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 OvenThe 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.
2. Recirculate Plenty of AirDuring 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 NeededThe 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.