[AlloyEng]

One of the critical design factors affecting overall atmosphere furnace operating costs and life is the cross-sectional shape of internal components, such as a muffle. The design of this cross-sectional shape has a profound effect on the structural strength, life and cost of a furnace component.

As temperatures rise, all materials lose structural strength. For example, an RA-330 alloy will lose about 90 percent of its creep strength as temperatures rise from 1,000 F to 1,600 F. To maximize structural strength at any given temperature, furnace component designers have three choices: increase material thickness, use a higher strength material or increase cross-sectional area using fabrication techniques. The first two alternatives result in significantly higher material costs; changing the cross-sectional shape does not.

The area moment of inertia is a characteristic of a cross-sectional area that relates to the ability of that shape to resist bending. For instance, doubling the thickness of a flat plate increases relative moment of inertial by a factor of eight; a rounded, sine-wave-shaped cross section increases relative moment of inertia by a factor of 23; while relative moment of inertial of a corrugated shape with flat, sloping sides and a flat top increases to 79 (as compared with the original thin, flat plate).

Using these values of relative moment of inertia in the equation for determining bending strength yields a load factor, or ability to resist bending, for each configuration. With a load factor of one for the thin, flat plate, doubling its thickness increases its load factor to four. Although this is a significant gain, it means twice the material costs. The sine-wave-like cross-sectional shape has a load factor of six; whereas the flat corrugated shape has a load factor of 12.

Although this last shape requires a higher labor cost and slightly more material to develop the corrugations and still maintain the specified overall component length, this design significantly increases strength while only slightly increasing overall costs.

More information available at Alloy Engineering, www.alloyengineering.com