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Role of Moisture in drying process


When the fabric is saturated in liquor, removed and held suspended in air, water starts dripping is known as ‘free water.

Free water by natural drying process
Free water by the natural drying process

When water stops dripping, further water can be removed by squeezing, padding or hydro-extraction.
This is known as ‘mechanical bound water’ available in interstices of yarns and fibres.
Mechanically bonded water
 Mechanically bonded water
Both free and mechanically bound are more than the equilibrium moisture content.
The amount of water freely held by fabric and the amount of water gained by fibres is independent of fibre content.
The main fabric property determines the amount of water a fabric freely picks up its thickness.

Thickness decides the amount of water picked up or water held by the fabric.
The time that it takes a fabric to dry is directly related to the amount of water which is in the material initially.
The time for the drying process is directly related to the amount of water that fabric has which again depends on the fabric types and previous chemical processes. (Amount of water) ~ (Time required for dry)
Owing to the high surface area, water evaporates more rapidly from fabric than from an equivalent volume of free water.
(Relative humidity: The moisture absorption capacity of air at present temperature is called relative humidity.)
During drying, the rising temperature of air lowers its relative humidity, since, although the absolute humidity of air changes very little, the saturation humidity increases enormously. Water must then evaporate from the fabric until it reaches equilibrium.
 As Temperature increases relative humidity decreases and drying rate increases i.e as temperature increases the rate of evaporation will also increase &  moisture removal is done.
For instance, ambient air's relative humidity is not zero. 
At 20°C, ambient air has a relative humidity of 50%; at 110°C, it has a relative humidity of 0.8%.
Achieving economy requires removing as much free water and mechanically bound water as possible to avoid energy use against latent heat. when it dries.
Squeezing in drying
Squeezing in drying
To achieve economical drying we have to squeeze the water from the fabric by padding mangles. Due to pressing max. The water we reduce water inside the fabric 
Hence, the energy required for drying will also be reduced. 
The below graph will show the relation between temperature, time and moisture content.
Typical drying curve
Typical drying curve

Relation between Temperature, Time and moisture content in the fabric.
 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 not necessary.
Most 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).

Among these three types, convection is widely used especially with hot air or direct combustion as a medium of drying.

Questions -
  1. What is the moisture in the fabric and how its impact on drying process?
  2. What is relative humidity?
  3. What is the difference between free water and mechanically bonded water?
  4. Explain the relation between temperature, time and moisture content for the fabric by the graph.

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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