The Three Types Of Impulse Turbine Engineering Essay

A turbine is a rotary engine that extracts energy from a fluid or air flow. The simplest turbines have one moving part, a rotor assembly, which is a shaft with blades where the moving fluid acts on the blades, or the blades react to the flow, so that they rotate and impart energy to the rotor. Examples of early turbines are windmills and water wheels. Turbines usually have a casing around the blades that contains and controls the working fluid. Working fluid contains kinetic energy (velocity head) and potential energy (pressure head) and these working fluids may be compressible or incompressible. A compressor or pump is a device similar to a turbine but operating in reverse.

The turbines produce almost all electric power on Earth. Most jet engines rely on turbines to supply mechanical work from their working fluid and fuel as do all nuclear ships and power plants. Aircraft engines also use the turbine powered by their exhaust to drive an intake-air compressor, a configuration known as a turbocharger (turbine supercharger). Turbines could also be used as powering system for a remote controlled plane that creates thrust and lifts the plane of the ground. They are as small as soda can, yet still strong enough to move objects with a weight of 100kg.

THE TURBINE PROCESS

If high-velocity steam is blown on to a curved blade and the steam direction changes as it passes across the blade. The steam will impart a force to the blade as a result of its change in direction across the blade. Now if the blades were free, it would move off in the direction of the force. The principle of steam turbine is where a number of blades were fixed around the circumference of a disc and the disc is free to rotate on a shaft. Steam is then blown across the blades which cause the disc rotates. To increase the rigidity of the blades, the top of the blades are connected together. By means of the nozzles, the high pressure steam is made to give up some of its energy to produce a large increase in kinetic energy of the steam. The steam thus leaves the nozzles at a high velocity. It passes from the nozzles over the blades and thus the turbine disc rotates. The power is then generated at the shaft. The number of nozzles which are in use act as a load to the turbine and so the higher the load requires that more steam must be used to sustain the load. Therefore, more nozzles are put into the used.

The turbine described is a simple turbine which is also known as de Laval turbine. This type of turbine usually rotates at a very high speed and this high speed will produce a centrifugal force. This turbine is usually small in size and, hence produces small power output. Due to the high speed of rotation, a direct drive between drive between the turbine disc and external equipment is not generally possible. For this reason, a reduction gear box is installed between and turbines of the turbine disc and external equipment.

A problem in steam turbine development has been to reduce the speed of rotation and at the same time to make full use of the energy in the steam, thus larger size and higher power output is produce. There are two basic types of turbines which is the impulse turbine and the reaction turbine.

THE IMPULSE TURBINES

These turbines change the direction of flow of a high velocity fluid jet and the resulting impulse spins the turbine and leaves the fluid flow with diminished kinetic energy. The pressure in the fluid of the turbine rotor blades remains constant. Before reaching the turbine the fluid’s pressure head is changed to velocity head by accelerating the fluid with a nozzle. Impulse turbines do not require a pressure casement around the runner since the fluid jet is prepared by a nozzle prior to reaching turbine. The transfer of energy for impulse turbines uses the Newton’s second law. There are three different types of impulse turbines which are the

Velocity compounding turbine

Pressure compounding turbine

Pressure-velocity compounding turbine

THE REACTION TURBINES

These turbines develop torque by reacting to the fluid’s pressure or weight. The pressure of the fluid changes as it passes through the turbine rotor blades. The reaction turbines require a pressure casement to contain the working fluid as it acts on the turbine stage or the turbine must be fully immersed in the fluid flow (wind turbines). The casing contains and directs the working fluid and, for water turbines, maintains the suction imparted by the draft tube. Multiple turbine stages may be used to harness the expanding gas efficiently for compressible working fluids. The transfer of energy in the reaction turbine uses the Newton’s third law.

Purple – Moving blades

Blue – Velocity

Red – Pressure

Brown – Fixed blades

THE VELOCITY COMPOUNDING TURBINE IN IMPULSE TURBINES

Steam is expanded in a single row or nozzles in this type of turbine. The high velocity steam leaving the nozzles passes on the first row of the moving blades where its velocity is only partially reduced. Then, the steam leaving the first row of moving blades passes into a row of fixed blades mounted in the turbine casing and this row of fixed blades serves to redirect the steam back to the direction of motion such that it is suitable for entry to the second row of moving blades. The steam velocity reduces partially in the second row of the moving blades. A slower turbine is resulted due to only part of the velocity of the steam is used up in each row of the blades.

Blue – Velocity

Red – Pressure

Green – Nozzle

Purple – Moving blades

THE PRESSURE COMPOUNDING TURBINE IN IMPULSE TURBINES

The steam enters a row of nozzles where its pressure is only partially reduced and its velocity is increased in this type of turbine. The high velocity steam passes to a row of moving blades where its velocity is reduced. The pressure is again partially reduced and its velocity is again increased when the steam passes into a second row of nozzles. The high velocity steam is then passed to a second row of moving blades where its velocity is again reduced. Next, the steam then passes into a third row of nozzles and so on. All pressure drops occur in the nozzles but the pressure remain constant in each turbine stage. The turbine run slower since steam velocities will not be so high due to only part of the pressure drop occurs in each stage. All stages, however, are coupled to the same shaft, with the result that there is no loss of output.

Green – Nozzle Purple – Moving blades

THE PRESSURE-VELOCITY COMPOUNDING TURBINE IN IMPULSE TURBINE

A combination of the pressure compounding turbine and the velocity compounding turbine will give a pressure-velocity compounding turbine. In this type of turbine, the steam is partially expanded in a row of nozzles where its velocity is increased. The steam then enters a few rows of velocity compounding turbine and then to a second row of nozzles where its velocity increases. The steam then enters another few rows of velocity compounding turbine and so on. All the pressure at the nozzles decreases.

Generally, the diameter from the inlet to the exhaust increases in all multistage turbines. This is because the specific volume increases as the pressure of steam falls. A greater area will be required to pass the steam for continuity of mass flow and this can be done by either increasing the diameter of the turbine discs or increasing the height of the blades. A greater area will be required to pass the steam in order to preserve the mass flow if there is depreciation in velocity.

Blue – Velocity

Red – Pressure

Green – Nozzle

Brown – Fixed blades

Purple – Moving blades

DIFFERENCES OF THE TURBINES

There are many differences that can be stated between the 3 types of impulse turbine. The 3 types of impulse turbine are the:

Velocity compounding

Pressure compounding

Pressure-velocity compounding

The differences between these turbines can be classified in terms of:

Structure of the turbine

The process of the turbine

The pressure change in the turbine

The velocity change in the turbine

Structure of the turbine

The structure of the velocity compounding turbine is it consists of a turbine then to a moving blade and a fixed blade. The structure then continues with a second row of moving and fixed blades. The structure of the pressure compounding turbine is it starts from a turbine and then to a moving blade then to a second row of turbine and moving blades and so on. Besides that, the structure of the pressure-velocity compounding turbine is the combine of both of the structure of the velocity compounding turbine and pressure compounding turbine.

The process of the turbines

High velocity steam from the nozzles passes thru the moving blades then to the fixed blade and the second row of moving and fixed blade in the velocity compounding turbine. In the pressure velocity turbine, the high velocity steam from the nozzles passes thru a moving blades and the low velocity of steam enters another turbine and then to a second row of moving blades and so on. Whereas in the pressure-velocity compounding turbine, the steam from the turbine enters a row of moving blades then a fixed blade and then another row of moving blades. The steam finally then enters another turbine and the process is repeated.

The pressure change in the turbine

The pressure in the velocity compounding turbine remains constant throughout. In the pressure compounding turbine, the pressure decreases partially when it passes the rows of turbine. Furthermore, the pressure in the pressure-velocity compounding decreases partially then it passes thru the row or turbine and remains constant until the second row of turbine where the pressure decreases partially again.

The velocity in the turbine

The steam velocity reduces partially in the rows of the moving blades in the velocity compounding turbine. A slower turbine is resulted due to only part of the velocity of the steam is used up in each row of the blades. Whereas in the pressure compounding, the velocity decreases partially when its pass thru the blades but increases back when passing the nozzles. Finally, in the pressure-velocity compounding turbine, the velocity decreases in the turbine and remains constant when passing the blades. The velocity is again decreased when passes thru a second row of turbine.

CONCLUSION

The steam turbine has greatly improved the energy conversation in our daily lives. There are still future developments oh the steam turbines in order to improve efficiency. Development are now developing turbine which requires a smaller input but produces a bigger output.