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.
ASME PTC 4.1-1964 (reaffirmed 1991) is a performance test code for steam generating units, defining direct and indirect methods for calculating boiler efficiency. It has largely been superseded by the updated ASME PTC 4-2013 standard. You can access a copy of the document through The American Society of Mechanical Engineers - ASME Fired Steam Generators - PTC 4 - ASME
