An air-cooling apparatus is provided which includes a securing mechanism for holding two or more separate electronics racks in fixed relation adjacent to each other, and a multi-rack door sized and configured to span the air inlet or air outlet sides of the racks. The securing mechanism holds the electronics racks in fixed relation with their air inlet sides facing a first direction, and air outlet sides facing a second direction. The door includes a door frame with an airflow opening. The airflow opening facilitates the ingress or egress of airflow through the electronics racks, and the door further includes an air-to-liquid heat exchanger supported by the door frame, and disposed so that air flowing through the airflow opening passes across the heat exchanger. In operation, the heat exchanger extracts heat from the air passing through the separate electronics racks.