What is the difference between rating and design calculations for heat exchangers?

Rating (performance) calculations: given an existing or specified exchanger geometry, determine the heat transfer rate and outlet temperatures for given inlet conditions. Uses NTU-effectiveness or iterative LMTD approach. Design (sizing) calculations: given process requirements (inlet/outlet temperatures, heat duty), determine the required geometry (surface area, tube count, shell diameter). Uses LMTD method with F-factor correction. Rating verifies if existing equipment meets requirements; design determines new equipment specifications.