Surface Condensers

In surface condensers, there is no mixing of exhaust steam and cooling water. The condensate can be re-used in the boiler: In a surface condenser, even impure water can be used for cooling purposes, whereas in jet condensers, cooling water must be pure. Although the capital cost and the space required are higher in surface condensers, it is still preferred due to its low running cost and high thermal efficiency of the plant.

Surface condensers can be classified into various types depending upon the position of the condensate extraction pump, the flow of condensate, and the arrangement of tubes.

1. Down Flow Condenser:

A down flow type condenser sectional view is shown in Fig. 1. In down flow condensers, exhaust steam enters at the top and flows downward. The cooling 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 type condenser.

2. Central Flow Condenser:

A central flow type condenser is shown in Fig. 3. In a central flow type 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 nest. Some of the exhaust streams, while moving towards the centre, meet the undercooled condensate and preheat the air, thus reducing undercooling.

3. Evaporation Condenser:

In the evaporation condenser (Fig. 4), the steam to be condensed is passed through a series of tubes, and the cooling water falls over these tubes in the form of a spray. Exhaust steam flows over the tubes to increase evaporation of cooling water, which further increases the condensation of steam.

Advantages:

The advantages of a surface condenser are:

1. The condensate can be used as boiler feed water.
2. Cooling water of impure quality can be used because the cooling water does not come in contact with steam.
3. High vacuum (about 735 mm of Hg pressure) can be obtained in the surface condenser. This increases the thermal efficiency of the plant.

Disadvantages:

The disadvantages of the surface condenser are:

1. The capital cost is high.
2. It is bulky and requires more space.

Requirements of a Modern Surface Condenser:

The requirements of an ideal surface condenser used for power plants are as follows:

1. The exhaust steam entering the condenser should be evenly distributed over the whole cooling surface of the condenser vessel with minimum pressure loss.
2. The amount of cooling water circulated in the surface condenser should be regulated so that the temperature of the cooling water leaving the condenser equals the saturation temperature of steam corresponding to the steam pressure.  This will prevent undercooling of condensate.
3. The deposition of dirt on the outer surface of tubes in surface condensers needs to be prevented. Passing the cooling water through the tubes and allowing the steam to flow over the tubes makes this happen.
4. There should be no air leakage into the condenser, as air destroys the condenser vacuum and thus reduces the work obtained per kg of steam. If there is any leakage of air into the condenser, the air extraction pump needs to be used to remove the air as soon as possible.