High-efficiency infrared electric liquid-heater

Abstract

An infrared electric liquid heater includes an elongated emitter unit contained within an outer containment tube to define an annular volume through which the to-be-heated water flows. The emitter unit includes a tubular silica-composition envelope that comes into direct contact with the water to be heated. A tungsten filament is contained within the envelope and is supported along the longitudinal axis by spaced-apart filament supports. The envelope is fabricated from a silica composition discovered to provide unique structural properties; the envelope drawn from a melt (by volume) of 99.88% to 99.94% pure silica (SiO.sub.2), 0.04% to 0.08% crystalline sodium borax (Na.sub.2 B.sub.4 O.sub.7.10H.sub.2), and 0.2% to 0.04% hydroxide of potassium (KOH). This composition has demonstrated a structural strength that allows for a high efficiency in-line, instant-on heater using only a single silica wall separating the electrically energized filament from the liquid being heated. The interior of the envelope is filled with an argon/krypton/hydrogen bromide gas mixture that re-deposits any metallic vapor arising from the filament back onto the surface of the filament to avoid undesired deposition of the material of the filament onto the inner surface of the envelope. The tungsten filament emits strongly between 1.mu. and 1.2.mu. in the near-infrared band. The containment tube is provided with a gold-deposited reflective surface to enhance the overall efficiency of the heater.