Designing Batch Ovens for Powder Coating Operations
Curing ovens are an important part of any finishing operation, and Col-Met’s John Owed says understanding key elements will assist in selecting the right oven for your application.
Q: We are in the early stages of purchasing a new gas-fired batch oven and want to get a good understanding of what items we need to consider prior to purchase. We apply a variety of powder coatings and are operating the oven at 375°F. The various parts we produce are in the oven for 20-30 minutes. What are the key elements we should consider when selecting an oven?
A: Curing ovens are an important part of any finishing operation. Understanding the key elements of a batch oven will assist in selecting the right oven for your application and is important to the quality and performance of your system. Batch ovens are typically used for curing “batches” of parts, just as the name implies. Batch ovens can be provided in a wide range of sizes and configurations based on part size, configuration and material handling needs.
The basic elements of an oven that need to be considered before purchasing include oven size, internal air flow design, operating temperature, heating capacity (design BTU), fuel type, door configuration and type, oven construction and controls.
In your case, because you are replacing an existing oven, you have probably already determined the size needed, which is dictated by the largest size part or cart (which may hold multiple pieces) with additional clearance above and along the sides. Generally, it is recommended to allow 18-24 inches from the ceiling or wall-mounted ductwork where the heated air is introduced to the parts to allow for proper airflow without disrupting the uncured powder coating. Additional space may be required, depending on the method in which the carts are moved in and out or through the oven.
Once the size of the oven is determined, you must assess the available floor space. The design and location of the duct work/airflow in the oven will have an impact on overall oven size. Ultimately, part configuration and size will dictate the airflow design in the oven. The more complex and geometric the parts, the more important the airflow will be. Options include a ceiling-mounted plenum which creates a downdraft that allows for top-down airflow, as opposed to wall-mounted input and exhaust plenums which create a cross draft. Variations to these designs and nozzles can also be incorporated to create more directional airflow, as required.
The next item to consider is the operating temperature of the oven, which is based on the coatings that are being cured. Most commonly, this will be in the range of 180-450°F. Most coating companies will specify the time at temperature by which their coatings must be properly cured. This requirement most commonly refers to the surface temperature of the part, not the air temperature of the oven. For example, a lightweight sheet metal part will quickly reach the oven set-point temperature, whereas a large cast-iron part will take much longer to reach “surface temperature.”
When purchasing an oven, it is critical to identify the composition and total mass of the product that will be cured, taking into consideration any carts or racks that will be used for parts handling. This allows your oven supplier to properly size the oven burner and internal airflow design so that proper curing can be achieved with minimal batch time consumed. You noted that you are operating your oven at a temperature of 350°F and curing parts for 20-30 minutes. This time does seem a bit long for curing powder; however, you did not mention the part type, composition or total mass in the oven.
Part conveyance into the oven can be done manually by cart or with more complexity using a floor track or conveyor. The oven’s door design is typically determined by part conveyance, available floor space or part size, and configuration. Solid, hinged double doors are the basic standard, if space is not an issue. When floor space is at a premium, a bifold swing door or a sliding door is a great option. Recent oven door innovations include metallicized fabric, insulated roll-up doors which offer many benefits, including reduced floor space requirements and options for “hands-free” operation. Ovens can be supplied for single entry or with dual sets of doors, which permit a pass-through design with doors at both ends to aid in workflow through the finishing operation.
An oven is comprised of a series of interlocking panels (with the thickness dependent on the operating temperature), a burner box and control panel. For your temperature range, a 4- to 6-inch wide panel would be provided with mineral wool insulation. Before purchasing an oven, it is important to understand the construction and how much labor/time will be required to assemble the oven in your facility. Depending on the size of the oven, it may be provided completely preassembled or need to be constructed on site. Ideally, if constructing on site, you want to ensure that the oven panels have been provided from the manufacturer preassembled with the insulation already installed. This will significantly reduce field installation time.
The control panel for an oven is often overlooked and many people settle for basic on/off and temperature settings. More recent PLC-based designs with touchscreen interface can offer a variety of features and benefits which can improve your process by reducing batch time, resulting in increased productivity and energy savings. Modern oven controls typically include programmable recipes that allow you to save specific oven settings (temperature and cure time) based on the part configuration or powder coating used. Built-in batch timers and hour meters simplify oven maintenance and troubleshooting. Communication can also be provided via Ethernet for remote monitoring.
To summarize, part size, presentation and complexity will dictate the design of the oven as related to airflow. Part composition and total mass per batch will impact burner sizing, so work with your supplier to find a design that best suits your process. Prior to purchasing, be sure to note available fuel source and plant operating voltage.
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