 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 that "Ua" will be greater than "Ub," simply by using shell side baffles. Given that the heat transfer rates are identical, the flow rates through each unit are the same and that the entering and leaving temperatures are equal, then the equation in Figure 2 may be derived.
This means that the heat exchanger that incorporates the use of shell side baffles may be designed to transfer the same amount of heat as the unit without baffles, but it does so with less heat transfer surface area. This does not mean, however, that the unit with less surface area is being overworked, it is simply an indication that this heat exchanger design has an improved mechanism for transferring the heat from one fluid to the other. In short, material integrity is maintained while the heat transfer characteristics are improved.
System Design ImplicationsThis example, while simple in application, may be used analogously to applications involving boilers of differing design. Again to cite the difference in design in steel bent watertube boilers available today vs. the original design from the 1950s, today's design provides for a true, forced, five-pass gas side flow pattern with gas side baffles for increased turbulence and contact, while the original design incorporates a loose gas side flow pattern which allows the products of combustion to exit the boiler through the path of least resistance.
And, while a minimum design standard, which limits the amount of surface area to no less than 5 sq ft/bhp, may be applicable to the 1950 design model, it is quite evident that the boiler with five passes on the gas side will have a higher overall heat transfer coefficient than its predecessor, and thus will not require the same amount of surface area to achieve the same rate of heat transfer.
And furthermore, the five-pass boiler design will be capable of transferring this heat without over-working the materials of which it is constructed. It has simply been designed with an improved heat transfer mechanism.
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