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 a 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 b 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 condenser
2. Central flow condenser
3. Evaporative condenser
1.Down flow condenser
Figure shows a sectional view of down 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. In this type of condenser, the cooling water and exhaust steam do not come in direct contact with each other as in case of jet condensers.
This is generally used where large quantities of inferior water are available and better quantity of feed water to the boiler must be used most economically. The arrangement of the surface condenser is shown in figure. It consists of cast iron air- tight cylindrical shell closed at each end as shown in figure. A number of water tubes are fixed in the tube plates which are located between each cover head and shell.
2. Central flow condenser.
Figure 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 the tube next. Some of the exhaust steam which moving towards the centre meets the under cooled condensate and pre-heats it thus reducing under cooling.
3. Evaporative condenser
In this condenser steam to be condensed in passed through a series of tubes and the cooling water falls 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.
These condensers are more preferable where acute shortage of cooling water exists. The arrangement of the condenser is shown in figure. Water is sprayed through the nozzles over the pipe carrying exhaust steam and forms a thin film over it. The air is drawn over the surface of the