The primary objective of this experimental investigation is to determine the in¯uence that dierent fan and heat exchanger characteristics as well as the plenum chamber geometry have on the ¯ow losses in the plenum chamber of a forced draught air-cooled heat exchanger (ACHE). The eect of air ¯ow maldistribution on the heat exchanger thermal performance is also investigated. A series of model tests are conducted employing dierent fans and heat exchangers. It is found that there is a critical minimum distance between the outlet of the fan and the heat exchanger. The heat exchanger loss coecient and its inlet geometry have a relatively strong in¯uence on plenum losses as well as on the air velocity distribution at the outlet of the heat exchanger. Other variables, including the position of the fan in the fan casing, fan-to-heat exchanger area ratio and fan characteristics, have a lesser eect on plenum performance. A plenum chamber recovery coecient is de®ned and expressed in terms of measured parameters. This coecient can be employed in the design of practical ACHEs. It is further found that air ¯ow maldistribution only has a small in¯uence on the heat exchanger thermal performance.