Steam Turbine Working and Types

What is Steam Turbine?

Steam Turbine Working and Types

Steam turbine comes under the classification of a mechanical machine that isolates thermal energy form the forced steam and converts this into mechanical energy. As the turbine produces rotatory motion, it is most appropriate for the operation of electrical generators. The name itself indicates the device is driven by steam and when the vaporous stream flows across the turbine’s blades, then the steam cools and then expands thus delivering almost the energy that it has and this is the continual process.

Working of Steam Turbines

As the name suggests, steam is required for the running of a steam turbine. This steam is produced by boiling water using a heat source which could be nuclear, gas or coal. This steam is then admitted towards the blades of the shaft. The blades of the turbine is in the shape of an airfoil. This shape aids in the rotation of the turbine as it provides a lift force. The turbine has a set of alternating fixed nozzle and blade arrangement. The nozzle is stationary and its area decreases from one side to the other, thereby increasing the velocity of steam flowing through it. This helps in increasing the speed of the blades as well. The turbine is connected to a generator which has a magnet that is wound by copper coils. The rotation of the magnet produces a strong magnetic field and this magnetic field causes the flow of electrons and thus electricity is generated.

Types of Steam Turbines

Steam turbines may be classified into different categories depending on their construction, working pressures, size and many other elements. But there are two basic types of steam turbines which are called impulse and reaction turbines. There are other types of steam turbines that are actually derivatives of these two main types.

1. Impulse Turbine

In this type of turbine, the superheated steam is projected at high velocity from fixed nozzles in the casing. When the steam strikes the blades (sometimes called buckets), it causes the turbine shaft to rotate. The high pressure and intermediate pressure stages of a steam turbine are usually impulse turbines. The entire pressure drops of steam take place in stationary nozzles only. Though the theoretical impulse blades have zero pressure drop in the moving blades, practically, for the flow to take place across the moving blades, there must be a small pressure drop across the moving blades also.

In impulse turbines, the steam expands through the nozzle, where most of the pressure potential energy is converted to kinetic energy. The high-velocity steam from fixed nozzles impacts the blades changes its direction, which in turn applies a force. The resulting impulse drives the blades forward, causing the rotor to turn. The main feature of these turbines is that the pressure drop per single stage can be quite large, allowing for large blades and a smaller number of stages. Except for low-power applications, turbine blades are arranged in multiple stages in series, called compounding, which greatly improves efficiency at low speeds.

2. Reaction Turbine

In this type of steam turbine, the steam passes from fixed blades of the stator through the shaped rotor blades nozzles causing a reaction and rotating the turbine shaft. The low pressure stage of a steam turbine is usually a reaction type turbine. This steam having already expanded through the high and intermediate stages of the turbine is now of low pressure and temperature, ideally suited to a reaction turbine.

In reaction turbines, the steam expands through the fixed nozzle, where the pressure potential energy is converted to kinetic energy. The high-velocity steam from fixed nozzles impacts the blades (nozzles), changes their direction, and undergoes further expansion. The change in its direction and the steam acceleration applies a force. The resulting impulse drives the blades forward, causing the rotor to turn. There is no net change in steam velocity across the stage but with a decrease in both pressure and temperature, reflecting the work performed in the driving of the rotor. In this type of turbine, the pressure drops take place in a number of stages, because the pressure drop in a single stage is limited.

The main feature of this type of turbine is that in contrast to the impulse turbine, the pressure drop per stage is lower, so the blades become smaller and the number of stages increases. On the other hand, reaction turbines are usually more efficient, i.e. they have higher “isentropic turbine efficiency”. The reaction turbine was invented by Sir Charles Parsons and is known as the Parsons turbine.

In the case of steam turbines, such as would be used for electricity generation, a reaction turbine would require approximately double the number of blade rows as an impulse turbine, for the same degree of thermal energy conversion. Whilst this makes the reaction turbine much longer and heavier, the overall efficiency of a reaction turbine is slightly higher than the equivalent impulse turbine for the same thermal energy conversion.