How to Reduce Temperature Variation

Every process has a "window," which is the spot where the optimum temperature, dwell time and heating/cooling ramp rates create a perfect product. In some cases it is open and forgiving, in others it is extremely small. Controlling temperature so that this process window never drifts is the key to lean and profitable manufacturing.

There are many different configurations of ovens, furnaces and freezers. Conveyorized processes have the same basic needs: a product must travel a given distance over a specific length of time through zones that pre-heat, rise to process temperature, hold for a specified amount of time, and then cool. Zones, temperature peak, dwell time (soak) and cooling are specific to each industry and product.

Whether heating up or cooling down, using a profiler puts you in control of the process. When the profile of the oven, furnace or other process is set, it can be saved along with the machine recipe. You can only improve upon the final result when you have a picture of what all products in the load experienced during the temperature cycle, says Ray Pearce, application and technical support manager at ECD Inc., in Milwaukie, Ore. Only time/temperature profiling can accomplish this, providing a true window into your process.

Rather than trailing long sensor wires to record temperature data inside your oven, some units conveniently ride behind your parts, attached with thermocouples. The resulting data provides a baseline profile, establishing the thermodynamic relationship between the load and the current condition of the heating system. From this data, you can make improvements using oven modeling, prediction and a host of integral and easy-to-use analysis tools and displays.

In a large curing oven, for example, a cold spot is often the result of inadequate baffling at a point where the product path reaches towards an outside oven wall, such as a switchback of a serpentine overhead conveyorized oven. Solution? Measure the entire length of the oven, and verify conveyor speed by observing a marker as it traverses a measured length with a stopwatch. The information then is entered into the oven model. For instance, a 98-ft. oven is divided into four even zones with a 5.50 ft/min line speed. One surface and one air sensor are mounted top, midway and bottom on a hanging workpiece for a total of 6 channels, a common orientation. The profiler, protected in a thermal barrier, hangs from the line and goes through the oven. When the data is downloaded, the oven operator spots any temperature variance of concern.

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Lateral Balancing for Baking Ovens

In bakery ovens, side-to-side heat transfer differential is the most commonly encountered oven temperature variance. Placing thermocouples across the breadth of the conveyor into the dough, left to right in combination with air sensors, gives you the ability to measure then correct such unevenness with burner, ducting and baffling adjustments.

You can also manage seasonal differences in output from large belt ovens located against an exterior wall. Such adjustments can be made in real time in a maintenance situation, when you are able to bump and stop the line while adjusting a given area. Judging the results on-screen as those adjustments are made is a powerful time saver.

Employing time/temperature profiling techniques for lateral balancing results in instant improvement in bread, cookie and other baked good yields and reduces waste that eats up your unrealized profits.