only taking 0.25 h for each. The induction method was next with 0.58 h to set up and 0.6 h to tear down. Resistance required the longest time with 1.5 h to set up and 0.37 h to tear down. Ease of setup and tear-down was also considered, and direct flame was the easiest. The direct flame method only required the valve to be rotated with a torch pointed at it, while the other methods required more complicated preparation. The only constraint with the propane method is if the part is too heavy for a turntable. The most difficult method was the resistance; with the reality that the operator must wire tie the pads to each other and in the desired position, as well as deal with hot pads once the part is preheated. Induction was significantly easier than resistance to set up, with the self-supporting coils and the additional advantage that the coil does not get hot. The quickest method of the three was propane, with induction a close second, and resistance a lagging third. Energy Efficiency Each method’s efficiency was analyzed based on energy (generated and consumed) as well as total energy used. For resistance and induction, the kilowatt hours (kWh) were recorded. For the flame test, the pounds of propane used were recorded for the preheat and preheat maintenance stages. In order to compare all three methods, the pounds of propane were converted to BTU1 and then to kWh2. The amounts of electricity used in the other tests were converted to BTU2 so that all three tests have kWh and BTU as values in relation to the temperature increase. Flame preheating was the least efficient, using 171 kWh and 585,000 BTU. Flame also had the quickest temperature drop once heat was removed, with a 12°F difference between the inside and the outside. The quick temperature drop was easily predicted because there was no insulation used for the propane test. The induction method was the most efficient, using 21.5 kWh and 73,000 BTU and had the smallest temperature drop once heat was removed, with only a 4°F difference. The resistance used 24.5 kWh and 84,000 BTU. The outside temperature dropped 34°F more than the inside. The difference can be linked to the requirement that the outside needed to be heated to 550°F in order for the inside to reach 500°F. Once the heat was removed, the outside and inside temperatures were still equalizing, and once the temperatures were the same they both started dropping. One of the most significant differences was the observation that the propane used 585,000 BTU compared to 73,000 BTU for induction. Therefore, 512,000 BTU (87.5%) of energy was wasted. Also, theoretically all 512,000 BTU went into heating the environment, meaning that in production situations, the wasted energy resulted in greater heat exposure to welders and other workers in the area. Induction proved to be the most efficient, using the least energy and having the slowest temperature drop — Fig. 4A–C. Safety Each method was analyzed to determine its level of safety based on the amount of handling and potential hazards. Safety was evaluated because it is one of the primary concerns in shop environments. Induction is the safest method out of the three. The part does not need to be on a turntable, which eliminated one part-handling operation. Also, the induction coils remain at room temperature at all times and with the part wrapped in an insulating blanket, the user has a very small chance of getting burned by the 500° F part. Resistance and propane are hazardous for multiple reasons, but propane is slightly more dangerous. With resistance and propane, the heating elements and torch are extremely hot during and immediately after preheating, and are only cooled by the air. With resistance, the pads are covered with an insulating blanket, but once the part is preheated it is difficult to move the hot pads. With WELDING JOURNAL 55 Fig. 5 — Cost per part while paying off the preheat equipment (1.5 years). Fig 6. — Cost per part after the preheat equipment is paid off.
Welding Journal | January 2014
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