A dual wall axial flow electric heater for leak sensitive applications provides an improved corrosion and leak resistant assembly and includes protective tubes over electrical heater rods, double tubesheets spaced apart by a plenum and leak detectors positioned to sensor leaks through the walls of the protective tubes. The design includes the option of two or more tube bundles with each inserted into opposite ends of a shell surrounding the tube sheets and heaters. The design provides ease of maintenance since each heater rod can be replaced independently while the unit is in service. Variable heat flux is provided from standard single flux heater rods by providing protective tubes of varying diameters. A built-in thermowell is provided to allow the rod temperatures to be monitored directly. Hot spots are avoided by the use of turning baffles and vibration is avoided by use of spider baffles to support the tubes.