Understanding How Temperature, Humidity, and Airflow Affect Batch Oven Performance
Improving the performance of your batch oven is essential for improving product yields and consistency, as well as process efficiency.
When trying to improve your batch oven, there are three points of control to look at:
- Temperature (dry bulb)
- Humidity (wet bulb)
- Airflow/Breakpoint
Making these three points of control work together will help refine your process.
We’ll walk you through how in this article.
The Basic Principles of Thermal Processing and Heat Transfer
There are two main heat transfer mechanisms involved in the thermal processing of food products:
- Convection
- Conduction
Heat transfer by convection occurs due to the motion and mixing of air as it flows over and around the product.
In general, industrial food ovens use “forced” convection that is created by the oven fan(s).
Heat transfer by conduction occurs when the atoms and molecules that make up the solid substrate (meat, cheese, fruits, vegetables, etc.) pass the heat load from one to the other.
Energy (heat) naturally transfers from a higher state to a lower state.
In the case of industrial food processing ovens, conditioned (hot) air is blown and moved across the cooler surface of the product thereby transferring its heat load to the product.
Conceptually, we use forced convection to transfer the heat load to the surface of the product and then rely on conduction to move the heat through the product.
The more accurately and efficiently we can move the conditioned air across the surface should improve yields, product consistency from top to bottom and side-to-side, and cook times.
Temperature/Dry Bulb
The ambient air temperature of the oven is measured by a probe, usually referred to as the “dry bulb temperature”.
Humidity/Wet Bulb
Humidity, or relative humidity, is the measurement of the amount of water vapor held in the air.
As air temperature increases, the amount of water vapor the air can support also increases. This is due to the higher amount of energy (caused by excited molecules) in the hotter air preventing the water from condensing.
Relative humidity is actually a calculation of the temperature difference between the dry bulb and the wet bulb. Similar to the dry bulb, the wet bulb is also a probe but it is draped in a saturated cloth, often called the “wet sock”. The higher the wet bulb temperature is relative to the dry bulb, the higher the relative humidity.
Humidity is one of the most important and often misunderstood tools available to oven operators.
Humidity can affect color, cooking times, drying times, yields, and product texture, just to name a few.
Humidity and Cooking Times
Cooking times can often be shortened through the proper use of humidity, as water is much more efficient at transferring heat energy to the product than air alone.
Also, the proper amount of humidity will eliminate evaporative cooling, which slows your heat transfer and therefore extends your cooking time. Evaporative cooling is where water is evaporating from the surface of the product and taking heat with it. By raising humidity we can keep more heat in the product.
Tech tip for those processors producing dried or dehydrated products: Humidity is your friend!
Oftentimes, the fastest way to remove water from the product is to:
- Use added humidity to increase your thermal exchange on the front parts of your cycle
- Then drop the humidity on the later end of the cycle
Just make sure you don’t go too low or you will over dry the surface of the product creating a condition called “case hardening”. The super-dried product surface will dramatically increase drying times as the water on the inside of the product will be mostly blocked from reaching the surface of the product.
Air Flow and Breakpoints
Most modern commercial batch ovens rely on some type of damper system to control the volume and velocity of airflow from one side of the oven to the other.
Different manufacturers have different configurations. Some use a single “wig-wag” type damper while most others use two dampers, one on each side of the oven, left and right that run the length of the oven.
For this discussion, we will focus on using a two-damper oven, but the single-damper ovens create the exact same conditions just in a different manner.
In a two-damper oven design, each damper is 90 degrees to one another and is constantly rotating. As they rotate, one is slowly restricting airflow on one side of the oven while the other is simultaneously opening and increasing airflow on the opposite side.
This creates an air stream on one side that is gradually becoming faster and stronger while the other is becoming equally slower and less powerful.
The strong, high-velocity airstream goes down the oven wall, under the product, and up the other side where it will eventually collide into the lower-velocity airstream.
This collision of high and low velocity causes a redirection of the airflow horizontally across your product. This is called a breakpoint.
As the dampers continue their rotation, the lower-velocity stream becomes stronger while the higher-velocity steam becomes weaker. This causes breakpoints to move up and down the product from top to bottom and bottom to top. Remember that because the dampers are constantly rotating, the high and low velocity air streams are constantly switching sides of the oven creating breakpoints up and down both sides of the oven and across the bottom.
For further clarification, if both dampers are half open, then the breakpoint should be at the bottom of the oven at the 6 o’clock position.
We explain more of how this process works with GIFs in our Oven Breakpoint Guide.
Without the benefit of the opposing airstreams constantly moving the breakpoint, the oven would dramatically overcook product in parts of the oven while dramatically undercooking others, meaning there would be no consistency in color, texture, or yields.
Adjusting Breakpoints
Ideally, we would “map” our ovens to locate the hot and cold zones and adjust where our breakpoints are to focus more time on the cold zones and less on the hot zones. This is part of “balancing” your oven. We will spend more time in the future discussing how to map and balance an oven.
Unfortunately, many oven manufacturers make adjusting airflows/breakpoints an arduous task that often takes much time, skill, tools, and many trips on and off your ovens to make adjustments.
Some manufacturers, though, make it relatively simple to balance your oven by giving you the ability to adjust your airflow through the recipe. This is especially important for operators who run multiple types of products, both hanging and screened, as they can optimize the airflow/breakpoint pattern for each specific product.
Our ovens make that process easy using our Total Flow Control system.
Improve Your Product Yield, Consistency, and Process Efficiency
Hopefully, by better understanding each of the three control points of a commercial batch oven, you should now have the basic tools needed to start improving your process efficiency, quality, and consistency.
Not quite sure how well your oven is functioning?
Our team can conduct an Oven Evaluation and give you recommendations on how to improve product yields and consistency.
Contact us today to book a time.
