In a drying process — food, pharmaceuticals, ceramics, timber or laundry — the drying rate changes with time and is limited by two different mechanisms.
The constant-rate period
At the start the product surface is wet. Evaporation occurs at the surface and its rate is set by how fast the air removes the moisture. The limiting side is the external one: air velocity, temperature and relative humidity.
In this period, increasing the air flow or raising the temperature helps directly. The surface temperature stays close to the wet-bulb temperature of the air because of evaporative cooling — a natural buffer protecting the product.
The falling-rate period
As the surface dries, the evaporation front retreats into the product. The moisture now has to reach the surface and the rate is set by internal diffusion.
In this period improvements on the air side have almost no effect. Supplying more flow only consumes energy. Worse, since the surface is no longer cooled by evaporation, the product temperature rises rapidly — scorching, cracking and quality loss occur precisely in this period.
The design consequence
A good dryer provides different conditions for the two periods: high flow and temperature at the start, then a staged reduction. A constant-condition dryer either passes the first period slowly or damages the product in the second.
Modelling
Evaporation is a mass and energy source. Vapour leaving the surface raises the humidity of the air and the latent heat cools the surface; the two must be solved together. Classical mass transfer relations are used for droplet or porous surface evaporation [1].
Distribution is critical. If every item in a batch is to dry identically, what matters is not the total flow but how that flow is distributed between the trays — a classic distribution problem.
The product side is modelled separately. Internal diffusion belongs to the product model rather than the flow solution; the two are joined at the surface.
References
- Ranz, W. E. & Marshall, W. R. (1952). Evaporation from drops. Chemical Engineering Progress, 48, 141–146 (Part I) and 173–180 (Part II).
- Shah, R. K. & Sekulić, D. P. (2003). Fundamentals of Heat Exchanger Design. John Wiley & Sons. DOI: 10.1002/9780470172605