A common misconception we encounter while working with customers to develop process heat systems is that the airflow transitioning from a small space to a larger one will expand to fill any volume. While this is true (to a point) for compressed air, which undergoes a rapid volume expansion as it transitions from a high pressure environment (80-100PSI) to an atmospheric environment, it is not true for blower-based systems where this pressure change does not exist.
Airflow from a blower-based system will always follow the path of least resistance; this means air will continue to travel straight away from an outlet unless forced to change course. There may be some spreading effect from the moving air traveling through still air and some rise if the air is hot; however, for practical purposes this effect is negligible. When designing a blower-based system, one must make specific design choices to equalize flow and avoid uneven heating patterns.
Here are a few problematic examples we see again and again:
In order for the air to diffuse as needed, it must be influenced and directed. This is most commonly done by strategically: restricting the outlet, installing deflectors and baffles within the system, or combining both of these solutions.
Restricting the Outlet
Restricting the outlet can be accomplished by both reducing the actual cross-sectional area of the outlet and by adjusting the outlet shape.
Reducing the outlet area constricts the airflow and generates some back pressure in the system which helps equalize the airflow rate across the outlet. Strategically reducing the outlet area can also influence airflow. A symmetrical wide slot nozzle (rectangular outlet) will often have more airflow from the center of the nozzle, but by strategically crimping the center portion of the nozzle and locally reducing the area, we can encourage more airflow to the edges of the nozzle.
Changing the outlet shape can be complex, consider this practical example: Imagine that a heated airflow exits a 30 mm diameter heater outlet (707 mm² area) and then travels through a wide slot nozzle with dimensions of 10 mm x 50 mm (500 mm² area). In this case, the nozzle is narrower than then heater diameter and the outlet area of the nozzle is about 30% smaller than the heater outlet. There will be some diffusion of the air across the nozzle outlet, but it is likely that there will be lower airflow at the edges. However, if the nozzle outlet is changed to 5 mm x 100 mm (still 500 mm² area), there will be far more restriction and the airflow will be much more equal across the nozzle.
Internal Baffles and Deflectors
Internal baffles function in much the same way as restricting the outlet. Strategically placed obstructions provide a restriction that will pre-diffuse the airflow before it reaches the outlet. As a result, the heated air is more evenly spread and the outlet shape is of smaller concern.
Internal deflectors, such as fins, act to change the course of the airflow in a desired direction. They can be used to help encourage portions of the airflow to split in different directions encouraging diffusion.
Internal baffles and deflectors are particularly useful for encouraging even airflow through a nozzle with multiple outlets. Air heated shrink tunnels often make use of these internal components to ensure hot air is diffused evenly throughout the sides and top of the tunnel in order to produce a good quality final product.
This article only scratches the surface of a complex topic. If you have questions or would like help in designing your process heat system, please call STANMECH. We are always happy to help.