Abstract: A multi tube evaporator with tubes having elongated side passages interconnected by an end crossover passage. The tube has oblong internal bumps in the side passages that have a low angle with the normal and partly spherical bumps in the crossover passage arranged so that heat transfer between the air passing past the tubes and the refrigerant passing through the tubes is by conduction through the walls of the tubes and the boundary layer of refrigerant moving along the inner walls of the tube and by forced convection of heat energy between the boundary layer and the main flow of refrigerant flowing therethrough. The low angle bumps in the side passages provide the least amount of drag of the refrigerant while effectively reducing the thickness of the boundary layer thereof to reduce its insular effect. The spherical bumps in the crossover are insensitive to the direction of flow and help provide a low pressure drop for effectively increasing efficiency of the evaporator.