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What is draft tube? The draft tube is an integral part of a reaction turbine, and its design criteria should be specified by the turbine manufacture.
It has two function: 1. One is to enable the turbine to be set above the tail water level without losing any head thereby.
A reduce pressure is produced at the upper end of the draft tube, which compensates for height at
Which turbine runner is set? Within limits the turbine can be set at different elevation without altering the net head By its use there is unbroken stream of liquid from headwater to tail water.
2. The second function of draft draft tube is to reduce the head loss at submerged submerged discharge to thereby increase The net head available to runner of turbine.
This is compensate by gradually diverging tube whose cross sectional area at discharge is larger larger than the cross sectional sectional area at entrance to the tube This invention relates to reaction turbines which are fed by water coming from a pressure conduit by means of an inlet conduit connecting the pressure conduit to a spiral tank surrounding the turbine wheel. Such turbines include a draft mounted axially of the turbine wheel for evacuation of the water. When this draft tube is curved, it is advantageous to flatten the stream of water after passage from the curved portion in order to reduce the loss of pressure which would normally result. For this purpose, the wall section is deformed transversely of the generally circular draft tube which necessitates the use of supporting means in order to assure the permanent shape of the deformed area. Page | 1
The present invention has for its object the provision of a draft tube for a reaction turbine wherein it is no longer necessary to provide support means previously employed to assure resistance of the wall against hydraulic forces which are present. The draft tube of our invention is characterized in that its wall comprises gutters connected along their edges by at least one dividing plate forming a beam.
It is an elevation, partly in section, of a turbine with a spiral tank including the draft tube. It is a transverse section of the draft tube on a larger scale. It is a longitudinal section on the line III--III of FIG. 2 on a still larger scale of the draft tube. It is a transverse section of an another embodiment of the draft tube on a larger scale. The turbine comprises a spiral tank connected to a conduit under pressure by a water inlet conduit 2. The turbine is provided at its upper portion with a curved draft tube 3 assuring evacuation of the water. As shown in the drawing, the draft tube is made of an assembly of two gutters of semi-circular cross-section connected at their edges by a dividing plate of a width less than the diameter of the gutters preferably the draft tubes of circular shape in transverse section particularly at its ends which are connected to the turbine. At the top of the curve and a relatively short distance beyond, the draft tube is preferably provided with the flat metal guide plates for guiding the stream of water. The plates are disposed opposite one another and are soldered along one of their edges to the dividing plate and at their other edges to related internal faces of the gutters. These metal guide plates are relatively thin and the space between them and the curved walls of the gutters can be filled with gravel in such a manner to transfer onto said walls the pressure exerted by the stream of water on the guide plates during its passage through the draft tube.
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Types of Draft Tubes As shown in Figure 5.23, the draft tubes are of the following three types: (1) Conical or divergent draft tube (2) Elbow type draft tube (3) Hydracone or Moody spreading draft tube
(1) Conical or divergent draft tube: The shape of the tube resembles that of a frustrum of a cone. It is commonly used in the Francis turbine. The cone angle varies from 4° to 8°. The efficiency of the conical tube is about 8.5% to 90%.
(2) Elbow type draft tube: It may be in the form of a simple elbow type or elbow tube with a circuit inlet and a rectangular outlet section. The latter type is used in the Kaplan turbine with an efficiency of about 70%.
(3) Hydracone or Moody spreading draft tube: This is a modification of conical tube and a solid conical cone is provided in the centre of the tube with a flare at the bottom end. Such an arrangement allows a large exit area without excessive length. The solid core at the centre enables full flow and reduces the eddy losses. The efficiency of the tube is about 85%.
Draft Tube Theory: Consider a turbine fitted with a draft tube (conical) as shown in Figure 5.24. Let y - distance of the bottom of draft tube from tail race, and pa - atmospheric pressure at the surface of tail race. Applying Bernoulli's equation to the section 2-2 (representing the runner exit or inlet of the draft tube) and the section 3-3 (representing the draft tube exit); assuming section 3"3 at the datum line, we have-
Where hf = loss of energy between sections 2-2 and 3-3. Rewriting the above expression (i) for p 2/w , we obtain
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The term (y 2 -y) which represents the vertical distance between the runner exit and the tail water level is called the suction head of draft tube and is denoted by H.
In equation it is less than atmospheric pressure. The efficiency of a draft tube ( ) is defined as the ratio of net gain in pressure head to the velocity head at entrance of draft tube. Thus
Where V2 = velocity of water at section 2-2 (inlet of draft tube), V 3 = velocity of water at section 3-3 (outlet of draft tube).