Calculator inputs
Use the form below to estimate external film transfer around particles or across a plate. The page remains single-column, while the form uses a responsive multi-column input grid.
Formula used
This calculator estimates the external film coefficient using common transport correlations. It then converts that coefficient into flux, total rate, resistance, and film thickness.
How to use this calculator
- Select the correlation that matches your geometry or available literature data.
- Enter transport properties in consistent SI units.
- Provide bulk and surface concentrations to define the driving force.
- Enter interfacial area to convert flux into total transfer rate.
- Press the calculate button.
- Review the displayed coefficient, flux, rate, Reynolds number, Schmidt number, and Sherwood number.
- Use the graph to see how velocity affects the current case.
- Download the result summary as CSV or PDF for reports.
Example data table
These worked examples show how different transport regimes affect the external film coefficient and resulting flux.
| Case | Correlation | Velocity (m/s) | Length (m) | Re | Sc | Sh | kc (m/s) | Flux (mol/m²·s) | Rate (mol/s) |
|---|---|---|---|---|---|---|---|---|---|
| Gas over spherical pellet | Sphere | 1.8 | 0.012 | 1440.0000 | 0.714286 | 22.3528 | 0.039117 | 0.082146 | 0.028751 |
| Water over short plate | Flat plate, laminar | 0.02 | 0.08 | 1596.8000 | 910.9127 | 257.2087 | 3.5366e-06 | 3.5366e-04 | 8.8415e-05 |
| Fast liquid over long plate | Flat plate, turbulent | 2.0 | 0.8 | 1.5968e+06 | 715.7171 | 30365.6525 | 5.3140e-05 | 0.0037198 | 0.0044638 |
Frequently asked questions
1) What does external mass transfer mean?
External mass transfer describes movement through the fluid film between a bulk phase and a surface. It excludes internal pore diffusion and reaction steps inside the solid.
2) Why does the calculator use Reynolds, Schmidt, and Sherwood numbers?
These dimensionless groups compress fluid dynamics and molecular diffusion into compact design relationships. They let engineers estimate a transfer coefficient without solving the full boundary-layer equations every time.
3) Which characteristic length should I enter?
Use the length required by your chosen correlation. For a particle, that may be diameter. For a flat plate, it is typically the streamwise plate length associated with average transfer.
4) Can the concentration difference be negative?
Yes. A negative value means the surface concentration exceeds the bulk concentration, so the computed flux points from the surface back into the bulk fluid.
5) What is film thickness in this tool?
The reported film thickness is an approximate equivalent thickness, estimated from D/kc. It is a practical interpretation aid, not a full velocity-profile solution.
6) When should I use the custom Sherwood option?
Use it when literature, experiments, or another validated model already gave you Sherwood number. The calculator then converts that value into kc, flux, rate, and resistance.
7) Does this calculator include internal diffusion or reaction kinetics?
No. It estimates only external transfer through the surrounding fluid film. If a process also has pore diffusion or reaction resistance, you should combine those separately.
8) Why are laminar and turbulent plate results so different?
Turbulent flow disrupts the concentration boundary layer more strongly than laminar flow. That usually raises Sherwood number and increases the external mass transfer coefficient significantly.