Page 249 - Basic Principles of Textile Coloration

238 DYEING MACHINERY

dyes on cotton, particularly in blends with polyester. The padded fabric passes
through a zone filled with saturated air-free steam for about 20–60 s. If the
polyester component has already been dyed by the Thermosol process, the dry
fabric already contains the dyes for the cotton. Before entering the steamer, it is
padded with chemicals for assisting the dyeing of the cotton. The steamer usually
has a bank of driven rollers at the top and a set of free rollers at the bottom, the
fabric being interlaced up and down through the two sets. A steamer typically
holds about 30–60 m of fabric. Again, good roller alignment is necessary to avoid
lengthways creases.

The roof of the steamer must be well insulated. In the roof, heated pipes or
plates circulating pressurised steam at a temperature above 100 °C prevent
condensed water from forming and dropping onto and marking the fabric. The
saturated steam in the box should be slightly superheated (105 °C). It is fed into
the sump water, which boils, the ceiling pipes or plates adding the required degree
of superheat. The amount of air in the steamer should be minimum, particularly
when dyeing with leuco vat dyes that are readily oxidised. The fabric enters
through a narrow slot and exits through a water seal that is often filled with warm
rather than boiling water.

12.6.4 Continuous washing units [4]

Continuous washing units are used both for washing after continuous scouring and
bleaching of grey cotton, and also for removing unfixed dye and chemicals after
continuous dyeing. The vertical types have horizontal banks of rollers along the
top and bottom. The upper rollers are driven and are not in the wash liquor. The
lower set are free-running and immersed. The fabric being washed therefore
undergoes a series of dips into the washing liquor. This flows countercurrent from
one washing box to the previous one, often by overflow of the compartment wall,
provided that the water is not overly contaminated. They usually operate at
temperatures up to around 95 °C. Direct steam injection into the in-coming
water is used to heat the washing solution. A number of devices are used to
increase the efficiency of such units to minimise heat and water consumption. The
dirty hot water discharged to the drain may pass through a heat exchanger to heat
up the in-coming water. Carry-over of dirty liquor between compartments, or
between adjacent washing units, can be minimised using a squeeze roller in
contact with the last top roller.