Transport Number
Dimensionless groups that represent the ratio of convective mass transfer to molecular mass transport describe the rate of evaporation or absorption. The sherwood number is used in textile drying calculations to evaluate the effectiveness of the air jets impinging on the wet fabric. It relates the mass transfer coefficient to the diffusion coefficient and a characteristic length.
This parameter is the mass-transfer analogue of the Nusselt number.
Mass Transfer
A high value of this group indicates that convective air currents dominate the transport of water vapor away from the textile web. The sherwood number increases with the turbulence of the air flow, which is represented by the Reynolds number. In stenter frames, the nozzles are designed to maximize this value by directing high-velocity air directly at the fabric.
This accelerates the removal of the vapor boundary layer.
Dryer Optimization
Industrial designers use this dimensionless transport number to calculate the required length of drying ovens for new production lines. By predicting the sherwood number under different fan speeds, they can determine the minimum time needed to dry the fabric. This prevents under-drying, which can lead to mold growth during storage, and over-drying, which wastes energy.
The calculation is essential for optimizing the production rate.
Boundary Condition
When the fabric surface becomes dry, the rate of mass transfer is governed by internal moisture diffusion rather than external convection. Under these conditions, the relevance of this dimensionless group decreases.
