How does a Crossflow Turbine work? | An-Overview

Crossflow turbine is a specialized impulse turbine designed for low and high heads, mini and micro-hydroelectric projects. A crossflow turbine is a low head turbine that operates at an extremely high flow rate. Therefore, it is a low-speed turbo engine. It was developed in 1903 by Anthony Mitchell.

Parts of a Crossflow Turbine:

1) Casing

Crossflow turbine casing made of structural steel. It is strong, resistant to impacts and frost. If the water has a high content of abrasive substances (e.g., sand, silt) or the actual composition of the water is considered aggressive (e.g., seawater, acidic water), all parts of the turbine associated with water are made of stainless steel.

2) Guide Vans

In a crossflow turbine, the working water guides by two force-balanced profile guide vans. The vans distribute the water beam and balance it, allowing it to easily enter the runner‌.

The vans are properly mounted on the turbine casing and serve as a closing device if the turbine’s head is low. There is no need to use a shutoff valve between the pressure piping and the turbine. Two guide vans fit with independently extended arms, to which automatic or manual control attaches. Guide vans are housed in high-resistance slide bearings that require no maintenance. When the turbine closes by gravity through additional loads on the ends of the arms.

3) Runner

The runner is a vital component of a turbine. It consists of blades made of profile steel polished by a well-proven method. Based on actual hydraulic data, structural steel or stainless steel uses for their construction. Both ends of the blade are attached to the runner disc and welded to the runner’s intermediate disc after a specific process. Depending on the size, the runner has 37 blades. Simply bent blades generate only small axial forces and therefore do not require reinforced axial bearings with complex fittings and lubrication. Multiple discs support the blades of wide runners. The runners must be carefully balanced and subjected to crack detection control prior to the final installation of the turbine.

4) Bearings

Crossflow turbine equips with self-adjusting roller bearings that have many advantages such as low rolling resistance and simple handling. The design of the bearing housing prevents water leakage into the bearings and the contact of the lubricant with the working water. In addition, the runner is centered on the turbine casing through the bearings.

5) Draft tube

Crossflow turbine is a free stream turbine similar to the Pelton turbine. However, in the case of medium or low heads, it is possible to place the draft tube to use the entire head. However, the water column in the draft tube must be controlled. This ensures by the control air valve, which affects the suction pressure in the turbine casing. Therefore, turbines with suction heads of 1 to 3 m may use favorably without any cavitation hazards.

 6) Shaft

The axial shaft is a part of the main structure of a crossflow turbine. In a set of cross-flow turbines, the same shaft usually has the same diameter for two different rotors but different lengths. Connecting the rotating shaft to the generator converts the mechanical motion into electrical energy.

The rotor usually rotates slower than the generator. Therefore, a gearbox or belt drive can be used to change speed. For large hydroelectric, the intermediate shaft is also used to facilitate the balance of radial forces acting on the components.

Advantages of Crossflow Turbines:

1) One of the advantages of a crossflow turbine is that it has self-priming property.

2) It is less expensive.

3) It has good control

4) As the water goes through the runner twice, it provides extra capacity.

5)  Its good thing is that when the water leaves the runner, it also helps to clean the runner from small debris and pollution.

6) The maximum capacity of the cross-flow turbine is somewhat less than that of the Kaplan or Franchise turbine.