Abstract: A vapor generator comprising an evaporator unit, which in turn includes an inner chamber for containing a first fluid in a liquid state, and further includes a preheated liquid inlet and a vapor outlet, the evaporator unit having a heating device therein which can be activated for vaporizing the first fluid contained in the inner chamber to generate vapor. The vapor generator further comprises a preheating tank defining an inner chamber and comprising a liquid inlet for injection of the first fluid in a liquid state in the inner chamber, and a liquid outlet. Moreover, the vapor generator comprises an opened liquid channel connecting the preheating tank liquid outlet to the evaporator unit liquid inlet, and establishing free and continuous fluid communication between the preheating tank inner chamber and the evaporator unit inner chamber.