Abstract: A composite tube composed of an inner tube extending through and spaced from an outer tube by radial partition walls is coiled helically. The helical composite tube coil is housed in a tank having an interior silvered reflective surface and evacuated to minimize heat loss through the tank wall. A carburetor supplies a combustible gas mixture to one end of the inner tube, the gas mixture burns in such tube, and a centrifugal blower draws the combustion gas through the tube. A vaporizable liquid is supplied under pressure to the end of the outer tube adjacent to the centrifugal blower for passage of the vaporizable liquid through the outer tube in the direction opposite the flow of combustion gas through the inner tube for vaporization of the combustible liquid under pressure, such as for producing superheated steam from water.