Abstract: A rotary regenerative air preheater has electromagnetic drive devices to adjust the relationship between an end face of the regenerative mass and a sealing frame on a duct part. Electrical control means are responsive to signals generated by contact between the sealing shoe (or special sensors) borne on the frame and the end face, to drive the electromagnetic devices and break the contact. The control may be on/off, a time response such that a desired maximum clearance is experienced, or may be responsive to drag (i.e., the angle of arc over which contact is experienced). The electromagnetic drive devices are inherently self adjusted by having an air gap in their flux circuit which varies in dependence on the position of the sealing frame.