A quench cooler or transferline heat exchanger for quenching the effluent from a thermal cracking furnace has an inlet connector between the cracking furnace tubes and the tubes of the quench cooler. The tubes of the quench cooler are arranged in a circular pattern of spaced tubes. The flow passage of the connector is configured to initially decelerate and then re-accelerate the gas. This involves a conical diverging diffuser followed by a radial diffuser and then an annular converging section. The cross-sectional transitions are smooth to avoid dead spaces and minimize pressure loss.