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LEL Automatic Recirculation Systems
DEC.FRS™

LEL Automatic Recirculation Systems play an essential role in enhancing safety and efficiency of a converting line, making them a key consideration when planning the installation of a VOC emission control system (e.g. a solvent recovery unit, or a thermal oxidizer).

Increasing energy costs are requesting our customers (flexible packaging and converting industries, using rotogravures, flexo and lamination machines) to explore and evaluate energy saving technologies, in order to increase their competitiveness in the global market.

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DEC.FRS™ (Flow Recirculation Systems) in drying hoods maximize efficiency, enhance safety, and minimize environmental impact in the converting industries: we provide a comprehensive and flexible solution to achieve these goals, empowering industries to optimize your VOC emission control systems and gain a competitive edge in the global market.

Drying hoods are essential components of converting processes, removing solvents from materials such as film, paper, and textiles. However, traditional drying hood designs often suffer from inefficiencies, safety hazards, and environmental concerns. These challenges stem from the suboptimal airflow patterns and inadequate control over VOC emissions.

Increased converting speeds, often associated with the application of special and background colors and heavy coatings (high ink coverage), lead to elevated solvent concentrations in the exhaust air stream. Obstructions in the inlet air path, such as those caused by dust accumulation or exacerbated by poor maintenance practices, can reduce airflow to the dryer, further increasing solvent concentration in the exhaust. High solvent concentrations pose a significant explosion risk in both the dryer and the exhaust system. Therefore, continuous monitoring of solvent concentration (LEL) in the converting line is essential.

DEC.FRS™ • LEL Automatic Recirculation Systems | how it works?

Automatic recirculation systems work in conjunction with LEL monitoring systems to continuously measure and optimize the solvent concentration in the exhaust air, ensuring safety and efficiency of the dryers. These systems use LEL sensors to analyze the gas sample and provide real-time data on solvent concentration levels.

Every drying station is equipped with its own dedicated supply (fresh-air inlet) and suction (exhaust) fans. Using an interconnecting duct between the dryers’ inlet and outlet, through modulating dampers, the DEC.FRS™ manages the recirculation of air based on the solvent concentration within the drying stations. This setup regulates the hot air being reused while continuously monitoring and maintaining solvent levels within safe limits.

When the solvent concentration in the exhaust air is below a set safety limit (typically set as LEL %), the system automatically recirculates a portion of the hot exhaust air back into the drying process: this reduces the need to heat the "fresh air", resulting in significant energy savings.

In drying hoods for packaging operations, recirculation rates can reach very high levels (up to 100% in specific configurations) where all dryer injected/exhausted air is returned to the 'fresh air' intake. Despite this, an outlet flow persists due to "false air". In modern systems, advanced machine design minimizes false air infiltration. However, because the printing element is maintained under slight negative pressure to reduce fugitive emissions, a continuous outlet flow remains, even at maximum recirculation efficiency. This flow, combined with the uncirculated stream, is directed to the VOC emission control system for treatment.

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DEC.FRS™ • LEL Automatic Recirculation Systems | benefits

  • operating costs: improved thermal efficiency, yielding energy savings of roughly 40-50% (ambient temperature dependent), translates to lower operating expenses for any converting operation.
  • safety and worker protection: proactive safety enhancements and worker protection measures create a secure work environment, minimizing accident potential and prioritizing employee well-being.
  • environmental compliance: reduction of the exhausted air flow (SLA), thanks to the increase of solvent concentration (g/m3), with consequent optimization of the investment (CAPEX) and operating costs (OPEX) for the VOC emission control system, activated carbon solvent recovery or the thermal oxidizer system;
  • competitive edge: gain a competitive edge in the global market by optimizing the converting processes, ensuring safety, and minimizing operational costs and environmental impact.

    • DEC.FRS™ • LEL Automatic Recirculation Systems | modularity

    FRS™ systems are engineered for modularity and compactness, with sensors that effortlessly integrate alongside a variety of ducts, ovens, dryers, or enclosures. These units are suitable for field mounting or panel installation, offering a streamlined, space-efficient design that eases maintenance. They can be fitted with either local or remote displays and alarms.

    DEC.FRS™ • LEL Automatic Recirculation Systems | compatibility

    Our retrofit solutions are compatible with all laminating, coating, flexo, and gravure presses. While some dryer ventilation system adjustments may be necessary, our expert engineers will conduct a thorough inspection to provide a detailed report. This report will include the total dudget investment, an estimation of the resulting discharge SLA flow rate reductions, estimated annual energy savings, and the project's payback period (ROI).

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    DEC.FRS™ • LEL Automatic Recirculation Systems | control loop

    The DEC.FRS™ system's response to a Lower Explosive Limit (LEL) exceeding the set threshold (H/HH) depends on the chosen configuration. Here's a breakdown of the three available options:

  • DEC.FRS_B™ (basic): H (high): triggers a visual alarm for operator awareness. HH (high-high): initiates an emergency line shutdown for critical safety situations.
  • DEC.FRS_A™ (active): H (high): automatically slows down the production line, potentially by reducing the rate of solvent evaporated. HH (high-high): initiates an emergency line shutdown for critical safety situations.
  • DEC.FRS_D™ (dynamic): continuously adjusts the recirculation system (adjusting the fresh air intake vs. the recirculation ratio) aiming to maintain the LEL value at the desired optimal set point. H (high): automatically slows down the production line, potentially by reducing the rate of solvent evaporated. HH (high-high): initiates an emergency line shutdown for critical safety situations.

    • DEC.FRS™ • LEL Automatic Recirculation Systems | single & multi-line

    Three different FRS™ control and recirculation processes are available:

  • DEC.FRS_SDR™: single dryer recirculation;
  • DEC.FRS_SLR™: single line recirculation (multiple dryers);
  • DEC.FRS_MLR™: multi lines recirculation.

    • DEC.FRS™ • LEL Automatic Recirculation Systems | sensors

    Discover the diverse array of sensor technologies available for implementation, including infrared, flame ionization detectors, and catalytic sensors.

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    DEC.FRS™ • LEL Automatic Recirculation Systems | compliance

    EN 1539:2015 (Dryers and ovens, in which flammable substances are released – Safety requirements) specifies safety measures for equipment where flammable vapors are generated. It mandates explosion protection strategies, including ventilation and monitoring systems to keep vapor concentrations below 25% of the LEL under normal conditions (or higher with additional safeguards). For control systems performing safety functions, EN 1539 often references EN ISO 13849-1 to ensure their reliability, particularly when automatic controls are integral to explosion prevention.

    DEC.FRS™ can optionally be provided in compliance with "PL d" of EN ISO 13849-1, featuring a durable, fault-tolerant design specifically engineered to address the explosion hazards detailed in EN 1539:2015. By incorporating highly reliable components and advanced diagnostics, the system guarantees safe performance in high-risk settings. DEC meticulously records every phase—risk assessment, design, and validation—to substantiate compliance, harmonizing operational efficiency with rigorous safety requirements.

    DEC.FRS™ • LEL Automatic Recirculation Systems | conclusions

    LEL automatic recirculation systems (DEC.FRS™) play a crucial role in ensuring safety and efficiency in printing and laminating processes. By continuously monitoring solvent concentration levels and automatically recirculating exhaust air, these systems help prevent explosions, reduce energy consumption, and improve operational efficiency.

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    Contact DEC for a consultation: we can help you assess your needs and recommend the best Advanced Flow Optimization technology for your operation.

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