Shell-and-tube exchanger construction and TEMA types

Why it matters

Shell-and-tube exchangers are the workhorse of refineries and chemical plants: coolers, heaters, condensers, reboilers and feed–effluent exchangers. Thermal design gives the area, but the mechanical arrangement — which fluid goes in the tubes, how the bundle copes with thermal expansion, whether it can be cleaned — decides whether the exchanger survives and can be maintained. TEMA designations are the shared language for specifying these choices.

Key ideas

Main parts.

• Shell: the outer pressure vessel; carries the shell-side fluid.
• Tube bundle: many tubes (commonly 19.05 mm or 25.4 mm OD) carrying the tube-side fluid, fixed into tubesheets by rolling (expansion) or welding.
• Front and rear heads (channels): distribute the tube-side fluid; pass partition plates in the heads create multiple tube passes.
• Baffles: segmental baffles (typically 20–35 % cut) force the shell-side fluid to flow across the tubes, raising h, and support the tubes against sagging and flow-induced vibration. Spacing is usually between about one-fifth of the shell diameter (minimum, often with a 50 mm floor) and the shell diameter; closer spacing raises h and pressure drop.
• Tie rods and spacers, impingement plate at the shell inlet nozzle, longitudinal baffle for two-shell-pass (F-shell) units.

Tube layout. Triangular pitch (30°) packs more tubes into a shell and gives higher shell-side h but cannot be cleaned mechanically from outside; square (90°) or rotated-square (45°) pitch leaves cleaning lanes. Pitch is usually 1.25 × tube OD.

Passes. One shell pass with two (or 4, 6…) tube passes (1-2 exchanger) is the most common. More tube passes raise the tube velocity and h but raise pressure drop steeply and reduce the LMTD correction factor F. A 2-shell-pass (F-shell, or two shells in series) is used when a temperature cross would make F too low.

TEMA designation. Three letters: front head type – shell type – rear head type.

• Front head: A (channel with removable cover, tubes cleanable without disturbing piping), B (bonnet, integral cover, cheaper), N, C, D (high pressure).
• Shell: E (one-pass), F (two-pass with longitudinal baffle), G and H (split flow), J (divided flow, low Δp), K (kettle reboiler with vapour space), X (cross-flow).
• Rear head: L, M, N (fixed tubesheet), U (U-tube bundle), S (floating head with split backing ring), T (pull-through floating head), P and W (packed floating heads). Examples: BEM (fixed tubesheet), BEU or AEU (U-tube), AES (split-ring floating head), AKT (kettle reboiler).

Thermal expansion — the key mechanical choice.

• Fixed tubesheet (BEM, NEN): cheapest, no internal joints, tube side cleanable; but the shell side cannot be cleaned mechanically and the bundle is not removable. With a large shell-to-tube temperature difference an expansion bellows on the shell is needed.
• U-tube (BEU): bundle free to expand, removable, cheapest removable design, good for high pressure on the tube side; but U-bends are hard to clean mechanically inside, individual tubes are hard to replace, and only even numbers of passes are possible.
• Floating head (AES, AET): bundle removable, both sides cleanable, expansion accommodated; most expensive, internal gasketed joint can leak.

Fluid allocation (rules of thumb). Put in the tubes: the more fouling or corrosive fluid (tubes are easier to clean and only the tubes and channels need expensive alloy), the higher-pressure fluid (small-diameter tubes are cheaper to make thick than a shell), cooling water, and the hotter fluid if heat loss matters. Put in the shell: the more viscous fluid or the one with the lower flow rate (cross-flow over baffles gives a better h at low Re), condensing vapours (low pressure drop), and the fluid with a tight pressure-drop limit.

Typical velocities. Liquids in tubes about 1–2.5 m/s (water ≥ 1 m/s to limit fouling); shell-side liquids about 0.3–1 m/s.

Formulas

n_p = ṁ/(ρ·v·π·d_i²/4) Tubes per pass for a chosen tube-side velocity; ṁ (kg/s), ρ (kg/m³), v (m/s), d_i tube inside diameter (m).

N_t = A_o/(π·d_o·L) Total number of tubes for outside area A_o (m²), tube OD d_o (m), length L (m).

v = ṁ·N_p/(ρ·N_t·π·d_i²/4) Tube velocity for N_t tubes and N_p passes.

h_i ∝ v^0.8; Δp_t ∝ N_p·L·f·v² with f ∝ Re^(−0.2) Scaling of tube-side h and pressure drop in turbulent flow (constant flow rate: v ∝ N_p, so Δp_t ∝ N_p^2.8).

q = U·A·F·ΔT_lm Thermal design equation linking construction (F) to area.

Worked examples

Example 1 (standard). Cooling water (25 kg/s, ρ = 995 kg/m³) goes through the tubes of an exchanger needing 60 m² of outside area. Tubes are 19.05 mm OD, 14.83 mm ID, 4.88 m long. Target water velocity is about 1.5 m/s. Choose the number of tubes and passes, and find the actual velocity.

1. Flow area of one tube: π × 0.01483²/4 = 1.727 × 10⁻⁴ m².
2. Tubes per pass for 1.5 m/s: n_p = ṁ/(ρ·v·A_i) = 25/(995 × 1.5 × 1.727 × 10⁻⁴) = 97.
3. Tubes for the area: N_t = A_o/(π·d_o·L) = 60/(π × 0.01905 × 4.88) = 205.4 → 206 tubes.
4. Passes: 206/97 = 2.1 → use 2 tube passes with 103 tubes per pass.
5. Actual velocity: v = 25/(995 × 103 × 1.727 × 10⁻⁴) = 1.41 m/s (acceptable, above 1 m/s).

Answer: 206 tubes in 2 passes (103 per pass), water velocity ≈ 1.41 m/s — a 1-2 exchanger.

Example 2 (GATE level). In the exchanger above, the designer considers 4 tube passes instead of 2 with the same tubes and flow, to raise the tube-side coefficient. Estimate the factors by which h_i and tube-side pressure drop change (turbulent flow, ignore return losses), and comment.

1. Doubling the passes halves the tubes per pass, so velocity doubles: v → 2.82 m/s.
2. h_i ∝ v^0.8: factor 2^0.8 = 1.74.
3. Pressure drop: path length doubles (×2), v² rises ×4, and f ∝ Re^(−0.2) falls ×2^(−0.2) = 0.871. Factor = 2 × 4 × 0.871 = 6.96 (≈ 2^2.8).
4. Comment: h_i rises 74 % but Δp rises about 7 times, and 2.8 m/s may cause erosion. Worth it only if the tube side controls U and pressure drop is available; otherwise keep 2 passes.

Answer: h_i × 1.74; Δp_t × about 7.

Common mistakes

• Thinking TEMA "types" are just BEM/BEU/AES; the code is three letters (front head, shell, rear head) and many combinations exist.
• Specifying a fixed-tubesheet exchanger for a dirty shell-side fluid or for large temperature differences without an expansion joint.
• Putting the high-pressure or corrosive fluid on the shell side, which makes the shell expensive.
• Choosing triangular pitch when the shell side must be mechanically cleaned.
• Adding tube passes without checking F and pressure drop.

For GATE CH

Expect conceptual questions on the purpose of baffles, pitch choice, fluid allocation, thermal expansion arrangements (fixed tubesheet, U-tube, floating head), TEMA letters, and numerical questions on tubes per pass, tube velocity, and how h and Δp scale with number of passes or baffle spacing. Practise the scaling relations h ∝ v^0.8 and Δp ∝ v^1.8·L.

Quick check

1. What do the three letters in "AES" stand for?
2. Which fluid usually goes in the tubes: cooling water or a viscous oil?
3. Why is a triangular pitch avoided for dirty shell-side fluids?
4. Name two functions of baffles.
5. Which design is cheapest when the bundle must be removable?

Answers: 1. A: removable-cover channel; E: one-pass shell; S: floating head with split backing ring. 2. Cooling water. 3. The tubes cannot be cleaned mechanically from outside (no cleaning lanes). 4. Direct shell-side flow across the tubes (higher h) and support the tubes against sagging and vibration. 5. U-tube.