In the Hysys calculations for heat exchangers, what is the Ft Correction Factor?

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Follow Sun Lanyi’s “Process Simulation Training” to complete Example 7.5, the calculation for a tubular heat exchanger. The heat transfer area is 20 m2, and the heat transfer coefficient is 230 W/(m2·°C), which is used to cool air at 50°C, with a flow rate of 5040 and a pressure of 200 kPa, down to 35°C. The cooling medium is cooling water at 25°C and 300 kPa. The pressure drops in the shell and tube sides are 15 kPa and 10 kPa respectively. Determine the outlet temperature and flow rate of the cooling water. At first, I tried to figure it out on my own without changing the value of Tube Passes Per Shell, using the default value of 2. As a result, the flow rate was 1289, and the exit temperature was 38.99. Meanwhile, the system indicated that the Ft Correction Factor was low. By changing the value of Tube Passes Per Shell to 1, as suggested in the book, the flow rate became 777.8, and the exit temperature was 48.35. If you think about it carefully, what the book actually implies is that this is a counterflow heat exchanger (which should have an impact on the heat transfer coefficient), and I just hadn’t noticed that before. What I want to know is: 1. What is the Ft Correction Factor? ； 2. How does Hysys determine that the Ft Correction Factor is too low? Thanks~~~~