Energy Bridge And Technical Core Of ORC Generator System For Heat Recovery Heat Exchanger
The low-grade heat source characteristics of the ORC system and the physical properties of the organic working fluid impose strict customized design requirements on the heat recovery heat exchanger, and its technical characteristics are mainly reflected in the following four aspects:
(1) Efficient heat exchange design: balancing waste heat utilization and system compactness
Low grade heat sources have small temperature gradients and low energy densities, requiring heat recovery heat exchangers to have ultra-high heat transfer efficiency. In engineering, the structure design of "finned tube+cross flow/counter flow arrangement" is usually adopted: high-frequency finned tubes are used to enhance heat transfer in the hot side channel, increasing the contact area with the waste heat medium; The cold side working fluid channel adopts reasonable channel allocation to achieve countercurrent heat transfer with the hot side medium, maximizing the heat transfer temperature difference. At the same time, ORC systems are often used in industrial sites or mobile devices (such as new energy heavy-duty trucks), and heat exchangers need to achieve maximum heat transfer area in a limited space. Therefore, compact designs (such as plate fin and microchannel structures) have become the mainstream choice, and their volumetric heat transfer coefficient can reach 3-5 times that of traditional shell and tube heat exchangers.
(2) Workfluid adaptability: addressing the unique physical and chemical properties of organic working fluids
There are significant differences in boiling point, viscosity, and corrosiveness between organic working fluids and water, which require special requirements for material selection and structural design of heat exchangers. For example, some organic working fluids (such as R134a) may experience significant volume expansion during phase transition, and it is necessary to design a reasonable flow channel cross-sectional area to avoid excessive pressure loss; Chlorine containing working fluids may decompose and produce corrosive gases at high temperatures, so the material of the heat exchanger should be 316L stainless steel or Hastelloy alloy with strong corrosion resistance; The phase transition characteristics of dry fluids (such as R245fa) and wet fluids (such as n-pentane) are different, and a targeted heat exchange process needs to be designed to avoid the generation of droplets at the outlet of wet fluids, which can cause damage to the turbine due to liquid impact.
(3) Temperature and pressure control: ensuring stable operation of the system
The evaporation temperature of organic working fluid in ORC system is usually between 60 ℃ -180 ℃, and the working pressure can reach 2-4MPa. The heat recovery heat exchanger needs to accurately control the outlet temperature and dryness of the working fluid - excessive superheat will increase system energy consumption, while insufficient superheat may lead to turbine failure. For this reason, heat exchangers usually adopt a segmented design, divided into preheating section, evaporation section, and superheating section. By optimizing the length of each flow channel and the distribution of heat transfer area, the dryness of the working fluid outlet is ensured to be stable at 0.95 or above. At the same time, the heat exchanger needs to have sufficient pressure resistance and sealing performance to cope with pressure fluctuations of organic working fluids during phase transition, and prevent safety hazards and energy loss caused by fluid leakage.
