A shell-and-tube heat exchanger is described by a three-letter TEMA code — for example, BEM, AES, or NEN. Each letter refers to a specific component: the front (stationary) header, the shell, and the rear header. Getting this code right at the specification stage determines cleaning access, thermal expansion accommodation, pressure rating, and maintenance cost over the equipment's life.
The TEMA Three-Letter Code System
TEMA (Tubular Exchanger Manufacturers Association) introduced a standardised nomenclature to eliminate ambiguity in specifying heat exchangers. The three letters describe, in order: (1) the front-end stationary head type, (2) the shell type, and (3) the rear-end head type. A fully specified exchanger also includes nominal diameter and tube length — for example, 20–192 BEM means a 20-inch shell, 192-inch tubes, BEM configuration.
Front-End (Stationary) Head Types
The front head is where the tube-side fluid enters. The most common types are:
- Type A — Channel with removable cover. The most common type. The cover can be unbolted for inspection and cleaning without disturbing the tube bundle or piping. Preferred when tube-side fouling is expected.
- Type B — Integral (bonnet) cover. Cheaper to fabricate than Type A because there is no separate channel cover flange, but the tubesheet must be unbolted to access tubes. Use only where tube-side fouling is low.
- Type C — Channel integral with tubesheet and removable cover. Similar to Type A but the channel is welded to the tubesheet. Used when the tube-side fluid is hazardous and a gasketed tubesheet joint is unacceptable.
- Type N — Fixed tubesheet, channel integral with tubesheet. No channel gasket. Both tubesheets are welded to the shell. Used for high-pressure, hazardous fluids where joint leakage cannot be tolerated.
- Type D — Special high-pressure closure. Used above 600 bar. The cover threads onto the channel barrel to handle extreme pressures.
Shell Types
The shell letter is the most performance-critical of the three. It determines flow path, pressure drop, and suitability for multi-pass or mixed-phase services.
| Shell Type | Flow Pattern | Best For | Limitations |
|---|---|---|---|
| E | Single-pass, one shell-side inlet and outlet | Most common. Clean services, sensible heat transfer | High pressure drop for viscous fluids; limited for temperature cross scenarios |
| F | Two-pass shell (longitudinal baffle divides shell) | True counter-current flow; handles temperature cross that requires 2 shells in series | Longitudinal baffle prone to leakage; bypass reduces thermal efficiency if poorly welded |
| G | Split flow — fluid enters at centre, exits at both ends | Horizontal thermosiphon reboilers; services with low allowable pressure drop | Low shell-side turbulence; not suited for fouling services |
| H | Double split flow — two inlets, two outlets | Very low pressure drop requirement; large-diameter exchangers | Complex piping; limited baffle options |
| J | Divided flow — single inlet, two outlets (or reverse) | Condensers, low pressure drop services, steam heating | Cannot be used where temperature crossover exists |
| K | Kettle type — enlarged shell with weir | Thermosiphon reboilers, steam generators, vaporisers | High cost; vapour disengagement requires adequate shell diameter |
| X | Cross-flow — no baffles, shell-side flows across tubes perpendicularly | Vacuum condensers, gas coolers with very low allowable pressure drop | Poor thermal performance; limited to applications where pressure drop governs |
Rear-End Head Types
The rear head determines how thermal expansion between tubes and shell is accommodated — this is often the deciding factor in the overall design.
- Type L — Fixed tubesheet (same as front A, but rear). Both tubesheets welded to shell. No differential thermal expansion accommodation; requires an expansion bellows in the shell if ΔT is significant. Lowest cost, highest tube-side and shell-side pressure rating.
- Type M — Fixed tubesheet, bonnet (integral cover). Similar to L, but with bonnet style rear head. Used for low-fouling, low-ΔT services.
- Type N — Fixed tubesheet, channel integral. Full welded design for hazardous, high-pressure services. No expansion accommodation.
- Type U — U-tube bundle. Tubes are bent into a U-shape. The bundle can be removed completely for shell-side cleaning. No rear tubesheet. Cannot accommodate individual tube replacement; tube-side cleaning by mechanical rodding is impossible. Lowest cost when differential thermal expansion is significant.
- Type S — Floating head with backing device. The rear tubesheet floats inside the shell, clamped by a split shear ring and backing device. Bundle can be removed. Most common floating-head type for hydrocarbon and chemical services.
- Type T — Pull-through floating bundle. The rear tubesheet is smaller than the shell ID, allowing the bundle to be pulled out of the shell without removing the shell cover. Most maintainable; largest bypass area between bundle and shell.
- Type W — Externally sealed floating head (packed gland). The rear tube sheet slides out through a packed gland. Allows expansion but gland packing is a potential leak path; used for low-pressure services only.
Common TEMA Configurations and Their Typical Applications
| TEMA Code | Description | Typical Application |
|---|---|---|
| BEM | Bonnet front, E shell, Fixed tubesheet rear | Clean, non-fouling, low ΔT services. Chemical coolers, water heaters |
| AEM | Channel front, E shell, Fixed tubesheet rear | Moderate fouling tube-side. Cooling water services where tube cleaning is needed |
| AES | Channel front, E shell, Floating head (S) | Refinery and chemical services with fouling on both sides and high ΔT |
| AET | Channel front, E shell, Pull-through bundle (T) | Heavy fouling, high-ΔT services where maximum bundle accessibility is needed |
| BEU | Bonnet front, E shell, U-tube rear | Clean tube-side, fouling shell-side services. Steam heaters, condensers |
| AKT | Channel front, Kettle shell, Pull-through bundle | Reboilers, vaporisers, steam generators |
| AJW | Channel front, J shell, Packed gland floating | Condensers with low allowable shell-side pressure drop |
| NEN | Fixed tubesheet, E shell, Fixed tubesheet | High-pressure, hazardous fluids (H₂, Cl₂, ammonia) where no gasketed joint is permitted |
How to Select the Right Configuration
Follow this logic in sequence:
- Step 1 — Determine shell type based on pressure drop budget, flow pattern requirement (counterflow?), and whether the service involves phase change (reboiling, condensing).
- Step 2 — Determine rear head type based on differential thermal expansion (tube metal temp minus shell metal temp). If ΔT > 40–50°C, a floating or U-tube design is strongly recommended over fixed tubesheet.
- Step 3 — Determine front head type based on tube-side fouling. If the tube-side needs periodic mechanical cleaning, specify Type A. If tube-side is clean and cost matters, Type B is acceptable.
- Step 4 — Check TEMA class requirement (R, B, or C — covered in a separate article) to determine whether additional mechanical requirements apply.
- Step 5 — Confirm material of construction for shell, tubes, baffles, and tubesheets based on fluid corrosivity and temperature.
Key Takeaways for Procurement Engineers
When issuing an enquiry or purchase order for a shell-and-tube heat exchanger, always specify: the TEMA code (three letters), nominal shell diameter, effective tube length, TEMA class (R/B/C), design pressure and temperature for both sides, fluid names and basic properties, and applicable codes (ASME Section VIII, TEMA 11th Edition). Omitting any of these forces the manufacturer to assume — and assumptions invariably result in a design that costs more, performs differently, or requires engineering clarification that delays delivery.
If you are unsure which TEMA code to specify, describe your service conditions (fouling nature, ΔT, phase change) and ask the manufacturer to recommend the configuration. A reputable manufacturer will explain the trade-offs.
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