Abstract: In thermal cracking of hydrocarbons, especially steam cracking to light olefins, a transfer line heat exchanger unit is provided in which cracked gas flows from a furnace into heat exchange tubes, which comprises a distributor having an inlet for said gas and two or three diverging branches forming with said distributor a wye or tri-piece for passage of gas, each branch having along its length a substantially uniform cross-sectional area and being in fluid flow communication with a respective cooling tube. Unfired residence time and pressure drop are reduced, thereby improving selectivity to ethylene.