Work out freezing time (plank) instantly with clear inputs, formula shown and shareable results.
Plank's equation gives freezing time for a slab as (rho x L / deltaT) x (a/2h + a^2/8k), where the first term is surface heat transfer resistance and the second internal conduction. Only latent heat is considered, so it underestimates total time; it is nonetheless the standard first estimate for freezer sizing.
Plank's equation for a slab
t = (rho x L / deltaT) x (a / (2h) + a^2 / (8k))
It ignores sensible heat removed above and below the freezing point, and assumes a sharp freezing front at a single temperature. Corrections such as Cleland and Earle's method address both.
For thin products and low air velocity, surface resistance dominates and increasing air speed helps. For thick products, internal conduction dominates and only reducing thickness helps.