Actually the centrifugal pump makes a draft. The turbine takes advantage of this and the diverging nozzle increases the vacuum. The more the nozzle is diverging the faster the air accelerates and the pressure decreases. There are also two large diverging nozzles from turbine to pump..... The velocity is again sped up and it hits the rotor in the pump. Then the volute shape and diverging nozzle exhaust speed it up again creating more vacuum.