These types of turbines are suitable for a wide range of water heads, normally from 10 to over 300 meters. This type of machine can also be used where the water head varies, even with percentages exceeding 50%, of nominal, for lake level fluctuation or in an aqueduct due to the effect of the growing head losses from capacity used. The capacity may also change and still keep a high level of working efficiency and reliability. However, it is advisable not to go down to levels below 35-40% of the designed machine values. The Francis turbine may also be produced with a single runner or in some cases with double outlet runner, opposite to each other with a single spiral casing. In this case, the wheel will be symmetrical and each half is calculated at 50% of the total machine carrying capacity. This makes it possible to increase the speed of the machine with evident cost saving also for the generator.