Short Answer:

Compounding in turbines is the process of dividing the total energy drop of steam into several stages to reduce the speed of the turbine rotor and improve efficiency. It helps in controlling the high rotational speed that occurs when the entire steam expansion happens in a single stage.

In compounding, the expansion of steam or the pressure and velocity of steam are controlled in multiple steps using a series of nozzles and blades. This ensures smooth operation, higher efficiency, and safer mechanical design in large power-producing steam turbines.

Detailed Explanation :

Compounding in Turbines

Compounding in turbines is an important method used to reduce the extremely high rotational speed of a simple impulse turbine and to make the turbine suitable for practical applications. When high-pressure steam expands completely in a single nozzle and strikes the blades in one stage, the velocity of the steam becomes very high, causing the turbine rotor to rotate at an excessively high speed (often more than 30,000 rpm). Such high speeds are difficult to manage mechanically and are not suitable for driving generators or machinery directly.

To overcome this problem, the total energy of the steam is divided into several stages. Each stage consists of a set of fixed and moving blades that gradually convert the steam’s energy into mechanical work. This multi-stage arrangement is called compounding. By using compounding, the steam’s energy is extracted in steps, the rotor speed is reduced to a practical limit, and efficiency is improved.

Purpose of Compounding

The main purposes of compounding in steam turbines are:

• To reduce the very high speed of rotation of the turbine shaft.
• To increase the efficiency of energy conversion.
• To minimize mechanical stress on blades and rotor.
• To obtain smooth, steady, and safe turbine operation.
• To make the turbine suitable for power generation and industrial applications.

Types of Compounding in Turbines

There are mainly three types of compounding used in impulse turbines and a combination type that uses features of both pressure and velocity compounding.

1. Velocity Compounding

In velocity compounding, the total pressure drop of the steam takes place in one set of nozzles. The high-velocity steam jet produced is passed over multiple rows of moving blades separated by fixed guide blades.

• The first row of moving blades converts part of the steam’s velocity into mechanical work.
• The fixed blades then redirect the steam flow toward the next set of moving blades.
• The steam’s velocity gradually decreases after each stage while the rotor continues to rotate.

This arrangement reduces the turbine speed compared to a single-stage impulse turbine.
Example: Curtis Turbine.

Advantages:

• Simple construction compared to pressure compounding.
• Reduced rotor speed suitable for practical applications.

Disadvantages:

• Moderate efficiency because some kinetic energy is lost in redirection.

2. Pressure Compounding

In pressure compounding, the total pressure drop of steam does not occur in a single nozzle but is divided into several stages. Each stage consists of one set of nozzles and one set of moving blades.

• The steam expands partially in the first nozzle, producing high-velocity jets.
• These jets strike the moving blades, transferring energy and causing rotation.
• The steam then enters the next set of nozzles, where it expands again, and the process continues.

Each stage works with a smaller pressure drop, making the operation smooth and efficient.
Example: Rateau Turbine.

Advantages:

• Higher efficiency due to staged pressure expansion.
• Lower velocity of steam in each stage reduces blade erosion.
• Suitable for large turbines.

Disadvantages:

• More complex construction and higher cost compared to simple designs.

3. Pressure-Velocity Compounding (Combined Compounding)

Pressure-velocity compounding combines both pressure and velocity compounding. The total pressure drop is divided into several stages (like pressure compounding), and within each stage, multiple rows of moving and fixed blades are used (like velocity compounding).

• In this method, steam expands partially in the first nozzle, then passes through several moving and fixed blade rows.
• This combination results in moderate steam velocity and reduced turbine speed.
• The process is repeated in multiple stages until the total pressure drop is completed.

Example: Curtis-Rateau Turbine.

Advantages:

• Very efficient due to gradual expansion and multiple velocity reductions.
• Produces moderate speeds suitable for generators.
• Offers smoother operation and reduced losses.

Disadvantages:

• More expensive and complex to design.
• Requires accurate blade alignment for smooth functioning.

Need for Compounding

Without compounding, the simple impulse turbine would rotate at an extremely high speed, making it impractical for power generation. Compounding reduces this speed to a safer range (usually between 3000 to 6000 rpm for power plant turbines). It also helps in distributing the work done by steam more uniformly across several stages, minimizing wear and tear and improving operational life.

In modern steam turbines, compounding not only helps in speed control but also in improving thermal efficiency by ensuring that the expansion of steam is smooth and controlled.

Applications of Compounded Turbines

• Power Plants: Used in thermal and nuclear power stations for electricity generation.
• Marine Engines: Suitable for propulsion systems requiring stable and continuous rotation.
• Industrial Plants: Used to drive pumps, compressors, and mechanical devices.

Conclusion

In conclusion, compounding in turbines is an essential technique used to divide the total energy of steam into several stages to achieve controlled speed, improved efficiency, and smooth operation. It allows turbines to convert high-pressure steam energy into mechanical work effectively while maintaining practical rotational speeds. Among all methods, pressure compounding, velocity compounding, and pressure-velocity compounding are the most common, each having specific advantages depending on the application. Therefore, compounding plays a vital role in the design and operation of modern steam turbines used for power generation and industrial purposes.