Economizer in Steam Power Plant

Economizer in Steam Power Plant

An economizer is used to raise the temperature of feed water with the help of waste heat energy of gases before it is supplied to the boiler. It is a heat exchanger mostly used in thermal power plants.

The temperature of gases is about 350˚C to 550˚C at the exit of the last heater or superheater a large amount of heat energy would have been wasted.

It reduces the amount of heat to be added to the steam in the boiler per kg of steam. It means, the rate of steam evaporation rises at the same rate of heat supplied by burning the fuel in the boiler.

An economizer is similar to a convective type superheater. In most cases, the economizer is placed between the convective superheater (CSH) and the air preheater.

In the case of low and medium-pressure boilers, the economizer is made of cast iron and in the case of a high-pressure boiler, it is made of steel.

The economizer is made of cast iron to avoid water hammering due to the boiling of water. Feed water temperature must be kept at about 40˚C below the saturation temperature of steam.

Advantages of Economizer

The advantages of economizer are listed below.

  • The economizers are used to increase the efficiency of the boiler. There is approximately a 1% rise in efficiency per 5˚C rise in feed water temperature.
  • Due to the reduction in temperature differential in a boiler, reduces the temperature stresses. The energy equation of economizer is;

    \[ Q = \dot m_w \dot C_{pw} (t_{wo}-t_{wi}) = \dot m_g C_{pg} (t_{g1}-t_{g2})  \]

    \[ Q = U_o A_o \Delta T_{lm} \]

Where, ∆Tlm = log mean temperature difference

  • With the help of fins on the gas side, the heating surface area A0 can be increased.

Design Aspects of Economizer

  • The gas should come in contact with the entire heating surface area.
  • To reduce the accumulation of dust, the heating surface must be polished.
  • Any steam formation with water in tubes should escape into the boiler drum without causing any water hammer.
  • It should be compact and easy to carry out cleaning and maintenance.
  • Water velocity should not be below 0.5 m/s in non-steaming economizers and below 1 m/s in steaming economizers.
  • The gas passages are designed to maintain constant gas velocity throughout the passages.

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