In the textile industry, stenter frames are used to dry, stretch and fix textile webs after the finishing process. The term textile finishing refers to the coating, laminating, dyeing and printing of textile fabric. Before entering the stenter frame, the textiles are often soaked with additives to achieve certain properties. In the stenter frame, the moist fabric is finally guided through several fields with very hot air flowing through them and dried.
The energy-rich exhaust air is released into the open atmosphere. Prior to this, part of the heat is extracted from the hot air in a heat reclaimer and used to preheat the supply air to the stenter frame. This reduces power consumption considerably but it involves a problem, however: When drying the textile webs – especially during the fixing and condensation process – oils and other aerosols are emitted from the textiles and from the applied additives in addition to dust and water vapour. You can reduce the pollution burden by washing the fabric, but even then, it is technically impossible to operate the stenter frame without emissions into the ambient air. Therefore, cleaning of the exhaust air is imperative.
A first cleaning process is performed in the heat reclaimer. A great part of the pollutants condensate to droplets during heat recovery. An air washer further cools the exhaust air and water-based pollutants are trapped in the washing water.
In the textile industry, commonly off-the-shelf air washers are used that have poor filter performance. This causes clogging again and again which interrupts the operation. An electrostatic precipitator fitted downstream can improve filter performance. However, these electrical filters are highly susceptible to disturbances and involve high operating costs. Short circuits often occur due to the high air humidity. The combination of air washer and electrostatic precipitator should therefore be critically evaluated.
Another solution is the use of ozone-generating UV tubes. Without efficient pre-separation, however, greasy deposits quickly form on the tubes. They must be removed and cleaned at regular intervals. This results in high maintenance costs and possible downtimes.
Thermal afterburning has also not proven its worth. It requires high investments and the operating costs are considerable. Textile companies have a lot of waste heat anyway, so an additional heat generator makes little sense.
Are there alternatives?
As an alternative, we recommend a special cyclone separator which is based on the patented X-CYCLONE® process (Fig. 2).
It is additionally equipped with a spraying device which wets all surfaces with a water film. The undesirable particles are washed out of the exhaust air to a high degree. Subsequently, they are ejected through rapid deflections in the X-CYCLONE® separator (Fig. 3).
The high turbulences improve the degree of separation on the one hand and strongly increase the particle transfer between the exhaust air and the washing water on the other hand. This provides for an improved cleaning performance. The combined filter works on a purely mechanical basis and practically cleans itself during operation. This system technology almost achieves the efficiency of an electrostatic precipitator and takes up only one third of the surface area of a conventional air washer.
Only if the air washer, the spraying unit and the cyclone separator are individually matched to the application, an optimum solution can be achieved.
Best practice example
The textile finisher Van Clewe in North Rhine-Westphalia recently renewed the air cleaning system on a Monforts stenter frame.
Company spokesman Ansgar van Clewe described the initial situation as follows: “As far as the air washing system is concerned, I believe we have had already all the available variations in our factory and unfortunately many problems occurred: insufficient mixing of the exhaust air via the atomisation, sticking and clogging in the separator, short circuits on the electrostatic precipitator, formation of deposits on the UV tubes, etc.
As the use of thermal afterburning does not really fit into our corporate concept – we have plenty of heat – I was looking for alternatives and came across REVEN. On the one hand the separation limit was important for me, i.e. 1µ droplets instead of 14µ with conventional separators, and the fact that separation and mixing of the exhaust air takes place in the separator itself.”
Van Clewe adopted the system configuration we recommended and dispensed with the electrostatic precipitator. The X-CYCLONE® separator replaces the initially provided electrostatic precipitator as well as the droplet separator. The new concept allows Van Clewe to save maintenance and energy costs. In addition to this, it provides for exhaust parameters that comply with legal workplace exposure limits.
Ansgar van Clewe confirmed: “The first air washer from REVEN has been working for three years now without any problems. We are very satisfied with the function and finally have our “problem stenter frame” under control. Otherwise we would certainly have had trouble with the authorities, which we could avoid this way.”
Until the final commissioning of the filter system, however, many difficulties had to be overcome, such as sticking and clogging of the separator blades. Numerous series of measurements were necessary to determine the final geometry and position of the X-CYCLONE® blades as well as the optimum spray pattern of the water nozzles. In order to minimise water consumption, the washing water is circulated with only a small addition of fresh water.
At the beginning there is the measurement
We have set up a special “particle measurement service” for this task. A measuring team puts the air hygiene in the customer’s factory in the focus of the sensor measurements. Highly sensitive measuring devices indicate the degree of contamination of the indoor air and the outgoing air and identify critical work zones with high loads. In addition, they reveal the repartition of the pollutant sizes. With the latest measuring set, we can also detect formaldehyde1and VOC (volatile organic compounds)2.
The customer receives a detailed analysis of the current status. The measurements allow the comparison of the separation efficiency of different filter combinations over the entire particle range. And during the acceptance test of the ventilation system, they provide evidence of the required separation efficiency and of compliance with workplace exposure limits.
1Formaldehyde is formed during all incomplete combustions and other oxidation processes. In industry, it is used as a raw material for plastics, as an adhesive in wood processing, as a “crease-resistant and easy-care finish” for textiles, and as a preservative in agriculture and the food industry. Formaldehyde is used as a disinfectant and is also found in cosmetics, body and oral care products and partly in paints, varnishes and carpets. The indoor workplace exposure limit is at0.37 mg/m³.
2 Volatile organic compounds (VOC) is the collective term for organic, i.e. carbon-containing substances that occur as a gas phase to an extent relevant for air pollution control. Volatile organic compounds in the indoor air can cause symptoms such as headaches, hypersensitivity reactions, fatigue, loss of performance, sleep disturbances and irritation of the respiratory tract. Especially people who have suffered premature illness are at risk. These symptoms are summarized under the term “Sick Building Syndrome”. According to the guideline value of the German Federal Environment Agency, the average total concentration of VOCs should be less than 0.3 mg/m³.