SURFACE CONDENSERS

    In surface condensers there is no direct contact between the steam and cooling water and the condensate can be re-used in the boiler: In such condenser even impure water can be used for cooling purpose whereas the cooling water must be pure in jet condensers. Although the capital cost and the space needed is more in surface condensers but it is justified by the saving in running cost and increase in efficiency of plant achieved by using this condenser. 
   Depending upon the position of condensate extraction pump, flow of condensate and arrangement of tubes the surface condensers may be classified
as follows:
1)Down flow type: Fig.1 shows a sectional view of dawn flow condenser. Steam enters at the top and flows downward. The water flowing through the tubes in one direction lower half comes out in the opposite direction in the upper half Fig.2 shows a longitudinal section of a two pass down-flow condenser.

2)Central flow condenser: Fig.shows a central flow condenser. In this condenser the
steam passages are all around the periphery of the shell. Air is pumped away from the centre of the condenser. The condensate moves radially towards the centre of tube nest. Some of the exhaust steams while moving towards the centre meets the undercooled condensate and pre-heats it thus reducing under cooling.

3)Evaporation condenser: In this condenser steam to be condensed is passed
through a series of tubes and the cooling waterfalls over these tubes in the form of spray. A steam of air flows over the tubes to increase evaporation of cooling water, which further increases the condensation of steam.

 The various advantages of a surface condenser are as follows: 

1. The condensate can be used as boiler feed water. 
2. Cooling water of even poor quality can be used because the cooling water does not come in direct contact with steam.
3. High vacuum (about 73.5 cm of Hg) can be obtained in the surface condenser. This   increases the thermal efficiency of the plant.

The various disadvantages of' the surface condenser are as follows: 

1. The capital cost is more.
2. The maintenance cost and running cost of this condenser is high.
3. It is bulky and requires more space.