Draft tube in a turbine
Basically, Draft tube is a conduit which connects the runner exit to the tail race where the water is being finally discharged at atmospheric pressure from the reaction turbine.
The primary function of the draft tube is to reduce the velocity of the discharged water at atmospheric pressure to minimise the loss of energy at the outlet. If the velocity will be more then K.E. will be more and losses will be more. In other words, We use draft tube to have more net available head across the turbine so that power produced by the turbine will be more an hence efficiency will be high.
Let's apply bernoulli's principle.
CASE : 1
When cylindrical draft tube is used
CASE 2 :
When conical (not tapered ) draft tube is used:
As V2 is greater than V4 , energy per unit weight is more in later case . So we use conical draft tube.
The primary function of the draft tube is to reduce the velocity of the discharged water at atmospheric pressure to minimise the loss of energy at the outlet. If the velocity will be more then K.E. will be more and losses will be more. In other words, We use draft tube to have more net available head across the turbine so that power produced by the turbine will be more an hence efficiency will be high.
Let's apply bernoulli's principle.
CASE : 1
When cylindrical draft tube is used
CASE 2 :
When conical (not tapered ) draft tube is used:
Function Of A Draft Tube:-
The primary purpose of the draft tube is to monitor the influx of water. The turbine has something called a "tail race," which is the lowest portion of a dam or construction project where water pools. The tube connects the turbine to this tail race, permitting the turbine to be outside of the water but still have access to the water.
Turbines need to have a minimum amount of water to propel them in order to produce enough energy. Without these tubes, the pressure could drop because of lack of water, and in turn, the entire turbine could fail to work and power could be lost.
Since the area needs to be increased,for the influx of water hence the tube is tapered. :)A draft tube at the end of the turbine increases the pressure of the exiting fluid at the expense of its velocity. This means that the turbine can reduce pressure to a higher extent without fear of back flow from tail race.
The primary purpose of the draft tube is to monitor the influx of water. The turbine has something called a "tail race," which is the lowest portion of a dam or construction project where water pools. The tube connects the turbine to this tail race, permitting the turbine to be outside of the water but still have access to the water.
Turbines need to have a minimum amount of water to propel them in order to produce enough energy. Without these tubes, the pressure could drop because of lack of water, and in turn, the entire turbine could fail to work and power could be lost.
Since the area needs to be increased,for the influx of water hence the tube is tapered. :)A draft tube at the end of the turbine increases the pressure of the exiting fluid at the expense of its velocity. This means that the turbine can reduce pressure to a higher extent without fear of back flow from tail race.
In turbines, water enters from very high head i.e. very high pressure.
As the water passes through turbine vanes, it gives its energy to vanes and pressure of water decreases to sub-atmospheric values so that we can have high energy output because of higher pressure difference across turbine. Now we just can't make turbine open to atm. because of low pressures otherwise air entrapment could cause problems like cavitation.
Now in order to again have water exiting at atmospheric pressure, we need to raise water pressure. To achieve this, we use draft tube and because of its tapering outward shape, it acts as diffuser since area of flow increases. Thus we have water coming out from turbine at atm. pressure.
Another function of draft tube is that it also acts as collector for water coming out of turbine and directs it to reservoir.
As the water passes through turbine vanes, it gives its energy to vanes and pressure of water decreases to sub-atmospheric values so that we can have high energy output because of higher pressure difference across turbine. Now we just can't make turbine open to atm. because of low pressures otherwise air entrapment could cause problems like cavitation.
Now in order to again have water exiting at atmospheric pressure, we need to raise water pressure. To achieve this, we use draft tube and because of its tapering outward shape, it acts as diffuser since area of flow increases. Thus we have water coming out from turbine at atm. pressure.
Another function of draft tube is that it also acts as collector for water coming out of turbine and directs it to reservoir.
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