Heat Exchanger Design Calculations (Step-by-Step)
Heat exchangers are critical components in industries such as oil & gas, chemical processing, and power generation. They facilitate the efficient transfer of heat from one fluid to another without mixing. The most commonly used type in high-pressure and high-temperature applications is the shell-and-tube heat exchanger.
This comprehensive guide walks through the full thermal and mechanical design of a shell-and-tube heat exchanger using steam as the hot fluid and hydrocarbon as the cold process stream. The example follows industry-recognized codes and standards such as ASME Section VIII Div. 1 and TEMA (Tubular Exchanger Manufacturers Association).
Types of Heat Exchangers
Heat exchangers come in many forms, but the most commonly used in oil and gas facilities include:
- Shell-and-Tube Exchangers – Cylindrical shell containing bundles of tubes. Fluid flows through both shell and tube sides. Suitable for high pressures and temperatures.
- Plate Heat Exchangers – Corrugated plates stacked to form narrow flow passages. Compact, efficient, but not ideal for fouling fluids.
- Air Cooled Heat Exchangers – Use ambient air for cooling. Common in refineries and remote facilities.
- Double Pipe Exchangers – Simple, used for small heat duties and clean fluids.
Applicable Codes and Standards
Proper design requires compliance with well-established industry codes and standards:
- ASME Section VIII Div. 1 – Governs pressure vessel design and calculation.
- ASME Section II Part D – Provides allowable material properties.
- TEMA – Standardizes heat exchanger types, tolerances, and configurations.
- API 660 – Common in oil and gas process exchangers; outlines mechanical and construction criteria.
Additional references include HTRI (Heat Transfer Research Inc.) data, and manufacturer datasheets for practical U-values and fouling factors.
Step-by-Step Thermal Design
1. Define Design Inputs
In our example:
| Parameter | Steam (Shell Side) | Hydrocarbon (Tube Side) |
|---|---|---|
| Inlet Temperature | 180°C (Saturated Steam) | 40°C |
| Outlet Temperature | Condensed at 180°C | 90°C |
| Pressure | 6 barg | 8 barg |
| Flow Rate | Sufficient for duty | 20,000 kg/hr |
| Specific Heat (Cp) | — (Latent Heat) | 2.1 kJ/kg·K |
| Latent Heat of Steam | ~2010 kJ/kg | — |
2. Calculate Heat Duty (Q)
Use the cold fluid side since the steam is condensing (constant temperature):
Q = m × Cp × ΔT = (20,000 / 3600) kg/s × 2.1 kJ/kg·K × (90 – 40) °C = 5.56 kg/s × 2.1 × 50 = 583 kW
Heat Duty = 583 kW
3. Estimate Temperature Difference – LMTD
For a condensing steam exchanger (constant temperature), use:
ΔT1 = 180 – 90 = 90°C
ΔT2 = 180 – 40 = 140°C
LMTD = (140 – 90) / ln(140/90)
≈ 113.7°C
4. Select Overall Heat Transfer Coefficient (U)
For steam-hydrocarbon shell-and-tube exchangers with light fouling, a typical U value is:
U = 500 W/m²·K (conservative value)
5. Calculate Required Heat Transfer Area (A)
Q = U × A × LMTD
=> A = Q / (U × LMTD)
= 583,000 W / (500 × 113.7)
= 10.26 m²
Required Heat Transfer Area = 10.26 m²
6. Select Tube Dimensions
Let’s use standard tubes:
- OD = 19.05 mm (3/4")
- Tube length = 3 m
- Number of tubes = A / Area per tube
Area per tube:
A_single = π × OD × L = 3.1416 × 0.01905 × 3 = 0.1796 m²
Number of tubes = 10.26 / 0.1796 ≈ 58 tubes
Use 60 tubes in a 1–2 pass layout.
Step-by-Step Thermal Design (Continued)
7. Velocity Check on Tube Side
High fluid velocity improves heat transfer, but too high may cause erosion and vibration. Recommended velocity: 1.0–2.5 m/s for hydrocarbons.
Let’s calculate:
Tube ID (assume thickness = 1.65 mm): ID = 19.05 mm – 2 × 1.65 mm = 15.75 mm = 0.01575 m Tube cross-sectional area: A_tube = π/4 × (ID)² = 0.000195 m² Total area for 30 tubes per pass (1-2 pass exchanger): A_total = 30 × 0.000195 = 0.00585 m² Mass flow rate = 5.56 kg/s Assume density of hydrocarbon = 750 kg/m³ Volumetric flow rate: Q_vol = 5.56 / 750 = 0.00741 m³/s Velocity (v) = Q_vol / A_total v = 0.00741 / 0.00585 ≈ 1.27 m/s
Tube side velocity = 1.27 m/s → Acceptable
8. Estimate Pressure Drop (Tube Side)
We use the Darcy-Weisbach equation:
ΔP = f × (L/D) × (ρ × v² / 2) Where: f = Friction factor (assume 0.03 for turbulent flow) L = Tube length = 3 m D = ID = 0.01575 m ρ = 750 kg/m³ v = 1.27 m/s
ΔP = 0.03 × (3 / 0.01575) × (750 × 1.27² / 2)
≈ 0.03 × 190.5 × (603.4)
≈ 3448 Pa = 3.45 kPa
Tube side pressure drop ≈ 3.45 kPa → Acceptable
Step-by-Step Mechanical Design
1. Design Conditions
- Tube Side: 8 barg, 90°C (Hydrocarbon)
- Shell Side: 6 barg, 180°C (Steam Condensation)
- Corrosion Allowance: 1 mm
- Design Code: ASME Section VIII Div. 1
2. Tube Thickness Calculation (External Pressure)
For external pressure design (as hydrocarbon is under internal pressure), we can use ASME formulas or charts.
- Material: SA213 TP304L
- Design Temp: 90°C → Allowable stress = 117 MPa
- OD = 19.05 mm, thickness = 1.65 mm
Using ASME charts or the equation:
t = (P × D_o) / (2 × S × E – 1.2 × P) Where: P = Design Pressure = 0.8 MPa D_o = 0.01905 m S = 117 MPa E = 1 (for seamless tubes)
t = (0.8 × 0.01905) / (2 × 117 – 1.2 × 0.8) = 0.01524 / 233.04 = 0.0000654 m = 0.0654 mm
Required thickness = 0.0654 mm → Provided thickness = 1.65 mm → OK
3. Shell Thickness Calculation
- Shell ID = 200 mm → Radius = 100 mm
- Material: SA516 Gr.70
- Allowable Stress at 180°C = 137 MPa
t = (P × R) / (S × E – 0.6 × P) Where: P = 0.6 MPa R = 100 mm S = 137 MPa E = 1.0
t = (0.6 × 100) / (137 – 0.6 × 0.6) = 60 / 136.64 = 0.439 mm Add corrosion allowance: 1 mm Minimum thickness = 1.44 mm → Use 5 mm (code min)
Shell thickness = 5 mm → Acceptable
4. Tubesheet Thickness
Follow TEMA guidelines for fixed tube sheet:
- Design differential pressure: 0.8 MPa (tube side higher)
- Pitch = 25 mm (triangular pattern)
- Tube count = 60
t_ts = (P × D × √3) / (2 × S × η) Where: P = 0.8 MPa D = 200 mm S = 117 MPa η = Ligament efficiency (typically 0.8)
t_ts = (0.8 × 200 × 1.732) / (2 × 117 × 0.8)
= 276.96 / 187.2
= 1.48 mm → Add 3 mm margin → Use 5 mm
Tubesheet thickness = 5 mm → Acceptable
5. Baffle Design (Shell Side)
- Baffle Type: Segmental, 25% cut
- Baffle Spacing: 20–50% of shell diameter → use 80 mm
- Supports tubes, promotes turbulence
6. Expansion Joint Check
If thermal stress between tube and shell is significant, include an expansion joint. In this case:
- Steam at 180°C, hydrocarbon at 90°C
- ΔT = 90°C → Check thermal growth
ΔL = α × L × ΔT Assume α = 17 × 10^-6 /°C ΔL = 17e-6 × 3 × 90 = 4.59 mm Since differential expansion is < 5 mm → Expansion joint NOT required.
Thermal expansion within limit → No expansion joint needed
Fabrication and Inspection Considerations
- Hydro test pressure: 1.5 × design pressure = 1.2 MPa (tube side), 0.9 MPa (shell side)
- Non-destructive examination (NDE): RT for shell welds, pneumatic leak test for tube-to-tubesheet joints
- Passivation for stainless steel tubes
Next: We’ll complete the example by preparing a layout, adding drawings, and discussing best practices.
Heat Exchanger Layout and General Arrangement
A well-planned mechanical layout ensures ease of maintenance, fluid routing, and support. The typical components of a shell-and-tube heat exchanger include:
- Shell (cylindrical pressure vessel)
- Tubes and tube bundle
- Tube sheets
- Channel heads and covers
- Baffles and support plates
- Nozzles (inlet and outlet)
- Support saddles or lugs
Recommended Layout Practices:
- Steam inlet on the top of shell-side to aid condensate removal
- Condensate outlet at bottom shell side
- Hydrocarbon (cold fluid) in tube side: inlet at lower side, outlet at upper side
- Drain and vent connections on both shell and tube sides
- Removable channel cover for tube bundle access
- Provision for thermowells and pressure taps
Drawings should clearly indicate:
- Tube layout and pitch pattern (triangular or square)
- Baffle spacing and orientation
- Nozzle size and rating (e.g., ANSI 150#)
- Welding details as per ASME Section IX
Best Practices in Heat Exchanger Design
Thermal Design Best Practices
- Always apply correct fouling factors (use TEMA recommendations)
- Choose counter-flow arrangement for maximum LMTD and effectiveness
- Keep tube velocity above 1 m/s to avoid laminar flow
- Account for tube-side pressure drop – balance with pump sizing
- Use U-tube bundle when thermal expansion between shell and tube side is significant
Mechanical Design Best Practices
- Use corrosion allowance based on process fluid aggressiveness (typically 1.0 mm)
- Select standard materials with known allowable stresses (ASME Section II Part D)
- Use TEMA R (refinery) class when operating in hydrocarbon service
- Consider seismic and wind loads if exchanger is outdoor
- Always add nameplate and lifting lugs
Fabrication & Testing Best Practices
- Perform hydrotest at 1.5 times design pressure
- Conduct air leak test on tube-to-tubesheet joints using soap bubble or halogen detection
- Use passivation for stainless steel surfaces after fabrication
- Inspect baffle edges for sharp corners – avoid vibration-related failures
Summary Design Checklist
Before finalizing the exchanger, verify the following:
- Thermal duty calculated and LMTD method validated
- U-value selected based on service, fouling, and flow type
- Heat transfer area determined and tube count selected
- Velocity and pressure drop acceptable on both sides
- Tube and shell wall thickness checked per ASME
- Tubesheet thickness and ligament efficiency verified
- xpansion joint considered if ΔT > 70°C
- Supports, lifting lugs, and drains included in drawing
- Inspection and testing procedures specified
Appendices
Appendix A: Common Overall Heat Transfer Coefficients (U)
| Service | U (W/m²·K) |
|---|---|
| Steam to Water | 800 – 1500 |
| Steam to Hydrocarbon | 400 – 800 |
| Oil to Water | 250 – 500 |
| Gas to Liquid (clean) | 100 – 250 |
| Gas to Gas | 20 – 100 |
Appendix B: Common Materials and Allowable Stress (at 150°C)
| Material | ASME Code Name | Allowable Stress (MPa) |
|---|---|---|
| Carbon Steel | SA 516 Gr. 70 | 137 |
| Stainless Steel | SA 213 TP304L | 117 |
| Low Alloy Steel | SA 335 P11 | 103 |
Appendix C: Units Conversion Table
| Quantity | From | To | Multiply By |
|---|---|---|---|
| Pressure | bar | Pa | 1e5 |
| Length | inch | mm | 25.4 |
| Heat Transfer Area | m² | ft² | 10.764 |
| Energy | kcal | kJ | 4.184 |
| Temperature | °C | °F | (×1.8) + 32 |
Conclusion
This detailed guide presents the complete methodology for designing a shell-and-tube heat exchanger using steam as the heating medium and hydrocarbon as the cold fluid. Following industry standards like ASME and TEMA ensures safe and efficient operation in critical industrial services.
From heat duty and thermal sizing to mechanical strength and layout, each design phase contributes to the long-term reliability of the exchanger. Always validate your calculations with software like HTRI or Aspen Exchanger Design & Rating for final approval.
Whether you're designing for a refinery, chemical plant, or offshore platform, this approach ensures your exchanger is ready for real-world operation.