Effect of Load Arrangement

Current practice requires engineers to have more than a feel for load heating patterns (time-temperature profiles). They must acquire an ability to determine the effects of many operating and design variables on various loads time-temperature curves. The Shannon Method, which enables one to calculate specific time-temperature curves, is discussed briefly several places in this book and then detailed in chapter 8. The reader is encouraged to adapt the Shannon Method for processes other than the steel reheat and forging cases illustrated here. 

The square billets in figure 3.6 were laid on a hearth so that the width of each empty space between them equaled the width ofeach billet (spacing ratio, C/W = 2/1 = 2), 

Example 3,2 Heat a load of three steel rounds, 24 (0.61 m) diameter, for forging in a furnace 8.5 ft (2.6 m) wide x 6 ft (1.83 m) high inside. Loads are on piers with centerlines 3.2 ft (0.98 m) apart. High-velocity burners fire through alleys between the pieces-enhanced heating). The center piece is the most difficult to heat because outer pieces shield it from side radiation and convection thus, it will govern the heating time required. 

Dividing the circumference of the center load into four quarters, each of which should theoretically receive 25% of the heat to that piece. (See figure 3.9.) Small numerals are the authors estimate of the true % received by each quadrant, totaling 60% with enhanced heating. (If enhanced heating had not been applied, the bottom quadrant would probably have received almost none, totaling only about 46%.) From fig. 3.8, for 60% exposure on a cylindrical shape, read a time-lag factor, F, of 1.25 thus, the time-lag will be 0.1 (1.25) (24) (24) = 72 min. 

By the Shannon Method explained in Chapter 8, a temperature-versus-time heating curve was calculated for the center piece, and the total heating time was found to be 23.5 hr. If the center piece were removed to give the two outer pieces better heat transfer exposure, the heating time for the two remaining pieces would be 20 hr. 

---