Mass-Transfer Biot Number: Internal and External Resistances Need Compatible Bases

Define the Material or Thermodynamic Model

A primary drying-model study defines Bim = k Lc/(ρw De), with k in kg/(m²·s), diffusivity De in m²/s and fluid density ρw. That density factor belongs to the stated coefficient convention. A gas-film coefficient in m/s cannot be substituted as the same k.

Review a Hypothetical Comparison

Assign k = 0.002 kg/(m²·s), Lc = 0.005 m, ρw = 1000 kg/m³ and De = 1 × 10−9 m²/s. Bim is 10. Recasting the coefficient as k/ρw = 2 × 10−6 m/s gives the same result through (k/ρw)Lc/De. Omitting density while retaining the original k would yield 10,000 with incompatible dimensions.

Keep the Evidence with the Decision

Write the boundary flux equation beside the coefficient and identify the driving concentration or moisture variable. Preserve the characteristic-length definition. Only compare internal and external resistance after the conversion follows that model.

The assigned coefficient is not a measurement of the cited material or an HM process. A different transport convention may require a partition or density conversion; there is no universal unit-free shortcut.

Does the dimensionless resistance comparison use a compatible surface coefficient and internal diffusivity definition?

This blank worksheet is for your own project. It contains no H M machine trial result.

Customer Questions

Primary References

These references support the technical principles discussed in this guide. The worked examples and review questions are educational.

• primary drying-model study
• Reference from hmpharmamachines.com