Temperature crossover and heat load issues when using EDR to calculate BKU-type heat exchangers

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Hello everyone. Recently, I was working on the calculation model for a generator; a kettle-type heat exchanger was chosen as the heat exchange device. The heat source is hot water at 126°C, which is used to heat ammonia water at 77°C to 95°C. Meanwhile, the temperature of the hot water drops to 85°C, at which point ammonia gas begins to evaporate. During the calculation, I found that if a single-pipeline approach is used, the calculation fails to succeed. I would like to ask why this is the case Later, I recalculated using a BKU-type heat exchanger, and the calculations passed, but it was noted that the heat transfer rate of this heat exchanger had exceeded its maximum capacity. However, no points where temperatures intersected in any local heat transfer areas were found in the temperature curve. Is there something wrong with these calculations? I hope someone experienced in this field can reply