Severe wall thinning is found on the tube of a low-pressure evaporator(LPEVA) module that is used for a heat recovery steam generator(HRSG) of a district heating system. Since wall thinning can lead to sudden failure or accidents that lead to shutdown of the operation, it is very important to investigate the main mechanism of the wall thinning. In this study, corrosion analysis associated with a typical flow-accelerated corrosion(FAC) is performed using the corroded tube connected to an upper header of the LPEVA. To investigate factors triggering the FAC, the morphology, composition, and phase of the corroded product of the tube are examined using optical microscopy, scanning electron microscopy combined with energy dispersive spectroscopy, and x-ray diffraction. The results show that the thinnest part of the tube is in the region where gas directly contacts, revealing the typical orange peel type of morphology frequently found in the FAC. The discovery of oxide scales containing phosphate indicates that phosphate corrosion is the main mechanism that weakens the stability of the protective magnetite film and the FAC accelerates the corrosion by generating the orange peel type of morphology.

In this study, the actual energy consumption of the secondary side of district heating system(DHS) with different hot water supply temperature control methods are compared. Two methods are outdoor temperature reset control and outdoor temperature predictive Control. While outdoor temperature reset control has been widely used for energy savings of the secondary side of the system, the results show that outdoor temperature predictive control method saves more energy. In general, outdoor temperature predictive control method is lowering the supply temperature of hot water, and it reduces standby losses and increases overall heat transfer value of heated spaces due to more flow into the space. During actual energy consumption monitoring, outdoor temperature predictive control method saves about 19.1% when it compared to outdoor temperature reset control method. Also, it is found that when partial load condition, such as daytime, the fluctuation of hot water supply temperature with outdoor temperature reset control is more severe than outdoor temperature prediction control. So, it proves that outdoor temperature prediction control is more stable even at partial load conditions.