A falling film freeze exchanger having a shell connected to vertically spaced apart horizontally arranged circular upper and lower tube sheets; a plurality of vertically positioned parallel tubes, with each tube extending through and connected to a hole in each tube sheet; an inlet to feed a liquefied refrigerant gas to the lower part of the shell side of the heat exchanger and an outlet to withdraw refrigerant vapor from the upper part of the shell side of the heat exchanger; a liquid distribution box positioned above the upper tube sheet and a conduit to deliver an aqueous feed liquid to the liquid distribution box; a solid walled sleeve, surrounding and spaced outwardly from each tube, joined to the upper tube sheet and extending downwardly a short distance below the refrigerant liquid/gas interface on the shell side thereby providing a refrigerant vapor space between each sleeve and tube; and a hole in and through the upper part of the sleeve wall near the upper tube sheet.