Transport Property
Rate constants that quantify the movement of vaporized solvent from a wet fabric surface into the surrounding air flow govern stenter drying efficiency. The convective mass transfer coefficient determines the velocity of evaporation at any given temperature and humidity. It is measured in metres per second and depends heavily on the turbulence of the air.
This property is important for optimizing the energy consumption of drying lines.
Boundary Layer
Air velocity profiles above the textile web govern the thickness of the boundary layer that resists moisture transport. A larger convective mass transfer coefficient corresponds to a thinner boundary layer, which allows moisture to escape more rapidly. Nozzles in the drying chamber must be positioned to maximize air impingement on the fabric.
This direct air flow increases the local turbulence and accelerates the drying process.
Industrial Control
Controlling the moisture profile of fabric emerging from a high-speed finishing line requires continuous adjustment of fan speeds and temperature zones. Operators monitor these variables to maintain the convective mass transfer coefficient at a level that prevents both under-drying and over-drying. Advanced stenters run simulations using these coefficients to dynamically adjust the line speed.
This maintains fabric quality without wasting thermal energy.
Drying Limit
Very dense fabrics with high internal moisture resistance limit the effectiveness of high external transfer coefficients. In such cases, internal diffusion becomes the rate-limiting step.
