The air-pumping characteristic of a 63 mm diam centrifugal atomiser running dry within a cocurrent spray dryer was studied extensively using a novel experimental technique which involved the direct measurement of the wheel vacuum pressure together with the power required to pump the air. Approximately 10% of the power consumed by the atomiser was utilised in pumping the airstream. A further 25% was consumed in shearing the air around the wheel and the remaining 65% was consumed by friction and motor losses within the drive train. Typically. the wheels considered pumped between 2 l s(-1) and 6.5 l s(-1) of air at a maximum rotational speed of 50 000 rpm which corresponded to between 5% and 16% of the primary air supply to the dryer. A dynamic closure of the wheel/atomiser body gap was observed occurring at 30 000 rpm to 40 000 rpm which was thought to be due to an upward acting force generated by the pressure differential either side of the wheel. This significantly affected the air-pumping behaviour of the wheel.