a vacuum pump comprises a first set of turbo - molecular stages , a molecular drag stage , a first inlet through which fluid can pass through the first set of stages and the molecular drag stage towards a pump outlet , second and third sets of turbo - molecular stages located between the first set and the molecular drag stage , a second inlet , the second and third sets being arranged such that fluid entering the pump through the second inlet is separated into two streams each flowing through a respective one of the second and third sets , and conduit means for conveying fluid passing through the first set and one of the second and third sets towards the outlet .