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Drying method - Conduction and Radiation


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 radiofrequency electromagnetic field).


Conduction or Contact drying

 

Steam is supplied at pressures ranging from 35 to 65 psi, heating a series of cylinders around which fabric is passed. Many different kinds of fabrics can be dried using cylinders. 

However, certain types of fabric, such as knitted and compressible terry, are more likely to deform when put under pressure and tension.

It is important to note that radiation and convection losses in cylinder dryers are significantly higher up to 35% of the total energy used. 

Conduction drying machine
Conduction drying machine


Drying at the Finishing stage :

For better outcomes and durability, it was anticipated that applied finishing agents in chemical finishing would penetrate the spaces between the fibres and even into amorphous areas. 

When the dry fabric is placed in a finishing bath, water transports finishing agents to the centre of the fabric in a one-way mass transfer process.

Drying at the Finishing stage


Wet-on-wet finishing is a two-way mass transfer process in which finishing liquor enters the fabric's interstices and ingested water exits the cloth. 

The fabric is treated with finishing liquor for 2 to 3 seconds when padding, which is insufficient time for the phenomena.


(1) One-way mass transfer - When the dry fabric is entered into the finishing bath and transfer takes place from the outer to the interior of the fabric where water carries Finishing agents in the core.


(2) Two-way mass transfer - When the wet fabric is entered inside the finishing bath and water inside the fabric shall come out and finishing agents or finishing liquor shall enter in interstitial space between fibres.

While padding, the fabric is treated in finishing liquor for 2-3 seconds which is a too short time for penetration of finishing liquor.



 

Radiation drying (Radio frequency drying) :

In the era of dropping rates, circulating hot air for drying loose fibres, hanks, or yarn packages is slow and ineffective. 

Evaporation occurs at the surface, and owing to the slow migration process, the surface no longer receives water from inside or core.

Radiofrequency drying
Radiofrequency drying

The 'volumetric' impact of radio-frequency (RF) heating provides the solution to this physical form of material that needs to be dried. 

The energy is rapidly transferred from the oscillating electric field to the water molecules. 

Within the yarn bundle or fibre lump, steam will develop and will At the beginning of heating, force liquid water to the surface.

Radiofrequency drying

Several industrial dryers use radio waves with frequency ranges between 27.13 and 13.56 MHz, however, 27.3 MHz is most frequently used for textile applications.

In bulk, water molecules oscillated 27 million times per second. 

In the RF field, polarised molecules vibrate and generate heat through friction. 

The rate of heating and evaporation is exactly related to the amount of provided electromagnetic energy, which is perfectly controllable.


Two types of dryers are being used in the textile industry:

(A) Infra-red dryer (IR dryer)

(B) Radiofrequency dryer(RF dryer)

Explanation

(A) IR Dryer

(B) RF Dryer

(1) It has high energy and high amplitude waves so, the evaporation rate is very fast


(2) It is used for fabric drying because it has high energy and water is only at the surface of the fabric which time of drying will be less hence, surface-level drying is only possible.


(3) Evaporation of water is fast due to high energy as well as water at the surface of the fabric.


(4) Time of contact is less due to high energy.


Application -

For fabric


(1) It has minimum energy, the highest wavelength and low frequency. The evaporation rate is slow.


(2) It is used for cone or cheese drying because its fibres contain water in the core as well as the surface fibres. It required low energy dryer 


(3) Water Evaporation is a slow process because water is at its core.

 

(4) Time of contact is more due to less energy.


Application -

For cone or cheese

 

Note: if we use an IR dryer for cone or cheese then core water remains inside & complete evaporation of water won't take place due to less time of contact with the IR Dryer.


In an IR Dryer, the fabric is vertically exposed to IR rays and 60 % of moisture gets removed in 1 second.

As Temperature increases the vibrations increase due to which the evaporation rate will be faster.

 

Questions -

  1. Which methods are useful for the drying process?
  2. Explain the conduction drying method with drying cylinders.
  3. Explain the contact drying method with drying cylinders.
  4. Explain Radiation drying with Radio frequency drying.
  5. What are the differences between IR and RF drying?


Continue read,

Part 1 Drying introduction 

Part 2 Role of Moisture in the drying process

Part 3 Drying method - Conduction and Radiation

Part 4 Drying method - Convection



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