Efficiency (η) is computed as:

[ L_2 = \frac9 \cdot H \cdot (h_fg + C_p,steam \cdot (T_fg - T_air))HHV \times 100 ]

From stoichiometry: ( W_dg \approx 17.5 ) lb dry gas / lb fuel ( C_p = 0.24 ) Btu/lb°F ( L_1 = \frac17.5 \times 0.24 \times (350-80)21500 \times 100 \approx 5.3% )

The standard provides empirical curves for radiation loss based on boiler load percent. These curves are from 1964 data. If you apply them to a modern fluidized bed boiler or a HRSG, you will get nonsense. The code allows you to substitute manufacturer data for L6, but you must document the deviation.

A critical warning: Many free websites offering a download of "ASME PTC 4.1.pdf" are hosting scanned copies of the 1964 edition with missing fold-out charts (specifically Figure 1, the Heat Loss chart).