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Drying method - Convection

Drying at the intermediate stage

Instead of ultimate drying, contact drying is typically utilised for intermediate drying. Since VDRs are used to dry clothes, controlling fabric width is unnecessary.

The majority of the time, pre-drying is necessary before entering. as in batch printing.

(A) Conduction or contact (drying cylinders)

(B) Convection (centre, tumble dryer),

(C) Radiation (infrared, microwave or radio-frequency electromagnetic field).

Convective drying

Circulating air, which is heated and then releases some of its heat to wet fibres at a lower temperature, is used to transport heat. 
The wet fibre temperature is primarily influenced by the humidity of the drying atmosphere, and the pace of drying is proportional to the difference in temperature between the air and the fibre. 
Most widely employed, a stenter is a convective type drying device used for finishing, heat setting, and the thermosol process.
Specific heat of air = 0.24 Kcal/kg

Drying stenter machine
Drying stenter machine

After some time interval removal of air takes place because, by continuous circulation of air, the ability of air to absorb moisture or water will be reduced or lost. Due to which air contains water. Air becomes wetter & for heating water more energy will be required as well as drying of fabric will be more difficult hence, we will replace that air with new air.
Stenter - Heating system
Stenter - Heating system
*When Moisture content = 10 % i.e 100 gm per kg fabric then air should be replaced.

Other important things in stenter:

At the selvedge, pins or clips fastened on an endless chain firmly hold the fabric in place. 
The chain can be adjusted for width, overfeed, or underfeed to regulate the resulting fabric's proportions. 
Hot air is blown on wet fabric by thermoelectric heaters or gas-fired stenters before being cycled again i.e Thermic fluid heaters or gas-fired stenters blow hot air against the wet fabric and then recirculated. 
The splitting up of the system in chambers, usually between three to eight, provides better control over the heating pattern. 
Stenter - Heating system

Optimize exhaust air humidity:

High air temperatures and low humidities were used to operate with higher drying speed when energy was cheap.
Large volumes of cold air were heated and discharged before they had taken up optimum amounts of water vapour.

Optimize exhaust air humidity
Substantial energy cost can increase when humidity in exhaust falls below 0.1 kg water/kg air.
It has been claimed that 30% energy saving is possible by increasing the humidity of stenter exhaust air from 5 to 10% with the additional advantage that over-drying is less likely to occur. 
Further increase in humidity of more than 10% in circulating air reduces the rate of drying, which is normally not accepted. The moisture content of air = 10 % then replaces the air.
Drying method - Convection

Questions -
  1. Which methods are useful for the drying process?
  2. Explain the conventional drying method with stenters.
  3. What is Optimize exhaust air humidity?

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