The purpose of this document is to justify and provide technical criteria for the rules of Part Unfired Heat Exchanger (UHX) of ASME Section VIII Division 1, 2013 Edition, devoted to the design of U-tube, Fixed and Floating head Tubesheet Heat Exchangers. The criteria document applies also to Paragraph 4.18 of Section VIII, Division 2, 2013 Edition, which is entirely based on Part UHX.