20-10-2012, 05:52 PM
AIR CIRCULATION IN STENTERS
AIR CIRCULATION IN STENTERS.pptx (Size: 802.58 KB / Downloads: 36)
INTRODUCTION
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.
CONSTRUCTION
Stenters usually have a closed circuit circulation system which prevents infiltration of air in the circulating system.
The air circulating system consists of a powerful blower(or fan) that pushes the air into two tapered ducts and through the precisely made nozzles to achieve high impact velocity at the fabric surface.
Jet nozzles are designed to obtain maximum evaporation and even air flow and specifically designed for different fabrics.
HEATING MEDIA
The most common means of heating is by direct gas firing with the burnt gas fumes being fed into the stenter oven.
A few units are also indirect gas fired but their efficiency is poor as compared to direct gas fired systems.
Thermal oil heating is also another method. It requires a small oil boiler which can be used for a wide range of temperature.
Oil itself can be used as a means of heating stenters. But because of the oil spots and the soot due to the incomplete combustion this is done via heat exchangers.
High pressure steam from boilers fed to the heat exchangers. A steam pressure of 1000KPa will provide an air temperature of around 165 degree celsius which is adequate for drying and curing cross linking reactants but inadequate for heat setting.
MECHANISM OF DRYING
Usually in fabric drying slightly more air is forced into the bottom ducts to support some of the weight of the fabric and because the bottom is wetter than the top.
Supporting the weight of the fabric prevents excessive “sagging”.
The hot air, after being forced against the fabric makes its way between the ducts to the sides of the stenter where it is collected by the fan and recirculated.
Where a pair of stenters is used for a 3m long compartment the fan motors are rated at 7.5KW each but for a 5 compartment stenter total electrical power requirement is 100KW.
The temperature probe fitted at the inlet of the duct regulates the operation of the burner or the flow of the air over the heat exchanger.
Dampers fixed at the top of the compartment regulates the rate at which exhaust gases are removed into an exhaust duct.