Fig. 7. Shown is the infrared sensor measurement and the load thermo- couple measurement vs. time.
volume of 375 cu. in. (Fig. 4). In each trial, the load also was instru-
mented with load thermocouples so the center temperature could be monitored. The effectiveness of on-heat detection by the OP-AID method was judged by: • the surface temperature of the load as measured by a load surface ther- mocouple;
• the center temperature of the load as measured by the center load thermo- couple;
• the time savings compared to the hour-per-inch control practices (time for furnace control thermocouple to reach the set-point temperature plus one hour per inch of thickness of load cross-section). The first trial involved heating the
49.25 x 7.88 x 3-in. rectangular billet, laid on its narrow side, to 1,600F. Fig. 5 displays the thermocouple and infrared sensor measurement for the rectangular geometry for the trial. Due to the thin width of the load, the center did not sig- nificantly lag behind the surface temperature. The infrared temperature signal consistently was slightly above the surface thermocouple measure- ment. The probable explanation for the discrepancy was that the surface thermocouple in the rectangular billet was embedded slightly below the surface. Also as expected, in the early stages of the heating process, the infrared measurement was artificially high due to the background radiation effects from the furnace environment, which reflects more energy than that generated by the load. However, as the surface of the load
MODERN CASTING / March 2010
Fig. 8. The infrared signal derivative for the 8-in. diameter cylinder oscillates between the limits after 4,943 seconds into the cycle.
approaches the furnace temperature, the infrared sensor becomes more and more accurate. This was characteristic of all the trials conducted. Despite the discrepancy between the
surface thermocouple and the infrared sensor measurement, the OP-AID method provided accurate results, as it relies on the relative measure of the rate of change of the infrared sensor measurement rather than on its absolute accuracy. The infrared signal derivative of the
rectangle surface temperature is shown in Fig. 6. The signal was maintained be- tween the upper and lower control limit from 3,960 seconds into the process onwards. This point in time represents when the OP-AID method would de- termine the load to be on-heat (under the assumption that the filtering delays
represented a 26.7% improvement over the current practice. Additionally, at the point in time at which the OP-AID method determined the load to be on heat, the surface thermocouple mea- sured 1,587F, which was 13F below the target temperature (1,600F). The center temperature of the load at this point in time was 1,580F which was 20F below the target temperature. The second trial involved an 8-in.
diameter cylinder placed in a 2,000F furnace. Fig. 7 displays the infrared sensor measurement and the load thermocouple measurement vs. time. In the later portion of the heating cycle, the infrared sensor measurement and the surface thermocouple measure- ment agreed more closely than for the rectangular geometry. Fig. 8 displays the results of
In addition to energy savings, tighter heat treatment process control and shorter heat treatment cycles could result in improved product quality and consistency, increased operational productivity and a reduction in greenhouse gas emissions.
are absorbed in the soak period). Using the conventional hour per inch rule, the load would have been determined to be on-heat after 5,400 seconds. In this case, the OP-AID method
provided a time savings over the hour- per-inch rule of 1,440 seconds. This
the infrared signal derivative for the 8-in. diameter cylinder. The signal oscillates between the limits from 4,943 seconds into the cycle onwards. With filtering delays ab- sorbed in the soak period, the OP- AID method would determine the load to be on-heat at this point in time. The hour-per-inch rule would dictate that the load would take 14,400 seconds to reach a uniform temperature before being declared on-heat. Thus the OP-AID method represented a time savings over the
hour-per-inch rule in this case of 9,457 seconds, a 65.7% improvement over the current practice. Additionally, at the point in time at which the OP-AID method determined the load to be on heat, the surface thermocouple measured 1,993F, which was 7F below the target tem-
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