Steam Turbine

Defining a Steam Turbine: Basic Principles

A steam turbine operates on the principle of thermodynamics. The basic method involves the generation of high-pressure steam in a boiler, which is then expanded through a steam turbine to perform work.

There are numerous designs and arrangements for steam turbines, but most of them work on the same fundamental principle. The steam's potential energy is first converted into kinetic energy in the form of high-speed steam. This kinetic energy then gets converted into mechanical energy as it moves the turbine's blades, which are connected to the turbine's shaft.

The formula for kinetic energy in thermodynamics is given as:

Where, m = mass of the steam v = velocity of the steam

Historical Context of the Steam Turbine

The history of the steam turbine can be traced back to the 1st century AD when Hero of Alexandria created the 'aeolipile' which used steam power to rotate a ball.

However, it wasn't until the 1880s, during the industrial revolution, that practical steam turbines capable of generating substantial power were developed. The invention of the modern steam turbine is credited to Sir Charles Parsons. His design dramatically increased the efficiency and power output of steam engines, paving the way for widespread industrial and maritime applications.

Today, steam turbines are used in a variety of applications, including power generation and propulsion systems, due to their high efficiency and reliability.

Various Real-world Steam Turbine Examples

Understanding the steam turbine concept might seem abstract without any practical examples. To bridge this gap, let's delve into the exploration of real-world applications where you can find simple and complex industrial steam turbines.

Simple Steam Turbine Example

Steam turbines are no stranger to simple applications. Let's take a look at one such instance - table-top demonstration models used in educational settings.

These miniaturised models are usually seen in science classrooms to give students a practical understanding of thermodynamics, specifically how heat can be used to produce mechanical energy. Each model features a small boiler, a turbine, and a rotating wheel demonstrating the conversion of thermal energy into mechanical work.

The steps to set up and demonstrate such a model include:

• Filling the small boiler with water
• Applying heat to the boiler to produce steam
• The steam then travels to the turbine
• The turbine rotor starts to spin, moving the attached wheel
• The wheel's rotation is the mechanical work derived from the heat

In such a model, there's no need for complex calculations. However, teaching the underlying principle, a crucial formula is merited:

This formula represents the kinetic energy acquired by the moving steam where 'm' is mass and 'v' is velocity.

This model offers a simple, easily understandable example of a steam turbine that underlines the basics of steam-to-mechanical energy conversion.

Complex Industrial Steam Turbine Example

For a complex instance of steam turbines, let's consider their use in thermal power plants. Here, turbines are an essential component in the production of electricity.

In a thermal power plant, the process involves several stages:

• Coal or natural gas is burnt to produce heat
• This heat is used to convert water into high-pressure steam
• The steam is then channelled into the steam turbine
• The steam turbine rotates, driving an attached generator
• This generator then produces electricity

In this instance, turbines are used to rotate a generator's rotor - a process that requires high-speed, high-efficiency turbine blades to optimise electricity production.

The turbines used in power plants are much larger and more complicated than the simple tabletop model, often consisting of several stages of blades to manage pressure changes efficiently.

To decipher the efficiency of these turbines, several complex formulas come into play, including the equation for turbine efficiency:

Where: \(\eta_t\) = Turbine Efficiency \(h1\) = Enthalpy of steam input in the turbine \(h2\) = Enthalpy of steam leaving the turbine \(h3\) = Enthalpy of steam if expanded isentropically

This example of industrial steam turbines demonstrates how engine design and thermodynamic calculations come together to deliver impressive outputs in a complex, real-world setting.

Grasping Steam Turbine Efficiency

In your exploration of the steam turbine, understanding efficiency becomes paramount. The efficiency of a steam turbine plays a crucial role in deciding the overall performance of power plants or any systems where they are utilised. But how is this efficiency calculated, and what factors influence it? Let's delve into these aspects.

The Science Behind Steam Turbine Efficiency

In simple terms, the efficiency of a steam turbine, often represented by the Greek letter \(\eta_t\) (eta), is the ratio of the useful work obtained from the turbine to the energy supplied in the form of steam. Simply put, it measures how well the turbine converts steam's thermal energy into mechanical work.

Several factors influencing this efficiency include blade configuration, steam conditions, the steam's expansion process, and power extractions for feed water heating. However, one key component stands out: the thermodynamic efficiency. Thermodynamic efficiency is primarily influenced by the pressure and temperature differences between the steam entering the turbine and the exhaust steam.

Steam turbine efficiency can be determined using the formulation:

Where: \(\eta_t\) = Turbine Efficiency \(h1\) = Enthalpy of steam input in the turbine \(h2\) = Enthalpy of steam leaving the turbine \(h3\) = Enthalpy of steam if expanded isentropically

Here, isentropic expansion refers to an ideal, reversible process where the steam expands without any loss or gain of heat and without any energy loss due to friction or turbulence. In reality, such an ideal expansion is unachievable, and the actual exhaust enthalpy (\(h2\)) is always greater than the isentropic exhaust enthalpy (\(h3\)). Therefore, \(\eta_t\) is always less than 1 (or 100%). The difference between \(h2\) and \(h3\) reflects any inefficiencies in steam expansion due to friction, heat loss, turbine blade geometry, etc.

Factors Influencing Steam Turbine Efficiency

The efficiency of steam turbines is a significant aspect to consider in their design and operation. Multiple factors contribute to the overall efficiency of a steam turbine, each playing a crucial role in its operation.

• Steam Properties: Properties of the steam, including pressure, temperature, moisture content and mass flow rate, affect how efficiently the steam energy is converted into mechanical energy. Turbines are most efficient when they operate at high pressure and temperature. However, the steam's moisture content should be low, as excess moisture can cause erosion of the turbine blades.
• Blade Design: The design, size and arrangement of the turbine blades significantly impact the steam expansion process and the turbine's overall efficiency. Blades need to be designed optimally to extract maximum energy from the steam.
• Turbine Staging: Typically, turbines are designed with multiple stages (sets of blades), to handle the large pressure drop from the steam inlet to the exhaust. This multi-stage design improves efficiency, but the number of stages and their design need to be correctly optimised.
• Material of Construction: The material used to construct the turbine, particularly the blades, has a bearing on its efficiency as it determines the strength, durability and resistance to heat and pressure loads.
• Maintenance: Regular maintenance of the turbine and its components helps ensure the system operates at optimum efficiency. Any irregularities, such as steam leaks or blade erosion, can significantly reduce efficiency and should be promptly addressed.

In conclusion, numerous factors contribute to the efficiency of a steam turbine. While some are inherent to the design and construction of the turbine and others relate to the application, it sets a goal for engineers and science enthusiasts alike – optimising these aspects for the highest levels of efficiency possible.

Exploring Application of Steam Turbine

When considering the sheer power and versatility of the steam turbine, you'll find an array of applications across multiple sectors. From power generation to the marine industry, the steam turbine has established an essential position.

Use of Steam Turbines in Power Generation

A central application of steam turbines is in the field of power generation, specifically in thermal power plants. Here, electricity production heavily relies on the efficient functioning of steam turbines. The process begins by heating water inside a boiler until steam is produced. This steam, carrying significant thermal energy, is then channelled into the steam turbine.

The primary benefit of utilising steam turbines in power generation is their high efficiency at large scales. This makes them ideally suited for bulk power generation at electricity plants where a few percentage points of efficiency can translate into significant cost savings.

To demonstrate this, consider these steps involved in power generation using a steam turbine:

• Burning of a fuel (coal, natural gas, etc.) heats up water in a boiler to produce high-pressure steam
• The high-pressure steam is funnelled into the steam turbine
• The steam causes the turbine's blades to rotate
• The rotational energy of the turbine is transferred to a generator
• This movement causes the generator to produce electricity

A vital concept here is the efficiency of energy conversion, which can be represented by the formula:

Where: \(\eta_t\) = Turbine Efficiency \(h1\) = Enthalpy of steam input in the turbine \(h2\) = Enthalpy of steam leaving the turbine \(h3\) = Enthalpy of steam if expanded isentropically (ideal expansion)

High turbine efficiency results in less waste, making the process more cost-effective and reducing environmental impact, given that less fuel is required to generate the same amount of power.

Role of Steam Turbines in Marine and Industrial Sectors

In addition to their significant role in power generation, steam turbines also find applications in the marine and industrial sectors. In the marine industry, steam turbines were traditionally used as the main propulsion systems for ships, particularly for large ocean-going vessels. This was mainly due to their high power output, reliability, and capability to use heavy fuel oil. The steam generation system, steam turbines, and the attached propellers formed a unified propulsion system.

To illustrate the application of steam turbines in the marine industry, consider the following steps:

• Water is heated in a boiler, producing high-pressure steam
• This steam is directed into a steam turbine
• The steam causes the turbine to rotate
• This rotation is transferred to a propeller shaft via reduction gearing, propelling the ship

It's important to note that modern ships have largely shifted towards diesel engines and gas turbines for propulsion due to their higher efficiency and lower maintenance requirements. However, steam turbines are still in use in some niche applications, such as LNG (liquefied natural gas) carriers, where steam can be generated using the excess heat from the gas.

In the industrial sector, steam turbines are employed in numerous applications. They are used in oil and gas industries for driving compressors, pumps, and other large-scale mechanical equipment. Industries that use steam in their processes, such as chemical and petrochemical industries, often employ steam turbines in a co-generation configuration, where the steam used in industrial processes is subsequently sent to a turbine to generate electricity. This allows these industries to maximise the efficiency of their energy use.

Here, the efficiency of these steam turbines also plays a crucial role. The same efficiency formula is applicable:

High efficiency in these applications not only translates to cost savings, but it also reduces the environmental impact by decreasing the industry's overall energy demands.

Comprehensive Guide on Classification of Steam Turbine

As you delve deeper into the intricacies of steam turbines, understanding the various types of turbines becomes essential. The classification of steam turbines is based on several different criteria, though, for the purpose of this explanation, the focus will primarily be on steam flow direction and compounding arrangement.

Classification Based on Steam Flow Direction

Steam turbines can be categorised based on the flow direction of steam through the turbine. This distinction is made using three primary classifications: axial flow, radial flow, and tangential or helical flow turbines.

For some steam turbines, the flow direction can be a combination of axial and radial flows, subject to the design requirements. Each type of steam turbine has its advantages and disadvantages, making them suitable for specific applications. Therefore, the choice of the steam turbine is driven by the desired output, efficiency, size constraints, cost factors, and the particular application it is designed for.

Classification According to Compounding Arrangement

'Compounding' in steam turbines refers to the method by which energy from the steam is extracted in stages or 'compounds'. The necessity to 'compound' arises from the sheer amount of energy in the high-pressure steam, which results in excessively high rotational speed if extracted in one stage. Compounding allows the pressure energy to be translated into velocity throughout a series of stages, maintaining a manageable rotational speed. Based on this, steam turbines can be categorised into two key types: velocity compounded and pressure compounded turbines.

The diagram below depicts a simple representation of a velocity compounded turbine. The fixed blades (stator blades) are shown in green and they guide the steam onto the moving blades (shown in blue). Here, N represents the nozzle where steam gains its initial velocity.

 --------------     --------------   --------------
 |            |     |            |   |            |
 |     N      |     |     N      |   |     N      |
 |            |     |            |   |            |
 --------------     --------------   --------------
      || || ||         || || ||        || || ||
   ____||_||_||________||_||_||_______||_||_||____
  |    __    __    __    __    __    __    __    |
  |   |  |  |  |  |  |  |  |  |  |  |  |  |  |   |
  |   |  |  |  |  |  |  |  |  |  |  |  |  |  |   |
  |   |__|  |__|  |__|  |__|  |__|  |__|  |__|   |
  |______________________________________________|
              (Moving Blades)

         (Fixed Blades- in green)
  _________     _________     _________
 |         |   |         |   |         |
 |    __    |   |    __   |   |    __   |
 |   |  |   |   |   |  |  |   |   |  |  |
 |   |  |   |   |   |  |  |   |   |  |  |
 |   |__|   |   |   |__|  |   |   |__|  |
 |_________|   |_________|   |_________|

In terms of efficiency, pressure compounded turbines generally have the upper hand, maintaining high efficiency even at partial loads. However, velocity compounded turbines tend to be simpler in construction, and their ability to handle higher velocities can provide greater power output.

Therefore, the selection between velocity compounded and pressure compounded depends on the specific requirements of speed, power output, efficiency, cost, among other factors.

Understanding Compounding of Steam Turbine

The compounding of a steam turbine is the method used to control and utilise the energy from the steam and convert it into mechanical work. Essentially, it's the system by which high kinetic energy of steam is efficiently converted into rotational force to drive a generator or mechanical equipment. Two key compounding methods in steam turbines are velocity compounding and pressure compounding. A third option, combined compounding, interfaces these two compounding methods for optimized performance. Let's delve deeper into these types.

Velocity Compounding: Steam Turbine

In a velocity compounded steam turbine, also known as the Curtis turbine, the steam expands in a series of fixed blades and moving blades, alternatively arranged. Firstly, the steam expands in a nozzle, achieving high kinetic energy. This high-speed steam is then directed over one set of moving blades, causing the blades to rotate and deflecting the steam in the opposite direction. Despite losing some of its kinetic energy to the blades, the steam still maintains a significant energy level.

Now, instead of letting the steam exhaust and waste the remaining energy, it's redirected using a set of fixed blades onto the next set of moving blades. Each set of fixed blades acts as a guide, directing the steam into the next set of moving blades, extracting more energy at each stage. This alternating setup of fixed and moving blades continues until almost all of the steam's energy is exhausted.

This method provides high-speed rotation as energy is extracted stage by stage, maintaining the steam pressure almost constant throughout the turbine. However, it can be mechanically complex and suffers efficiency losses due to high blade speeds and subsequent friction.

Pressure Compounding: Steam Turbine

In a pressure compounded steam turbine, often referred to as the Rateau turbine, steam expands in a series of nozzles instead of fixed blades. Each nozzle provides a staged pressure drop and releases high-speed steam onto its own dedicated set of moving blades. The pressure compounded turbine, therefore, has an independent nozzle and moving blade set for each stage, extracting energy from the steam.

The pressure in the steam drops down in stages, making the process energy efficient and reducing the stress on the turbine blades. This compounding method is typically used in larger turbines where maintaining high efficiency at all lengths and loads is crucial. Despite its improved efficiency, the design and construction of pressure compounded turbines are complex and costly compared to their velocity compounded counterparts.

Combined Compounding: Steam Turbine

Notably, it's possible to combine both velocity and pressure compounding to form a combined compounded steam turbine. This turbine uses both the Curtis stage (velocity compounded stage) and the Rateau stage (pressure compounded stage). The high-pressure steam first enters the Curtis stage, where it partially expands and imparts kinetic energy to the moving blades. The still energetic steam then enters the Rateau stages, where it undergoes further expansion and energy extraction.

The advantage of the combined compounded turbine lies in its versatility. The Curtis stage can handle the initial high-pressure steam and high velocity, obtaining a high power output, while the Rateau stages extract energy more efficiently at lower pressures. This combination provides a balance between power output and operational efficiency, making combined compounded turbines suitable for a wide range of applications.

Thus, the selection of compounding method depends primarily on the balance between power output, operational efficiency, cost implications, and specific application requirements. It's always crucial to understand the fundamental principles of each type, making it easier to decide which best fits a specific context.