Comprehensive Coil Coating and Air Pollution Control System for an Automotive Parts Manufacturer
The Challenge
This large automotive industry manufacturer already had a liquid coating process line mounted in the facility. As the production output requirements grew and environmental legislations became stricter, the automotive production team faced several problems with the existing equipment and process design.
First off, finished automotive parts are especially unique as they are exposed to a wide range of external conditions: tremendous temperature and pressure range, mechanical impacts, and water exposure, and their mechanical properties need to remain constant in all scenarios. The automotive lines current liquid coating process could no longer reach the latest impact resistance standards and coating durability requirements. Their in-house research and development team invented several coating formulations; however, none had the durability and performance to satisfy the primary requests regarding mechanical features and product lifespan to resolve those issues with the existing equipment in place.
In addition, achieving uniform peak temperatures during the curing phase was a challenge due to the curing oven configuration and demands. The customer required a line speed of 85 FPM to satisfy the expected product throughput. The curing oven’s dwell time needed to be between 20 and 40 seconds to ensure consistent coating with the stiff bond to metal. To satisfy the curing time at that speed, the oven’s length had to be approximately 50 feet. However, the greater the oven length the more difficult it is to have uniform and efficient heating. In the counter flow regime of the air and conveyor belt, the air inlet temperature varies significantly from the air outlet temperature, leaving the strip heated unevenly. It is crucial to ensure that every part of the metal surface reaches the target peak metal temperature (PMT) during dwell time. The PMT required for this coil coating application was 480-500F depending on the thickness of the aluminum strips. Within that short period, the greatest challenge for the new design parameters is to achieve the desired coating durability by maintaining the uniform temperature distribution across the oven’s length, through specialized thermal dynamic controls.
From the environmental regulations perspective, the greatest issue represents the emission of the (VOCs) that are by-products of the organic coating’s application. The highest VOC emission occurs in the coating rooms, followed by the curing ovens, where their evaporation occurs due to higher ambient temperatures. The type VOCs released depends on the used polymer, but all VOCs are highly toxic for humans if inhaled. According to EPA standards, all VOCs from manufacturing facilities must be completely destroyed before exhaust gases emission enter into the atmosphere. Thermal oxidization is the most commonly used technology for VOC removal due to exceptional efficiency, but this often causes overwhelming additional costs since required operating temperatures are around 1500F.
The Solution
After a detailed analysis of the process requirements, Epcon’s engineering team developed the custom-designed coil coating system composed of coating rooms and curing ovens interconnected with heat exchangers and a regenerative thermal oxidizer (RTO) via an internal piping network. The custom-designed coating line provides an automated liquid covering of metal coils by passing through a series of rollers, ensuring the constant film thickness across the metal surface area, while the thermal oxidizer breaks down the VOCs in the exhaust gas so that no harmful emission enter the atmosphere.
A typical organic coil coating line consists of decoilers, entry strip accumulator, cleaning, chemical pretreatment, primer coat application, curing, final coat application, curing, exit accumulator and recoilers. Therefore, the aluminum sheet passes through the curing oven at two junctures; after the primer coat application and again after the final coat application.
Curing ovens are specially designed with a no heat flash tunnel and two independent temperature zones to meet exact process requirements. The operator is enabled to define the setpoint in both zones corresponding to the specific composition of organic coating applied to the metal surface. The accomplishment of two different temperature setpoints during the continuous process enabled optimal polymer atomic structure transition while forming stiff and durable bonds. Both oven zones are comprised of one combustion chamber installed on the side of the oven equipped with a gas fired burner and gas train. Each combustion chamber has a burner and a 30,000 CFM recirculation fan that evenly transfers heat across the sections’ length enabling the uniform temperature distribution with minimal deviations. The Flash tunnel is located at the entry of the oven with insulation, and void of recirculation or heating capabilities. The oven has top and bottom mounted supply plenums through which heated inlet air enters the oven while being evenly distributed through each section by forced flow, induced via recirculation fans. The heated air from the secondary heat exchanger coming to the oven has a sufficient temperature with only a minimal heat requirement, added by automatically controlled natural gas combustion to reach the oven’s setpoint.
After the aluminum strips pass through the oven, it enters the cooling chamber, which is a push-pull type and used to reduce the processing strip temperature. The cooling chamber is mounted at the end of the curing oven, and it is provided with one ambient air supply fan that cools the strip via a convection mechanism and a 55,000 CFM exhaust fan. The inclusion of the cooling chamber is essential for enabling instantaneous material handling and continuous process resumption.
The VOC laden exhaust air then flows from the coater room and curing ovens through interconnected pipes to the primary heat exchanger. There, the exhaust air is pre-heated by treated clean air coming from the oxidizer outlet. After passage through the primary heat exchanger, the temperature difference between the thermal oxidizer’s setpoint and VOCs rich exhaust gas temperature is significantly reduced. The required additional heat is supplied from the natural gas booster burner mounted perpendicularly to the exhaust gas airflow in the oxidizer’s combustion chamber. The perpendicular assembly and specially designed nozzles enable the air’s turbulent flow, mixing all fluid layers and reaching a required temperature of 1500 Fahrenheit to ensure the successful and complete oxidation takes place.
The regenerative thermal oxidizer (RTO) structure design was quite complex, and Epcon’s engineering team had to determine the thermal oxidizer’s required dimensions according to the client’s predefined VOCs evaporation rate from both sources, ovens, and coater room. The 2 chamber RTO’s length and capacity are specifically designed to maintain the 1 second residence time, taking into account the volume flows of the exhaust air from the process sources, and additional air intake provided by natural gas burners. The mentioned residence time is crucial to secure complete VOCs oxidation without any residual emissions. Components of a traditional RTO (regenerative thermal oxidizer) include a system fan, motor, burner, heat exchange media, flow control valves, electronic & automatic system controls, temperature recorder, and exhaust stack. The system is typically lined with ceramic or media fiber insulation, while its exterior is comprised primarily of steel.
The Results
The applied coatings meet the highest impact strength and scratch resistance standards due to optimal curing of the applied coating with temperature uniformity in the curing oven within -/+5F. The specially designed and energy-efficient thermal oxidizer removes more than 99% of VOCs while simultaneously providing thermal energy for curing ovens and even some other processes in a manufacturing facility. Therefore, the whole coil coating operation costs are tremendously reduced due to carefully engineered thermal integration throughout the system, requiring a minimal heat input with more than 85% thermal recycling throughout the system.
All process variables are constantly adjusted and maintained with Epcon’s pretested control system, made up of thermocouples to measure temperature, fans and spray nozzles adjusted by variable frequency drives, a custom PLC, and operating interface panel. The negative feedback control loop parameters are tuned by the Epcon engineering team according to defined setpoint temperatures in each oven section and thermal oxidizer as well. The most notable control system performance is reflected in the fastest speed of setpoint achievement enabling short oven’s dwell time while forming bonds of the highest strength and quality. Most importantly, users can in real-time assign process variables, control and monitor complete integrated units remotely from one place using user-friendly software.
The described integrated unit represents the highest quality, economically viable, and ecologically compatible solution for the coil coating process and VOCs removal. For this part manufacturer, the presented design provides the ability to successfully meet enormous production capacity demands in the continuous process that is a must in the modern automotive industry.