Many coating systems (e.g. coil coating, lacquering, laminating, gravure printing, flexo printing, offset, …) use solvents. After the coating of the substrate a dryer evaporates these solvents using hot air or infrared radiation. The solvent vapors generated in the dryer must be diluted with air to avoid an explosive mixture in the dryer which could be ignited by a source of ignition which is often present in dryers.
Standards such as the European EN 1539:2015 “Dryers and ovens, in which flammable substances are released – Safety requirements” prescribe safety measures to ensure that the atmosphere in the dryer will not explode. These measures include a minimum ventilation rate (which needs to be monitored), the declaration of hazardous zones and the use of gas detection systems to measure the concentration of solvent vapors in % LEL:
- Ventilation rates: ventilation rates must usually be set according the maximum solvent intake at the highest production speed to ensure that the dryer is safe in all operating conditions. This would mean that at lower production speeds or with coatings that contains fewer solvents the ventilation rate is higher that necessary: since the ventilation air must heated and usually fed into a thermal oxidizer this will use up more energy (and increase costs).
- Declaration of hazardous zones can help to increase the safety of the dryer but is not sufficient.
- A solvent vapor monitoring system measures the solvent vapor concentration in % LEL and generates alarms when the concentration surpasses the allowed maximum concentration. This concentration alarm will then trigger measures to put the dryer into a safe state (e.g. stop of the coating, increase of ventiliation rate, shutdown of the heating system, …).
Coating processes which do not inherently prevent the excess input of solvents into the dryer, require the use of a LEL monitoring system according to EN 1539:2015. A mere monitoring of ventilation rates which are chosen based on the normal solvent intake cannot detect this error nor prevent an explosion.
Using LEL monitoring systems allows to optimally adjust ventilation rates according to the actual solvent concentration in each drying zone. This will increase solvent concentrations to the allowed limits with a number of benefits:
- energy savings by using less drying air. Depending on the drying temperature these saving can be quite significant.
- increase of production speed. This is an alternative to saving energy where the speed of the web is increased.
- saving money on the oxidizer: increasing solvent concentration might finally allow you to run the oxidizer in an autothermal mode and use less natural gas. The lower volume of waste gas from the dryer might offer the opportunity to connect another emission source to the oxidizer or allow you to use a smaller oxidizer.
The adjustment of the ventilation rate to actual solvent concentration can be done in a completely automatic way with continuous damper control, or instead by using a recipe based system with fixed damper settings for each product.
In many coating processes the complex and changing mixtures of solvents require an LEL monitoring system that can not only measure all of the solvents but in particular will be able to measure them all with (almost) the same sensitivity (response factor). If the response factors versus different solvents variey significantly then, in order to be safe, the readings for some of the solvents will be quite exaggerated and might lead to early, unnecessary alarms. This would require higher ventilation rates and thus counteract the whole idea of the installation of the LEL monitoring system.
PrevEx FTA analyzers are the measurement system with the least variation of response factors for solvents and are therefore optimally suited to be used for the LEL monitoring of complex and variable – sometimes even unknown – solvent mixtures.
Coating processes are often running 24/7 and a shutdown of the production line due to a failure of the LEL monitoring system can be expensive. A high availability even in demanding environments is therefore a must. The PrevEx FTA analyzer of the 670 series are rugged devives which have been specially developped for use in industrial environments and have proven their superior performance over decades. Heating all parts in the sample train (up to 250°C) and mounting the analyzer directly online at the sampling location allows to deal with difficult, high dew point vapors in the sample gas by effectively preventing condensation.