DRAFT TUBE: Types of Draft Tube, Conical Draft Tube, Simple Elbow Tubes, Moody Spreading tubes, Elbow Draft Tubes with Circular inlet and rectangular outlet, The conical draft tubes, Draft tube theory and Efficiency of Draft Tube

DRAFT TUBE:

The draft tube is a pipe of gradually increasing area, which connects the outlet of the runner to the tail race. It is used for discharging water from the exit of the turbine to the tail race. This pipe of gradually increasing area is called a draft tube. One end of the draft tube is connected to the outlet of the runner and the other end is submerged below the level of water in the tail race. The draft tube in addition to save a passage for water discharge has the following two purposes also:

1. It permits a negative head to be established at the outlet of the runner and their by increase the net head on the turbine. The turbine may be placed above the tail race without any loss of net head and hence turbine may be inspected properly.

2. It converts a large portion of the kinetic energy rejected at the outlet of the turbine into useful energy. Without the draft tube the kinetic energy rejected at the turbine will go waste to the tail race.

Hence by using the draft tube, the net head on turbine increases. The turbine develops more power and also the efficiency of the turbine increase.

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 a reaction turbine fitted with a draft tube. Also without draft tube the kinetic energy rejected at the outlet of the will go water to the tail race.

Types of Draft Tube:

1. Conical Draft Tube

2. Simple Elbow Tubes

3. Moody Spreading tubes

4. Elbow Draft Tubes with Circular inlet and rectangular outlet

The conical draft tubes and moody spreading draft tubes are most efficient while simple elbow draft tube and elbow draft tubes with circular inlet and rectangular outlet require less space as compared to other draft tubes.

Draft tube theory:

Draft tube theory
Draft tube theory
Draft tube theory

Efficiency of Draft Tube: 

the efficiency of a draft tube is defined as the ratio of actual conversion of kinetic head in to pressure in the draft tube to the kinetic head at the inlet of the draft tube.

Efficiency of Draft Tube


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