Handbook-Heat & Heat Treating

1 HEATING AND HEAT-TREATING Considered solely from the standpoint of cost per unit of energy supplied, the oxy-acetylene flame is an expensive source of heat. To use oxy-acetylene flames to boil water or heat a room would be grossly extravagant except in an extreme emergency. Virtually every economically-justified use of the oxy-acetylene flame is based on its extremely high temperature, and on the rapid rate of heat transfer which that temperature, and the concentration of the flame, makes possible. In the case of several processes which will be covered in this chapter, high temperature and high heat transfer rate are absolutely essential. In other applications of the oxy-acetylene flame a third advantage – the ability to do the job faster – also becomes significant. Here’s a simple example: You wish to put a sharp bend in a steel bar that’s about 25 mm (1 in.) thick. The high temperature of the oxy-acetylene flame is not required to get the bar hot enough to bend. You could even rig up a miniature blacksmith’s forge – if you had some firebrick, charcoal, and a bellows – and do the job. But the fastest way to do the job, and in most respects, the best way, is to put the biggest head you have on your welding torch and use the oxy-acetylene flame to heat the bar. The time saving will more than make up for any extra fuel expense. The larger the head, the faster you can do the job, and the less acetylene you will use. For a job like this, a head which burns 100 cubic feet of acetylene per hour is more economical than a head which burns 40 cubic feet of acetylene per hour. It pays to have such a head as part of a welding outfit, even though it may never be used for welding.* *You may recall that in Chapter 3 we said that acetylene should not be withdrawn from a cylinder at an hourly rate greater than one-seventh of full cylinder capacity. You may therefore ask: is it not wrong to operate a 100 cfh head from a single cylinder? The answer to this: Yes, you can withdraw acetylene at seemingly excessive rates for 5-10 minutes at a time (and the job we cited shouldn’t take longer) if you restrict such high withdrawal to rather short periods (not more than 10 minutes) and don’t repeat such demands more than once every 30 minutes. For continuous operation of a 100-cfh head, at least two large cylinders of acetylene should be manifolded.