The HTRI Design Manual is considered the "bible" of thermal design and rating for heat exchangers. It contains the proprietary methods, correlations, and equations used by the HTRI software suite (Xist, Xace, Xphe, etc.).
HTRI incorporates specific correlations for tube patterns (30°, 60°, 90°, 45° layouts). For example, a 45° rotated square pitch generally offers a balance between heat transfer and pressure drop, while a 90° square pitch is preferred for cleaning but results in lower thermal performance. Htri Design Manual Pdf 123