DRYING AND SETTING IN THE TEXTILE INDUSTRY

Technical University of Liberec

DRYING - INTRODUCTION

Textile materials are usually wetted and dried several times in preparation, dyeing and finishing processes.

Water, used in various phases of textile processing, accumulates in fabrics, and the excess moisture must be removed.

Moisture can be removed from textile materials to some extent by mechanical methods such as squeezing, centrifugal extraction or vacuum extraction.

For the rest moisture - evaporative heating must be used.

How is water content u (%) expressed?

Oven dry mass (dry weight of the material) is gained by heating a sample in an oven under given temperature - for textiles 105 °C - for at least 3 hours, or as long as the sample mass does not change in time.

Mass of water is then

WATER IN TEXTILE FIBRES

100mass of water

uoven dry mass

mass of sample oven dry mass

WATER IN TEXTILE FIBRES

The water retention value is the amount of water which the fibre retain without dripping.

How is water bound to the solid substrate?

1) Free moisture – water is on the substrate surface2) Macro capillar bound moisture3) Micro capillar bound moisture4) Absorbed moisture (hygroscopic moisture)5) Chemically bound water molecules in substrate

Drying removes water under items 1 to 3.

Water under items 4, partially 3, is regained back after drying process by physical mechanisms.

Water removed in items 5 desintegrates the substrate.

WATER IN TEXTILE FIBRES

How is the moisture content evaluted in the air?

Moist air is mixture of dry air and water vapor

Specific humidity

x is related to 1 kg of humid air

ma mass of dry air

mv mass of vapor in air

Relative humidity R.H.Water vapor content relative to that of fully saturated air.It is mostly expressed in per cents.

mass of water vaporx

mass of dry air

MOISTURE IN AIR

Relation between x and R.H.

where p is atmospheric pressure of moist airps.v. is partial pressure of saturated vapor at given

temperature

pa is partial pressure of dry air

R.H. must be used in decimal numbers

Pressure conditions in moist air shows the

diagram at the right.

pa

pv

p

SPECIFIC AND RELATIVE HUMIDITY

Values of saturated vapor pressure are tabulated by temperature. Excerption from such tables is shown.

SATURATED VAPOR PRESSURE

Example

Determine specific humidity of moist air if R.H. is 25 %, temperature 70 °C and atmoshperic pressure 98 kPa.

ps.v. from saturated pressure table is 31 120 Pa

p is given 98 000 Pa

R.H. is given 25 % = 0.25

Use of equation from previous slide yields x = 0.0536

Note: Values of moist air can be read from various diagrams. Carrier diagram in Anglo Saxon countries and h-x Mollier diagram in Europe. Calculated value of x is dimensionless. The air at given conditions contains 53 g water vapor per each kilogram of dry air, actually.

CALCULATION OF SPECIFC HUMIDITY

EQUILIBRIUM MOISTURE CONTENT

Equilibrium moisturecontent depends on:

• ambient temperature• relative humidity of outer air• atmospheric pressure.

Varies greatly with the type of material.

EQUILIBRIUM MOISTURE CONTENT

Effect of temperatureon the equilibrium moisture content of cotton.

0F =( 0C x 1.8) + 320C =( 0F – 32) : 1.8

e.g. 600C = 1400F

EQUILIBRIUM MOISTURE CONTENTAny material dried below equilibrium moisture content regains its equilibrium moisture content back.

HOT AIR DRYING THEORY

1) Very short settling down period, where the textile is heated up.

2) Constant rate period, where free and macro capillar water is removed

3) Falling rate period, where micro capillar water is removed

Dependent on water bonds, the moisture is removed from material during the process of drying. Practically three periods are detected in textile drying.

Hot air drying process

HOT AIR DRYING THEORY Drying curveu vs. time relationship is called a drying curve.

It shows moisture content decline during drying process.

Obviously the process should end when equilibrium moisture content is achieved. Critical moisture is a separation point between constant and falling rate periods.

HOT AIR DRYING THEORY

After initial heating, the rate of evaporation is constant.Then the drying rate is reduced as diffusion and capillary forces control it.If this material is over dried, (say up to 2% moisture), it absorbs the moisture from atmosphere and stabilises at a level called equilibrium moisture content (e.g. about 7% for cotton).

Critical and equilibrium moisture content (in %)

HOT AIR DRYING THEORY

Temperature-timedrying diagram.The diagram shows surface temperature of dried textile and temperature of drying substance.

During constatnt rate period the surface temperature is constant, temperature of wet bulb approximately. In falling rate period the surface temperature rises approaching the temperature of drying substance.

tem

pe

ratu

re

Symbol is for mass flow. After rearranging, the basic equation for air consumption is gained

Material equilibrium

The water in a liquid phase is transeferred from the dried textile to the air as a vapor.

HOT AIR DRYING THEORY

1 1 2 2s A s sm u m x m u m x

1 2

Mass ratio of water and dry air is equal to the difference of specific humidity of out- and incoming air.

m

2 1w

A

mx x

m

DRYING TECHNOLOGY To lower the moisture content in the textile material :

1. Mechanical water removing ( MR )

2. Thermal drying ( TD )

E.g. Process sequence for cotton drying

Mechanical drying is the more economical, costing less in terms of energy.Thermal drying only until the fibre’s standard residual moisture content.

T D100 - 200 % 60 - 80 % 7 - 11 %

M R

Mechanical water removing methods

1) Squeezing (mangling) –continuous process

2) Vacuum extraction –continuous process

3) Centrifugation –discontinuous process

MECHANICAL DEWATERING

Perforated basket rotate at high speed (over 1000 rpm).Centrifugal force - forsing surplus water through the perforations.

MANGLING

Squeezing effect depends on:

1. pressure of rollers

u

The water retention is reduced by increasing the pressure.

Squeezing effect depends on:

2. velocity of rollers

3. diameter of rollers

MANGLING

The water retention is reduced by reducing fabric speed.

Sqeezing mangle with swimming rollHigh-efficiency water mangle with swimming roll KUSTERS. Roll is pressed by oil which is inside the cylinder.

For water extraction after open-width washing.

HIGH EFFICIENCY DEWATERING

ROBERTO ROLL The solid rubber roller(below) presses the fabric, from which water is to beremoved, against the Roberto roll (above) which has an outer layer of deformable hygroscopic material and which removesadditional water due to the capillary forces within this material.

HIGH EFFICIENCY DEWATERING

sponge-like covering

Surface water adhering to woven fabrics is easily removed through suction extraction.The wet fabric passes over a suction slot.

VACUUM EXTRACTION

A vacuum pump draws air through the fabric and the water removed is withdrawn via tap.

Average rates of residual moisture achieved when vacuum technology is applied.

VACUUM TECHNOLOGY

COMPARISON OF MECHANICAL METHODS OF REMOVING WATER

Mangle : the cheapest

Vaccum extractor: the most expensive.

Centrifuge : very useful on account of its versatility and it is capable of dealing with a large variety of fabrics.

Disadvantage: discontinuous process.

It is expedient to remove as much water as possible by mechanical means as the process is relatively cheap compared to drying by evaporation.

METHODS OF MOISTURE MEASURING

Principles of moisture measuring devices.

The measuring units are based on:

Electrical resistance detection

The resistance increases exponentially as the residual moisture decreases.

Can be used for contact measurement of running fabric.

METHODS OF MOISTURE MEASURING

Principles of moisture measuring devices (cont.)

Microwave absorptionContact free measurement.

Warning:Moisture measurement of non homogenous materials, such as curtains, give erroneous results. The same problem is with textiles where moisture is unevenly distributed.

A semiconductor oscilator transmits microwawe energy through the web. Part is absorbed by water. The not absorbed part of the energy is measured by a sensor.

METHODS OF THERMAL DRYINGMethods of

dryingAdvantages Disadvantages

Convectiondrying(hot air)

- Small risk of fabric thermal degradation- High flexibility in process regulation- Based on any source of energy

- Big energy consumption

Conduction(contact)drying(drums)

- Low energy consumption- Maintenance reduced to theminimum

- Very poor handle of the fabric- High time contact of the process regulation

RadiationdryingInfra-red

-Very flexible regulation to the surface of fabric

- Risk of burning- Can be based only on gas and electricity.

The stenter (stenter frame, tenter) is a heated oven, with the fabric passing through on a chain drive, held in place by either clips or pins. Air is circulated above and/or below the fabric, before being exhausted to atmosphere.

STENTER DRYING

STENTER DRYING

Air circulation scheme in a stenter drying chamber.

Stenter transport mechanism

Fabric is fed onto pins on two endless chains.

The distance between these chains is adjustable so that the tension (weftways) in the fabric is accurately controlled.

STENTER DRYING

Stenter transport mechanism

STENTER DRYING

Two driven chains carrying pins or clips to hold the fabric edges for passage through the enclosed drying compartments.

Clip stenters are useful for dense and heavy fabricsthat tend to damage pins, but overfeeding is not possible.

Heating of stenters

• Steam (low and high pressure)• Gas• Circulating oil heating• Electricity (mostly as a supplementary heating)

STENTER DRYING

Heating of stenters

1) STEAM

Until the 1950s, stenters were mainly heated by steam. Stenters were primarily used for drying fabrics made of natural fibres.

The increasing use of man-made fibres - called for circulating air temperatures of 180 - 220 °C in stenters.

With a saturated steam pressure of 0.8 to a maximum of 1 MPa, customary in textile factories, this was not possible.

STENTER DRYING

2) GAS HEATING

Direct gas heating

With direct gas heating, the combustion gases are directly mixed with the air circulating in the stenter.

Indirect gas heating

With indirect gas heating, the combustion gases do not come into contact with the circulating air. The heat exchange between the combustion gases and the circulating air takes place via heat exchangers.

STENTER DRYING

Stenters for elastomer fibres

Stenters used to treat fabrics with an elastomer fibre content (greater that 10%) cannot be directly heated with gas.

The spinning oils, softeners and knitting oils used in the production and processing of elastomer fibres caused heavy yellowing of the fabrics (nitrogen oxides NOx).

• Indirect gas heating

• The circulating air temperature measured across the width of fabric may deviate only within a range of 1 °C.

STENTER DRYING

CALCULATION OF ENERGY FOR DRYER

How much energy is needed for a dryer?

Maximum evaporation capacity of a convective dryer is e.g. 600 kg/hour. Estimate energy supply.

For evaporation of 600 kg water per hour is needed

kW (1W=1J/s)

where enthalpy of water at 30 °C

enthalpy of vapor at 120 °C

values of enthalpy were found in thermodynamic tables. Temperatures 30 and 120 °C are selected deliberately as average values.

Resulting value is

1

3600w w end startP m h h

123 /starth kJ kg

2700 /endh kJ kg

430wP kW

CALCULATION OF ENERGY FOR DRYER

Convective dryer needs approximately 10 kg of dry air for each kg of evaporated water. Thus energy for air heating is

kW

kg/s

kJ/kg.K is specific heat capacity of dry air at constant

pressure

tend and tstart are temperatures of input water and output vaportaken from the previous slide

Resulting value is

Approximately 20 % of total input for evaporation and air heating make heat loss through insulation, base, input - output air leakage etc.

a a pa end startP m c t t 1

103600a wm m

1,006pac

151aP kW

116lP kW

On the other hand about 40 % of power for air heating is regained in a heat recovery system.

CALCULATION OF ENERGY FOR DRYER

. . 60h rP kWFinal estimation of power needed for given dryer is

Rounded up 700 kW

Note:

Nominal evaporation capacity of a dryer is guaranteed under specific conditions. Basicly drying of pure cotton of surface density 105 g/m2, width of 1.2 m, plain woven fabric, unbleached, and other conditions eventually. Real capacity is much lower than the nominal one.

. . 637TOT w a l h rP P P P P kW

CONTACT DRYINGDrying on a series of steam-heated cylinders. The rate of drying is very rapid.

CONTACT DRYING

The fabric can be taken directly from the wet finishing range, avoiding a costly passage on a separately operating air dryer.

Advantages: up to 40% energy saving compared to conventional air dryers

Combination of contact and convection drying

FLEISSNERS DRYERS

First patent for through-air drying technology in 1929

Based on a combination of perforated drum and radial fan. The fan sucks air out of drum and returns it over heating elements to the ouside of the drum.

Through-air principlePerforated drum dryers

FLEISSNERS DRYERS

Combination of contact and convection drying

Advantages Disadvantages

FLEISSNERS DRYERS

• less floor spacerequirementscompared to stenters

• relatively gentle anduniform drying

• setting of fabric width moredifficult than on the stenter

• relatively high investment costs

• unsuitable for pile fabrics

• marking off with dyed fabrics

RELATIVE ENERGY COSTS OF DE-WATERING AND THERMAL DRYING

METHOD ENERGY COST

Mangling 1.5 Centrifuging 2.5Suction slot 4.5

Drying cylinders 25Stenter 35IR radiation 120