1. Draft tube is used to increase the pressure from
the low turbine exit pressure to the pressure of
the surrounding to which the fluid is rejected.
2. The draft tube is pipe of gradually
increasing area which connects the
outlet of the runner to the tail race. It is
used to discharging water from the exit
of the turbine to the tail race. This pipe
of gradually increasing area is called
Draft tube. One end of the draft tube is
connected to the outlet of the runner
while the other end is sub-merged
below the level of water in the tail race.
3. PURPOSES OF DRAFT TUBE
1. It permits a negative head to be established at the outlet
of the runner and thereby increase the net head on the
turbine. The turbine may be placed above the tail race
with out any loss of net head and hence turbine may be
inspected properly
2. It converts a large proportion of the kinetic
energy(V2
2/2g) rejected at the outlet of the turbine into
useful pressure energy. Without the draft tube, the
kinetic energy rejected at the outlet of the turbine will go
waste to the tail race.
4. Hence by using draft tube, the net head on
the turbine increases. The turbine developed
more power and also the efficiency of the
turbine increases.
If a reaction turbine is not fitted with a draft
tube, the pressure at the outlet of the runner
will be equal to atmospheric pressure. The
water from the outlet of the runner will
discharge freely into the tail race. The net
head on the turbine will be less than that of
reaction turbine fitted with a draft tube.
Also Without the draft tube, the kinetic energy
rejected at the outlet of the turbine will go
waste to the tail race.
6. 1. Conical diffuser or straight divergent tube-
This type of draft tube consists of a conical diffuser with
half angle generally less than equal to 10° to prevent flow
separation. It is usually employed for low specific speed,
vertical shaft Francis turbine. Efficiency of this type of
draft tube is 90%
7. 2. Simple elbow type draft Tube-
It consists of an extended elbow type tube. Generally,
used when turbine has to be placed close to the tail-race.
It helps to cut down the cost of excavation and the exit
diameter should be as large as possible to recover kinetic
energy at the outlet of runner. Efficiency of this kind of
draft tube is less almost 60%
8. 3. Elbow with varying cross section-
It is similar to the Bent Draft tube except the
bent part is of varying cross section with
rectangular outlet. The horizontal portion of
draft tube is generally inclined upwards to
prevent entry of air from the exit end
9. Use of draft tube in reaction turbine
To reduce velocity/ kinetic energy of water making
exit thereby converting kinetic energy to pressure
head allowing turbine to be installed above tail pool
level.
What is the function of draft tube
Due to high KE at the exit of reaction turbine,
there would be a loss of overall reduction in KE. So
to prevent this loss, the draft tube is provided
which produces net efficiency
10. purpose of draft tube in hydraulic turbines
It makes possible the installation of the turbine above
the tail race level without the loss of head
the velocity of water at the runner outlet is very high.
By employing a draft tube of increasing cross sectional
area, the discharge takes place at a much lower velocity
and thus, a part of the kinetic energy that was going as
a waste is recovered as a gain in the pressure head, and
this increases the efficiency of the turbine.
The draft tube prevents the splashing of water coming
out of the runner and guides the water to the tail race.
11. What turbine does not require draft tube?
Draft tube is used to increase the pressure from the
low turbine exit pressure to the pressure of the
surrounding to which the fluid is rejected.
Only reaction turbines require a draft tube as there is
low pressure at the exit of the turbine, where as in
impulse turbine the pressure at the inlet and the exit is
essentially the same, which is above the pressure to
which the fluid rejected and thus it require no draft
tube.
12. Consider a capital draft tube
Hs = vertical height of draft tube
above the tail race
Y = distance of bottom of draft tube
from tail race
13. Applying Bernoulli’s equation to inlet (1 – 1) and outlet (2 – 2)
of the draft tube and taking section 2 – 2 as the datum line
We get
(P1/ ρg) + (V1
2/2g) + (Hs + y) = (P2/ ρg) + (V2
2/2g) + 0 + hf ----------------(1)
Where hf = loss of energy between section 1-1 and 2-2
But
(P2/ ρg) = atmospheric pressure head + y
= (Pa/ ρg) + y
14. Substituting this value of (P2/ ρg) in equation (1), we get
This equation (P1/ρg) is less than atmospheric pressure
16. Actual conversion of kinetic head into pressure head
Kinetic head at the inlet of draft tube
Efficiency =
Actual conversion of kinetic head into
pressure head =
Theoretical conversion of kinetic head
into pressure head in draft tube =
17. EFFICIENCY =
V1= Velocity of water at inlet of Draft tube
V2= Velocity of water at outlet of Draft tube and
hf= Loss of head in the Draft tube
