Table 1 — Summary of Test Results Method InducȀon Resistance Flame Equipment Miller Pro Heat 35 PDS Bartech Belchfire torch Total propane used, lb N/A N/A 27 Total electricity used, kWh 21.6 24.6 N/A Total energy used, BTU 73,000 84,000 585,000 Avg. temperature drop in 1 h/outside diameter vs. 36/4 56/34 76/12 inside diameter temperature differenȀal, °F Time to preheat inside to 500°F, h 0.60 1.75 1.10 Set up Ȁme, h 0.58 1.50 0.25 Tear-down Ȁme, h 0.60 0.37 0.25 Total Ȁme, h 1.78 3.62 1.60 Total cost* $150.34 $287.57 $187.69 *Costs based on the following values: labor @ $65/h; electricity @ $0.064/kWh; propane @ $0.652/lb. direct flame, the part is not covered at all so there is a large part that will be at the preheated temperature that the operator has no protection from. Also, with direct flame, there is an open flame as well as hoses filled with combustible gas leading to a cylinder of gas or a manifold system. The torch can be knocked over or inadvertently pointed at something or someone that could be burned. Also, the propane torch heats the room creating a less desirable work environment. Induction is the safest method, having a heating element that does not get hot, heating the valve while it is wrapped in insulating blanket, and requiring no part handling. Furthermore, since the part is heated from the inside, induction heating results in less radiant heat exposure. Cost The cost of each method was analyzed based on cost of labor, electricity, propane, and personnel usage. An analysis using $65/h for labor, $0.064/kWh for electricity, and $0.652/lb of propane revealed that resistance preheating costs the most to preheat a valve, costing $287.57. That breaks down to $164.67 in labor to preheat the valve, $121.33 in labor to set up and tear down, and $1.57 in electricity. Direct flame preheating was the next most expensive, costing $187.68; $137.58 in labor to preheat the valve, $32.50 in labor to set up and tear down, and $17.60 in propane. Finally, induction preheating was the cheapest, costing $150.34. That breaks down to $72.04 in labor to preheat the valve, $76.92 in labor to set up and tear down, and $1.38 in electricity. If the shop is air conditioned, there will be extra electricity used to dissipate the 512,000 BTU put in the room by the propane, $4.29 of electricity if the unit is specifically sized for this amount of heat. Also, if preheat labor is not taken into account when the shop preheats offline, preheating one part while at the same time welding another, direct flame becomes the cheapest method, followed by induction. The induction method proved to be the most efficient. With induction making the best use of the operator’s time, using the least electricity, and having a very fast uniform heating pattern. Cost of the unit is another factor in calculating the cost to preheat each valve. The induction unit costs $39,000 while the resistance unit costs $15,000, and the flame torch costs $1,200; but with the time savings with induction, the cost is offset. Preheating with induction will save the user $37 per part over direct flame and $137 per part over resistance, once the equipment has been paid for. Conclusions and Recommendations Based on this study, the induction method was the best in most categories (Table 1). Induction heating required the least amount of time to preheat, was the most energy efficient, safest, and most cost-effective. It used less energy than the resistance and the electricity cost less than the propane used. Induction heated the valve the fastest and was quicker to set up than the resistance. The induction method also was the safest for the user, with the whole valve being insulated and heating coils that do not get hot. If offline heating is employed, propane is the cheapest, but with an open flame this adds heat to the room that adds extra cost, more safety concerns, and creates a less-desirable work environment. With preheat labor not included, induction was the second cheapest. Resistance was the slowest and most expensive in every scenario, due to setup time and the amount of time it took to preheat. Perhaps the most important result from this study is the fact that many variables need to be evaluated. While the cost of induction heating equipment is greater than that for either the resistance or direct flame method, the efficiencies offered will offset the added cost. Perhaps more importantly, the fact that the induction method creates a safer environment for the worker will help to optimize both productivity and quality.♦ References 1. www.flameengineering.com 2. Google® calculator 56 JANUARY 2014 Change of Address? Moving? Make sure delivery of your Welding Journal is not interrupted. Contact Maria Trujillo in the Membership Department with your new address information — (800) 443-9353, ext. 204; mtrujillo@aws.org.
Welding Journal | January 2014
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