Draft Tubes -Types,Working and Efficiency[Complete Guide]

Draft Tube is a connecting pipe which is fitted generally at the outlet or exit of turbines which and convert kinetic energy of water at outlet of turbine to static pressure. It helps to avoid wastage of kinetic energy of water that flow from the outlet of turbines.
It is generally fitted in the power turbines like reaction turbines, Kaplan turbines or Francis turbines.
The diameter of draft tube is small near the inlet and large near its outlet.The outlet of draft tube is always submerged in water.

The draft tube increases the pressure of the exiting fluid in expense of its kinetic energy and velocity. It increases the efficiency of turbines.

It is located just under the runner and allow to decelerate the flow velocity exit from the runner.
Materials that are used to create draft tube are cast steel and cemented concrete.
Efficiency of a draft tube is the actual kinetic energy that is converted to static pressure.

Types of draft tube :-

1 Conical diffuser draft tube
2 Simple Elbow Draft Tube
3 Elbow draft tube with varying cross section
4 Moody Spreading Draft Tube

1 Conical Diffuser Draft Tube:
In this type of draft tube the flow path is straight and divergent. This draft tube is fabricated with mild steel plates. It is tapered in shape and the outlet diameter is larger than inlet diameter of this draft tube.

The tapered angle of draft tube should not be too large as it will cause the sepeartion of flow from the wall of draft tube .This angle should not be too small also as it will require a longer draft tube which causes a significant loss of kinetic energy.
So the taper angle is always nearly 10 degrees.

2 Simple Elbow Draft Tube:

In Simple Elbow draft tube , the tube is of elbow shaped. It is mainly used in kaplan turbine.
In this type of draft tube, the cross section area remains same throughout the length of draft tube.The inlet and outlet of the draft tube is circular shaped.
This draft tube is generally used at places of low head and the turbine is 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 outlet of runner.

3 Elbow draft tube with varying cross section:
Elbow draft tube with varying cross section is an improvement of simple elbow draft.
In this type of draft tube the inlet is circular and the outlet is of rectangular shape. The horizontal part of the draft tube is generally inclined upwards to prevent entry of the air from the exit end.
The cross-section area of this type of draft tube changes from inlet to outlet. Outlet of this draft tube is always below the tail race.
It is genrally used in Kaplan Turbine and efficiency of this type od draft tube is about 70 percent.

4 Moody Spreading Draft Tube:
In this type of draft tube, the outlet of the daft tube is divided into two parts. It is similar to conical draft tube and is provided with the central core part which divide the outlet into two parts. There is one inlet and two outlets of this draft tube.
This type of draft tube is mainly used to reduce the swirling action of water.
It is used in vertical shaft turbine.
Efficiency of this type of draft tube is nearly 88 percent.

Working Of Draft Tube:

In case of turbines like Kaplan Turbine and Francis Turbine the head available at the inlet is generally low, hence the turbine is placed much closer to tail race to obtain maximum head.

As most of the pressure of water is converted to mechanical energy of the turbine, the pressure head at the outlet of the turbine is below the atmospheric pressure.
Since the exit of turbine is placed near tale race and the pressure of water at the exit of turbine is less than atmospheric pressure, so it can cause a backflow of water. This happens because water flows from high pressure to low pressure and pressure at exit of turbine is less than atmospheric and at tail race there is atmospheric pressure.

Backflow of water causes serious damage to the turbine and its different parts and can stop the turbine from working.

To avoid this problem of backflow a draft tube is is used between the outlet of turbine and tail race. Draft Tube increases the pressure of water to atmospheric pressure.

Applying Bernoulle’s Principle at section 1-1 and 2-2

[Pressure Head + Velocity Head + Elevation Head]_1-1 = [Pressure Head + Velocity Head + Elevation Head]_2-2


Let
P₁ = pressure of fluid at section 1-1 (inlet of draft tube)
V₁ = velocity of fluid at section 1-1 (inlet of draft tube)
Similarly,
P₂ = pressure of fluid at section 2-2 (outlet of draft tube)
V₂ = velocity of fluid at section 2-2 (outlet of draft tube)
ρ = density of flowing fluid
g = gravitational force
h_f = loss of head (energy) in draft tube
H_s = vertical height of draft tube above the tail race
y = distance of bottom of draft tube from tail race.
P_a = atmospheric pressure of fluid.

( P₁ / ρg ) + ( V₁² / 2g ) + ( H_s + y ) = ( P₂ / ρg ) + ( V₂² / 2g ) + ( 0 + h_f )
( P₁ / ρg ) = ( P₂ / ρg ) – ( H_s + y ) + ( V₂² / 2g ) – ( V₁² / 2g ) + h_f

Pressure head at section 2 – 2 is equal to atmospheric pressure head and distance y.
( P₂ / ρg ) = ( P_a / ρg ) + y
( P₁ / ρg ) = ( P_a / ρg ) + y – H_s – y + ( V₂² / 2g ) – ( V₁² / 2g ) + h_f( P₁ / ρg ) = ( P_a / ρg ) – H_s + ( V₂² / 2g ) – ( V₁² / 2g ) + h_f

Converting the equation for our requirement (i.e., in the middle of R.H.S taking “-” common)
( P₁ / ρg ) = ( P_a / ρg ) – H_s – [ ( V₁² / 2g ) – ( V₂² / 2g ) – h_f ]

In the above equation [ ( V₁² / 2g ) – ( V₂² / 2g ) – h_f ] is called kinetic head.
Here [ ( V₁² / 2g ) – ( V₂² / 2g ) ] is the dynamic head.
From the above equation we can write
( P₁ / ρg ) < ( P_a / ρg )
So        P₁ < P_a

Pressure head at inlet of draft tube or outlet of turbine is less than the atmospheric pressure. So the net head on turbine with draft tube increases.

Efficiency of Draft Tube:

Efficiency of draft tube Ƞ_d = Actual conversion of kinetic energy to pressure heat at the inlet of draft tube.

Therefore,
Ƞ_d = (Actual conversion of kinetic energy into pressure head)/(Total Kinetic energy present at inlet of draft tube)

Ƞ_d= [ ( V₁² / 2g ) – ( V₂² / 2g ) – h_f ] / [ ( V₁² / 2g ) – ( V₂² / 2g ) ]

Actual conversion of kinetic head into pressure head = [ ( V₁² / 2g ) – ( V₂² / 2g ) – h_f ]
Theoretical conversion of kinetic head into pressure head = [ ( V₁² / 2g ) – ( V₂² / 2g ) ]