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


Concept of the Dyeing Trough

The patented Benninger trough is a closed trough with a yarninlet and outlet. Both inlet and outlet are equipped with an oxygen trap. Thetrough is split into two compartments. The yarn passes first the dipping andwetting compartment with a passage length of 6.6m, then through the flexiblesqueeze rollers before entering the penetration chamber with a passage lengthof 6 m. Both compartments are, during the indigo dying process, flooded with98% 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 dyeliquid 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 overflowcompartment, 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 havea capacity 'of 80 I/min. In this case, the dye liquid exchange rate per troughis every 4.5 min. This exchange frequency is only possible due to the oxygenfree atmosphere in the dye troughs. The dye liquid back flow is achieved by theslightly lower level in the mixing tank in comparison to the theoretical levelin the dye troughs. The total indigo volume required for a 6 trough dyeingrange is 3600 I (including mixing tank), which is substantially less incomparison to conventional slasher dyeing ranges with 8000 to 12000.


Nitrogen Supply


For an indigo dyeing range installation with six dyetroughs, the following nitrogen supply setup is required: A compressor with acapacity of 170 Nm3/h at a pressure of 10- 11 bar, to supply compressed air forthe complete dyeing range including the nitrogen generator. The nitrogengenerator 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 nitrogencontains less than 2% oxygen which is sufficiently pure for the dyeing process.The nitrogen is produced via a three-stage filtration process and is electronicallymonitored.


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

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Originallypublished in New Cloth Market: November 2009