• The function of feed water heaters is to raise the temperature of the feed water by means of bled steam before the feed water is supplied to the boiler from the hot well.

• There are two main types of feed water heaters: Open or contact type and closed or surface type.

OPEN OR CONTACT HEATERS

• These heaters are usually constructed to remove non-condensable gases from water and steam along with raising the tempera-ture of feed water and such an heater is termed as "Deaerator".

• Oxygen and Carbon dioxide dissolved in the feed water greatly corrode the inner surfaces of the boiler unit and the pipelines.

• Therefore, the feed water should be delive-red to the boilers after it has undergone deaeration or degasification.

• The amount of gas dissolved in water depen-ds upon its temperature. This decreases sharply with increasing temperatures and drops to almost zero at the boiling point.

• Hence thermal deaeration of the feed water is usually practised.

• Open heaters are of two types: Tray type and Jet type.

Vertical tray type open heater

• In the jet type, there is no vent condenser but a preheater section in the top of the shell. In this section, the water is atomised with the help of jet nozzles and most of O2 is removed. Remaining non-condensable gases are separated in the air separating trays.

• The direct contact feed water heaters (open heaters) have got the following advantages:

1.Complete conversion of steam to water is accomplished.

2.Non Condensable corrosive gases are removed from the feed water.

3.The removal of impurities in the water is possible.

4.The water is brought to the temperature of the steam.

5.The heater acts as a small reservoir.

• Because of the stress limitations of the heater shell, the steam pressure is limited to a few N/m2 above atmospheric pres-sure although pressures to about, 5 bar absolute have been used.

• The feed water is rarely heated above 105°C.

• To prevent failure from boiling in the suction connection of the feed water pump, the deaerators should be mounted 8 to 9 m above the pump axis.

CLOSED OR SURFACE HEATERS

• Such heaters consist of closed shell in which there are tubes or coils through which either steam or water is circulated.

• Usually, the water is circulated through the tubes and the steam and water may flow either in same direction or in opposite direction.

• The closed heaters can be of vertical or horizontal type.

Vertical closed heater

• In closed water heaters, the feed water can never be heated to the temperature of steam.

• To maintain a high overall heat transfer for the heater, the water velocity should be high but pumping costs will limit the velocity. A balance will result in water velocities of about 1 to 2.5 m/s.

Arrangement of Feed Water Heaters

 

• A power plant steam cycle contains typi-cally three low pressure feed water heaters and two or three high pressure ones.

• The steam cycle normally contains at least one open feed water heater (other heaters are closed type) that operates at a pres-sure slightly higher than atmospheric pres-sure. This heater is called the 'Deaerating heater‘.

 

• Steam for l.p. heaters is bled from l.p. tur-bine and that for h.p. heaters and deaerator, from i.p. and h.p turbines

 

Feed water heaters in a 120 MW unit

Desuperheating heaters in a 120 NW set