Why U stamps on PHEs?

Hello all,


I am looking for some specific information on why plate and frame heat exchangers are typically provided with an ASME U stamp. It seems very odd when hanging, air cooling evaporators, which are not so different in function, do NOT require a U stamp (and thus do not require a safety relief valve). I have a sneaking suspicion that the U stamp is not really necessary (and thus relief valves) on PHEs are not really necessary, but I could be mistaken. In any case, I would really like to understand why PHEs are considered pressure vessels, if there is, in fact, a compelling reason. As a note, I am in the USA, so I'm looking for requirements based on codes in the states. I really appreciate any direction here!

Thanks!

J