Field of the Invention
The present invention generally relates to thermal processing ovens, more specifically to thermal processing ovens having pressurized air streams, and particularly to methods for controlling the pressurized air streams within the thermal processing oven.
Description of the Related Art
Thermal processing ovens (smokehouses) are the standard equipment within the Food Processing Industry used to dry and cook food products using controlled heat and humidity. Their principle uses are either for the treatment of pet food for consumption, which falls under FDA guidelines, or for the treatment of food for human consumption, which falls under the guidelines of the USDA's public health agency the Food Safety Inspection Service (FSIS). These agencies require documentation through all stages of the food production, and particularly through the thermal processing step. This documentation is called Hazard Analysis and Critical Control Points or HACCP.
Existing thermal processing ovens are borderline in their compliance to Federal regulations due to internal blockages in their treated airflow patterns. These blockages are caused by product support structures (shelves/racks) and by the product being processed itself. This is most evident when processing horizontally laid out products, such as jerky, where the blockages within the oven cavity create inconsistent product yields, inconsistent texture (including color and density), and issues with product safety and shelf life (moisture content/cooking variances).
The product to be processed is typically positioned centrally within the oven cabinet during processing and is delivered to the oven via a fixed rack system for stationary processing (Batch Style) or is conveyed through the oven via mobile racking or a conveyor system (Continuous or Intermittent Style). The product itself is generally either hung (vertical design) or laid flat on the racks or conveyer belt (horizontal design). The product arrangement primarily depends on the type of product being processed, wherein links and sticks are usually hung vertically suspended from supports, and wherein laid-out products, such as jerky, are arranged horizontally on screens.
Thermal processing ovens often have heating and humidity controlled by a programmable logic controller (PLC). In addition to temperature and humidity, the PLC may also control the operation of a main recirculation fan which can deliver a pre-determined volume of air, under pressure, to a system of ductwork. In prior art ovens, this airflow from the main recirculation fan is typically delivered internally to the sides of the oven, flows around the product and exits the oven from the top of the oven in a central location below the main recirculation fan. The air is then retreated and recirculated. The pressurized airflow discharging off the main recirculation fan and flowing down the sides of the oven may be controlled by one or two dampers which further restrict the airflow. These damper(s) may be opened in varied proportions and limit the volume of airflow from one side of the oven to the other.
Vertical Airflow Design (Approximately 90% of Existing Oven Designs)
In a vertical airflow thermal processing oven, dampers are used to control the volume of airflow to the insides of the oven. That airflow then is returned in an upward movement back to the main recirculation fan. The dampers of the prior art may be set to either rotate or modulate through a fixed timed cycle, thereby cyclically increasing and decreasing the air's volume and velocity entering the oven and moving the treated air from side to side. These dampers are often found in pairs and are set to rotate perpendicularly to one another, meaning the dampers are staggered at a 90-degree offset. This creates a slow side to side motion, at approximately 1 cycle per minute. The prior art damper system typically creates two airstreams entering the oven sides, both of which are fed by the main recirculation fan. If the paths of the two airflows meet they create a turbulent airflow. The contact point of the two airflows, called a breakpoint, varies in location as the dampers vary the airflow delivered to the oven. Since the dampers are rotating with a perpendicular offset the air entering the internal ducts around the damper blades has an unstable pressure within the oven's internal ductwork. This creates different pressures as the airflow enters the oven. In prior art ovens these dampers are typically linked via a chain which is driven by a single motor with gear reduction, or by individual motors with gear reduction. The motor(s) or linked chain drives move the dampers through a full 360 degrees of rotation. Oftentimes, the two dampers are placed into a staggered angular orientation before being chained together in the system to create the variance in the flow from side to side as the motor(s) turn the dampers. This causes the airflow combination location to move in a repeated and fixed course within the oven on a path which cannot be varied. This fixed path is the cause of overcooking in some areas of the oven and undercooking in other areas.
Since the dampers' cycles are fixed by the nature of the connected drives and/or chain driven dampers, the treated air's breakpoint is delivered at the bottom of the oven's floor of the oven approximately one-third of the total cycle time. Since the dampers are connected to a fixed movement, the breakpoint cannot be adjusted to any other location outside of the dampers' set cycle time. This yields a system wherein the product, more so a horizontally laid product, will block a resulting airflow occurring at the base of the oven as the rising air is immediately blocked by the racks and the bottommost product.
When the airstream breakpoints are concentrated at the bottom of the oven and are blocked by the bottommost product, the bottom third of the product in the oven becomes overcooked while the top third of the product could be under cooked within the normal time frame provided by the oven's controller. The oven's PLC cannot correct this situation, so the oven operator must manually change the oven's PLC program to allow the whole product load to remain in the oven until the top products are fully cooked. This further dries and overcooks the bottom product leading to even greater yield losses and uniformity issues. This in turns leads to package give away (having to pack a higher weight per package since the product weights are varied) and increased product rework (having to reprocess some of the product due to being out of spec) as well as throwing away some product which is not acceptable. Further, this problem also creates a longer cooking cycle which reduces product outputs (volumes).
Horizontal Airflow Design (Approximately 10% of Existing Oven Designs)
As in the case of the vertical airflow design, the horizontal airflow design also uses a PLC temperature controller, a main recirculation fan and air flow dampers. The airflow however is concentrated and delivered horizontally once it enters the oven through the use of fixed internal air plenums which are located on either side of the oven's left and right walls. These fixed sidewall plenums accept the fan's full air volume which is then turned sideways and discharged out of a series of openings toward the horizontally laid products. However, when the airflow is discharged from sidewall plenums, the treated air is discharged through the series of openings into the oven at a certain velocity. However once the air leaves the series of openings and enters the large cavity of the oven, the pressure abruptly drops, causing the resulting velocity of the airflow to drop. The airflow's outward velocity further decreases as it moves away from the original discharge point. The universal fan laws state; Velocity (FPM)=Volume (CFM)/Area (sq. ft.). As the Area increases and the volume remains the same, velocity decreases. This promptly creates a reduced and laminar airflow from the supply plenums. This situation exists even if the blower fan were reversed, pulling air into the plenums. The result is that the product located closest to the sidewalls will cook faster than the product located centrally to the oven. This results in undesired temperature variations, relative humidity differences and moisture content dissimilarity causing food safety concerns and product variations.
Other Failed Attempts to Solve the Air Flow Problem.
A few other prior art references are noted: U.S. Pat. Nos. 4,603,491 and 4,250,917 are both directed to reversible cross (horizontal) flow. Both of these design suffers the same issues as the horizontal oven mentioned above in that it uses laminar airflow flowing from a fixed position.
U.S. Pat. No. 6,943,321 is directed towards horizontal airflow having two adjacent blowers creating two zones within an oven. Since the pressurized air streams do not interact with each other they do not create a turbulent interaction between airflows.
U.S. Pat. No. 9,107,422 is directed towards changing airflow patterns within an oven utilizing a forward fan and a reversing fan. These fans are positioned in different areas of the oven to apply heated air over different areas of a continuous belt. These airstreams do not meet one another.
The invention described herein solves the above disadvantages by providing a method for manipulating an air stream within a thermal processing oven. In the instant invention two or more airflows converge creating a new turbulent airflow which envelopes the product rendering better uniformity. This is accomplished by using one or more dampers which are independently manipulated to control the airflow of two or more treated airstreams within the oven. Individualized damper control allows the converging airflows to be positioned anywhere along walls of the oven creating a turbulent airflow which exits their convergence point in a direction different from that of the uncombined flows. This resulting third flow can be caused to pause (dwell) anywhere along the walls of the oven and can be adjusted to direct airflow across the product in areas where overcooking or undercooking is prone to occur.
Further, this invention will position the airflows' breakpoints within a food processing oven in programmed orientation, including vertically, diagonally or horizontally and may be done so under computer control. This allows the airflow to be programmed to move up or down, sideways or across the interior of the oven over a programmed time frame and can dwell at any location, for any programmed length of time. With this ability to position the airflows' breakpoints, both vertical hung and horizontally laid products benefit by positioning the airflows under precise control.