How Boilers Are Used in Power Generation: Turbines

March 21, 2014

In the first article in this series about how boilers are used for power generation we looked at their combustion system and how they extract stored energy from a fuel source. In this article we will take a closer look at their core function: boiling water into steam. It is the thermodynamic energy present in this steam that is ultimately used to power the turbines that generate the power.

What Are Steam Turbines?

Steam turbines are large-scale mechanical devices that are designed to convert the thermal energy from steam into kinetic energy which can then be used to do mechanical work. The mechanical work is focused on a rotating output shaft and this rotary motion is then in turn used to rotate an electrical generator. Though there are other ways to create electricity this is by far the most pervasive technique. It is estimated that about 90% of the United States’ electricity is created using steam turbines. Meanwhile, steam turbines account for about 80% of electricity worldwide.

The Thermodynamics of Steam Turbines

Steam turbines operate on the principle of thermodynamics. The superheat or dry saturated steam vapor enters the turbine after having been heated by the boiler at a high temperature and pressure. It then passes through a nozzle on the turbine and as it exits the nozzle it is directed at a high velocity towards the turbine’s blades, thereby turning them.

Steam Turbines and Generators

For electrical power generation the steam turbine will be attached to an electrical generator. The generator needs to rotate at constant synchronous speeds that vary depending on the frequency of the electrical station. The most common speeds are 3,000 RPM for systems that use 50 Hz and 3,600 RPM for systems that use 60 Hz.

A Brief History of Steam Turbines

The modern steam turbine was invented by Sir Charles Parsons in 1884 and it generated only 7.5 kW of electricity during its first iteration. Though it had a modest start, this invention soon revolutionized the world, allowing for a cost-effective, plentiful way to create electricity. In addition to its impact on power, the steam turbine also revolutionized marine transport. Part of the effectiveness of the steam turbine came from the fact that it could be easily scaled up in size and output. Within Parsons’ lifetime the steam turbine he created had already been scaled up by a factor of about 10,000 and could then produce up to about 50,000 kW in capacity.

Types of Steam Turbines

Steam turbines are used for a variety of applications. In addition to power generation, steam turbines are also used for process steam applications in refineries, plants, and other industrial settings as well as on steam-powered ships and locomotives. Due to this wide variety of applications, steam turbines vary and may be categorized in several different ways. Some of the most common ways include:

Blades and Stages – Turbines are comprised of blades and nozzles and the blades are often arranged to go through a series of stages. This series of stages is called compounding and helps to improve the efficiency of the steam turbine, especially at lower speeds. If the turbine is composed of fixed nozzles alternating with blades it is called an impulse turbine. If the turbine is composed of moving nozzles alternating with fixed nozzles then it is called a reaction turbine.

Pressure-Compounded Turbine – In a pressure-compounded turbine, fixed nozzles followed by a row of moving nozzles divide the pressure drop from the steam inlet and the resulting exhaust into several small drops. This type of turbine is also known as a Rateau turbine since Rateau was the inventor.

Pressure-Velocity Compounded Turbine – In a pressure-velocity compounded turbine there are several velocity-compounded impulse stages that consist of fixed nozzles followed by moving blades alternating with fixed blades. This arrangement divides the velocity drop into smaller drops. The velocity-compounded stage is often called a Curtis Wheel after its inventor.

Steam Supply and Exhaust Conditions – Turbines categorized by their steam supply and exhaust conditions include condensing, non-condensing, reheat, extraction and induction.

Condensing Turbines – Condensing turbines are the type most commonly found in use in electrical plants. Their exhaust steam is in a partially condensed state as it leaves a boiler.

Non-Condensing Turbines – Non-condensing turbines are commonly used in process steam applications. Their exhaust pressure is regulated by a valve to suit the needs of the particular steam process. This type of turbine may be found in pulp and paper plants, district heating units, desalination facilities, and refineries.

Reheat Turbines – Reheat turbines are also commonly found at power plants. Reheat turbines feature a steam flow which exits from the turbine and enters the boiler where additional superheat can be added.

Extraction Turbines – Extraction turbines feature steam released in various stages from the turbine to be used for industrial processes, or to be sent back to the boiler’s feedwater heater to improve the overall efficiency of the system.

Induction Turbines – Induction turbines add low pressure steam at an intermediate stage. This produces additional power.

Measuring Turbine Efficiency

A turbine’s efficiency is often measured based on what is called its isentropic efficiency. The isentropic process, also known as the constant entropy process, is when the entropy of the steam that enters the turbine is perfectly equal to the entropy of the steam that leaves the turbine. This would be considered an ideal steam turbine because there would be no entropy loss. However, an actual ideal turbine couldn’t exist, instead the ratio of how close it comes to this ideal output is said to be its isentropic efficiency. This efficiency may range anywhere from 20% to 90% depending on the turbine and its application.

A fuel source is combusted to free the store energy. That energy is then used to heat water into steam. The thermodynamic energy in steam is then used to power a turbine. The turbine is connected to a generator and the generator creates the electricity. Though the boiler itself doesn’t generate the power, boilers are nevertheless a fundamental, indispensable part of the power generation process.