When determining how best to recirculate the air in an industrial oven, one should consider the load configuration along with the available options to evaluate how the airflow may affect the parts. The goal is to provide the most even and effective heating for the product while considering part geometry, size and orientation. Care must be taken with any airflow type to avoid blocking or impeding the heated air from being delivered to the load.
There are several airflow configurations and louver styles from which to choose. In most cases, the more common styles are more cost-effective to build. This article will look at the most common airflow styles.
By far the most common airflow design, combination airflow is well suited for loads that allow the recirculation air to pass both horizontally and vertically up through the load. This airflow style is used for both batch and conveyor oven applications.
With combination airflow (figure 1), supply ducts are located on both side walls, running from the very front to the very back of the work chamber. The ducts introduce the heated air into the chamber near the bottom. This helps ensure the heated air is distributed evenly throughout the length of the work chamber.
After coming into contact with the load, the air rises past the product vertically to completely bathe the product in heated air. The air then is returned to the heating plenum through an adjustable return duct — typically located above the work chamber — to better guide the path of the air. Having the return duct over the entire width and length of the chamber provides the best control of the air and air temperature uniformity. Some oven designs have a small return opening above the center of the heating chamber, and this may not provide an even distribution of the heated air or allow adjustability.
When using combination airflow, it is important to note that loads will heat up more effectively if they are elevated above the oven floor. Elevating the load allows the recirculation air to get beneath the load.
This type of airflow design does not work well with the product loaded on multiple solid shelves where the heated airflow cannot be directed vertically through the product. In addition, load cars that have a solid base and do not allow air to pass up through the load are not compatible with combination air. This cart design will seriously slow the load heatup rate and the ability of the load to be evenly heated.
Horizontal-style airflow is well suited for applications where there are multiple horizontal tiers of product or trays loaded such that air cannot easily pass vertically up through the product (figure 2). This airflow style also is used for both batch and conveyor oven applications.
Air is supplied into the work chamber through a fully adjustable boxed duct — louvered openings are factory preset — located on the side and along the length of the work chamber. After the heated air passes through the product and the work chamber, it is collected in a similar duct located on the opposite wall of the work. The air then is returned to the heat and recirculation plenum, where it is reheated and reintroduced into the work chamber.
There is usually a slight part-temperature variance across the chamber width because the air cools as it delivers the heat to the parts. This effect is exacerbated as the width of the chamber increases. Bidirectional horizontal airflow can be provided to equalize the part heating by placing dampers in the supply and return ductwork to periodically change the direction of the airflow (right-to-left, then left-to-right, etc.). Typically, the dampers are programmed to change the airflow direction using a programmable setpoint controller. This reduces the temperature difference between the left and right sides of the product.
The top-down style airflow is popular for flat-belt and spindle (chain-on-edge) conveyors when the product height is less than 39" (1 meter). The distance between the top of the product and the supply duct should be minimized to ensure sufficient heat transfer to the load. For powder coating applications, however, enough distance must be maintained to prevent the air from blowing the powder off the parts.
In a top-down airflow arrangement, the air usually returns along the sides, below the conveyor (figure 3). Belt conveyor ovens also may utilize a return air duct below the conveyor belt to ensure the air passes through the conveyor belt more evenly. Spindle conveyor ovens usually have the air returned along the sides.
Because the chamber height generally prohibits good air control, top-down airflow is not typically recommended for overhead conveyor ovens or batch ovens. Unless the chamber is designed with a return duct under the load, the heated supply air loses velocity after leaving the supply duct louvers. Then, it gets drawn to the return plenums located at the bottom sides of the chamber. This does not allow the air to reach the bottom center of the load, causing cold spots. This effect worsens as the chamber width and height increase.
Bottom-up style airflow is an excellent choice for continuous conveyor applications (e.g., belt type, overhead trolley, etc.). The air is supplied from below the load, allowing the air to flow vertically upward, through and past the product. This ensures proper heating — provided the belt, part carrier and load do not impede the airflow as it travels vertically upward. For improved temperature uniformity, a full return sheet with manually adjustable louvers usually is provided above the load to guide the air after it exits the supply duct. For ovens with open ends during normal operation, the full return also will reduce cold air from entraining as far into the chamber and further disrupting air temperature uniformity (figure 4).
Bottom-up airflow also can be utilized in batch ovens. In those applications, the oven is placed over a pit with a bar grating surface (mounted flush with the factory floor) above the supply ductwork or has structure mounted over the duct to support the load.
Bottom-up airflow also is an excellent choice for overhead trolley conveyor ovens. Regardless of the product height, the hot air fully bathes and engulfs the product as it rises.
Top-Down and Bottom-Up Airflow
Top-down and bottom-up style airflow patterns — also known as top-bottom airflow — are primarily used in flat belt conveyor applications (figure 5). This type of airflow is particularly effective with heavy loads of densely packed small parts or thicker flat loads, where it is difficult — or impossible — to pass the air through the load. Supply ducts are located both above and below the conveyor to deliver the heated air to the top and bottom of the load.
When fast heating rates are desired, impingement-style louvers are utilized to disrupt the boundary layer of cold air located just on the surface of the parts being heated. This design uses high velocity air exiting the supply duct. After striking the load, the air returns back to the heating system between the supply air nozzles.
An array of supply and return louver designs exist for all of the airflow styles mentioned. Some supply louvers are meant to discharge the heated air in a diffuse pattern (e.g., H shape, C shape, round holes, etc.). Others are meant to impinge the air onto the load (e.g., slotted louvers, cones, tubes, etc.) for enhanced heating.
An experienced oven manufacturer will have several louver styles from which to choose, depending on your application. In some cases, the oven may have a combination of supply louver styles (e.g., H-shape with slotted louvers).
Companies that utilize computational fluid dynamics (CFD) analysis software to design their equipment are able to optimize airflow, heat transfer and overall oven performance (figure 6). CFD is effective for optimizing the duct design inside an oven. Potential issues can be corrected in the design phase of manufacture, which results in a better design with reduced setup and testing times.
When reviewing your oven project with an oven manufacturer, be prepared to discuss topics such as part size and orientation, load cart design, load carrier design, temperature uniformity and part-heatup rates. These variables should be discussed prior to receiving a firm proposal, and taking them into account will help you select the proper airflow style to achieve a successful, cost-effective outcome.
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