Crossflow turbine is a specialized impulse turbine designed for low and high heads, mini and micro hydroelectric projects. Cross-flow 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.
Hydropower is considered to be one of the most desirable sources of electrical energy because it is environmentally friendly. Research in the field of small hydropower plants has been on the rise in recent times. Many types of small hydroelectric turbines include radial, axial and propeller type turbines. There are a lot of sites suitable for low head hydro turbines. Low-head hydro turbines include the Kaplan turbine, the axial inlet and axial outlet turbines, the cross-flow turbine. Nowadays, crossflow turbines are getting popularity in low head and small water flow rate installations. Because these turbines have simple deign and ease of construction at the power plant site.
Parts of a Crossflow Turbine:
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.
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.
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.
How does a Crossflow Turbine work?
The crossflow turbine consists of a drum-like rotor with a solid disc at each end and a gutter-shaped rotor blade connecting the two discs. Water directs into the rotor by an inlet guide, which is first pressurized by the rotor, transferring some of its velocity through the drum interior, and then from the turbine to the rotor at ambient pressure. The crossflow turbine then converts water flow and pressure into mechanical energy. This action reduces the very low pressure on the blades of the turbine, thereby increasing its efficiency. Most of the energy is collected by the upper blade (approximately 75%) and the remaining 25% by the lower blade.
Due to the angular momentum at which water flows into the turbine rotor blades, the kinetic energy of the water is converted into torque on the output shaft, which can be used to drive an electric generator in a small-scale hydropower system. The cross-flow turbine then turns on a generator, which is connected to an electric load to power a track.
Advantages and disadvantages of Crossflow Turbine
- One of the advantages of a crossflow turbine is that it has self-priming property.
- It is less expensive than reaction turbine.
- It has good control
- As the water goes through the runner twice, it provides extra capacity.
- Its good thing is that when the water leaves the runner, it also helps to clean the runner from small debris and pollution.
- The maximum capacity of the cross-flow turbine is somewhat less than that of the Kaplan or Francis turbine.
- Difficulties in self-initiation
- Low power coefficient.