Heat Exchangers

This lesson covers heat-exchanger hardware and specification. Learn TEMA shell-and-tube types, tube layout and bundle sizing, tube-side and shell-side velocities, baffles, fouling allowances, materials, plate and air-cooled exchangers, and the exchanger datasheet.

Thermal calculations give the required area; mechanical design turns it into a real exchanger with a shell, tubes, baffles, nozzles and heads that can be built, cleaned and inspected. Shell-and-tube exchangers are the most common because they handle high pressures and temperatures and are easy to maintain.

Good specification avoids fouling, vibration and leaks for decades of service.

An analogy

A shell-and-tube exchanger is like a bundle of drinking straws inside a pipe: one fluid flows through the straws, the other snakes around them, guided by baffles.

TEMA types

Designated by front head, shell and rear head letters (e.g. AES, BEM, AEU). Fixed tubesheet (cheap, no shell-side cleaning), U-tube (handles expansion, hard to clean tube bends), floating head (removable bundle, cleanable, costlier). TEMA classes R (refinery), C and B.

Fluid allocation

Put on the tube side: fouling, corrosive, high-pressure or cooling-water streams (easier to clean, cheaper alloy). Shell side: viscous fluids or condensing vapours where baffles improve transfer. Tube-side water velocity ≈ 1–2.5 m/s limits fouling and erosion.

Baffles and vibration

Segmental baffles (20–35 % cut) direct shell flow across tubes, spaced 0.2–1 shell diameters. High shell velocities can cause flow-induced tube vibration — checked per TEMA. Impingement plates protect tubes at inlet nozzles.

Alternatives and specification

Plate exchangers (compact, high U, close approach, limited pressure), air coolers (no water, large footprint), spiral and printed-circuit exchangers. Datasheets (TEMA/API 660) record duty, fluids, pressures, temperatures, materials, fouling factors and allowable pressure drops; HTRI/Aspen EDR perform rating.

Formulas

• Tube count: N_t = A / (Ï€ d_o L)
• Bundle diameter: D_b = d_o (N_t / K₁)^(1/n₁) (1-pass, triangular: K₁ = 0.319, n₁ = 2.142)
• Tube-side velocity: v = ṁ / (ρ · N_t/n_p · π d_i²/4)
• Overall coefficient with fouling: 1/U = 1/h_o + R_fo + wall + (R_fi + 1/h_i)(d_o/d_i)

Worked example: Tube count and bundle

Given: Required area 50 m²; Tubes 19.05 mm OD × 6 m, 1 pass, triangular 25.4 mm pitch

Find: Tubes and bundle diameter.

• Area per tube = Ï€ × 0.01905 × 6 = 0.359 m² → N_t = 139
• D_b = 19.05 × (139/0.319)^(1/2.142) = 325 mm

Answer: 139 tubes, ≈ 325 mm bundle (shell ≈ 350–380 mm)

Worked example: Tube-side velocity

Given: 30 kg/s water, 139 tubes, ID 15.75 mm, 1 pass

Find: Velocity.

• Flow area = 139 × Ï€/4 × 0.01575² = 0.0271 m²
• v = 0.030 m³/s/0.0271 = 1.11 m/s

Answer: 1.1 m/s ✓ (within 1–2.5 m/s)

Common mistakes

• Wrong: Putting dirty cooling water on the shell side. — Right: Tube side is easier to clean — allocate fouling fluids there.
• Wrong: Low tube velocities to cut pressure drop. — Right: Low velocity promotes fouling — keep water ≥ 1 m/s.

Applications

• Refineries: Crude preheat trains, overhead condensers, reboilers.
• Chemicals & power: Feed/effluent exchangers, surface condensers.
• Food & pharma: Plate and tubular exchangers with hygienic design.

Key takeaways

• TEMA types trade cost vs cleanability and expansion.
• N_t = A/(Ï€d_oL); bundle ∝ √N_t.
• Tube-side water 1–2.5 m/s.
• Specify fouling, materials and allowable ΔP.

Going further

• Flow-induced vibration: Vortex shedding and fluid-elastic instability checks (TEMA Section 6).
• Bell–Delaware method: Shell-side coefficients with leakage and bypass corrections.
• Heat-exchanger networks: Pinch-based retrofit of preheat trains.