A system for heating a process fluid having an elongated horizontal shell filled with a heat transfer liquid, an elongated horizontal firetube and an elongated horizontal process coil of heat conducting pipe through which process fluid passes, the firetube being positioned within a lower interior portion of the shell and the process coil being positioned within an upper interior portion of the shell above the firetube and a baffle system between the firetube and the process coil forming an elongated higher elevation up flow passageway and a spaced apart elongated lower elevation down flow passageway so that when heat is supplied to the firetube convection flow of heat transfer liquid is continually channeled from the shell lower portion through the up flow passageway into the shell upper portion and past the process coil and channeled from the shell upper portion through the down flow passageway into the shell lower portion.