb & super heaters absorb sensible heat from flue gases.
  3. The economizer is composed of low-temperature convection pass surface.
  4. Economizer is placed in series with the feed water heaters of the regenerative system.
  5. The function of the economizer is, to raise the temperature of feed water downstream of the highest pressure feed water heater to the saturation temperature corresponding to the boiler pressure.
  6. After absorbing the heat from flue gases to the super heater and reheater by steam some heat is present in flue gas this heat is absorbed by sub cooled feed water which is flow in side economizer before entry of boiler drum.
  7. With this type of additional heat absorb by the working fluid(feed water) efficiency of the boiler also increased.
  8. Economizer is a bank of tubes over which flue gases pass.
  9. It is a forced flow convection heat exchange through which feed water is supplied at a pressure higher than the pressure at the evaporating section.
  10. Economizer size is also governed by the feed water temperature at the economizer inlet and the temperature of the exit flue leave from the economizer.
  11. With very low feed water temperature a chance of corrosion of economizer external surface due to flue gas which is passing over it and economizer surface tubes temperature falls bellow dew point temperature of flue gas.
  12. If feed water contains a significant amount of dissolved oxygen (0.7 ppb in high-pressure boilers) pitting and corrosion may take place inside the economizer tubes.
  13. For this deaerating of the feed water is done in deaerator & maintaining a pH value near around 9.
  14. Coal ash in flue gas at temperatures <600°C is no longer sticky and hence fouling of tubes is not as much an issue as in SH.
  15. If ant ash is adhesive on the economizer tubes then it is easily removed from soot blowing operation.
  16. But at the operation of oil guns then ash are stick on tubes of economizer.
ECONOMISER
Fig.- ECONOMIZER IN BOILER SECOND PASS



ARRANGEMENT IN BOILER
  1. In large size boiler counter flow economizer are used.
  2. In it flue gases in flow down across the economizer tubes in second pass of boiler & feed water enter at the bottom of the economizer flow upward in the tubes.
  3. If downward flow is used in economizer then steam bubbles can cause flow stagnation and disrupt the flow separation.
  4. Upward flow of water eliminates unstable water flow & gives the most uniform gas distribution.
  5. The economizer tubes are bare tubes, in it there are no any fins are mounted.
  6. The tubes are arranged in line & stage to allow passage of large ash through the tube bank.
  7. The minimum clear space between economizer tubes ranges from 2- 4 inches.
  8. A pressure droop is present in between inlet and out let of feed water.

LOCATION OF ECONOMIZER IN BOILER IN 200 MW UNIT
Fig.- Location of economizer in 200 MW boiler



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