My question is regarding backflow total temperature that need to be set to avoid backflow. I am simulating flow through heat exchanger, where at process outlet I have given pressure outlet, hot fluid is entering shell at 493k and utility fluid at 338K through tubes. Solver is giving me the reverse flow is happening. My question here is what should be the backflow temperature when I don't know temperature at outlet.

It should be near atmospheric or it should be higher to avoid backflow.

(Physically there is no backflow at outlet)