Weight and size characteristics of heat exchange equipment of hybrid Flash/ORC power plants: case of application in WHR cogeneration plants

Abstract

Calculations of the characteristics concerning the heat exchange equipment of the Flash/ORC power plant for the usage in waste heat recovery (WHR) cogeneration plants are presented. The use of hybrid power plants with water steam and organic working fluids, on the one hand, leads to an increase in the mass of the heat exchange equipment, and on the other hand, it ensures the generation of electrical energy and heat at high temperatures of the heat source, when the use of high-temperature working fluids is limited their thermal stability (for example, the thermal stability of silaxanes is about 300 ºC). Calculations are performed for a heat source with temperatures of 250ºC and 350 ºC. The generated electrical power of the power plant at a heat source temperature of 350ºC is 460 kW. The generated thermal power by capacitors is 1755.3 kW. The mass of the capacitors of the second and third circuits is 5000 kg and 14500 kg, respectively. It is shown that when the temperature of the heat source decreases to 250ºC with a double-circuit design, the mass of the heat exchange equipment decreases. Thus, the mass of the condenser is 12700 kg, and the generated thermal power is 1430.2 kW. The values of the heat exchange surface area of a direct-flow steam generator with flue gases, and an intermediate steam generator between the stages of the thermal circuit of a regenerative heat exchanger, as well as a water-cooled condenser are given. When the condenser is cooled by atmospheric air, the surface area of the condenser increases significantly. A characteristic feature of organic working fluids is the low efficiency of heat exchange in heat exchangers, especially at low temperatures. The heat transfer coefficient in heat exchangers with organic working fluids is about 1500-2800 W/(m2•K), which causes an increase in the heat exchange surface area. In this case, it is necessary to reduce the diameters of pipes in the heat exchangers of the steam generator, to use the pipes with internal ribs, and coil pipes in order to reduce the pipe length. The use of steam generators with coil steam-generating channels, characterized by a higher heat exchange intensity, is promising. The selection of the optimal structure and parameters of the ORC installation is carried out taking into account the mutual influence of steam turbine units on the efficiency of working processes in general.