SEMINAR ON AIR CIRCULATION IN STENTERS BY: ARADHIKA AGGARWAL B.TECH TEXTILE CHEMISTRY SEM 5 ROLL NO:890114

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.

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.

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.

CROSS SECTION OF A DIRECT FIRED STENTER

Air distribution in the fabric from top to bottom nozzles is regulated through the dampers fixed in the circuit. To offer better control in the stenters, these are spilt into a number of compartments usually between 2 and 8 three meter sections each fitted with a temperature probe, heat exchanger, fans and exhaust. The temperature in the air chambers can be as high as 200 degree celsius. Usually air temperatures in the region of 130-150 degree celsius are used for high drying rates without excessive losses in the exhaust.

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.

AIR FLOW IN STENTER

EFFECT OF THE VARIATION IN STEAM CONTENT IN STENTERS It was discovered that the steam injection in stenters greatly reduced yellowing. This was probably because of steam replacing oxygen and reducing oxidation. It has been observed that 10% steam in exhaust air is about to proportion for when drying. If the steam content is reduced to 4%, two and a half times as much air is required. Presence of superheated steam is known to produce a softer handling fabric and enables setting to be carried out at low temperatures.

NEW TECHNOLOGIES Modern stenters also have “SPLIT AIR” technology where each half chamber of the dryer is equipped with a twin blower system which incorporate individual invertor driven blowers for top and bottom set of nozzles. This results in infinitely adjustable air balance on each face of the fabric which is very useful in processing a wide range of fabrics like knits, coated fabrics and piled fabrics.

“DUO-THERM” technology has been devised to set the circulating air temperature above and below the fabric web differently of each other. This is particularly useful in case of nonwovens having two different sides. “SUPRA-FLOW” technology is another special feature for energy efficiency and process control in air stenters. It is a combined transport system with a horizontal circulating roller chain and an air permeable transport belt with an air circulating system where air flows from top to down through the nonwovens and transport belt.

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