Steam turbine types

Steam design and types.

1.Condensing

The primary type of steam turbine is the condensing steam turbine, which has been used for large drive applications above a certain power rating limit (say, as a very rough indication, above 8 MW). These steam turbines exhaust directly to one or more condensers that maintain vacuum conditions at the discharge of the steam turbine. An array of tubes with cooling water condenses the steam into water (liquid) in the condenser.

The condenser vacuum occurs as the near-ambient cooling water condenses the steam (the turbine exhaust) in the condenser. Because a small amount of air is known to leak into the system when it is below atmospheric pressure, a relatively small compressor is usually used to remove non-condensable gases from the condenser. Non-condensable gases may include air, a small amount of the corrosion byproduct caused by the water-iron reaction and hydrogen.

A condensing steam turbine’s processes produce the maximum mechanical power and efficiency from the steam supply. However, the power output of condensing steam turbines is sensitive to ambient temperature. Condensing steam turbines are expensive, large, complex and less suitable for mechanical drive applications. Steam turbines, especially for small or medium-sized machines, leak steam around blade rows and out the end seals. When an end is at low pressure, as is the case with condensing steam turbines, air can leak into the system. The leakages cause less power to be produced than expected.

2.Back-pressure

Another steam turbine type is a back-pressure steam turbine, which is the most suitable equipment for mechanical-drive applications, such as the drivers of compressors or pumps. The term back pressure refers to steam turbines that exhaust steam at above atmospheric pressures. The discharge pressure is usually established by the specific application of the steam in a plant. Lower pressures are frequently used in small and large low-pressure (LP) applications, such as heating systems, and higher pressures are often used when supplying steam to industrial processes.

Industrial processes often include further expansion for other smaller mechanical drives, using small steam turbines for driving rotating equipment (for instance, lubrication oil pumps) that continuously runs for long periods. Significant mechanical power generation capability is sacrificed when steam is used at appreciable pressure rather than being expanded to vacuum in a condenser. Discharging steam into a steam distribution system at 10 Bar gauge (barg) can sacrifice around half the power that could be generated when the inlet steam conditions are around 50 barg and 420°C, typical of small and medium steam turbines.

Between the mechanical power output of a condensing steam turbine and the power and steam combination of a back-pressure steam turbine, essentially any ratio of power-to-heat output can be supplied. Back-pressure steam turbines can have many different back pressures, further increasing the variability of the power-to-heat ratio.

3.Reheat Steam Turbine

Reheat turbines are also used almost exclusively in thermal power plants. All turbines, that have high-pressure turbine and low-pressure turbines use a steam reheat between these stages. Reheat allows to deliver more of the heat at a temperature close to the peak of the cycle (i.e. thermal efficiency increases). This requires the addition of another type of heat exchanger called a reheater.  The use of the reheater involves splitting the turbine, i.e. use of a multistage turbine with a reheater.  It was observed that more than two stages of reheating are unnecessary, since the next stage increases the cycle efficiency only half as much as the preceding stage.

4.Extraction
Extraction type turbines are common in all applications. In some applications, when required, steam can be extracted from turbine before steam flowing through the last stage, named extraction turbine. As in back-pressure turbines, extracted steam can be used for many industrial processes or it can be used to improve the efficiency of thermodynamic cycle. The second case is usually known as the heat regeneration.