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.
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 expressed as LEL%), the DEC.FRS™ system automatically recirculates a portion of the hot exhaust air back into the drying process. This reduces the need to heat "fresh air," resulting in significant energy savings and a reduced volume of exhaust SLA (Solvent Laden Air) flow.
In drying hoods for packaging operations, DEC.FRS™ can reach very high recirculation rates (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" inlet. 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.
DEC.FRS™ • LEL Automatic Recirculation Systems | benefits
- operating costs: improved thermal efficiency, stemming from the reduced need to heat fresh air, leads to considerable energy savings—typically 40-50%, depending on the ambient temperature, directly translating to lower operating expenses (OpEx) 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).
Our retrofit solutions are exclusively designed for existing converting lines: we do not offer or apply retrofits to recent/new converting lines. Integrating such systems into a new line requires deep interfacing with the original operating software, and any unauthorized modifications could void certifications and potentially compromise safety. For new converting lines, any desired modifications or upgrades should be performed directly by the original manufacturer (OEM) of the converting line.
DEC.FRSx™ • 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.FRS1™ (basic): H (high): triggers a visual alarm for operator awareness. HH (high-high): initiates an emergency line shutdown for critical safety situations.
- DEC.FRS2™ (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.FRS3™ (dynamic): continuously adjusts the recirculation system (adjusting the fresh air intake vs. the recirculation ratio - 0 to 60% or 0 to 100% depending on LEL sensor configuration) 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.FRSx_SDR™: single dryer recirculation;
- DEC.FRSx_SLR™: single line recirculation (multiple dryers);
- DEC.FRSx_MLR™: multi lines recirculation.
DEC.FRS™ • floor sweep management
To reduce fugitive emissions, floor sweep can be managed with different approaches:
- connected to the fresh air inlet of the converting line;
- connected to a RBC™ • Rotary Bed Concentrator before being treated by the main VOC abatement system. This solution reduces the volume and increases VOC concentration, leading to a smaller VOC emission control system and guaranteeing the lowest operating costs (OPEX);
- connected directly to the Central Exhaust Duct • CED™ for processing by the VOC abatement system. This is the least preferred option, as it increases the size and operating costs of the VOC emission control system.
DEC.FRS™ • LEL Automatic Recirculation Systems | sensors
Discover the diverse array of sensor technologies available for implementation, including infrared (IR) and flame ionization detectors (FID).
The use of catalytic LEL sensors must be avoided because the catalyst can be poisoned by specific compounds within the chemicals (e.g., inks, varnishes, adhesives), including functional additives, amines, and general impurities.
DEC.FRS™ • LEL Automatic Recirculation Systems | compliance
EN 1539 (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 references ISO 13849-1 (Safety of machinery, safety-related parts of control systems) 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 ISO 13849-1, featuring a durable, fault-tolerant design specifically engineered to address the explosion hazards detailed in EN 1539. 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 | challenges
Retrofitting a solvent drying hood with a Flow Recirculation System (FRS) controlled by a Lower Explosive Limit (LEL) sensor is a significant technological and safety upgrade. This system recycles a portion of the solvent-laden air back to the dryers, reducing the volume of fresh air needed. This leads to substantial savings in energy consumption both heating and VOC emission control system (oxidizer or solvent recovery). However, implementing such a system requires a planned shutdown of the production line(s), which presents several challenges that must be addressed.
A successful drying hoods retrofit hinges on the careful planning and execution of a controlled shutdown. This is necessary to perform critical modifications that cannot be completed while the lines are operational. The key challenges include:
- Downtime and Production Loss: the most significant challenge is the required shutdown of production lines. The installation of a new FRS involves extensive work, including modifying existing ductwork, replacing dampers, and installing new control systems and sensors. This process can take several days, leading to a direct loss of production and revenue. A detailed project schedule must be established to minimize this period, with all materials and labor pre-arranged.
- Mechanical and Structural Modifications: the new system requires changes to the existing ductwork. This may involve cutting into existing ducts, adding new sections, and ensuring all connections are properly sealed to prevent leaks. The existing dampers, often simple manual or pneumatically controlled units, must be replaced with new, motorized, and often LEL-controlled dampers that can precisely regulate airflow. This mechanical work is labor-intensive and requires skilled technicians.
- Electrical and Control System Integration: the new LEL control system needs to be seamlessly integrated with the existing machine controls. This includes wiring new LEL sensors, installing control panels, and programming the Programmable Logic Controller (PLC) to manage the recirculation process safely and efficiently. The new system must be configured to respond instantly to LEL readings, automatically adjusting damper positions to maintain solvent concentration below a safe threshold, typically around 25% of the LEL.
- Safety and Regulatory Compliance: a crucial challenge is that any modification to the machine will likely void its existing certification. If such a certification is a mandatory legal requirement in the Country of implementation, the retrofit may not be feasible unless the entire machine can be re-certified after the installation. During the shutdown, strict safety protocols must be followed, especially when working on or near potentially explosive solvent vapors. All work must comply with local and international safety regulations, to meet relevant safety standards.
Any initial budget for a retrofit project is an indicative proposal only. The actual costs can vary significantly based on the specific conditions of each machine and facility. Factors that influence the final price include:
- the age and condition of the existing equipment;
- the complexity of the existing ductwork layout;
- the number of drying hoods and LEL control points to be retrofitted;
- the integration challenges with the existing control systems.
Therefore a site visit to perform a detailed inspection of the machinery and site layout is mandatory. This allows for a dedicated assessment of the required modifications and a reliable quotation. This ensures that all major challenges and opportunities are identified and addressed, providing a clear and reliable path forward for this essential upgrade.
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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FAQs • Frequently Asked Questions
LEL Automatic Flow Recirculation Systems • DEC.FRS™
What is a DEC.FRS™ LEL automatic flow recirculation system?
DEC.FRS™ is an automatic recirculation system that uses a Lower Explosive Limit (LEL) sensor to safely return a portion of solvent-laden exhaust air from a drying hood back into the drying process, cutting the volume of fresh air that needs to be heated.
How much energy can a DEC.FRS™ system save?
Depending on ambient temperature and recirculation rate (up to 100% in specific configurations), DEC.FRS™ typically cuts thermal energy consumption by 40-50% by reducing the amount of fresh air that must be heated.
What's the difference between DEC.FRS SDR, SLR and MLR?
DEC.FRS_SDR™ recirculates air for a single dryer, DEC.FRS_SLR™ recirculates air across all dryers on a single converting line, and DEC.FRS_MLR™ extends recirculation across multiple lines.
What's the difference between DEC.FRS1, FRS2 and FRS3 control loops?
DEC.FRS1™ (basic) triggers a visual alarm at the high threshold and an emergency shutdown at high-high; DEC.FRS2™ (active) automatically slows the line at the high threshold; DEC.FRS3™ (dynamic) continuously adjusts the fresh-air/recirculation ratio to hold the LEL value at an optimal set point.
Which LEL sensor technology does DEC.FRS™ use?
DEC.FRS™ uses infrared (IR) or flame ionization detector (FID) sensors; catalytic sensors are avoided because they can be poisoned by compounds found in inks, varnishes and adhesives.
Can DEC.FRS™ be retrofitted onto an existing converting line?
Yes — DEC.FRS™ is designed as a retrofit for existing laminating, coating, flexo and gravure lines, though it requires a planned shutdown for the ductwork, damper and control modifications; it is not offered for new/recent lines, where upgrades should come from the original equipment manufacturer (OEM).
What safety standards does DEC.FRS™ comply with?
DEC.FRS™ can be supplied in compliance with "PL d" of ISO 13849-1, supporting the explosion-protection requirements of EN 1539 for dryers and ovens releasing flammable vapors.
How do I get a quote for a DEC.FRS™ retrofit?
Because costs depend on the existing ductwork, the number of drying hoods and LEL control points, and the integration challenges with existing control systems, DEC performs a site visit and inspection before issuing a reliable quotation.
