A machine that improved the impulse system significantly was invented by Samuel Knight, a millwright from California, in 1866. He was inspired by the high pressure jet systems that were used to blast away rocks and soil in hydraulic mining operations in the gold fields. He designed a wheel with buckets around its rim that could capture the energy of a jet of water that had been accelerated by falling from a great height (hundreds of vertical feet) through a pipe or penstock. This type of turbine is called an impulse or tangential turbine because the water hits the buckets at a tangent to the wheel's rotation. The water's velocity, which is about twice as fast as the speed of the bucket's edge, changes direction by 180 degrees in the bucket and then leaves the runner at a low speed.
Impulse turbines are well suited for high head applications, where the water pressure is much greater than the atmospheric pressure. They can operate with a high efficiency over a wide range of flow rates and heads. However, they are not very effective for low head applications, where the water pressure is close to the atmospheric pressure. In such cases, the water jet loses a lot of energy due to air resistance and splashing.
To overcome this limitation, another type of turbine was developed that used the water pressure itself to drive the runner. This is called a reaction or radial turbine. In this type of turbine, the water enters the runner through a nozzle that reduces its cross-sectional area and increases its velocity. The water then passes through curved blades that change its direction and pressure. The pressure difference between the inlet and outlet of the blades creates a force that pushes the runner in the direction of rotation. The water leaves the runner at a lower pressure and velocity than it entered.
Reaction turbines are more efficient than impulse turbines for low head applications, where the water pressure is relatively high and constant. They can also operate with a higher specific speed, which means they can produce more power with a smaller runner diameter. However, they are less efficient than impulse turbines for high head applications, where the water pressure varies significantly along the runner. They also require more careful design and maintenance to prevent cavitation, which is a phenomenon where bubbles form and collapse in the water due to low pressure.
There are many types and variations of impulse and reaction turbines, depending on the specific application and design. Some of the most common ones are:
Pelton turbine: This is an impulse turbine that uses one or more jets of water to strike buckets on the rim of a wheel. The buckets have a split in the middle that deflects the water to the sides, creating a large impulse force. Pelton turbines are widely used for high head and low flow applications, such as mountainous regions.
Turgo turbine: This is an impulse turbine that is similar to the Pelton turbine, but uses a smaller wheel and larger jets of water. The water strikes the buckets at an angle, creating a smaller impulse force but a higher specific speed. Turgo turbines are suitable for medium head and medium flow applications.
Cross-flow turbine: This is an impulse turbine that uses a cylindrical drum with curved blades on its surface. The water flows through the drum twice, first from the outside to the inside, and then from the inside to the outside. The water transfers its momentum to the blades as it changes direction. Cross-flow turbines are used for low head and high flow applications, such as irrigation systems.
Francis turbine: This is a reaction turbine that uses a spiral-shaped casing to guide the water to the runner. The runner has radial blades that curve backward. The water enters the runner radially and leaves it axially. The water pressure decreases as it passes through the blades, creating a reaction force that drives the runner. Francis turbines are widely used for medium head and medium flow applications, such as hydroelectric power plants.
Kaplan turbine: This is a reaction turbine that is similar to the Francis turbine, but uses axial blades that can be adjusted to optimize the performance. The water enters and leaves the runner axially. The water pressure decreases as it passes through the blades, creating a reaction force that drives the runner. Kaplan turbines are used for low head and high flow applications, such as rivers and canals. 061ffe29dd