Heat Exchanger
Sizing (Excel)

A heat exchanger sizing calculation determines the heat transfer area, tube count, shell ID, baffle arrangement, and pressure drop for a shell and tube heat exchanger to deliver the required duty within process and mechanical constraints. It records process data on each side (composition, temperature, flow, pressure), the heat duty, LMTD with F factor correction, overall heat transfer coefficient U, fouling resistance per TEMA, tube geometry, number of tubes and passes, shell ID, baffle arrangement, pressure drop, material, and applicable design code (ASME Section VIII Div 1 / Div 2 or PED).

Required columns: sizing reference, exchanger tag and service, TEMA type (BEM / AES / NEN / etc.), process stream identifier shell and tube side, fluid composition and phase, inlet / outlet temperatures, mass flow rates, operating and design pressure and temperature, heat duty, LMTD with F factor, overall heat transfer coefficient U, required heat transfer area, fouling resistance per TEMA Section 9, tube OD / wall / length, tube pitch and layout (triangular or square), number of tubes, number of tube passes, shell ID and shell passes, baffle type / spacing / cut, shell and tube side pressure drop, material of construction, and design code reference per ASME Section VIII.

Pathnovo, an engineering document intelligence platform, builds the heat exchanger sizing by extracting process data from the heat and material balance and the stream summary, reading service conditions from the P&IDs, and applying TEMA / API 660 geometry rules to produce a thermal-hydraulic sizing draft. The platform computes LMTD, applies F factor correction for multi-pass, estimates U from correlation data or HTRI-style coefficients, sizes the area, and selects tube count, shell ID, and baffle arrangement. Drafts are reviewed by the process engineer and feed the equipment datasheet. See the P&ID extraction workflow for upstream data capture.

LMTD (Log Mean Temperature Difference) is the standard driving force for heat transfer: LMTD = (dT1 - dT2) / ln(dT1 / dT2), where dT1 and dT2 are the terminal temperature differences. For multi-pass shell and tube exchangers (e.g., 1 shell pass 2 tube pass), the F factor correction accounts for the non-counter-current flow and is read from TEMA Figure T-3.2 charts as a function of the dimensionless groups P and R. The effective driving force is F x LMTD. F factor below 0.75 indicates the configuration is unsuitable and a different TEMA type or multiple shells in series is required.

TEMA (Standards of the Tubular Exchangers Manufacturers Association) covers the mechanical standard for shell and tube heat exchanger types, nomenclature, and tolerances. It defines TEMA class R (refinery, most stringent), class C (commercial, general process), and class B (basic, light duty), and gives the standard type designation (BEM, AES, NEN, etc.). API 660 (Shell and Tube Heat Exchangers) is the refinery and petrochemical purchase specification that references TEMA class R as the baseline and adds API-specific requirements for materials, inspection, and testing. EPC scope for refineries uses both standards together. PED (Pressure Equipment Directive) applies for European installations.

Fouling resistance (Rf) is added to the clean overall heat transfer coefficient (Uc) to give the dirty (or service) overall heat transfer coefficient (Ud): 1 / Ud = 1 / Uc + Rf-shell + Rf-tube. TEMA Section 9 (Recommended Good Practice) gives indicative fouling values for typical refinery and petrochemical services: crude oil 0.0004 to 0.0009 m²-K/W, cooling tower water 0.0002 to 0.0004 m²-K/W, steam 0.00009 m²-K/W. The sizing template captures Rf-shell and Rf-tube as separate inputs and uses Ud for area sizing. The cleanliness factor (Cf = Ud / Uc) is typically 0.7 to 0.85 for refinery services.

On a revamp project, the existing exchanger is the starting point. The sizing is re-run with the new operating conditions (revamped throughput, new feedstock composition, modified heat integration). If the existing shell can deliver the duty at the new conditions, only the tube bundle may need replacement (re-tubing with different metallurgy, different layout, or different number of tubes). If the existing shell cannot, a new exchanger or an additional shell in series or parallel is required. Particularly important on Indian PSU refinery revamps where the existing TEMA class R exchangers may date to the 1980s and require IBR re-certification per the IBR compliance reference.