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end of pipe
The flooded tube boiler
        The flooded tube boiler is a boiler in which gravity draws liquid water to where the heat is added, and steam and oil rise away from the heat area to power the steam engine.  Water is not circulated, it is stagnant especially at the bottom of the boiler.  This allows sediment to settle out and oil to rise and separate.  The flooded tube boiler features no hot spots that can burn oil, the entire boiler surface is at the same temperature.  In this variation, the boiler also includes an air circulation system that takes heat from the inside surface of an evacuated tube and deposits it on the boiler.
        The boiler consists of an E shaped structure of galvanized water pipe, 3/4 inch diameter.  The long pieces are 6-7 feet long and are spaced nominally 2 feet apart.  A structure holds the pipes one foot over the surface of the array and parallel to the mirrors.  The ends of the E are capped and sealed carefully, leaks are catastrophic.  At one end of the E, a line draws steam upwards.  Another line brings in water from the water pump.  A level sensor detects the water level within the pipe.  The long pieces will nominally be at about 45 degrees from vertical, actually matching the slope of the array which normally matches the slope of a roof.  The short pieces of the E will be close to level or tilted slightly so that the end which draws steam off is highest.
        Around each of the long pieces,  four lengths of 4 foot long 1/4 inch diameter rods are suspended.  The rods each have a hole drilled along each end, wire goes through the hole and supports the rods equally spaced around the iron pipe.  The wire must not extend to the perimeter of the rods, the outer surface of the rods must be flush.  The wire is stabilized against the pipe and rods with high temperature epoxy such as J&B weld.  The top two rods are especially important.  There must be a gap of a few inches between the rods and the end caps, and a similar gap at the top.
        When the array is completed, one carefully slides the evacuated tube over the rods in a similar fashion to sliding a cover over something else long and rigid and skinny.  For example the cap on a pen, that is truly the proper metaphor.  The evacuated tube sits on the top two rods with its weigh supported by the long straight line of contact with the rods.  The tube is hung by wires strung around hooks mounted on the evacuated tube, but most of its weight is supported by the contact with the aluminum rods.
        The top two rods form a channel between the evacuated tube and the iron pipe.  This channel is cold, compared to the bottom, since there is less sunlight on the top surface. The cooler air will sink through this channel, causing the hotter air on the bottom side of the tube to rise.  This air circulation causes heat to be removed from the glass and deposited on the cooler iron pipe.  Much safer than hot oil which is the normal heat transfer mechanism used in steam systems.  This thermosiphon is good for gain of 5-10 suns.  Higher will require a more advanced mechanism.
        Here is a handy calculator for radiated losses.  The coefficient is higher for visible light than infrared light in the evacuated tube collectors.