Page 61 - Advances in Textile Biotechnology

40 Advances in textile biotechnology

fabric enters the bath until the point where the fabric leaves the bath. The
α-factor of the bath is an indication of how much of the fabric is stagnant
and how much is convective in the padding bath. It must be noted that each
system has its own value of the squeezing factor α. The value of α is related
to the value of the forces or pressures that are exerted onto the load while
passing over rollers or through squeezers. Figure 2.5 shows a schematical
relationship between α and the pressure forces.
The deformation parameter only gives information about the fraction of

pores in which some flow can be generated as a result of the deformation.

It does not give information about the magnitude of the flow in the pores.
This value is dependent on the rate of deformation and not on the magni-
tude of deformation. The rate of deformation in represented by the Peclet
number. This dimensionless number is defi ned as:

P
Pe = convective mass transfer = vL P [2.7]
diffusive mass transfer D
−1
is the mean liquid velocity in the pores (m s ) and L P is the
in which v p
length of the pores (m). This parameter is related to the liquid velocities
generated in the pores because these depend directly on the deformation
rate. The higher the rate of deformation, the higher the induced pore veloc-
ities are. It can be argued that the pore liquid velocities that are in fact the
liquid displacement velocities should be proportional to the pore deforma-
tion rate as drawn schematically in Fig. 2.6.
If we now consider an enzyme padding processes with a particular value
of the squeezing factor α and of Pe, then the curve that describes the con-
centration of enzymes in the fabric as a function of time looks as sche-
matically drawn in Fig. 2.7.

1

a

0
Pressure force
2.5 Expected relation between the squeezing factor α and the pressure
force exerted onto a textile.

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