Aluminum FurnaceMaintenance Tips

Load Thermocouple & Test Load Instructions

Many years of experience have proven that the uniform heat and air distribution in aluminum furnaces result in uniform temperatures and uniform heating rates throughout the work space. Unsatisfactory results from test surveys are typically caused by many preventable factors. 

An unbalanced load will not give good surveys. Coils or ingots of non-uniform size, coils of non-uniform tightness, coils with telescoped sides, or coils with loose outside wraps, etc., cannot be considered good loads.  Load arrangements that do not allow uniform air flow will not give good results.  Incorrect placement of load thermocouples will give incorrect and misleading data. For coil annealing, the thermocouples must be placed approximately 1" (25 mm) from the corner of a coil. Thermocouples located under the first wrap of a coil or between loose wraps will not indicate true coil temperature.  Poor quality load thermocouples will give incorrect and misleading data. Thermocouples should be made from accurate thermocouple lead wire so that the errors due to thermocouple calibration will be minimum. For critical surveys, the thermocouples should actually be calibrated in a test oven so that the readings from those thermocouples can be corrected for the thermocouple error. Thermocouples should be properly insulated.

Thermocouple Lead Wire

Proper care must be taken when stringing the thermocouple lead wire. Present day furnaces have increased internal circulation as compared to furnaces built several years ago. This circulation causes considerable movement of the thermocouple lead wire. If the wires are not properly tied down, the excessive movement may wear through the insulation and short out or break the wires. A grounded wire will read the temperature of the surface it is contacting, rather than the work temperature. Grounded thermocouples may read above or below the actual work temperature. The need for properly tying down the thermocouple lead wires and insulating the wires from the surfaces they contact cannot be over-emphasized. Improper stringing of lead wire has been the major cause of unacceptable surveys.

Thermocouple Placement

After thermocouples are placed in coils, the coil must be peened so that the tip of the thermocouple is in contact with the work. Thermocouples placed in ingots should be placed in drilled holes with the holes packed with foil or peened in.

Load Sketch

Before running a survey, an accurate sketch of the load must be made. This sketch must include the dimensions of the spaces between the coils or ingots, and space from the end of the car to the load, etc. This sketch must also accurately locate and identify all of the load thermocouples. The sketch should show the exact location of each thermocouple in the individual coils or ingots. It is usually necessary to make side view as well as plan views of the load to show all of these thermocouple locations.

Coil inside diameter
Coil outside diameter
Coil width
Coil weight
Thickness of the sheet
Designation of the aluminum alloy
Vertical or horizontal axis of coil
If the coil has a mandrel, show inside diameter, outside diameter, thickness, length, and weight.

Running the Survey

During survey, all air and load thermocouple temperatures should be read and tabulated on a data sheet every half hour. This tabulation must include the starting of "light off" time. Notes should be made covering any unusual occurrences during the survey which may affect the results.

All instruments used to record the load temperature during must be carefully calibrated so that all instruments read the same. Instruments used to control furnace temperature must also be calibrated to assure correct readings. At the end of the survey it is essential that all load thermocouples be read at least once on the same instrument. If this is a multipoint instrument, it is also essential that the load thermocouples which are reading the highest and lowest temperatures be connected one at a time to one point on the multipoint recorder. Occasionally, a multipoint recorder will introduce inaccuracies which are not consistent on the various points.

When evaluating survey results, the temperature spread between the hottest and the coldest load thermocouples is the significant information. Usually all of the load temperatures will be below the furnace set point. The exact relationship of furnace set point to load temperature spread is affected by instrument calibration and other factors, and is not important when evaluating furnace performance.

When re-running a survey, a new load should be used. If the same load must be re-run, the coils or ingots should be repositioned in general. Specifically, the coils that were hot and cold should be interchanged to prove if the cause of the hot and cold coils is in the coils themselves. If a load with re-arranged coils is to be re-run, a new load and thermocouple location sketch must be prepared. The new sketch should have all of the coils numbered so that a comparison of the two surveys can be made and a specific coil can be located on either of the two surveys. If a given coil is hot or cold on both surveys, even though it is run in different locations, that particular coil has different heating characteristics than the other coils, and is causing the undesirable results.

Usually it is not possible to re-use thermocouple lead wire for a second survey. The re-use of thermocouple lead wire usually leads to failure of many of the thermocouples, and occasionally to incorrect readings.

If partial loads are being run for tests or for ordinary production, the load should be distributed equally in all of the zones. Uniform spacing should be maintained. Partial loads may heat slower than full loads because of lower air velocities through the work caused by the increased free flow area above partial loads. If the load is not spaced uniformly there may be considerable variation in the heating rate of various portions of the load.

The latest design, materials and equipment specifications should be obtained from the company before any reliance is placed on the enclosed since changes may occur due to product improvement.