Environmental Engineering Reference
In-Depth Information
and then flows out again axially, which causes a redirection of the flow. In con-
trast to the Kaplan turbine the Francis turbine (Fig. 8.11) runner blades are not
movable. The flow is only regulated via the guide vanes; slow and fast rotating
turbine wheels are differentiated here (so-called low-speed and high-speed run-
ners). In larger machines with higher head, the water flows through a spiral to the
guide vanes. In smaller machines the guide vanes are situated in a simple turbine
pit.
In general, high rotational speed of the runner has to be aimed for, as this re-
sults in low torques at the turbine axis and thus smaller machine dimensions; thus
the turbine costs, and in some cases the costs of other power station components,
can be reduced. Francis turbines can normally be run from 40 % of their maxi-
mum power (Fig. 8.9). In the case of high-speed runners, high efficiencies only
occur at about 60 % of the maximum flow.
Fig. 8.11 shows a run-of-river power station with a Francis-shaft turbine as an
example. The adjustment of the guide vanes is done via a rotating shaft behind the
turbine shaft that enables opening and closing the vanes.
Older small hydroelectric power stations often have Francis shaft turbines at
heads as low as 2 m (see Fig. 8.8). If these plants were newly built nowadays,
double regulation Kaplan tubular turbines or S-turbines would be used. With their
very good efficiency curve over a broad range of discharges, they guarantee an
optimum exploitation of energy. If old plants are reactivated, the entire in and out-
flow areas would have to be adjusted. The work involved in this is often so ex-
pensive that Francis machines are put in again, although they have a slightly less
favourable efficiency curve.
Fig. 8.11 Power station with a vertical Francis turbine (see /8-10/)
Pelton turbines. The Pelton turbine - an impulse turbine - has a runner (Pelton
wheel) with fixed buckets. It is regulated by one or several nozzles with spear-
type valves that control the flow and direct the water jet tangentially to the wheel
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