Short Answer:
The purpose of a condenser in the Rankine cycle is to condense the exhaust steam from the turbine into water by removing its latent heat. This process reduces the back pressure on the turbine and allows the condensed water to be reused in the boiler.
In simple words, the condenser helps improve the efficiency of the Rankine cycle by maintaining a low pressure at the turbine exit and conserving water. It enables continuous operation of the cycle by recycling the working fluid, making the system more efficient and economical.
Detailed Explanation :
Purpose of a Condenser in Rankine Cycle
The condenser plays a very important role in the Rankine cycle, which is the basic thermodynamic cycle used in thermal power plants for electricity generation. The main purpose of a condenser is to convert the exhaust steam from the turbine into liquid water by removing the latent heat of vaporization. This is achieved by passing cooling water through tubes inside the condenser, which absorbs the heat from the steam.
The condenser not only helps in recovering water but also creates a low-pressure environment at the turbine outlet. This low pressure increases the expansion ratio of the steam, which in turn improves the overall efficiency and power output of the cycle. Without the condenser, the Rankine cycle would not operate efficiently, and a large amount of steam and water would be wasted.
- Function of Condenser in Rankine Cycle
The main function of a condenser is to maintain the continuity and efficiency of the Rankine cycle. Its specific functions include:
- Condensation of Exhaust Steam:
The condenser converts low-pressure exhaust steam from the turbine into water (condensate) by cooling it. The heat removed from the steam is transferred to the cooling water circulating through the condenser. - Maintaining Low Pressure in the Turbine:
The condenser operates under vacuum conditions. This low pressure at the turbine exit allows the steam to expand more, producing additional work and increasing the power output of the turbine. - Reusing Condensate Water:
The condensed water is collected and pumped back to the boiler for reuse. This reduces the need for fresh water and makes the system more economical. - Heat Rejection to the Environment:
The condenser removes waste heat from the cycle and transfers it to the environment, usually through a cooling tower or water body. This is essential to maintain a temperature difference for continuous operation.
Thus, the condenser performs the dual role of heat rejection and water recovery in the Rankine cycle.
- Working Principle of Condenser in Rankine Cycle
The working of the condenser is based on the principle of heat exchange and phase change. When the exhaust steam from the turbine enters the condenser:
- The steam comes into contact with tubes carrying cold water.
- The steam transfers its latent heat to the cooling water and begins to condense.
- As condensation occurs, the steam changes from vapor to liquid form (water or condensate).
- The condensate collects at the bottom of the condenser and is removed using a condensate extraction pump.
- The cooling water absorbs the heat and exits the condenser at a higher temperature.
This process maintains a low pressure in the condenser because the steam occupies much more volume than water. As steam condenses, its volume decreases significantly, maintaining the vacuum. This low pressure increases the steam expansion ratio in the turbine, improving the cycle’s efficiency.
- Importance of Condenser in the Rankine Cycle
The condenser is essential for efficient operation of the Rankine cycle. Its importance can be summarized as follows:
- Increases Turbine Efficiency:
By maintaining low exhaust pressure, the condenser allows more energy to be extracted from the steam in the turbine. The greater the pressure difference between the boiler and condenser, the higher the turbine’s work output. - Improves Thermal Efficiency:
The low condenser pressure ensures that the steam expands more completely, increasing the overall thermal efficiency of the Rankine cycle. - Conserves Water:
The condensed water is reused as boiler feedwater, reducing the need for external water supply. This is especially beneficial in areas with limited water availability. - Reduces Operational Cost:
Reusing condensate water saves both fuel and water, as the condensate is already preheated and requires less energy to convert into steam again. - Maintains Continuous Operation:
By converting the exhaust steam into water and returning it to the boiler, the condenser ensures the smooth, continuous operation of the Rankine cycle. - Prevents Back Pressure on Turbine:
The condenser maintains vacuum conditions that prevent excessive back pressure, which could otherwise reduce turbine efficiency and mechanical performance.
Thus, the condenser is essential not just for condensation, but also for efficiency improvement, economy, and operational stability.
- Types of Condensers Used in Rankine Cycle
There are two main types of condensers used in the Rankine cycle:
- Surface Condenser:
- Steam and cooling water do not mix.
- Steam condenses on the outer surface of tubes carrying cooling water.
- The condensate is pure and can be directly reused in the boiler.
- Commonly used in large thermal power plants.
- Jet Condenser:
- Steam and cooling water come into direct contact.
- The steam is condensed by mixing with cold water.
- Condensate cannot be reused directly due to impurities.
- Used in small plants or where water availability is high.
Among these, the surface condenser is most widely used in modern power plants because it maintains a high vacuum and allows condensate reuse.
- Effect of Condenser Pressure on Rankine Cycle Efficiency
The condenser operates at very low pressure (below atmospheric). The lower the condenser pressure, the higher the efficiency of the Rankine cycle.
However, if the pressure becomes too low:
- The condenser size must be increased.
- Moisture in the turbine may increase, causing blade erosion.
Therefore, an optimum condenser pressure is maintained to balance efficiency and practicality.
- Heat Rejection and Cooling Requirement
The condenser rejects the waste heat from the Rankine cycle. This heat is removed using a cooling water system. The amount of cooling water required depends on the quantity of steam condensed and the temperature difference between inlet and outlet water.
If the condenser is not properly cooled, the cycle efficiency will drop because of high back pressure in the turbine. Hence, cooling towers or large water sources are essential parts of condenser operation.
Conclusion
The purpose of a condenser in the Rankine cycle is to condense exhaust steam into water, maintain a low pressure at the turbine exit, and recycle the condensate for reuse in the boiler. It helps in improving the thermal efficiency of the cycle, reducing water wastage, and ensuring continuous operation of the power plant. By providing a means to remove waste heat and maintain vacuum conditions, the condenser becomes an essential component of the Rankine cycle. Without it, the efficiency and sustainability of the system would be greatly reduced.