Page 240 - Basic Principles of Textile Coloration

MACHINES FOR DYEING FABRIC 229

chemicals are added through this tank. From its bottom, the small pressure pump
feeds the solution back into the machine. Complete circulation of the liquor takes
about 45–120 s. The liquor ratio is usually around 10:1.

The fabric will then usually be scoured but the beam machine does not
effectively remove solid impurities that are retained in the layers of fabric. Heating
is by high-pressure steam passed through pipes running along the bottom of the
autoclave. Cooling is achieved using the same tubes. Some larger beam dyeing
machines have an external heating system.

Fast, even liquor flow through the roll of fabric is essential. This is usually in the
in-to-out direction, but it can be reversed. Out-to-in flow can compress the material
causing flattening and glazing, particularly on the inner layers. The main pump is a
high capacity circulating pump. A secondary pump pressurises the vessel even when
dyeing at temperatures below the boil. The major disadvantage of in-to-out flow is
the risk of unlevel dyeing, with the inner layers of fabric having a deeper colour than
those on the outside of the roll. The rate of exhaustion must be below about 2% per
cycle of liquor. The high liquor flow rate helps to control the strike. The flow rate
should be as high as possible without disturbing the layers of fabric. It depends on
the pump, the fabric permeability, and the number of layers in the roll. Unlevel
dyeings are difficult to correct unless readily migrating dyes are used.

Sampling during beam dyeing remains a problem. Small samples placed in the
liquor flow can be withdrawn through a pressure lock but have not been exposed
in the same way as the layers of fabric in the roll. In many cases, it may be
necessary to cool, drain the liquor to storage, sample the roll and then refill with
additional dye solution. This is expensive.

12.4.4 Jet dyeing machines [2]

The first jet dyeing machine, based on a design by Burlington Industries in the
USA, was introduced by the Gaston County company in 1961. Jet machines
essentially all use the same principle (Figure 12.6). Dye liquor is injected at high
speed from an annular orifice around a rope of fabric as it passes through a venturi
tube. This is a tube with a constriction. The high linear velocity of the jet of
liquid, and its flow around the fabric rope in the venturi, pull the fabric along
towards the back of the machine. Fabric speeds are usually about 200–
2 5 0 m m i n–1 but can be much higher. The fabric then falls into the back of
machine and slowly moves to the front where it rises to the jet again. A typical
cycle takes about 1 min.