Application
In a Gravure/ Flexo printing or Solvent based lamination process, solvents are used as a carrier for inks or adhesives. Once this mixture is applied on the web, the web passes through a dryer where hot air is blown on the web. This hot air evaporates the solvents only and the ink/adhesive adheres to the web. There is an exhaust provided for the dryer which sucks the solvent laden hot air from the dryer and throws out. In a printing or laminating process, a mix of various types of solvents are used. Each solvent has their individual low explosion level (LEL) value. The ink/adhesive coverage (gsm of wet and dry weight) decides the percentage of ink/adhesive to solvent. This in turn decides the concentration of the solvents going out in the exhaust air. In today’s world we see a lot of special colours, background colours, heavy coating. This means higher solvent concentration in the exhaust air. We also see the use of various types of solvents where the LEL values are lower. At times the inlet path of air gets choked due to dust collection. This reduces the air going in the dryer and in turn increases the solvent concentration in the exhaust air. High concentration of solvents in the exhaust, especially where the solvents LEL values are low are a big risk towards explosion in the exhaust. Hence it is necessary that one should continuously monitor the solvent concentration in the exhaust ducts.

LEL monitoring system is an analytical system that continuously monitors the solvent concentration in the exhaust air at each dryer during the printing process. A sample of the gas in exhaust is extracted from the exhaust channel and analyzed by an Infrared (IR) sensor. The IR sensor signals are transmitted to the analytical unit. These signals are calibrated to show the solvents concentration in gm/m3 as well as in %LEL Safety Many times, in absence of the safety measures, there could be dangerous situations in the ovens due to the high solvent concentration. The system generates pre-alarms to inform the machine operator that the concentration is increasing towards danger levels. If due to some reasons, the solvent concentration reaches beyond the set limit, the machine speed either slows down or it completely stops.

Saving in the thermal energy (Fuel) consumption The heating system consumes a lot of thermal energy in the process of heating up the fresh air up to the required temperature. And this air goes out in the atmosphere after evaporating the solvents and taking it along in the atmosphere. The exhaust air is still hot with a lot of thermal energy due to its high temperature. The recirculation percentage is based on the actual solvent concentration. Lower the solvent concentration, higher will be the percentage recirculation. But in any case, the system always ensures that the solvent concentration is kept below a set % (safe%) of LEL. With the help of an automatic recirculation system, a part of the air in the exhaust can be recirculated back into the inlet. The calibrated signal coming out of the IR sensor analytical unit is interfaced with the damper control unit. This is done online and automatically. An automatic recirculation system can save us a lot of fuel and money due to the safe and optimum recirculation of the hot air. Investment in LEL monitoring system has a very attractive return on investment.