Cure Oven Inspection Report
Appears in Print as: 'Ovens: Into the Fire'
Terry Ray has seen nearly everything in his years inspecting e-coat paint cure ovens as a paint systems manager at KMI Systems Inc. (Crystal Lake, IL), a company that designs, builds and installs paint and porcelain finishing systems for customers such as General Electric, Whirlpool, Weber-Stephen and Maytag, among others.
And he knows that poor oven operations can cost customers thousands of dollars in additional energy costs and hurt performance, including organic vapor condensation inside the oven that leads to ‘dirt’ rejects as well as higher clean up costs, condensation of flammable tar-like residues in the oven roof and wall panel seams, and high natural gas costs due to high operating temperatures as a result of a short residence time inside the cure oven.
“Over years of use, oven performance and operation can be negatively affected by many factors,” Ray says. “Items such as poor maintenance change in product mix, change line speed and oven function can contribute to various issues.”
That’s why Products Finishing magazine decided to tag along with Ray as he crawled his way through a 29-year-old e-coat cure oven to show specific examples on how to improve its performance. He filed the following report for his customer, but others may get an idea on how keeping cure ovens in top shape can some money in the long run.
The E-Coat cure oven was originally installed and placed into production during the second half of 1981 to cure PPG Cathodic E-Coat paint applied to automotive components. In addition to the oven’s current condition, we analyzed the oven’s current conveyor speed rate, the product throughput, exhaust rate, operating temperature and theoretical heat loads to determine if improvements can be implemented that will increase efficiency and/or oven throughput.
Observations and Readings
Cool Air Infiltration into the Oven: When the e-coat oven was cold it was determined that several thousand CFM air is being drawn into the oven through the conveyor entrance bottom sealed opening in the north-east corner of the e-coat oven and that air was being pushed out the conveyor exit end bottom sealed opening on the west end of the e-coat oven. The air velocity in and out of these openings will drop off slightly once the burner is firing and the oven reaches operating temperature, but cool air will still be drawn up into the entrance opening and likely be pushed out the exit opening. This is because the oven burner modulates from “high fire” during heat up to a lower “steady state” fire during normal production, there will always be a slight influx of air drawn into the oven through the conveyor openings since more air is being exhausted than is replaced at the heater box. Air balance improvement can be accomplished by cleaning up the fresh air inlet opening and damper in the east side of the cure oven heater box and then open the damper slightly more so more fresh air enters the heater box and is subsequently preheated before entering the main body of the oven.
E-Coat Cure Oven Burner: The burner is showing signs of deterioration. The burner sleeve is starting to warp from the heat and several of the air/gas jet holes are starting to break out. The west side of the south wall of the heater box also needed some immediate repairs.
Oven Recirc Fan: The airflow through the recirculation ductwork was measured using both an anemometer and compared to the original recirculating fan design airflow. Data was recorded and indicates that the total air being recirculated in the e-coat cure oven is significantly less than what is indicated on the original design drawings. The following are a few reasons why this might be:
- The fan belts might be loose and slipping
- The discharge slide dampers in the floor mounted distribution ductwork might be choked down too much
- There is organic dirt build up on the fan blades
- The recirculated air volume could have always been less than that listed on the drawings, possibly because the static pressure calculation for the ductwork was originally miscalculated.
Organic Vapor Condensation in the Oven Panel Joints: As the oven continues to age, the oven panel joints and flashings have loosened and small gaps have appeared in the flashing joints as a result of the repeated expansion and contraction of the oven as it is heated up at the start of production each day and then cooled at the end of production. Since organic vapor is released into the oven as a result of the e-coat paint curing process, hot vapor laden air migrates through the leaky oven joints where it came in contact with cooler air and metal surfaces. The temperature of the vapor then drops below its dew point leading to condensation inside the oven corner packing and oven panel interlocking joint packing.
Oven Temperature Profile: Temperature profile of the oven was recorded with the oven temperature set at 400ºF and the conveyor running at 16 fpm and aluminum parts were being run. Based on the graph and following data points, the oven air temperature in the path of the air probe on the part being measured reached 400ºF after approximately 5.5 min and stayed at 400ºF for only about 5 minutes, then the air temperature dropped as the air probe passed through a cool area in the e-coat oven. The cool area is a result of cool fresh air being drawn into the e-coat oven through the part entrance opening. The following are a few solutions that address this issue:
- Reduce the amount of fresh air drawn into the oven through the conveyor openings by drawing more air in through the heater box fresh air intake
- Relocate or extend the exhaust intake duct so it draws air more from the middle of the oven.
- Readjust the floor mounted supply ductwork so more hot air is discharged into the east end of the oven and then readjust the return air ductwork accordingly.
- Install a new internal partition wall that extends from the west end of the entrance opening to approximately 30 feet to the west. This will force the infiltration air to travel directly west when the air first enters the oven. The infiltration air will then warm up before traveling past parts that have already been heated up to temperature.
Oven Panel Surface Temperature: The infrared report for the e-coat cure oven indicates that there are sections of wall panels where the average temperature exceeds 110ºF on a mild 52ºF day which is indicative of two problems:
- The area just inside the oven where the hot spots where measured is much hotter than they should be, which can become a bigger issue if condensed organic deposits get too hot and spontaneously combusts.
- The hot spots can also be the result of either settling of the insulation in the oven panels or organic build up which results in a reduced “R” value for the panels.
Oven Original Design vs. Current Operations
The biggest change to the parameters is that the conveyor speed keeps climbing over the years and as it does so, the dwell time in the oven continues to shrink, which eventually will lead to borderline cure conditions unless the e-coat paint evolves and can cure in less time and at lower temperatures.
Comments and Recommendations
Exhaust Fan Capacity: It is recommend that the oven exhaust be set to approximately 7,100 ACFM in order to reduce the concentration of organic vapors in the oven atmosphere. This will reduce the amount of condensation into the oven panel joints and onto cooler surfaces in the bottom seal entrance and exit tunnels. Increased exhaust can also have positive impact on color control, if combustion products or vapors released by the paint are causing the paint color to shift.
Changes to the Fresh Make-Up Air: Increase the amount of fresh make-up air drawn directly into the heater box so the amount of fresh air plus combustion products is within 2% to 3% of the total exhaust SCFM (standard cubic feet per minute, which is the volume that the air would be at 70 ºF before being heated up to the oven operating temperature). Increasing the fresh air as recommended will minimize the amount of cool air infiltrating into the oven entrance and exit and thus reduce condensation in the vicinity of those areas. There is one potential draw back however, which is there could be a little more condensation inside the heater box near the fresh air inlet.
Add Partition Walls: It is recommended that a new internal partition wall that extends from the west end of the entrance opening to approximately 30 feet to the west be installed. This will force the infiltration air to travel directly west when the air first enters the oven. The infiltration air will then warm up before traveling past parts that have already been heated up to temperature.
Temporary Repair of Oven Internal Flashings & Joints: The existing top inside corner trim angles (flashings between the oven roof and wall panels) should be re-sealed by replacing missing or backed out sheetmetal screws with new bigger screws. This will close up the leak paths into the oven joints and slow down further organic vapor migration into the oven corner packing and oven panels.
Replace Oven Flashings & Insulated Wall & Roof Panels: It appears that all of the oven joints and a significant number of the oven panels have some build up of organic residue. Because of this finding it may be best to wait and replace the wall and roof panels in addition to rebuilding the joints and reflashing the joints. It is recommend that any new oven panels be 6” thick with 4 pcf density mineral wool insulation and 20 gauge aluminized steel inner and outer skins instead of lesser quality panels made with 20 gauge galvanized steel outer skins and 22 gauge aluminized steel inner skins.
Increase Part Residence Time in Oven: Since production has increased from the original design speed of 14 FPM to 24 FPM today, the residence time in the oven has gradually been reduced from 21 minutes to less than 13 minutes, which in turn necessitates operating the oven at higher operating temperatures (approximately 40º - 50ºF hotter). This increases natural gas consumption by 15% to 18% depending on the outside temperature and also increases the likelihood of a fire in the wall and roof joints due to condensed organic tars being heated up to their spontaneous ignition temperature. It is strongly recommend that you consider lengthening the oven residence time by making use of the dehydration pass of conveyor, which would allow you to reduce the oven temperature and save gas.
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