Introduction to Condensers and Advantages of Condensers

1. In order to develop the maximum work, according to the Carnot’s principle, the heat must be supplied at the maximum possible pressures and temperatures and the heat should be rejected at minimum pressures and temperatures.
2. However, the heat supplied depends upon the pressure of steam to be generated in a boiler and its design, while the limit of temperature is governed by the availability of materials.
3. Therefore, to increase the work output and to improve the efficiency of the power plant, the aim is to reject the heat at minimum pressure and temperatures.
4. The basic power plant cycle (Rankine Cycle) has already been discussed. The steam from the steam turbine or the steam engine could be exhausted to atmosphere in such acase the back pressure would be above the atmosphere pressure as shown in Figure by the process (4-5) of the cycle (1-2-3-4-5-1).Such a cycle represents the cycle for a non-condensing steam engine.
5. It should be noted that the energy of exhaust steam, fully or partially, cannot be reutilized.If the back pressure could be reduced below the atmospheric pressure, the new cycle is represented as (1-2-3-6-7-1) for a condensing steam engine.
6. The reduction in back pressure is achieved by a device called condenser. The advantage of using condensing steam engine is to increase the work output equal to shaded area (4-5-7-6).
7. Therefore, the condenser is a heat transfer device in which the exhaust steam of a turbine or an engine is condensed by means of cooling water at pressures below atmospheric. The condensed steam, called the condensate, can be again returned to the boiler. It will save the cost of water.
8. Therefore, the advantages of a condenser are :

• It increases the work output per kg of steam supplied to the power plant. Alternatively, it reduces the specific steam consumption, therefore, it reduces the size of the power plant of given capacity.
• It improves the thermal efficiency of the power plant.
• Since the condensate is returned to the boiler, it affects the saving in cost of water to be supplied to the boiler.
• Since the pure feed water is available for the boiler, it saves the cost of water softening plant.

The provision of a condenser increases the capital cost of the power plant. Since it needs a condenser, the allied pumps to operate the condenser, the hot-well and the cooling tower. However, the increased capital cost is offset in a very short time due to increased efficiency of the power plant. The lowest practical back pressure for the steam engines is about 0.2 to 0.3 bar since it is an intermittent flow machine, while for the steam turbines the lowest back pressure is upto about 0.04 bar since it is a continuous steady flow machine.