Power Plant Engineering

372 POWER PLANT ENGINEERING

Comparing the equations (a) and (b) the extra work done per kg of water due to draft tube is

given by

∆W = W 2 – W 1 =

2

2

d

f

HhV

g



−−



2

2

H Vd

g



−



=

22

2

VVcd

g

−

• hf

=

22

VVcd

g

−

if hf = 0. ...(c)

The head on the turbine (H) remains same as before, W increases with the decrease in velocity Vd.
The velocity Va can be decreased by increasing the outlet diameter of the draft tube.

The outlet diameter of the draft tube can be increased either by increasing the height of the draft
tube or by increasing the angle of draft tube as shown in Fig. 11.27.
The increase in height for increasing the diameter without increase in angle is limited by the
pressure at the outlet of the runner (at point C). This will be discussed later in detail.
An increase in draft tube angle (2a) for increasing the diameter without increase in height is
limited by the losses in the draft tube.

The flow in the draft tube is from low
pressure region to high pressure region. In
such flow, there is a danger of water parti-
cles separating out from main stream and
trying to flow back resulting in formation of
eddies which are carried away in main stream
causing losses. The maximum value of a is
limited to 4. The gain in work by increasing
an angle a above 4 will be lost in eddy losses
and separated flow.

Sometimes in order to decrease the length of draft tube, the diverging angle has to be made more
than 4° and under such cases to reduce the losses due to separation, the air is sucked from the inside
surface of the draft tube.

Prof. Ackeret has shown that the efficiency of draft tube was raised from 50 to 80% by air
sucking process. However, water equal to 5°Ia of the total quantity is also withdrawn with the air. The
work done by the draft tube is further increased by decreasing hf. This is generally done by proper lining
the draft tube and by proper designing the shape and size of the draft tube.

The efficiency of the draft tube is given by

η = 2

c /2

W

Vg

∆

=

22

2

cd

c

VV

V

−

=

2

1 d

c

V

V



−



The chief advantages of using draft tube are listed below :

(1) It allows the turbine to be set above the tailrace water level where it is more accessible and yet

does not cause any sacrifice in the head of turbine. It also prevents the flooding of generator and other

equipment during flood period when the tailrace, water height goes up.

(2) It converts part of the velocity energy of the water leaving the turbine into the pressure energy

and increases the overall efficiency of the plant.

d

α

dd

α α

D D

D 1

D 1

D 1 > D D 1 > D

α 1 > α

Fig. 11.27