The rotor spins in air or another type of gas at atmospheric pressure. The friction between the gas and the spinning of the rotor increases rapidly causing the power needed to drive the rotor to increase rapidly. This increase causes the temperature of the rotor to rise. This is where the temperature control device comes in. If the temperature rises too much it can cause the substance in the tube to remix and disturb sedimentation.
Temperature control allows ultracentrifuges to have high rotational speeds.