The area of the fire tube is normally calculated based on a heat flux rate of 10,000 Btu/hr-ft2. The fire-tube length can be determined from:
where L = fire tube length, ft
q = total heat duty, Btu/hr
d = fire tube diameter, in.
A burner must be chosen from the standard sizes in Table 2-12, For example, if the heat duty is calculated to be 2.3 MMBtu/hr, then a standard 2,5 MMBtu/hr fire tube should be selected.
Any combination of fire tube lengths and diameters that satisfies Equation 5-7 and is larger in diameter than those shown in Table 2-12 will be satisfactory. Manufacturers normally have standard diameters and lengths for different size fire tube ratings.