What is "evaporating surface"

Evaporate \E*vap"o*rate\, v. t.

1. To convert from a liquid or solid state into vapor (usually) by the agency of heat; to dissipate in vapor or fumes.

2. To expel moisture from (usually by means of artificial heat), leaving the solid portion; to subject to evaporation; as, to evaporate apples.

3. To give vent to; to dissipate. [R.]

   My lord of Essex evaporated his thoughts in a sonnet.
   --Sir. H. Wotton.

   Evaporating surface (Steam Boilers), that part of the heating surface with which water is in contact.