All thermal power plants, be they coal fired or nuclear, use the modified Rankine steam cycle. The steam exiting from the steam turbine condenses in a condenser and then is reused in the steam cycle. Almost all thermal power plants use a surface condenser for cooling the steam. The only exception is in a geothermal plant where a direct contact condenser is used.
In a surface condenser, the steam flows over a tube bundle. The condenser cooling water flows through the inside of these tubes. In a large power plant, the condenser will have about 15,000 tubes.The heat transfer takes place through the surface of these tubes.
In a direct contact condenser, cooling water mixes with the steam. The evaporation of the water cools and condenses the steam.
The circulating water system consists of an intake canal, the pumps, piping, cooling towers and an outfall system.
There are two different systems based on how the water is sourced and recycled.
Open Cooling system.
In an open circulating water system, water from a large water body like the sea, or a river or a lake is pumped to the condenser and is returned back to the same source. Since the sea is a free and large open source of water, we see many power plants located on the seacoast.
The advantages of an open system are:
- Large quantity water is freely available.
- The water temperature is almost constant. Ambient temperatures do not affect the cooling capacity.
The disadvantages are :
- That plants can be located only near the sea.
- Organic and aquatic growth can plug the condenser tubes reducing the efficiency. This requires the use of chemicals like Sodium Hypochlorite to kill the organisms. This also requires methods to clean the tubes mechanically.
- The large quantity of water used can destroy aquatic life sucked into the system. Higher temperature water discharge also destroys many smaller species and fish eggs. Since the natural ecosystem is interdependent, destruction of one species can lead to eventual destruction of the entire ecosystem.
- Seawater is corrosive. This requires special materials like Titanium tubes in the condenser, rubber lining for the piping, and corrosion resistant impellers for the pumps.
- It requires a properly designed intake and an outfall to eliminate environmental impact.
Closed Cooling System.
The second is the closed cooling system where Circulating water is in a closed circuit. The Circulating water removes the heat from the condenser and flows to cooling towers. In the cooling towers an airflow, natural or forced, cools the water and the water returns to the condenser. Power plants located away from large sources of water utilise this type. The large concrete hyperbolic towers that you see near thermal power plants are used for cooling the circulating water.
The advantages are :
- The raw water requirement is considerably less. It requires only about 5% of the water requirement of the open cycle system. This is to make up for losses due to evaporation and blowdown.
The disadvantages are :
- The system requires higher capital investment, increased operating costs for the cooling towers, and more maintenance requirements and spares.
- Since air is the secondary cooling medium, ambient wet bulb temperature is a limiting factor in achieving the cooling water temperature. This will affect the performance of the power plants.