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issue: September 2006 APPLIANCE Magazine

Product Design
Optimized Boiler Design

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by Jeff Jelinek and Ryan Hardesty, Product Engineering, Weil-McLain

Weil-McLain turned to computational fluid dynamics (CFD) during the design of a new boiler and ended up taking substantial cost and time out of the product development process.

HVAC OEM Weil-McLain used computational fluid dynamics (CFD) to simulate the performance of boiler design iterations, and the company’s engineers say it helped them zero in on an extremely efficient and economical boiler design. What’s more, the use of simulation saved U.S. $150,000 to $300,000 in prototyping expenses and made it possible to bring the new product to market 6 months to 12 months faster than if traditional build-and-test methods were used. The company says that, in the 3 months since introduction, market reception of the new product has exceeded expectations.

Weil-McLain (Michigan City, Indiana, U.S.), a manufacturer of residential and commercial boilers used to provide hot water and steam heat, recently made the decision to develop a new three-pass, oil-fired, horizontal-flue water boiler.
Water boilers with vertical flues have comprised the bulk of the U.S. market for many years. In this standard design, gases are combusted in a chamber, then go through multiple parallel vertical passageways arrayed with pins used to transfer heat to the water, and finally pass out the stack. Weil-McLain made the decision to develop a new multipass design in which the combustion gases go through three separate horizontal passages, arrayed with fins, in order to increase heat transfer efficiency and reduce the combustion gas pressure drop. Another advantage of the horizontal design is that it is easier to clean the horizontal fins, since they can be accessed simply by opening a front panel.
The company started the design of the new water boiler using traditional design methods, developed from the many years of experience of company’s engineers in designing boilers based on judgment and handbook formulas.
Because of the lead-time involved in building the mold for the boiler casting, it takes about 6 months to build and test a prototype. The prototype of the first design met all performance requirements but the prototyping process showed that it would be difficult to manufacture. The passageways between the fins in the design were very narrow, which meant that very tight manufacturing tolerances were required in order to avoid clashes. It proved to be impossible to hold these tolerances on a consistent basis with a manufacturing process that would meet the company’s cost targets.
Engineers then developed a new design that overcame the manufacturability issues by opening up many of the passageways to the point that interferences would not be a concern. Still, they were anxious to avoid prototyping another design only to discover that it was unworkable.
The company had already been looking at CFD technology as a possible tool to improve the traditional design process. A CFD simulation provides fluid velocity, temperature and the distribution of other quantities throughout the solution domain for systems with complex geometries. As part of the analysis, a designer may change the geometry of the system or the boundary conditions and view the effect on fluid flow patterns and heat transfer effectiveness. For these reasons, CFD makes it possible to visualize equipment problems far more comprehensively than physical experiments. It also allows the analyst to evaluate the performance of a wide range of different configurations in a shorter amount of time and at a lower cost.

CFD Design Simulation

Since this was Weil-McLain’s first experience with CFD, engineers decided to initially work with a consulting organization and selected Fluent Incorporated (Lebanon, New Hampshire, U.S.). The HVAC maker chose the company because of its success developing CFD software and its demonstration of the ability of its software to accurately simulate similar products.
Weil-McLain engineers and Fluent consultants worked closely on the project so the design process could move along at maximum speed and help Weil-McLain engineers quickly come up-to-speed on CFD technology. The decision was made to model a more manufacturable design with the simulation being driven by laboratory data of the initial prototype.
Weil-McLain engineers provided Fluent consultants with a solid model of the design, which had been developed using Pro/ENGINEER computer aided design software from Parametric Technology Corporation (Needham, Massachusetts, U.S.) The consultants imported the geometry into Fluent’s Gambit pre-processor and simplified it to prepare it for analysis.
While mechanical design is primarily concerned with the solid areas of the part, CFD is mostly concerned with the passages where fluids flow. By slightly simplifying the solid details, they could build a fluid flow model that would save on computational time.

Simulation Results

Simulation results showed that the performance of the second design had decreased considerably in relation to the first prototype. Analyzing the simulation results helped engineers determine that there was not enough surface area for the primary combustion gases to contact the fins and move heat to the circulating water. This showed up in the simulation as a relatively high flue outlet temperature.
Obtaining this information from the simulation saved the need to build and test another prototype at a cost of approximately $150,000, as well as the 6 months that would have otherwise been spent waiting for the prototype to be built.
The simulation of the second prototype highlighted the benefits of CFD and convinced Weil-McLain engineers to use it to guide a series of rapid design iterations intended to achieve the company’s design objectives, particularly high efficiency and low manufacturing cost. Engineers used the results of the simulations, particularly the temperature in each of the three passageways and at the flue outlet, to determine how individual areas of each heat exchanger performed. The information at this level of detail was impossible to obtain from physical testing because of the difficulties in measuring combustion gas flow patterns, temperature and pressure in internal passages and on the physical limits on the number of sensors that could be used.

Optimizing One Passage at a Time

The approach that engineers found to be most useful was to focus their attention on the first passage of the hot combustion gas from the burner through the heat exchanger. The relative efficiency of each passage could be easily determined by taking the difference between the inlet and the outlet temperatures. Engineers evaluated the reasons for the performance of a particular design by looking at the flow of combustion gas through these passageways. In particular, they set out to maximize the use of the surface area in each passage by loading each passage evenly. The loading for a particular passage could easily be determined by plotting combustion gas mass flow rate across a cross-section of a flue passage. The goal was to evenly distribute the mass flow rate throughout the flue passage.
In optimizing the performance of each passageway, engineers primarily tried changing the width, length and height of the fins and the cross-sectional area of the passages. The results of these changes provided considerable understanding of the sensitivity of the design to changes in these critical design parameters—for example, the effect of making the fins taller and thinner on the flow velocity and convective heat transfer.
Engineers also considered the question of whether or not to add baffles to the third passageway. Baffles are used in many vertical flue designs to increase combustion gas velocity. However, by individually optimizing the passages in the new design, they were able to eliminate the need for baffles, which reduced manufacturing costs, easing installation and reducing maintenance labor. The overall size of the casting is limited by manufacturing process considerations so engineers, as a final step in the design process, distributed the available volume and weight among the three passes to obtain further increases in efficiency while minimizing weight.
Using this iterative process guided by the simulation results, Weil-McLain engineers and Fluent consultants were able to increase the efficiency of the heat exchanger to a near-industry-leading 86+ percent (87 percent is considered the efficiency ceiling with this type of equipment due to the condensing point of corrosive flue gases). At the same time, manufacturing costs were kept low enough to allow the unit to be competitively priced with other high-efficiency water boilers.
Simulation provided far more information on the performance of the various design iterations than physical testing, yet each iteration took much less time and expense than building and testing a physical prototype. As a result, simulation made it possible to increase the performance of the design to a much higher level than would have been possible with traditional design methods. The cost and lead-time involved in developing the product were also substantially reduced.

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