Use of Condensers in Thermal (Steam) Power Plants

The use of condensers in thermal power plant or steam power plant improves the efficiency of the power plant by decreasing the exhaust pressure of the steam below the atmosphere. Another advantage of the condenser is that the steam condensed may be recovered to provide a source of good pure feed water to the boiler and reduces the water softening plant capacity to a considerable extent.

Why Low Pressure or Vacuum at Condenser:

The maximum possible thermal efficiency of the power system is given by (T1-T2)/T1

Where T1 is the supply temperature

            T2 is the exhaust temperature

The expression of the efficiency shows that the efficiency of the system increases with increase in supply temperature (T1) and decrease in exhaust temperature (T2). The maximum value of the supply temperature T1 of the steam supplied to the prime-mover is limited by the material consideration. The temperature T2 (temperature at which heat is rejected) can be reduced if the exhaust of the steam prime-mover takes place below atmospheric pressure. This is because there is a definite relation between the steam temperature and pressure. Low exhaust pressure means low exhaust temperature. The steam cannot be exhausted to the atmosphere if it is expanded in the engine or turbine below atmospheric pressure. Under this condition, the steam is made to exhaust in a vessel known as condenser where the pressure inside is maintained below the atmospheric pressure by condensing the steam by circulating the cold water. A closed vessel in which steam is condensed by abstracting the heat from steam and pressure is maintained below atmospheric pressure is known as condenser. The steam condenser is one of the most essential components of all modern steam power plants

The different components of the steam condensing plant are:

Condenser, Supply of cooling water, Condenser cooling water pump, Condensate extract pump, Hot-well, Boiler Feed Pump, Air extraction pump, Cooling tower and make-up water pump  

Advantages of Condensers:

The advantages obtained by employing a condenser in steam power plant is listed below:

• The condensed steam, from the condenser is used as feed water for boiler. Using the condensate as feed for boiler reduces the cost of the power generation as the condensate is supplied at high pressure to the boiler and it reduces the capacity of the feed water cleaning system.
• The efficiency of the thermal power plant increases as the enthalpy drop increases by increasing the vacuum in the condenser. The specific steam consumption of the power plant also decreases  as the availability enthalpy drop or work developed per kg of steam increases with the decrease in back pressure by using the condenser
• The deposition of the salt in the boiler is prevented with the use of condensate instead of using feed water from outer source which may contain salt. The deposition of the salt in boiler shell also reduces the boiler efficiency. Thus is particularly important in marine steam power plant
• The use of condenser in steam power plant reduces the overall cost of the generation by increasing the thermal efficiency of the power plant. The efficiency condenser plant must be capable of producing and maintaining a high vacuum with the quantity of the cooling water available and should be designed to operate for prolonged periods without trouble

The desirable features of good condensing plant are:

• Minimum quantity of the circulating water
• Minimum cooling surface area per kW capacity
• Minimum auxiliary power
• Maximum steam condensed per m² of the surface area