The heat exchanger sizing calculator estimates the heat duty, LMTD and correction factor F, required surface area and a preliminary shell-and-tube layout for sensible, condensing and boiling services. It adds Bell-Delaware shell-side checks, pressure drop, fouling, TEMA screening, cost optimisation and a datasheet export.
How heat exchanger sizing works
Sizing starts from an energy balance and ends with an area. Everything in between is about finding a realistic overall heat transfer coefficient and the true mean temperature difference.
1. Heat duty
Q = m × cp × ΔT for sensible streams, or Q = m × hfg for condensing and boiling. The hot-side and cold-side duties must agree within the energy-balance tolerance.
2. Log-mean temperature difference
LMTD = (ΔT1 − ΔT2) / ln(ΔT1 / ΔT2), with ΔT taken at each end of a counter-current exchanger.
3. Correction factor F
Multi-pass shell-and-tube units are not purely counter-current, so the effective mean difference is F × LMTD. F is calculated from the dimensionless ratios R = (Th,in − Th,out) / (Tc,out − Tc,in) and P = (Tc,out − Tc,in) / (Th,in − Tc,in). Designs below about F = 0.75–0.80 are usually rejected in favour of more shell passes.
4. Area
A = Q / (U × F × LMTD), where U is the service (fouled) overall coefficient: 1/U = 1/ho + Rfo + wall resistance + (Rfi + 1/hi) × (Do/Di).
Worked example: water-to-water cooler, 1-2 shell and tube
Illustrative inputs: hot water 10 kg/s cooled from 80 to 60 °C; cooling water 15 kg/s entering at 25 °C; cp = 4.18 kJ/kg·K; service U = 1000 W/m²·K.
- Duty = 10 × 4.18 × 20 = 836 kW.
- Cold outlet = 25 + 836 / (15 × 4.18) ≈ 38.3 °C.
- LMTD (counter-current) with ΔT1 = 41.7 K and ΔT2 = 35.0 K ≈ 38.2 K.
- F for one shell pass and two tube passes, with R = 1.50 and P = 0.242, ≈ 0.97.
- Area = 836 000 / (1000 × 0.97 × 38.2) ≈ 22.6 m², before design margin.
Typical overall coefficients for first estimates
| Service | Typical U (W/m²·K) |
|---|---|
| Water – water | 800–1500 |
| Steam condensing – water | 1500–4000 |
| Light oil – water | 350–900 |
| Heavy oil – water | 60–300 |
| Gas – gas (low pressure) | 10–50 |
| Organic vapour condenser – water | 700–1000 |
These are order-of-magnitude ranges from common design references; the calculator replaces them with film coefficients once geometry is defined.
Assumptions and limitations
Built-in fluid properties are reference values, and the mechanical and TEMA checks are screening only. Final thermal and hydraulic design should be completed with rigorous software (for example HTRI or Aspen EDR), vendor data and ASME VIII / TEMA mechanical design by a qualified engineer.
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Frequently asked questions
How do you size a heat exchanger?
Calculate the heat duty from the energy balance, find the log-mean temperature difference and its correction factor F for the chosen arrangement, estimate the overall heat transfer coefficient U including fouling, then divide: area = duty / (U x F x LMTD). Add a design margin and check pressure drop on both sides.
What is LMTD and why is a correction factor needed?
LMTD is the log-mean of the temperature differences at the two ends of the exchanger and is exact for pure counter-current or co-current flow. Shell-and-tube units with multiple tube passes are partly co-current, so the effective mean difference is lower; the correction factor F accounts for this and is calculated from the R and P ratios.
What is a good LMTD correction factor?
Most designers aim for F of 0.8 or higher, and treat values below about 0.75 as unacceptable because the design becomes very sensitive to small changes in temperature. A low F usually means a temperature cross, which is fixed by adding shells in series.
What fouling resistance should I use?
Typical fouling resistances are around 0.0001 to 0.0002 m2K/W for treated cooling water and clean light hydrocarbons, and 0.0003 to 0.0009 m2K/W or more for heavy oils, crude and untreated water. Use the values in your project specification or TEMA tables where specified.
What is the difference between clean and service U?
Clean U excludes fouling and represents a new exchanger. Service U includes the fouling resistances and is the value used for sizing so that the exchanger still meets duty at the end of its run between cleanings.
Is this calculator suitable for final heat exchanger design?
No. It is a preliminary design and screening tool for duty, area, layout and cost studies. Final thermal design should be verified with rigorous rating software, vendor data and ASME Section VIII and TEMA mechanical design.
Written by Muhammad Raza
Founder of Aienginear. Engineering and digital solutions professional with 26+ years managing projects, plant shutdowns and turnarounds in oil & gas, petrochemical and power generation.
