Abstract: A heater includes a convection section with columns and tube sheets coupled to the columns with tubes received in the tube sheets. The convection section includes a space between the tube sheets associated with corresponding pairs of columns. A structural frame is coupled to the columns and positioned in the space to slidably receive one or more catalyst support beds for loading or unloading a catalyst into the convection section through a lateral side of the convection section of the heater. The structural frame may include beams, struts, slide plates, and other frame elements that assist with supporting the catalyst support beds and enable sliding of the catalyst support beds with respect to the structural frame.