An apparatus for the vaporization of a heavy hydrocarbon feed mixture with steam, said apparatus comprises an outer relatively elongated shell comprising a downstream outlet, an outlet section, a converging/diverging tubular section, an upstream inlet section and an upstream inlet, (a) said upstream inlet section comprising in fluid communication: (i) a frusto-conical inlet section for directing a hydrocarbon feedstock and dilution steam mixture received from the upstream inlet to an inner tubular pipe section, and (ii) said inner tubular pipe section for the passage of said hydrocarbon feedstock and dilution steam mixture from said frusto-conical inlet section through a plate separating the frusto-conical inlet section from the converging/diverging tubular section, said upstream inlet and said upstream inlet section are located at an upstream end of said apparatus; (b) said converging/diverging tubular section comprising (i) an opening section, a converging section, a throat section, and a diverging section, the walls of said diverging section extending to connect to said elongated shell, (ii) at least one steam inlet for receiving steam from an opening disposed along said elongated shell into an outer annular region defined by said plate on the upstream end, said elongated shell, and said diverging section on the downstream end, (iii) wherein said opening section forms an inner annular region around said inner tubular pipe section for receiving steam from said outer annular region and said hydrocarbon feedstock and dilution steam mixture from said inner tubular pipe section and discharges the mixture into the converging section, (iv) wherein said throat section receives the effluent from the converging section, and (v) wherein said diverging section receives the effluent from the throat section; and (c) said outlet section in fluid communication with the diverging section of said converging/diverging tubular section, said outlet section is in fluid communication with said downstream outlet at a downstream end of the apparatus. (a) said upstream inlet section comprising in fluid communication: (i) a frusto-conical inlet section for directing a hydrocarbon feedstock and dilution steam mixture received from the upstream inlet to an inner tubular pipe section, and (ii) said inner tubular pipe section for the passage of said hydrocarbon feedstock and dilution steam mixture from said frusto-conical inlet section through a plate separating the frusto-conical inlet section from the converging/diverging tubular section, said upstream inlet and said upstream inlet section are located at an upstream end of said apparatus; (i) a frusto-conical inlet section for directing a hydrocarbon feedstock and dilution steam mixture received from the upstream inlet to an inner tubular pipe section, and (ii) said inner tubular pipe section for the passage of said hydrocarbon feedstock and dilution steam mixture from said frusto-conical inlet section through a plate separating the frusto-conical inlet section from the converging/diverging tubular section, said upstream inlet and said upstream inlet section are located at an upstream end of said apparatus; (b) said converging/diverging tubular section comprising (i) an opening section, a converging section, a throat section, and a diverging section, the walls of said diverging section extending to connect to said elongated shell, (ii) at least one steam inlet for receiving steam from an opening disposed along said elongated shell into an outer annular region defined by said plate on the upstream end, said elongated shell, and said diverging section on the downstream end, (iii) wherein said opening section forms an inner annular region around said inner tubular pipe section for receiving steam from said outer annular region and said hydrocarbon feedstock and dilution steam mixture from said inner tubular pipe section and discharges the mixture into the converging section, (iv) wherein said throat section receives the effluent from the converging section, and (v) wherein said diverging section receives the effluent from the throat section; and (i) an opening section, a converging section, a throat section, and a diverging section, the walls of said diverging section extending to connect to said elongated shell, (ii) at least one steam inlet for receiving steam from an opening disposed along said elongated shell into an outer annular region defined by said plate on the upstream end, said elongated shell, and said diverging section on the downstream end, (iii) wherein said opening section forms an inner annular region around said inner tubular pipe section for receiving steam from said outer annular region and said hydrocarbon feedstock and dilution steam mixture from said inner tubular pipe section and discharges the mixture into the converging section, (iv) wherein said throat section receives the effluent from the converging section, and (v) wherein said diverging section receives the effluent from the throat section; and (c) said outlet section in fluid communication with the diverging section of said converging/diverging tubular section, said outlet section is in fluid communication with said downstream outlet at a downstream end of the apparatus.