8.1.1 Classification

Turbines can be crudely classified as high-head, medium-head, or low-head machines, as shown in the table below.  

Electricity generation usually requires a shaft speed as close as possible to 1500rpm to minimize the speed change between the turbine and the generator. Since the speed of any given type of turbine declines with head, low-head sites need turbines that are inherently faster under a given operating condition.

Turbines are also divided by their principle of operation and can be either impulse or reaction turbines.

The rotor of the reaction turbine is fully immersed in water and is enclosed in a pressure casing. The runner blades are profiled so that pressure differences across them impose lift forces, just as on aircraft wings, which cause the runner to rotate.

In contrast an impulse turbine runner operates in air, driven by a jet (or jets) of water.

There are 3 main types of impulse turbine in use:  the Pelton, the Turgo, and the Crossflow (or Banki) turbines. The two main types of reaction turbine are the propeller (with Kaplan variant) and Francis turbines.

The approximate ranges of head, flow and power applicable to the different turbine types are summarised in the chart below (up to 500kW power). These are approximate and depend on the precise design of each manufacturer.

Impulse and Reaction Turbines

Turbine Type		Head Classification
	High (>50m)	Medium (10-50m)	Low (<10m)
Impulse	Pelton	Crossflow	Crossflow
	Turgo	Turgo
	Multi-jet Pelton	Multi-jet Pelton
			Francis (open-flume)
Reaction		Francis (spiral case)	Propeller
			Kaplan

 

Head-flow ranges of small hydro turbines