Solvent based inks in different colors and patterns are applied to paper, plastic, film and foil substrates. Finished products include all sorts of wrapping paper, wall covering, candy wrappers, flexible food packages, shrink labels, high quality magazines and many, many more. These industries employ ovens or dryers to remove the solvents from the finished product.
The key hazard is an explosion from the buildup of solvent vapors in the oven or dryer atmosphere. This buildup could occur by a malfunction in the production process such as: a sudden or improper amount of coating, a change in ventilation controls or excessive speed. In any of these cases, the concentration of flammable vapor has an opportunity to rise above safe levels, creating a potentially explosive mixture of vapor in air.
So, how do you combat these hazards? The design specification for safe operation of a solvent oven or dryer is outlined in safety and fire codes, specifically, NFPA 86, the Standard for Ovens and Furnaces:
Safety Ventilation is Required to Stay Below 25% LFL
To prevent solvent vapor buildup, NFPA 86 requires continuous ventilation of the oven. It states the safety ventilation rate of continuous process ovens shall be designed, maintained, and operated to prevent the vapor concentration in the oven exhaust from exceeding 25 percent of the LFL.
Continuous Solvent Vapor Monitoring Allows Operation Above 25% LFL
NFPA also states the safety ventilation rate shall be permitted to operate at a safety ventilation rate lower than that specified where a continuous solvent vapor concentration indicator and controller is provided in accordance with 11.6.10. For such installations, the continuous indicator and controller shall be arranged to alarm and shut down the oven heating systems or operate additional exhaust fans at a predetermined vapor concentration that shall not exceed 50 percent of the LFL. In other words, when a continuous LFL monitor is installed, the oven can operate above 25% LFL but must shut down before exceeding 50% LFL.
There is enormous benefit in knowing the actual concentration of solvent vapor present in the oven zone. Rather than relying on calculations, the vapor monitor provides real-time, actual conditions. Should the concentration rise due to a failure or malfunction in the process, the operator is notified immediately and the monitoring system can shut down the process before an explosive concentration is achieved.