Abstract: A solar fluid heater consisting of a solar collector and a storage vessel. The solar collector includes a supply outlet and a return inlet, a supply conduit, leading the heated fluid from the supply outlet to the vessel, and a return conduit, returning the cold fluid from the vessel to the return inlet. The supply conduit comprises two contiguous portions, a first portion substantially exposed to the same night temperature as in the collector, and extending between the supply outlet and a transition zone. The first supply conduit portion takes an altogether descending course from the supply outlet to at least the transition zone, and a second portion is adapted to maintain it at night at a temperature substantially equaling that of the vessel contents and extends from the transition zone to the outlet opening of the supply conduit inside the vessel, whereby, during the night, the temperature of the first return-conduit portion is substantially equal to the temperature of the vessel contents.

Abstract: Hollow ceramic fibers are prepared by melt spinning an organic silazane polymer, infusibilizing the fiber to form an infusible layer on the surface, and pyrolyzing the fiber to obtain a ceramic fiber having an empty interior. The infusibilizing step includes two steps of treating the fiber with a gas containing the vapor of an infusibilizing silicon, boron, phosphorus or metal compound and further treating with a water vapor-containing gas or ammonia-containing gas. The surface infusible fibers are efficiently sintered into hollow ceramic fibers without fusion bond.