**4. Conclusions**

To investigate the damage of a natural gas heat exchanger, the numerical simulations of the flow fields of the original and modified designs are performed. It is found that there are two reasons for the damage. First, at the required operating conditions, the forced convection is weak, and the natural convection is strong and comparable with the forced convection. These two actions are perpendicular and compete to each other. As a result, strong unsteadiness in the flow field is induced. Second, the whole assembly is mounted horizontally and the flow exit pipe is located at the lowest position. Consequently, the high-temperature or low-density fluid is trapped in the upper portion of the vessel. The trapped fluid is continuously heated by the heating elements located in the upper region of the vessel and eventually exceeds the allowed service temperature of the steel pipe.

The numerical results and analysis suggest that the heat exchanger assembly should be mounted vertically and the exhaust pipe should be located at the top of the exchanger. With these modifications, the flow parameters become more or less uniform at each vertical cross section, the flow field becomes stable, the methane temperature at the exit reaches the designed value, and the vessel wall temperature remains the same as the surrounding gas. This new design has been trouble-free used up to now.
