If your chiller is single pass (and I presume it is since it is a fixed tubesheet), you must also consider that the heat flux and vapor generation rate at the hot tubesheet may be much higher than at the cold end. This will further increase your actual horizontal vapor velocity, as it would not...
Suggest you obtain a copy of ASME Document PVP20100-25131 (Proceedings of the ASME 2010 Pressure Vessels & Piping Division / K-PVP Conference, Bellevue Washington, July 18-22) entitled "Evaluation of Thermal Contact Resistance at the Interface Composed of Dissimilar Materials".
