Drying happens when liquid is vaporized from a product by the application of heat. Heat may be supplied by convection (direct dryers), by conduction (contact or indirect dryers), radiation or by placing the wet material in a microwave or radio frequency electromagnetic field. Over 85 percent of industrial dryers are of the convective type with hot air or direct combustion gases as the drying medium. Over 99 percent of the applications involve removal of water.
This is one of the most energy-intensive unit operations due to the high latent heat of vaporization and the inherent inefficiency of using hot air as the (most common) drying medium.
The figure above is a typical curve, in the initial period, drying rate is high; later, drying rate decrease because of interference of free moisture on the surface of the product.
Type of Dryers
1. Hot Air Dryer- Stentor
Fabric drying is usually carried out on either drying cylinders (intermediate drying) or on stenters (final drying). Drying cylinders are basically a series of steam-heated drums over which the fabric passes. It has the drawback of pulling the fabric and effectively reducing its width. For this reason it tends to be used for intermediate drying. The stenter is a gas fired oven, with the fabric passing through on a chain drive, held in place by either clips or pins. Air is circulated above and below the fabric, before being exhausted to atmosphere. As well as for drying processes, the stenter is used for pulling fabric to width, chemical finishing and heat setting and curing.
Contact Drying- Steam Cylinders/Cans
This is the simplest and cheapest mode of drying woven fabrics. It is mainly used for intermediate drying rather than final drying (since there is no means of controlling fabric width) and for predrying prior to stentering. Fabric is passed around a series of steam heated cylinders using steam at pressures varying from 35 psi to 65 psi. Cylinders can be used to dry down a wide range of fabrics, but it does give a finish similar to an iron and is therefore unsuitable where a surface effect is present or required
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