Page 233 - Basic Principles of Textile Coloration

222 DYEING MACHINERY

typical liquor-to-goods ratio for a package dyeing machine is around 10:1, when
the machine is fully flooded. Filling to just cover the packages, or even only to
cover the circular base, decreases the liquor ratio. In the latter case, only flow up
the spindles and out through the packages is possible. Dyeing at lower liquor ratios
offers considerable savings in water, steam, chemicals and time.

Many machines have an expansion tank. This accommodates the excess liquor
as it expands on heating the filled machine. This tank is used to add solutions of
dyes and any required dyeing assistants, which are then pumped back into the
machine. Almost all machines are pressurised, and can operate up to 135 °C.
Even when apparently dyeing at the boil, the machine is under pressure since the
lid is closed. Thus, more extensive safety precautions are required. Dyeing under
pressure, and placing the pump at the machine base, where the liquid pressure
head is highest, reduces cavitation. This arises from the formation of steam
bubbles behind the pump blades where the pressure is less, and their subsequent
explosive collapse when the pressure suddenly increases. This can cause noise and
vibration, pitting of the blade surfaces and a loss of pump efficiency.

One problem with pressurised package dyeing machines is the difficulty of
sampling to test that the dyeing has the correct shade. Some machines have an
isolated pot containing a single package that can be easily depressurised, cooled
and the yarn examined. Dyeing of a batch of yarn in package form, so that every
package is uniformly dyed throughout and to the desired shade, still requires
considerable experience. In package dyeing, it is difficult to reproduce the liquor
flow conditions from batch to batch and the shade reproducibility is sometimes not
completely satisfactory. For this reason, weaving or knitting of fabric with yarns
from different dye batches should be undertaken with caution.

Beams of warp yarn are dyed using the same principle, in simple machines with
unidirectional flow. Some machines may have four vertically mounted warp
beams. The perforated beam is initially wrapped with a non-absorbent non-woven
fabric before winding. This acts as a solution disperser and filter. As with other
types of package, the uniform winding of the yarn onto the beam is a prerequisite
for successful level dyeing.

Drying packages has always been a problem. Centrifuging packages in a basket
removes much of the loosely held water but complete drying requires a lengthy
period in a hot air oven. This type of process often takes as long as 24 hours to
completely dry the yarn on the inside of the packages. An improved method dries
the packages while they are still on the carrier frame. This is removed from the
dyeing machine and fits onto a connection to a hot air blower. More recently, the