Short Answer:

A surface condenser is a type of condenser in which steam and cooling water do not mix directly. The exhaust steam from the turbine flows over a large number of tubes carrying cold water inside them. The heat from the steam is transferred through the tube walls, causing the steam to condense into water.

Surface condensers are commonly used in thermal and nuclear power plants because the condensate obtained is pure and can be reused in the boiler. This type of condenser operates under vacuum and provides high efficiency with continuous and smooth operation.

Detailed Explanation :

Surface Condenser

A surface condenser is a non-mixing type condenser used in steam power plants to condense the exhaust steam from a turbine into water without mixing it with the cooling water. It consists of a shell containing a large number of small tubes through which cooling water circulates. The steam flows over the outer surfaces of these tubes, gives up its latent heat, and gets condensed into water.

In a surface condenser, the steam and cooling water are separated by metallic walls (usually made of brass or copper tubes). This arrangement ensures that the condensate (condensed steam) remains pure and can be used again as feed water in the boiler, thereby saving water and improving the overall efficiency of the plant.

Working Principle

The working of a surface condenser is based on the principle of heat transfer by conduction and convection.

1. Exhaust steam from the turbine enters the condenser shell under vacuum conditions.
2. Cooling water is circulated through the tubes at low temperature.
3. As the steam comes in contact with the cool tube surfaces, it loses its latent heat of vaporization.
4. The steam condenses into water (called condensate) and collects at the bottom of the condenser.
5. The condensate is then pumped back to the boiler by a condensate extraction pump.

An air extraction pump is used to remove non-condensable gases (like air or vapor) from the condenser to maintain the vacuum. The vacuum improves the pressure difference and enhances turbine efficiency.

Construction Details

The main components of a surface condenser are as follows:

1. Condenser Shell:
   The outer body or casing that holds the exhaust steam and cooling tubes. It is made of steel or cast iron and is designed to withstand vacuum pressure.
2. Tube Bundles:
   Thousands of thin metallic tubes (usually brass, copper, or stainless steel) are arranged horizontally or vertically. Cooling water flows through these tubes, and steam condenses on the outer surfaces.
3. Tube Plates:
   These plates hold the ends of the cooling tubes and keep them tightly sealed to prevent leakage between steam and water chambers.
4. Water Boxes:
   Located at each end of the condenser, these boxes distribute the cooling water into the tubes and collect it after heat exchange.
5. Baffles:
   Used to guide the steam uniformly over the tubes and ensure maximum heat transfer.
6. Air Extraction Pump:
   Removes air and other non-condensable gases to maintain the vacuum inside the condenser.
7. Condensate Extraction Pump:
   Removes the condensed water from the bottom of the condenser and returns it to the feedwater system of the boiler.

Operation of Surface Condenser

The exhaust steam from the turbine enters the condenser shell at low pressure and high volume. The steam comes into contact with the outer surface of the cooling tubes carrying cold water. Heat is transferred from the steam to the water through the metal tubes, and the steam condenses into liquid water.

The condensed steam (condensate) collects at the bottom and is continuously removed by the condensate pump. Meanwhile, the air extraction pump removes any trapped air or gases to maintain the vacuum. This vacuum helps reduce the back pressure on the turbine, increasing its efficiency.

The cooling water, after absorbing heat, leaves the condenser at a higher temperature and is either discharged to a cooling tower or recycled through a cooling system.

Advantages of Surface Condenser

1. Condensate Reuse: The condensate is pure and can be reused as feedwater in the boiler.
2. Improved Efficiency: The vacuum maintained in the condenser increases turbine efficiency.
3. No Mixing: Cooling water and steam do not mix, preventing contamination.
4. Suitable for Large Plants: Used in high-capacity thermal and nuclear power stations.
5. Better Temperature Control: The cooling process is uniform and can be easily regulated.

Disadvantages of Surface Condenser

1. High Initial Cost: More expensive due to complex construction and large number of tubes.
2. Maintenance Requirement: Needs regular cleaning of tubes to prevent scaling or fouling.
3. Large Space Requirement: Occupies more area compared to jet condensers.
4. Cooling Water Demand: Requires a continuous supply of cooling water.

Types of Surface Condensers

Surface condensers are generally classified according to the direction of steam flow:

1. Down Flow Surface Condenser:
   Steam enters at the top and flows downward over the cooling tubes. The condensate collects at the bottom.
2. Central Flow Surface Condenser:
   Steam enters from the sides and moves towards the center, where the condensate is collected.
3. Evaporative Surface Condenser:
   Uses a combination of water and air for cooling, where a part of the water evaporates to remove heat.

Each type is selected based on plant size, steam pressure, and available cooling medium.

Applications of Surface Condenser

• Used in thermal and nuclear power plants for efficient condensation of exhaust steam.
• Found in marine engines and industrial steam turbines.
• Employed in large refrigeration and process industries requiring recovery of pure condensate.

Importance of Vacuum in Surface Condenser

A strong vacuum is essential in surface condensers to allow maximum expansion of steam in the turbine and to lower back pressure.

• The vacuum is created by removing air and gases continuously using an air extraction pump.
• A high vacuum increases the pressure difference between the turbine and condenser, improving energy conversion efficiency.

Conclusion

In conclusion, a surface condenser is a vital component in modern power plants where steam and cooling water are kept separate for efficient condensation and condensate reuse. It works under vacuum to improve turbine efficiency and conserve water. Although it has higher cost and maintenance requirements compared to jet condensers, its ability to produce pure condensate and maintain high efficiency makes it the preferred choice for large-scale and continuous power generation systems.