Catching up to better boilers: does the design guideline of '5 sq ft/bhp' truly apply to boiler technology of today?

Over the past few decades, forced-draft boiler design has undergone many advances in technology. Improvements have been made in firetube design, going from the conventional two-pass type of arrangement to boilers such as the three-pass wetback designs.

Similarly, in the cast iron market, section design has been transformed both in extended surface configurations, as well as in materials engineering.

Likewise with steel bent watertube boiler design, changes have been made, taking the original design from a loose gas side flow pattern boiler to one which has a forced, five-pass gas side flow pattern. To varying extents, all of these technologically advanced products have seen years of field application to support research and tests performed during their developmental stages.

With these advances come improved heat transfer characteristics of the boiler, increased efficiencies, and reliable operation while not compromising equipment life or material integrity. And, while design engineers have, for the most part, kept pace with these advances in their engineering specifications, there still exists certain "minimum design standards" that have been in place since the technology of the 1950s. The most prevalent of these is the requirement that a boiler be designed with "a minimum of 5 sq ft of heat transfer surface area per boiler horsepower." When studying this requirement, the question begs to be asked: why?

The most common answer to this question is that a boiler must be designed with a certain minimum amount of surface area so as to protect the materials of its construction against damage due to "overworking" of the boiler.

What is really meant by overworking is that, if a boiler, or any heat exchanger, is designed with a very low surface area for a given amount of heat input, it will transfer as much heat as its design allows, but due to the low amount of surface, the material wall temperatures on the hot side will be elevated to a point that approaches the critical temperature of the material being used.

This, in turn, will result in material degradation and fatigue, causing premature failure of the material and subsequently failure of the boiler. Another consideration that plays into allowing a certain minimum amount of surface area relates to the fact that all heat exchangers, boilers included, will foul. This fouling creates an additional boundary layer to heat transfer, which in turn can elevate material wall temperatures.