Introduction
Heat exchangers transfer thermal energy between two fluid streams without mixing. Selecting the right type — shell-and-tube, plate-frame, air-cooled, or other — requires matching the fluids, temperatures, pressures, fouling tendency, and duty to the technology's strengths.
Type Comparison
| Type | Max Pressure | Max Temperature | Fouling Tolerance | Compactness |
|---|---|---|---|---|
| Shell-and-tube | > 300 bar | > 550°C | Excellent | Poor |
| Plate-and-frame (gasketed) | 25 bar | 180°C | Fair | Excellent |
| Welded / brazed plate | 40 – 100 bar | 400°C | Poor | Excellent |
| Air-cooled (fin fan) | > 300 bar | 500°C | Very good | Poor |
| Spiral | 25 bar | 400°C | Excellent (slurries) | Good |
| Double-pipe / hairpin | 300+ bar | 600°C | Good | Poor |
Shell-and-tube is the workhorse of refineries and chemical plants — robust, well understood, tolerant of high pressures and dirty services. Plate-frame dominates HVAC, dairy, and clean-fluid duties where compactness and easy cleaning matter. Air-cooled removes heat when cooling water is unavailable or expensive.
The LMTD Method
For simple counter-current or co-current flow, the log mean temperature difference (LMTD) is:
Where ΔT1 and ΔT2 are the terminal temperature differences at each end of the exchanger.
The heat duty is:
Where U is the overall heat transfer coefficient (W/m²·K), A is the heat transfer area (m²), and F is the LMTD correction factor for multi-pass arrangements (typically 0.75 – 1.0).
Typical Overall U-Values
| Service | U (W/m²·K) |
|---|---|
| Water — water | 850 – 1700 |
| Water — light oil | 350 – 900 |
| Water — heavy oil | 60 – 300 |
| Steam — water | 1000 – 4000 |
| Steam — light oil | 400 – 900 |
| Condensing hydrocarbon — water | 500 – 1100 |
| Gas — water | 30 – 300 |
| Air-cooler (gas service) | 100 – 350 (based on bare tube) |
TEMA Designations for Shell-and-Tube
Shell-and-tube exchangers follow the TEMA (Tubular Exchanger Manufacturers Association) nomenclature: three letters designating (1) front head, (2) shell, (3) rear head.
| Front | Shell | Rear |
|---|---|---|
| A: channel with cover | E: one-pass shell | L: fixed tubesheet |
| B: bonnet | F: two-pass with longitudinal baffle | M: fixed tubesheet |
| C: channel integral with tubesheet | H: double split flow | P: outside packed floating |
| N: channel integral with tubesheet | J: divided flow | S: floating head with backing device |
| D: high-pressure closure | K: kettle reboiler | T: pull-through floating |
| X: cross flow | U: U-tube bundle |
Common types:
- BEM — fixed tubesheet, cheap, non-removable bundle, for clean-clean service
- AES — floating head, most flexible, tubes accessible for cleaning
- AEU — U-tube, no differential expansion, but tube-side cleaning is limited
- AKT — kettle reboiler, common in distillation column reboilers
Sizing Procedure
- Set the duty: Q = m × cp × ΔT (or m × Δh for phase change).
- Determine terminal temperatures — both cold and hot inlet/outlet from process requirements.
- Compute LMTD and F factor for multi-pass.
- Estimate U from tables or from resistances 1/U = 1/hi + Rfoul,i + t/k + Rfoul,o + 1/ho.
- Solve for A = Q / (U × F × ΔTLM).
- Verify velocities and pressure drop — tube-side liquid 1 – 2 m/s, shell-side 0.5 – 1 m/s.
- Iterate the geometry until pressure drop is within budget (typically 0.2 – 0.7 bar per side).
Worked Example
Duty: Cool 50 t/h of process oil (cp = 2.2 kJ/kg·K) from 120°C to 60°C, using cooling water 30°C in, 40°C out.
- Heat duty: Q = (50,000/3600) × 2.2 × (120 − 60) = 1833 kW.
- LMTD (counter-current): ΔT1 = 120 − 40 = 80; ΔT2 = 60 − 30 = 30. LMTD = (80 − 30)/ln(80/30) = 51.0 K.
- Assume 1-2 shell/tube exchanger: F ≈ 0.87.
- Estimated U (water-oil, moderate fouling) = 400 W/m²·K.
- Area: A = 1,833,000 / (400 × 0.87 × 51) = 103 m².
- Select a BEM or AES exchanger with ~110 m² area for margin.
Choosing Between Types
Summary
Heat exchanger selection begins with the process conditions and ends with a technology whose materials, geometry, and cleaning provisions match the service. LMTD-U-A sizing gives the required area; TEMA type governs the mechanical design; realistic fouling assumptions keep the exchanger performing over its design life. Always cross-check with vendor thermal software before finalizing.