During design and model selection, if the flue gas temperature is 180°C and the heat pipe wall temperature approaches the critical temperature of the working fluid, can a conventional heat pipe heat exchanger still be used? The answer depends on the verification results of the heat pipe operating temperature constraints, rather than simply selecting a model based on flue gas temperature. The following outlines several key points that need to be confirmed, starting from common on-site questions.
The heat transfer capacity of a heat pipe heat exchanger is limited by the operating temperature range of the working fluid. When the flue gas temperature is 180°C, if the pipe wall temperature has approached the critical temperature of the working fluid, the working fluid may enter a supercritical or near-critical state, resulting in unstable heat transfer characteristics. In this case, the difference between the pipe wall temperature and the critical temperature should be calculated, and consideration should be given to adjusting the type of working fluid or reducing the heat load, rather than directly applying a conventional model.
Before model selection, the corrosive components of the flue gas and the heated medium must be confirmed. When corrosivity is relatively strong, the grade of pipe material and weld material should be correspondingly increased to avoid early failure caused by thinning or weld defects. This item is equally important as the temperature verification and cannot be judged solely based on experience.
Fin spacing and structural arrangement directly affect pressure drop and occupied space. The allowable pressure drop range and available installation dimensions should be provided on site so as to achieve a balance between heat transfer area and resistance. When pressure drop requirements are strict, the fin spacing can be appropriately increased, but it is necessary to simultaneously verify whether the heat transfer area meets process requirements.
If procurement personnel find during on-site inspection that the temperature on the heat source side is not low, but the temperature difference between the middle and edge of the tank body can reach more than ten degrees Celsius, this usually indicates uneven heat pipe arrangement or airflow organization. During the model selection stage, the heat pipe layout should be verified in conjunction with the tank body dimensions, and if necessary, the pipe spacing should be adjusted or flow guide structures should be added to reduce the temperature gradient.
Based on the above key points, the following should be confirmed item by item during model selection: medium corrosivity level, operating temperature and fluctuation range, allowable pressure drop and installation space, and tank body temperature uniformity requirements. For working conditions where the flue gas temperature is 180°C and the pipe wall temperature approaches the critical temperature, it is recommended to first verify the heat pipe operating temperature, and then determine whether to use a conventional heat pipe heat exchanger or adjust the working fluid scheme.
