The continuous dyeing of yarn on slasher dyeing ranges with open dye troughs has been practised for more than 20 years. The potential for improvements on open denim warp dye troughs are, however, limited. For the economical and ecological demands of today, Benninger has developed and successfully introduced the dyeing of indigo in closed nitrogen filled troughs. The main features and advantages of this technology are the concerns of this article.

Concept of the Dyeing Trough

The patented Benninger trough is a closed trough with a yarn inlet and outlet. Both inlet and outlet are equipped with an oxygen trap. The trough is split into two compartments. The yarn passes first the dipping and wetting compartment with a passage length of 6.6m, then through the flexible squeeze rollers before entering the penetration chamber with a passage length of 6 m. Both compartments are, during the indigo dying process, flooded with 98% pure nitrogen. The nitrogen flow is regulated by adjusting a flow valve. The nitrogen flow volume per trough is approximately 50 - 70 I/min. The dye liquid volume per trough is 360 I at the level of the overflow weir. Additionally to the 360 I in the trough are approximately 70 I in the overflow compartment, which results in a total dye liquid volume of 430 I per trough.

Indigo Dye Liquid Circulation flow schematic

Each trough is filled by a centrifugal pump. The pumps have a capacity 'of 80 I/min. In this case, the dye liquid exchange rate per trough is every 4.5 min. This exchange frequency is only possible due to the oxygen free atmosphere in the dye troughs. The dye liquid back flow is achieved by the slightly lower level in the mixing tank in comparison to the theoretical level in the dye troughs. The total indigo volume required for a 6 trough dyeing range is 3600 I (including mixing tank), which is substantially less in comparison to conventional slasher dyeing ranges with 8000 to 12000.

Nitrogen Supply

For an indigo dyeing range installation with six dye troughs, the following nitrogen supply setup is required: A compressor with a capacity of 170 Nm3/h at a pressure of 10- 11 bar, to supply compressed air for the complete dyeing range including the nitrogen generator. The nitrogen generator needs a minimal input capacity of 144.6 Nm3/h at 10.5 bar pressure. The nitrogen generator requires an output of 34.6 Nm3/h. The produced nitrogen contains less than 2% oxygen which is sufficiently pure for the dyeing process. The nitrogen is produced via a three-stage filtration process and is electronically monitored.

The distribution and regulation of the nitrogen flow to the dye troughs, indigo stock solution storage tank, Indigo dye mix tank and for the vatting reactor (optional), is controlled by air flow meters with a minimum/ maximum capacity from 10 - 70 I/min. The required nitrogen flow is set with the aid of a gaseous oxygen measuring detector. In the indigo dyeing trough, a residual amount of 4 - 6% oxygen in the dipping as well in the penetration section is absolutely sufficient for the dyeing process. Each trough can be separated into a single dyeing unit with its own liquid circulation circuit. This enables the multi functionality of the troughs for a variety of processes, which are independent of the main dyeing process.

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Originally published in New Cloth Market: November 2009