A modular mobile fract pump unit having for a radiator with a heat exchanger vertically for generally horizontal flow of air through the radiator and a duct provided on an air exit side of the heat exchanger to direct air exiting the heat exchanger upwardly. Preferably, the duct comprises a retractable duct for movement between an operative position for use in parked cooling operation and a storage position for transport of the unit as a road worthy vehicle, within the storage position the duct not increasing the overall width of the unit and in the operative position the duct extending laterally to the side of the unit beyond the storage position and increasing the overall width of the unit.