In case of reaction turbines, the water works on the turbine and while doing so, it imparts energy to the vanes and runner. Thus the water pressure reduces than the atmospheric pressure i.e. vacuum. As the water flows from higher pressure to lower pressure, since the water pressure has dropped below the atmospheric pressure, the water cannot come out of the turbine. Thus, a divergent tube is connected to the end of the turbine. This tube is called as a draft tube.
The draft tube is a divergent tube one end of which is connected to the outlet of the turbine and the other end is immersed below the tailrace i.e the water level.
Draft tube performs the function of increasing the pressure from the inlet to the outlet of the draft tube as it flows through it and hence increases it to more than atmospheric pressure. It reduces the velocity of the discharged water to minimize the loss of kinetic energy at the outlet. This permits the setting of turbine above the tail race without any appreciable loss of available head.
Apart from the above function, the draft tube performs the function of discharging the water that has worked on the turbine into the tailrace safely.