In surface condensers there is no mixing of exhaust steam and cooling water. The condensate can be re-used in the boiler: In surface condenser even impure water can be used for cooling purpose whereas in jet condensers cooling water must be pure. Although the capital cost and the space required is more in surface condensers but it is still preferred due to its low running cost and high thermal efficiency of plant.
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.
Central flow type condenser is shown in Fig. 3. In 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 tube nest. Some of the exhaust steams while moving towards the centre meets the undercooled condensate and pre-heats air thus reducing under-cooling.
In evaporation condenser (Fig. 4) steam to be condensed is passed through a series of tubes and the cooling waterfalls over these tubes in the form of spray. Exhaust steam flows over the tubes to increase evaporation of cooling water, which further increases the condensation of steam.
The advantages of a surface condenser are:
- The condensate can be used as boiler feed water.
- Cooling water of impure quality can be used because the cooling water does not come in contact with steam.
- High vacuum (about 735 mm of Hg pressure) can be obtained in the surface condenser. This increases the thermal efficiency of the plant.
The disadvantages of’ the surface condenser are:
- The capital cost is high.
- It is bulky and requires more space.
The requirements of ideal surface condenser used for power plants are as follows:
- The exhaust steam entering the condenser should be evenly distributed over the whole cooling surface of the condenser vessel with minimum pressure loss.
- The amount of cooling water being circulated in the surface condenser should be regulated that the temperature of cooling water leaving the condenser is equivalent to saturation temperature of steam corresponding to steam pressure.
This will prevent under cooling of condensate.
- The deposition of dirt on the outer surface of tubes in surface condensers need to be prevented.
Passing the cooling water through the tubes and allowing the steam to flow over the tubes makes this happen.
- There should be no leakage of air into the condenser because presence of air destroys the vacuum in the condenser and thus reduces the work obtained per kg of steam. If there is any leakage of air into the condenser air extraction pump need to be used to remove air as soon as possible.