Design Factors that Affect the Breakpoint
Wall to Floor Design Angle
The design of how the side walls and floor meet will either enhance or degrade the velocity of the air streams, resulting in either a stronger or weaker breakpoint, respectively. Many oven manufacturers use a 90° or 45° angle between the walls and floor, which results in the air stream degrading once hit hits those angles.
A radius, or cove, design along the bottom of the walls gently pushes the airstream along the floor, maintaining the air stream velocity and resulting in a stronger
breakpoint to make it across the rack and evenly cook product.
Supply Duct Design
The design of the supply duct plays a large role in enhancing or degrading the breakpoint. The typical cone design found in most ovens allows the high and low velocity airstreams to bounce around the cabinet, resulting in overcooking product on the edges and bottom of the cart.
A knife-point design with a width of around 1” keeps the high and low velocity airstreams along the oven walls to maintain velocity and make the breakpoint stronger when the airstreams collide.
Return Duct Design
A study sponsored by the National Institute of Food and Agriculture, found when one supply slot was fully open and the other was closed, the Venturi effect actually pulled the air across the cabinet with only a slip-stream pulled up into the return duct.
This discovery led them to develop a re-designed return duct that extended the duct down into the cabinet to reduce the Venturi effect. The new extended return duct not only reduced the pull of the Venturi, it also unexpectedly produced the most uniform, highest velocity airflow of all the test runs. The anemometer and fog machine data showed that the extended return duct favored pulling air through the historically “cold-spot” that exists at the top-center of conventional designs — generating the most uniform airflow and higher velocity air through all areas of the cabinet, including the “cold-spot.”
Using the extended-return duct in combination with the variable-width supply slots created much more uniform air velocities through the “cold-zone” at the top-center of the cabinet than conventional designs. Also, when used in further combination with the new high-volume air kit, the air velocities were measurably more uniform and almost double those of conventional designs.
Oven Truck Design
Placing a truck in your oven affects the flow of the high and low velocity airstreams that create the breakpoint. Any obstruction in the oven cabinet will cause the air flow to change — meaning the design and placement of the trucks you use in your oven is critically important.
Oven Truck Guidelines:
Space Between Carts
Proper airflow in an oven requires a 12” gap on the side walls and along the bottom of trucks to maintain the high and low velocity airstreams in order to create the breakpoint. The gap between carts in a double wide over should be NO MORE than 12”, otherwise a chimney effect is created and the airstreams move up to the return duct before creating the breakpoint.
Proper Truck Loading
How you load the trucks in an oven affects the high and low velocity airstreams as well. The optimal loading pattern is to fully load the oven; however, when only partially loading an oven, you should follow the guidelines laid out below to maintain the proper airflow to create a strong breakpoint.
The design of the rack used in the smokehouse can greatly affect airflow in the oven, and hinder the formation of the breakpoint.
As mentioned above, the high velocity airflow travels down the wall of the oven and across the floor to meet the low velocity airflow. In other words, it must travel under the rack in the oven.
Any obstructions in the path of the high velocity airflow can slow down or stop the airflow.
A smokehouse rack designed with a low clearance (less than 12”) will cause an airflow obstruction in the oven. This low design essentially creates a “wall” on the floor of the oven, breaking the high velocity airflow and affecting where the breakpoint occurs.
In order to keep the high velocity airflow from being obstructed, the rack should have minimal obstructions up to 12 inches from the floor.