flash boiler

from The Collaborative International Dictionary of English v.0.48

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.
   [Webster 1913 Suppl.]