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1、Analysis of the factors influencing dye uptake on jet dyeing equipmentIntroduction An important objective of sample colorations in a textile dyehouse laboratory is to simulate production-scale conditions. While typical laboratory equipment, e.g. a rotating beaker-type machine, is useful for colour m
2、atching purposes, a more detailed process analysis in the laboratory might require machines that operate more closely like those in bulk production. One important question in this context is if pilot-scale machines can be used to accurately predict the dye exhaustion of bulk dyeing machines.The infl
3、uence of the liquor flow rate on the exhaustion rate, initially established on package dyeing machines, has been known for a long time and has been confirmed several times since then, also for other types of dyeing equipment . There are at least three possible factors, which alone or in combination,
4、 can explain this sensitivity of the dye-uptake rate. All three factors contribute to a reduced dye supply at the fibre surface, thereby possibly leading to a process in which the ratedetermining step is no longer the diffusion of the dye inside the fibre (film diffusion-controlled process) but the
5、diffusion of the dye from the dyebath to the fibre surface (liquid-diffusion limited). All three factors also gain in importance as the fluid velocity drops. In the first case, it is known that as the fluid velocity drops, a diffusional boundary layer of increasing thickness develops at the substrat
6、e surface which reduces masstransfer to the individual fibre . In the second case, the primary cause is a much diminished fluid velocity near the fibre surface compared with the macroscopic average dyebath flow rate. This is caused by flow resistance in the fibre assembly (microscopic depletion) . T
7、he resistance can occur, for example, in the intra-yarn pores or at the intersection of yarns in assemblies. This phenomenon is not to be confused with boundary layer effects which, when flow direction is perpendicular to the fibre axis, vary little with flow rate. Thirdly, the dyebath depletes as i
8、t traverses the package or fabric pile, thus lowering the dye supply, and consequently the uptake rate of subsequent substrate layers (macroscopic depletion). In the third case, it is the macroscopic dye supply within the entire package or pile, as opposed to the dye supply with the yarn in the seco
9、nd case, which limits the overall uptake rate. Thus, the second phenomenon can arise in a single fabric layer, or even a single yarn, whereas the third phenomenon requires the existence of several layers of fabric or yarn. In this study, the term liquid diffusion effects is used when reference is ma
10、de to all three factors, without making specific reference to either factor.In a jet dyeing machine, fabric and dyebath are circulating. A fabric circulation in a jet dyeing machine may be divided into the following three phases:Passage through the jet nozzle and, possibly, an attached tube: In the
11、nozzle, fabric, water and air-flows meet to create a situation of high fabricliquor interchange. The nozzles design and the fluid velocity determine how much kinetic energy is transferred to the fabric. Fabric propulsion on bulk machines, especially at higher fabric speeds, is mainly achieved by the
12、 impulse transfer in the jet with the reel providing only a minor contribution. In order to ensure adequate propulsion of the fabric, the liquor speed in the nozzle must be higher than the fabric speed. Retention, normally partially flooded, in the storage chamber: In the storage chamber, there is s
13、ome fabric liquor interchange as the liquor travels over and through the packed fabric to the suction side of the main circulation pump, from where the dyebath is propelled back into the nozzle. On modern low liquor ratio machines, which allow the processing of cotton at liquor-to-goods ratio of aro
14、und 5:1, the dyebath level is so low that only a fraction of the fabric in the storage chamber is submerged. Additional dye transfer occurs between the surfaces of the wet, compressed fabric layers. Acceleration from the storage chamber over the reel and towards the nozzle: On its way from the stora
15、ge chamber, where the speed is zero or almost zero, to the reel, the fabric is accelerated quickly to its maximum speed. This acceleration and the gentle squeezing effect of the reel reduce the amount of water in the fabric rope, which in the case of cotton, for example, is around three times the fa
16、bric weight. Both actions contribute to a better overall fabricliquor interchange as mechanical agitation assists dye penetration and because more water is subsequently replenished in the nozzle.Based upon this general description of the fabric path in the machine, it would be expected that the following machine parameters could influence the dye-exhaustion rate.Jet nozzle size and designThe water exiting the nozzle hits the fabric rope at an angle, but interrupting the dyeing process a
