Understanding Steam Bleeding for Efficient Feedwater Heating

In power plants that use steam turbines, improving efficiency is crucial. One common method to enhance the thermal efficiency of the steam power cycle is by heating the boiler feed-water using steam extracted from the turbine at various stages during its expansion. This process is known as steam bleeding.

Let's look at the options provided in the context of power plant operations:

• Bleeding: This term specifically refers to the process of drawing off or extracting steam from intermediate stages of a steam turbine before it completes its expansion to the condenser pressure. The primary purpose of this bled steam is to heat the feed-water that is supplied to the boiler. By heating the feed-water, the amount of heat that needs to be added in the boiler is reduced, leading to an improvement in the overall cycle efficiency. This aligns directly with the question asking about draining steam for heating feed-water.
• Governing: Governing in a steam turbine system refers to the mechanism used to control the speed or power output of the turbine. It involves regulating the flow of steam into the turbine, typically by adjusting valves. Governing is related to operational control, not the process of using extracted steam to heat feed-water.
• Cooling: Cooling, in the context of a steam power plant cycle, usually refers to the process of condensing the spent steam from the turbine exhaust back into water in the condenser. This is done using cooling water. Cooling is the opposite of heating feed-water; it removes heat from the exhaust steam.
• Reheating of steam: Reheating is a process where steam, after partial expansion in a high-pressure turbine stage, is returned to the boiler or a separate reheater to be heated again before expanding further in intermediate or low-pressure turbine stages. Reheating increases the work output and can improve efficiency, but it involves heating steam itself, not using extracted steam to heat feed-water.

Based on the definitions, the process of draining steam from the turbine for the purpose of heating the feed-water is correctly identified as bleeding.

Revision Table: Power Plant Processes

Additional Information: Feedwater Heating Systems

Feedwater heating using bled steam is a standard practice in modern thermal power plants. The extracted steam is directed to feed-water heaters, which are heat exchangers where energy is transferred from the steam to the feed-water. This increases the feed-water temperature before it enters the boiler, reducing the energy required from the fuel in the boiler.

There are two main types of feed-water heaters:

• Open Feed-water Heaters: These are direct contact heaters where bled steam mixes directly with the feed-water. They operate at the pressure of the bled steam.
• Closed Feed-water Heaters: These are shell-and-tube heat exchangers where the steam and water are kept separate. The steam condenses on the outside of tubes through which feed-water flows, transferring heat without mixing.

Using multiple stages of bleeding at different turbine pressures further optimizes the feedwater heating process and contributes significantly to the overall efficiency gains of the power plant.