I know you're requesting assistance in the gasket designs, but I'm curious why you decided to use an u-tube design (shell & tube) for air to water application? Most of the time you need more surface area on the air side to properly transfer energy from one boundary to another. Most of the applications use a circular fin-tubes on the outer surface of the tube when water is moving inside of the tube. And circular fin-tubes do not work very well within the shell to tube applications.
Back to designing your gasket when you say "Pass Partitions Gaskets" are you referring to water separators located in the header? And What is your operating pressure and test pressure?
Is the pressure drop on the water-side? 0.5MPa or 72 psi
Jack @ Thermal Transfer Technology, LLC (Florida)