A simplified rating system for commercial shell-and-tube heat exchangers is developed based on the fluid-flow-pattern concepts derived by the author for the General Discussion of Heat Transfer, London, England, 1951. Recommended heat-transfer and friction characteristics for practical design purposes are presented which are based on previously presented research data and certain findings of the ASME—University of Delaware Heat Exchanger Research Project. Where possible, without serious sacrifice in accuracy, certain resistance-factor approximations and assumptions have been introduced in the interest of practical simplicity for the derivation of formulas of simple form for evaluating the fluid-flow fraction through the crossflow area of the tube bundle. Simple tabulations of heat-transfer and pressure-drop characteristic “rating numbers” may be readily developed from these formulas for various styles or series of heat exchangers having consistent design proportions. These rating numbers permit a rapid rating of the heat-exchanger shell-side performance. The system is responsive to the major effects of unit size, tube size, tube pitch and flow orientation, baffle spacing and cut, and the principal leakage and bypass clearances of the heat-exchanger design. The system developed is accurate well within practical commercial requirements, and is very flexible so that as more accurate resistance factors are developed by research for specific cases, they may be employed in the system for more precise results when desired.