Get the most out of your batch heating process by achieving and maintaining the best possible temperature uniformity.

Figure 1. Heated air is supplied into the work chamber through fully adjustable ducts located along the length and on each side of the work chamber, near the bottom of the chamber. This helps ensure the heated air is distributed evenly throughout the length of the work chamber.

Temperature uniformity is the measure of the temperature tolerance of the heating process performed inside an oven. In the same way there are dimensional tolerances when parts are machined, there also are temperature tolerances for heating. The allowable tolerance is determined by the process being performed in the oven. For paint or powder cure ovens, a common tolerance is +/-10 oF (+/-5.6oC). This means throughout the oven heating chamber, the temperature must not vary more than 10oF from the temperature setpoint. Many metallurgical specifications require +/-5 oF (+/-2.8oC). For drying water from parts, +/-20oF (+/-11.1oC) typically is acceptable.

How is Uniformity Measured? In order to determine the temperature uniformity inside an oven, temperature measurements must be taken at several locations. This is referred to as a temperature survey. A common benchmark for batch ovens is the standard nine-point test. Test thermocouples are placed near each corner of the work chamber, with another at the very center. After the oven is heated to the desired setpoint and soaked for a period of time, the nine points are measured and documented, typically with a chart recorder or datalogger. If any of the points are out of tolerance, the oven must be shut down and adjustments made to correct the problem.

Here are seven ways to improve the temperature uniformity of your oven if it is found out of spec. One or more adjustments may be necessary to get the heating profile you require.

1. Balance Your Oven

The relationship of the pressure inside an oven to the outside (ambient) conditions is referred to as the pressure balance. To operate efficiently and achieve good uniformity, the pressure inside the oven must be neutral or slightly positive. The proper balance prevents cold air from being drawn in around gaskets, under the door or elsewhere and impairing the uniformity. To check an oven for pressure balance, use a pressure gauge calibrated in inches of water column (w.c.) mounted through the oven wall. A well-balanced oven will show a positive pressure of 0.0" w.c. to 0.2" w.c. in relationship to the ambient pressure in the factory.

A simpler way to check the pressure balance is to hang a small piece of thread at the bottom of the door in front of the door sweep. If the thread hangs limp or is blown slightly outward by air escaping from below the door, the oven balance is probably acceptable. If the thread is sucked in, the oven has a negative pressure, which is undesirable. Correct balance is achieved by adjusting the exhaust and air inlet dampers. If the oven pressure is negative, open the inlet damper or close the exhaust damper until balance is achieved.

Figure 2. Supply ducts located on the side walls and a return duct on the roof provide excellent uniformity. H-pattern-style adjustable duct louvers allow adjustment of the airflow.

2. Recirculate Plenty of Air

During the initial purchase of your oven, make sure it has sufficient recirculation airflow. The recirculated air is what delivers the heat from the source (burner or heating elements, for example) to the work chamber. An oven with insufficient air will not achieve proper uniformity. Generally, the greater the volume of air circulated, the tighter the uniformity will be. Reputable oven vendors will be able to calculate the expected tolerance. You also can improve the uniformity of an existing oven by increasing the recirculation rate. This is done by increasing the blower RPM or replacing the blower altogether.

3. Deliver the Air Where It's Needed

The most common airflow design for batch ovens is combination airflow (figure 1), where the air is delivered along the full length of both sides of the work chamber via supply ducts. This ensures the heat is distributed evenly along the length of the chamber. In addition, a return duct should be located in the roof to evenly remove the air after it passes over the product. Both the supply and return ducts should have adjustable louvers -- H-shaped bendable flaps, for example (figure 2) -- that are adjusted to achieve the best uniformity.

Another type of airflow is horizontal flow, where all the air is delivered to one side of the heating chamber, flows across the parts, and into the return duct located on the other side wall. This design is used when there are multiple levels of shelves that would prevent air from passing vertically through the load.

Figure 3. Large ovens will not uniformly heat small loads. The oven must be sized properly for the load being processed.

4. Use Sufficient Insulation

A portion of the heat generated inside an oven is constantly lost through the insulated walls. To ensure the temperature is uniform throughout the heating chamber, this loss must be minimized. Industrial ovens operating up to 500oF (260oC) should have at least 4" of insulation, typically mineral wool. For ovens operating at temperatures from 500 to 1,000oF (260 to 538oC), a minimum of 6" is required. When tighter uniformity is required, an additional 1" or 2" often is used. It is common for older ovens to have areas where the insulation has sagged or broken down, as indicated by hot areas on the oven exterior. These areas need to be repaired to maintain proper uniformity.

5. Properly Arrange the Load in the Oven

To maximize even heating, the parts must be arranged in a manner that allows the heated air to reach all areas. If racks or shelves are used, be sure they elevate the load off the floor and do not block the air. There also must be sufficient space between each layer for air to circulate and transfer the heat properly. This sounds obvious, but overloading is frequently a hidden cause of heating and uniformity problems. In addition, the cart or racks should be constructed from as lightweight materials as possible to minimize the energy required to heat them.

6. Select the Correct Oven Size

Avoid a one-size-fits-all solution to your heat processing project. To ensure good temperature uniformity and even heating, the oven heating chamber should only be slightly larger than the load being heated (figure 3). A large oven will not uniformly heat a small load because the air will pass around the load, not through it. The result will be poor uniformity between the outside of the load and the center.

7. Certify your Oven Regularly

As an oven ages, heating elements burn out, gaskets wear, louvers are damaged, etc. All of these contribute to a loss of temperature uniformity. In order to obtain good results as your equipment ages, it is important to periodically certify (measure and document) the uniformity. This can be as often as monthly for metallurgical aerospace applications, or yearly for general-use cure ovens. PH

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