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?
Minimums To Be MetThe 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.
While these are valid considerations, it does not mean that certain boiler, or heat exchanger, designs are unable to safely transfer a given amount of heat with lower amounts of heat transfer surface area. The amount of surface area required for a given heat exchanger depends primarily on the actual design of the heat exchanger itself. In addition, heat exchanger design may be related directly and mathematically by the overall heat transfer coefficient (U) of the particular unit (Figure 1).
To use the equation in Figure 1, the value for "U" must first be determined. To calculate this value, many variables and factors are considered, but suffice it to say that this coefficient represents the so-called "heart" of the heat exchanger design.
A high value for "U" represents a heat exchanger with excellent heat transfer characteristics, such as high velocities, turbulence, good contact, low fouling factors, and extended residence time. Certain design strategies may be employed to increase the value of this coefficient, such as designing the heat exchanger with a multiple pass arrangement for the fluids, either on one or on both sides (hot and/or cold side), the incorporation of baffles or the use of extended surfaces.
Less As MoreBy using these heat exchanger design methods, a unit may be designed with less heat transfer surface area while still transferring the same amount of heat as a unit of a more basic design. Consider the simple example of two single-pass, straight tube, shell-and-tube heat exchangers with water as the fluid on both sides. Both units are identical in design (i.e., tube diameter, shell diameter, nozzle sizes and so forth), with the same materials of construction.
However, the first heat exchanger (HE-a) is designed with shell side baffles while the second unit (HE-b) has no baffles. In this scenario, it is known t