What is "flash boiler"

Flash boiler \Flash boiler\ A variety of water-tube boiler, used chiefly in steam automobiles, consisting of a nest of strong tubes with very little water space, kept nearly red hot so that the water as it trickles drop by drop into the tubes is immediately flashed into steam and superheated.

A flash boiler is a type of water-tube boiler. The tubes are close together and water is pumped through them. A flash boiler differs from the type of monotube steam generator in which the tube is permanently filled with water. In a flash boiler, the tube is kept so hot that the water feed is quickly flashed into steam and superheated. Flash boilers had some use in automobiles in the 19th century and this use continued into the early 20th century.