Kachan:
You are obviously a student, so you have difficulty understanding the complexity of a coil-wound heat exchanger (CWHE) as opposed to what we normally see and use – a TEMA-based shell and tube heat exchanger. There is a world of difference in the technology and methods employed and one has to be very mindful of the objectives and challenges that confront a CWHE design. If you are dealing with a Linde CWHE, then you have probably one of the most arcane and proprietary limited designs in heat exchangers. This type of design is primarily applied in cryogenic projects – but very big and expensive, world-scale projects. This is a heat exchanger application that consumes the time and efforts of perhaps several teams in a typical world-scale LNG plant design. You will never find any HTRI program to design this type of heat transfer unit – and much less in HySys. There are only a few companies that can be trusted, in my opinion, with a credible and dependable design – such as Air Products, Linde, and a few others. And they are not going to share their hard-earned and expensive expertise with anyone.
I believe you are dealing with an LNG or similar application and I don’t see what you can prove by developing an academic calculation (or “estimate”) when no one in academia is going to be able to determine or guess if your estimate (or guess) is correct or not. All you can do is guess at what you think is an appropriate overall “U” and leave it at that. Who is going to challenge it? Besides, there is little that you can do with an overall “U” when it comes to a CWHE because you can’t accurately determine the physical shape and size of the ultimate exchanger. You can’t do this because of the very complex constraints and demands that you have to confront with respect to thermal expansions and stresses that are built up due to cryogenic temperatures as well as the very tight and narrow temperature approaches that you must meet in order to make the finished product capable of meeting very efficient heat transfer demands. You are not going to find the tools to solve these problems in a text book - or in a classroom lecture.
If I were you, I would simply make an educated guess at a conservative overall “U”, guess at the overall dimensions and weights, and leave it to others to prove otherwise. No one will contradict or challenge you – unless they have done the real thing before, which I highly doubt will be the case.