The present invention is directed towards a fired heater for heating a process fluid that uses U tubes provided with increased internal heat transfer surface to reduce tube metal temperatures and coking and at the same time is not prone to plugging from coke. The fired heater comprises a radiant section enclosure with a number of U tubes in the radiant section. The U tubes are formed by connecting one or more tubular sections, and the U tubes are provided with internal generally longitudinal fins. The invention also is directed towards a process utilizing a fined heater with U tubes as disclosed for producing olefins from hydrocarbon feedstocks.