TEMA Standard — Shell-and-Tube Heat Exchanger Design, Fouling & Tube Sheet Calculator

TEMA (Standards of the Tubular Exchanger Manufacturers Association) is the primary industry reference for the mechanical design, classification and rating of shell-and-tube heat exchangers. It defines three design classes (R, B, C) for different service severity, tabulates fouling resistances (dirt factors) for common process fluids, specifies minimum tube pitch and baffle-spacing rules, and establishes the three-letter type code (e.g. AES, BEU, NEN) used on datasheets and purchase orders worldwide. Thermal design methods — LMTD F-correction, NTU-effectiveness — are applied alongside TEMA mechanical rules.

Class R is the most stringent, intended for severe petroleum refinery and related process service where reliability and long service life are critical. Class B (general process) is slightly less conservative and covers most chemical and petrochemical applications. Class C (commercial) has the lightest requirements and is used for low-pressure, non-fouling, light-duty industrial service. The classes differ in minimum tube wall thickness, corrosion allowance, baffle and tube sheet thickness, and bundle-to-shell clearance. Class B is the default for most process-plant datasheets.

The LMTD F-correction factor accounts for the fact that a shell-and-tube exchanger with multiple tube passes is not in pure counterflow — some tube passes are in co-current flow with the shell fluid. The counterflow LMTD overstates the effective mean temperature difference, so F ≤ 1.0 scales it down. TEMA guidance (following the Bowman-Mueller-Nagle 1940 charts) recommends F ≥ 0.75 for a single shell pass; below that the required area becomes very sensitive to small changes in the terminal temperatures and adding a shell pass is more economical.

TEMA's fouling tables list a resistance R_f (m²·K/W) for each fluid type — for example cooling tower water on the tube side is typically 0.000 176 m²·K/W. This resistance is added to the clean overall resistance 1/U_clean on both the hot and cold sides to give a fouled overall coefficient U_fouled. Since U_fouled < U_clean, the designer must specify more heat-transfer area than the clean calculation indicates — the cleanliness factor CF = U_fouled / U_clean (always ≤ 1) quantifies how much larger the exchanger must be to maintain its duty after fouling has developed.

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