High steam economy is obtained by recompression of the vapor generated from the boiling product. This is done by adding energy to the vapor by a steam jet. Part of vapor separated to be used together with boiler steam, to the thermo compressor. Boiler steam (usually at a pressure of 3.5-10 bar) is introduced through a nozzle creating a steam jet in the mixing chamber whereby vapor from the evaporator is sucked into the mixing chamber. The thermo compressor uses the high pressure steam to increase the kinetic energy and the steam ejects at high speed through the nozzle. The velocity of the mixture of boiler steam and vapor is reduced in diffuser and consequently, the pressure and temperature are increased, making the mixture suitable as a heating medium in the calandria.