Purpose. To study development of contact heating of a process liquid basing on the main principles of thermodynamics in terms of specially developed equipment. Methodology. The research of efficient operation of plants for contact heating of process liquids was based on analytical and laboratory studies. The analytical studies relied on determining heat and material balance of the process on the basis of quantitative parameters of the obtained heat, basing on contact heating of water (process liquid) as the end (intermediate) heat carrier. Test studies were carried out in terms of special plants for modelling thermodynamic processes of contact liquid heating. Findings. The efficiency of operation of the special equipment was substantiated by improving its design that makes it possible to preserve the balance between temperatures of liquids on the inlet and outlet of a special heating plant. Basing on the averaged value of the parameters characterizing thermodynamic transformations, time periods of the process liquid heating were identified. Originality. Dependences and numerical values of changes in maximum equilibrium temperature (boiling temperature) and relative amount of the evaporated water on the specified excess-air coefficient during the natural gas combustion in the submerged combustion devices were obtained. Parameters of the temperature field distribution in the heating system were obtained basing on the design features of a heating plant. The research data were aimed at identifying the efficiency of system operation depending on water consumption at the device inlet. The research was carried out in terms of one-stage and two-stage heating of a process liquid. Practical value. Design of a test plant for thermochemical water heating was improved; that helped simplify a process of the heating plant control to get maximum amount of heat energy. The efficiency of its operation was substantiated by controlling the temperature field distribution in the heating devices.