Textile Drying
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.
Drying Curve
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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