Batch Furnace Heating Capacity Practice

Example 3.4 Data for a furnace such as shown in fig. 3.16. Inside dimensions = 18 X 12 X 10 high. Load = 12 000 pounds of steel weldments to be stress relieved at HOOF. 

Solution to Example 3.4 For parallel planes, third case on p. 97 of reference 51, find the emissivity factor, Fe, to use with an arrangement factor of Fa = 1.0 in formula 4/la on p. 81 and with a black body radiation rate from the table on page 82, as follows  

For 1600 F tube temperature and 1100 F load temperature, find that the black body radiation rate is 20 700 Btu/ft hr. 

Total radiation heat transfer rate for eight W-tubes = 13 393 x 224 ft = 3 000 000 Btu/hr, or for one W-tube = 375 000 Btu/hr. The reader can estimate that the flue gas exit temperature with an average tube outside surface of 1600 F will be 1800 F. From an available heat chart for natural gas, at 1800 F and 10% excess air, read 48% available heat. Therefore, each of the sixteen regenerative burners should have a gross input capacity of 375 000 / 0.48 = 781 000 gross Btu/hr. 

Heat transfer in batch-type furnaces is limited by the same variable factors as in all other furnaces (e.g., furnace temperature, refractory radiation, gas radiation, convection, scale on the load, hearth heat loss, and location of the control temperature measurement). See also the list of improvements that can help furnace productivity in sections 4.6.1,4.6.1.2, and 4.6.1.3. Tables B.3 and B.4 in reference 52 give heat requirements for drying. 

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A continuous furnace may be heated so that the temperature of its zones is practically the same across the furnace. This temperature uniformity can be obtained by lengthwise firing in several zones (as illustrated by fig. 4.2), or by roof firing or side firing in several zones (as shown in fig. 4.3). In such furnaces, the heating capacity of a continuous furnace will equal or exceed the capacity of a same-size batch furnace. 

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