Sometimes it can happen, that with high heat exchanger surface and low duty the temperature of cold and hot side of a heat exchanger cross. For this there is a way implemented to handle, but it may not be a very sophisticated way. A different approach could be taken, for example with penalty terms as described here: https://orbi.uliege.be/bitstream/2268/96436/1/PhD_Thesis_Dissertation.pdf
Sometimes it can happen, that with high heat exchanger surface and low duty the temperature of cold and hot side of a heat exchanger cross. For this there is a way implemented to handle, but it may not be a very sophisticated way. A different approach could be taken, for example with penalty terms as described here: https://orbi.uliege.be/bitstream/2268/96436/1/PhD_Thesis_Dissertation.pdf