hat "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.
This same discussion may be related not only to the bent watertube design, but also to the firetube and the cast iron designs. The required heat transfer surface area for any given boiler, or heat exchanger, is primarily a function of the design of that particular boiler.
Without doubt, a new boiler design must be researched, tested, and then operated in field application over a period of time to verify that performance in practice does in fact approximate closely design calculations made during development of the product.
Design calculations must also take into account all variables that may affect equipment integrity and material life. However, once this has been done, which is the case with today's bent watertube, firetube, and cast iron boiler technology, then minimum design standards which have been in place for half a century should be reviewed and amended, thus allowing for the use of improved boiler design and technological advancement. ES
EDITOR'S NOTE: The images associated with this article do not transfer to the Internet. To review the figures, please refer to the print version of this issue.