A centrifugal separator for separation of at least two components of a fluid mixture which are of different densities, which centrifugal separator comprises a frame, a hollow spindle rotatably supported by the frame, a centrifuge rotor mounted to a first end of the hollow spindle to rotate together with the spindle around an axis (x) of rotation, wherein the centrifuge rotor comprises a rotor casing enclosing a separation space in which a stack of separation discs is arranged, a duct arranged to be through-flown by process medium during operation of the centrifugal separator and extending through said hollow spindle and in fluid contact with said separation space, at least one liquid outlet (11,12) for discharging a separated liquid phase from said separation space, a plurality of peripheral ports extending from the separation space through the rotor casing to a surrounding space for discharging a phase from the periphery of said separation space, wherein said centrifuge rotor comprises a sliding bowl bottom movable between a closed position, in which the peripheral ports are closed, and an open position, in which the peripheral ports are open, an inlet channel for supplying hydraulic fluid to a closing chamber between the sliding bowl bottom and the rotor casing in order to hold the sliding bowl bottom in the closed position, wherein the centrifugal separator further comprises at least one hermetic seal (18, 29) at a second end, other than the first end, of the hollow spindle, whereby said inlet channel for supplying hydraulic fluid to said closing chamber extends through the hollow spindle and is further arranged such that said hydraulic fluid is in thermal contact with said at least one hermetic seal (18, 29) when said hydraulic fluid is being supplied to said closing chamber. a frame, a hollow spindle rotatably supported by the frame, a centrifuge rotor mounted to a first end of the hollow spindle to rotate together with the spindle around an axis (x) of rotation, wherein the centrifuge rotor comprises a rotor casing enclosing a separation space in which a stack of separation discs is arranged, a duct arranged to be through-flown by process medium during operation of the centrifugal separator and extending through said hollow spindle and in fluid contact with said separation space, at least one liquid outlet (11,12) for discharging a separated liquid phase from said separation space, a plurality of peripheral ports extending from the separation space through the rotor casing to a surrounding space for discharging a phase from the periphery of said separation space, wherein said centrifuge rotor comprises a sliding bowl bottom movable between a closed position, in which the peripheral ports are closed, and an open position, in which the peripheral ports are open, an inlet channel for supplying hydraulic fluid to a closing chamber between the sliding bowl bottom and the rotor casing in order to hold the sliding bowl bottom in the closed position, wherein the centrifugal separator further comprises at least one hermetic seal (18, 29) at a second end, other than the first end, of the hollow spindle, whereby said inlet channel for supplying hydraulic fluid to said closing chamber extends through the hollow spindle and is further arranged such that said hydraulic fluid is in thermal contact with said at least one hermetic seal (18, 29) when said hydraulic fluid is being supplied to said closing chamber.