The steam condensers are broadly classified into two types:
- Surface condensers (or non-mixing type condensers). In surface condensers, there is no direct contact between the exhaust steam and the cooling water.
- Jet condensers (or mixing type condensers). In jet condensers there is direct contact between the exhaust steam and cooling water.
The surface condenser is designed to condense and deaerate the exhaust steam from the main turbine and provide a heat sink for the turbine bypass system. In surface condensers, there is no direct contact between the exhaust steam and the cooling water. The exhausted steam from the LP turbines is condensed by passing over tubes containing water from the cooling system. The steam condenses when it comes in contact with the cold surface of the tubes and due to the heat transfer to cooling water by conduction and convection. These tubes are usually made of stainless steel, copper alloys, or titanium depending on several selection criteria (such as thermal conductivity or corrosion resistance). Titanium condenser tubes are usually the best technical choice, however titanium is very expensive material and the use of titanium condenser tubes is associated with very high initial costs. In general, there are two types of surface condensers:
- water-cooled surface condenser
- air-cooled surface condenser
In thermal power plants, where cooling water is in short supply, an air-cooled condenser can be used. An air-cooled condenser is however, significantly more expensive and cannot achieve as low a steam turbine exhaust pressure (and temperature) as a water-cooled surface condenser.
The water gets warmed in the condenser is discharged into the cooling system (i.e. cooling tower, river, sea, or cooling pond). The condensate collected from these condensers is reused as feedwater in the boiler. Since the cooling water and steam do not mix, the condensate is recovered and any kind of cooling water can be used. In comparison to jet condensers, in surface condensers a high vacuum can be maintained, therefore greater thermal efficiency can be achieved. On the other hand, surface condensers are bulky, requires large area and high capital costs. But these capital costs can be recovered by the improved thermal efficiency (i.e. higher )saving in running cost.
Thus, these condensers are most suitable for modern thermal power plants. These are generally used where a large quantity of inferior water is available and better quality of feedwater is to be supplied to the boiler.
In jet condensers, the cooling water is sprayed on the exhaust steam and there is direct contact between the exhaust steam and cooling water. The process of condensation is very fast and efficient, but here cooling water and condensed steam are mixed up. The condensate then cannot be reused as feedwater to the boilers. The temperature of the condensate is same as that of the cooling water leaving the condenser. Due to more intimate mixing of steam and cooling water jet condenser requires less quantity of cooling water for the condensation of steam. In general, jet condensers require less building space and they are simpler in construction and lower in capital cost. Despite these advantages jet condensers are not usual in thermal power plants especially due to the loss of condensate.