Vapor generator

In combination with a solar heat source, tubular means, mounted on a tower, the tower being located eccentrically in a heliostat field, for passing a fluid in indirect heat absorbing relation with the heat source, the tubular means including first heat exchanger means, second heat exchanger means disposed downstream fluid flow-wise of the first heat exchanger means, third heat exchanger means disposed downstream fluid flow-wise of the second heat exchanger means, wherein the second heat exchanger means is interposed between at least a portion of the first and third heat exchanger means and said heat source, wherein the tubular means of said first, second and third heat exchanger means are respectively grouped into upright panels with fluid inlet means disposed at the bottom of the tubular panels, the panels of the first and third heat exchanger means being disposed in side-by-side side relation to form a nearly cylindrical tubular wall, the panels of the second heat exchanger means being disposed in a side-by-side relation and in spaced surrounding relation with at least a portion of the tubular wall and having the tubular means in the panels of the first and third heat exchanger means laterally spaced and rigidly united by metallic webs to produce a structure impermeable to light from the heliostat field, the second heat exchanger means having tubes with varying lateral spacing to provide a relatively uniform, low level, heat flux pattern to the first and third heat exchanger means and including associated means for pump assisted natural circulation.