When scouring or bleaching chemicals are
applied to wet fabric i.e., un-dried from preceding washer. Water is added
to the saturator chemicals by the fabric. If the entry expression Ei of the fabric coming in to the
saturator is higher than the exit expression Eo of the fabric leaving the saturator,
the chemicals will become more dilute and the saturator will eventually
overflow. The entry expression must therefore be, less than the exit
expression to prevent this happening and to give some allowance for chemical
addition. The minimum difference is 10%. The larger the difference then
more dilute is the feed liquor. A 15 to 20% difference is usually found in
practice. To achieve this difference a high efficiency nip is used in
front of the saturator.
The actual pick-up of chemicals, called the
effective pick-up, is the difference between entry and exit expression ( Eo
- Ei ) and the amount by which the incoming
water is exchanged for pad liquor. This may be written:
Effective pick-up = ( Eo
- Ei ) + (Ei x F),
where F is the interchange factor or the
fraction by which the in-coming water is exchanged for saturator solution.
The value F can only be determined by finding the concentration of
chemical on the fabric after padding and relating this to mangle
expressions and concentration of chemicals in the pad liquor. It is
usually about 0.7 to 0.8 and these value can be used for starting up and
then adjusted when F has actually been measured.
The effective pick-up replaces
"%" pick-up in equation , concentration
= % o.w.f. X 1000 / % pick up and so allows calculation of saturator
concentrations g/l for wet on wet application.
Example: A cotton fabric is to
be padded with 3% peroxide (50%) o.w.f. 90 % expression . Exit expression at
75% wet on wet. What is the
concentration of the pad liquor in g/l ? Assume F = 0.7
Calculation:
Effective pick-up = ( Eo
- Ei ) + (Ei x F)
= ( 90 - 75 ) + ( 75 x 0.7)
= 67.5%
The chemical concentration
required will be = % o.w.f. X 1000 / % pick up
= (3 x 1000)/ 67.5
= 44.4 g/l hydrogen peroxide 50% w/w
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