It is known to use oil as a transfer medium in heat exchangers for steam production. Thus, U.S. Pat. No. 2,222,575 discloses cooling of hot oil by direct contact of the oil with cooling water which immediately evaporates, thereby generating steam for use. The hot oil is pumped into a chamber having a temperature of approximately 343.degree. C., and water is sprayed over its surface from a plurality of nozzles. As the water evaporates, the oil is cooled and replaced by new, hot oil. U.S. Pat. No. 4,164,202 also discloses cooling of hot oil. In this reference, hot oil (about 350.degree. C.) and water are both sprayed into the interior of a vessel in the form of little droplets. When the droplets meet, the water evaporates and the oil, cooled to some extent, falls down and collects at the bottom. U.S. Pat. No. 4,207,840 discloses a steam generator comprising a bath of oil in a spherical vessel which is continuously heated from below by wood combustion. In order to generate steam, water is injected into the bath underneath the heated oil. As the water comes into intimate contact with the heated oil, it evaporates and rises to the surface of the oil where it is discharged from the spherical vessel as steam. Thus, in the prior art, the oil does not take part in producing, but only in transporting heat. Claim 1 refers generally to this process in its preamble.