Thermal power plants utilize water as the medium of converting heat energy from coal or other fuels to mechanical rotational energy in the turbine to produce electricity. Water on heating in a boiler forms steam at high pressure and temperature. The steam then expands in a turbine to rotate it. After the steam leaves the turbine, it is then condensed and reused again.
The condensation takes place in the condenser. The process removes the Latent heat and changes water in the vapour form to liquid form. This requires a medium to cool the steam. Water is the cooling medium in the condenser. This cooling water is termed Circulating Water or Condenser cooling water.
As the steam condenses to water, the volume reduces drastically, to one by ten thousandth of the steam volume. This creates a vacuum in the condenser. The water then collects in the bottom part of the condenser called the hot well. The vacuum enables the steam to expand more to get higher work output from the turbine. The continuous condensing and removal of the water helps maintain the vacuum.
Power plants operate with condenser vacuum in the range of 0.1 to 0.15 bar absolute. This is the maximum vacuum practically possible. The heat rejected in the condenser is almost 25 % of the heat input to a power plant. This constitutes the biggest loss in a thermal power plant.
This makes the Vacuum in the condenser one of the most critical operating parameters that affects the efficiency of a thermal power plant. All power plants critically monitor the condenser vacuum continuously.
The quantity and temperature of the Circulating Water is the main factor that affects the condenser vacuum and consequently the power output.
How much Circulating Water?
The quantity of Circulating Water required is very high. Based on a simple heat balance, the requirement will be almost 65 to 70 times the steam flow entering the Turbine.
A 600 MW thermal power plant uses around 1800 tons per hour of steam from the boiler. This means the circulating water requirement in tropical areas will be almost 120,000 cubic metre per hour. With a 15 Meter head, this requires a pumping power of around 4 MW.
This huge quantity of water, the second biggest input in a power plant after fuel, decides the location of a thermal power plant, including nuclear power plants.