For many years five (5) square feet per boiler horsepower was the standard in the design of the traditional multi-pass fire tube boiler. In these cases about 40 - 50 % of the burner's heat energy was transferred in the furnace (first pass) as radiant energy, and the balance was transferred in the convection passes which consisted of 2 or 3 additional passes using 2 - 3" tubes some of which may have had turbulators to increase the heat transfer coefficient. Today, with the call for smaller fire tube footprints, and the use of sophisticated computer algoythms, i.e., Computational Fluid Dynamics (CFD) with Finite Element Analysis (FEA), boiler designers are able to produce smaller fire tube boilers with the same integrity and life as the old 5 square foot boiler; using a larger furnace (60-70% radiant heat absorbed), proper matching of burner to furnace, and extended surfaces in the convective tube passes which has now been reduced from 2 or 3 additional passes to 1. This not only preserves or enhances fuel to steam efficiency in a smaller package, but reduces the blower motor brake horsepower as well. A good example of the design is Cleaver Brooks CBEX Elite or Premium models which can be found on their website.
Copied from Linkedin.: Steve Connor, Director Marketing Services at Cleaver-Brooks