What is Compounding of steam turbines?

Combining steam turbines is a method of extracting steam energy in multiple stages rather than in a single stage of steam turbines. Compound turbines have multiple stages, with more than one set of nozzles and rotors.They are arranged in series and are either keyed to a common shaft or fixed to the housing.The result of this arrangement allows steam pressure or jet velocity to be absorbed by the turbine in multiple stages.Compound steam turbines are used to reduce rotor speed to achieve optimum operating revolutions per minute. The steam produced in the boiler has a sufficiently high enthalpy when superheated.In all turbines, the blade speed is directly proportional to the speed of steam passing through the blades.

Now, if the full energy of the steam is extracted in one stage, i.e. if the steam expands from boiler pressure to condenser pressure in one stage, then its velocity will be very high.Consequently, the speed of the rotor (the blades are keyed) can reach around 30,000 rpm, which is too high for practical use due to the very high vibrations.Also, at such high speeds, the centrifugal force is very strong and can damage the structure.Therefore, compounding is required.The high velocity steam hits only a single ring of the rotor, resulting in 10% to 12% steam wastage.To overcome the waste of steam, a combination of steam turbines is used.

Types of steam turbines

1.Impulse:There is no change in pressure when steam passes through the moving blades.Only the steam flow rate changes.

2. Reaction:As steam flows over the moving blades, both pressure and velocity change.

Composite types

In impulse turbines, recombination can be achieved in the following three ways:

1.Speed Composite.

2.Pressure compounding.

3.Pressure speed compound.

In a reaction turbine, compounding can only be achieved by pressure compounding.