Enter Heat Exchanger Data
Required fields calculate Uo and Ui. Area and LMTD are optional, but both are needed for heat duty.
Example Calculation Data
| Input | Example value | Unit |
|---|---|---|
| Inside convection coefficient, hi | 1000 | W/m²·K |
| Outside convection coefficient, ho | 800 | W/m²·K |
| Tube diameters, di / do | 20 / 25 | mm |
| Tube wall conductivity, kw | 16 | W/m·K |
| Inside / outside fouling resistance | 0.0002 / 0.0001 | m²·K/W |
| Outer area / LMTD | 12 / 25 | m² / K |
Formula Used
The calculator provides coefficients based on the tube outside area and tube inside area. Diameter values are first converted from millimetres to metres.
Outside-area overall coefficient
1 / Uo = 1 / ho + Rf,o + [do × ln(do / di)] / (2kw) + (do / di) × [1 / hi + Rf,i]
Inside-area overall coefficient
1 / Ui = (di / do) × [1 / ho + Rf,o] + [di × ln(do / di)] / (2kw) + 1 / hi + Rf,i
Heat duty estimate
Q = Uo × Ao × LMTD
Use the same surface basis for U and area. Apply a correction factor separately for suitable multipass configurations.
How to Use This Calculator
- Enter inside and outside convection coefficients from a reliable source.
- Enter actual tube inside and outside diameters. The outside value must be larger.
- Enter tube wall conductivity and fouling resistance for both surfaces.
- Provide outer heat-transfer area and LMTD only when estimating heat duty.
- Select Calculate coefficient to display Uo, Ui, resistances, and optional duty.
- Use Uo with outside area or Ui with inside area. Do not mix them.
Heat Exchanger Performance Essentials
What the coefficient represents
An overall heat transfer coefficient combines thermal resistances between two fluids. It gives designers one practical value for sizing and performance checks. The coefficient includes convection inside the tube and outside it. Tube-wall conduction adds another resistance. Deposits add fouling resistance. A small result signals restricted heat flow. A high result signals efficient thermal communication. Engineers combine U, area, and temperature difference to predict duty. The coefficient needs a stated area basis. Outside-area U differs from inside-area U. Both values describe the same equipment. Their numerical values still differ. This calculator reports both surface-based values. It prevents mistaken equipment-data comparisons. Use consistent units throughout calculations. Coefficients use watts per square metre kelvin. Fouling resistance uses square metre kelvin per watt.
Resistance sources during operation
Convective resistance is often the largest controllable term. Fluid speed influences convection. Viscosity also changes fluid-side performance. Surface condition can reduce coefficient values. Higher turbulence usually raises convection. That can improve heat transfer. It can also raise pumping power. Good design balances these effects. Tube-wall resistance matters for low-conductivity materials. Thin metal tubes usually add little resistance. Thick walls can add more resistance. Polymer tubes can also matter greatly. Fouling grows during normal operation. Scale and oil films reduce performance. Corrosion products can have similar effects. Biological growth can also restrict flow. Design fouling allowances protect required duty. Excessive allowances can oversize equipment. Inspect operating history before selecting values. Use plant standards when available. Record every assumption for future troubleshooting.
Choosing the correct surface basis
The outside-area equation begins with external convection. It then adds outside fouling resistance. Cylindrical wall conduction follows next. Inside terms are converted to outside area. The diameter ratio handles this conversion. The inside-area equation reverses those conversions. Neither coefficient basis is universally better. Match each coefficient with its matching area. Use Uo only with outside area. Use Ui only with inside area. Mixing bases produces incorrect duty estimates. The calculator also applies Q equals U times A times LMTD. It works when area and LMTD are supplied. LMTD represents the effective temperature-driving force. It suits steady exchangers under standard conditions. Counterflow often creates greater driving force. Multipass units may require correction factors. Apply corrections before estimating final duty. Check carefully.
Applying the calculation
Begin with dependable fluid-side coefficients. Use correlations, tests, or specifications. Enter the tube-side coefficient first. Enter the exterior coefficient next. Provide actual inside and outside diameters. The outside diameter must be larger. Enter material conductivity for the tube. Add fouling resistance for each side. Use zero for a clean side. Submit the form for both coefficients. Results appear above the input grid. Review the displayed resistance breakdown carefully. A dominant term identifies improvement priorities. Higher velocity may improve convection. Cleaning may reduce fouling resistance. Material changes may reduce conduction resistance. Enter area and LMTD for duty. The calculator then estimates thermal power. Compare predicted duty with process needs. Always use a conservative fouled case. Recheck dimensions and units after changes.
Frequently Asked Questions
1. What does the overall heat transfer coefficient measure?
It measures the combined ability of all thermal paths to transfer heat between two fluids. It includes convection, wall conduction, and fouling resistance.
2. Why does the calculator show Uo and Ui?
The same exchanger can use inside or outside surface area. Each basis gives a different numerical coefficient. Both are correct when paired with their matching area.
3. Can I calculate a clean heat exchanger case?
Yes. Enter zero for one or both fouling resistance values. This gives a clean-condition estimate using the remaining convection and wall resistances.
4. Why must the outside diameter exceed the inside diameter?
A physical tube wall exists between the two diameters. The cylindrical conduction term requires a positive wall thickness and a valid logarithmic diameter ratio.
5. Why does the wall formula use a logarithm?
Heat moves radially through a cylindrical wall. The logarithmic term accounts for changing area from the tube inside surface to the outside surface.
6. What is fouling resistance?
It represents insulation caused by deposits, scale, corrosion products, oil films, or biological growth. Higher fouling resistance lowers the overall coefficient.
7. Is LMTD required for the coefficient calculation?
No. LMTD is only needed when estimating heat duty. The overall coefficient depends on the resistance network and tube geometry.
8. Can I use Celsius for LMTD?
Yes. A Celsius temperature difference has the same numerical size as a kelvin difference. Use either unit consistently for the temperature difference.
9. What commonly lowers the coefficient?
Low fluid velocity, viscous fluids, low-conductivity walls, thick tube walls, and fouling deposits commonly increase resistance and lower the coefficient.
10. Can this tool replace detailed exchanger rating software?
No. Use it for transparent estimates and checks. Detailed rating also considers pressure drop, flow distribution, phase change, and specialised correlations.
11. How can I validate the result?
Compare inputs with trusted correlations, plant data, or vendor documentation. Reliable inputs create trustworthy decisions and safer equipment choices.