Axial Flow Reaction Turbines Propeller and Kaplan Turbines

Axial Flow Reaction Turbines Propeller and Kaplan Turbines

• The propeller and kaplan are propeller shaped axial flow reaction turbines in which the water flows parallel to the axis of the shafts.
• These turbines were developed to meet the requirement of using available large quantity of water at low heads (upto 80 m).
• In the year 1910, Prof. V. Kaplan developed a propeller turbine having fixed runner blades of aerofoil section fixed to the hub of the shaft. Subsequently, Kaplan modified the design of propeller turbine in the present form of Kaplan turbine which has adjustable blades against the non-adjustable blades.
• The runner of a kaplan turbine is shown in Figure A.

Figure A

• It has a vertical hollow shaft which is enlarged at the bottom in the shape of propeller called hub or boss.
• The vanes are fixed on the hub which acts as the runner of Kaplan turbine.
• The runner vanes are 3 to 6 in numbers (upto 8) made of stainless steel. These vanes are adjustable and can be rotated about pivots fixed to the hub of runner.
• These vanes are adjusted automatically by governor mechanism according to the load on the turbine. Due to proper adjustment of runner vanes during the running of turbine, Kaplan turbine gives high efficiency over wide range of load conditions.
• Other constructional details of a Kaplan turbine shown in Figure B are similar to Francis turbine Kaplan turbine has the scroll casing, guide mechanism and draft tube similar to Francis turbine except that it has different type of runner.
• Water from penstock enters the scroll casing into guide vanes. The water from guide vanes turns through 90° and enters the runner vanes axially.

Figure B

• In case of Francis turbine, the water enters the runner radially while in Kaplan turbine the water strikes the runner axially.
• Kaplan turbine has very few moving vanes compared to Francis turbine. It reduces the friction resistance due to reduced surface contact of water with vanes.
• Kaplan turbines with sloping guide vanes taken the place of Francis turbine for certain medium installations The difference between Francis and Kaplan turbines are appended below :

Sr. No.	Francis turbine	Kaplan turbine
1.	Water enters the runner radially.	Water enters the runner axially.
2.	Number of blades used are generally.	Number blades used are generally to 3 to 8.
3.	Frictional resistance is high.	Friction resistances less due to reduced number of vanes.
4.	Only guide vanes are adjustable.	Guide vanes and moving vanes are both adjusted.
5.	High efficiency is obtained only at high loads.	High efficiency is obtained even at part load.
6.	It is big in size.	It is compact in size.
7.	Specific speed ranges from 50-250.	Specific speed range from 250-850.