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issue: November 2008 APPLIANCE Magazine

HVAC - 45th Annual Report
Cutting Humidifier Design Time with CFD

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Design engineers at Aprilaire turn to computational fluid dynamics (CFD) to help walk the thin line between too much and too little humidity.

For humidifier OEM Aprilaire (Madison, WI, U.S.; www.aprilaire.com), humidity is a direct by-product of airflow. Unlike misting or atomizing humidifiers, Aprilaire humidifiers use a natural evaporation process to generate humidity.

The company sees the process as the equivalent of nature’s way of humidifying: combining “just the right temperature with the right amount of moisture.” It sounds simple until you consider that an Aprilaire Whole-Home Automatic Humidifier measures the outdoor temperature and indoor humidity more than 86,000 times a day, continually adjusting the indoor humidity to maintain optimum levels.

Introducing humidity to the air as water vapor prevents minerals from entering the air and potentially the lungs of those in the home. A distribution tray in the system provides an even flow of water across the Aprilaire Water Panel, a patented honeycombed pad that allows air to pass over and evaporate water to create humidity. Humidified air is distributed via the home heating and cooling system.

A New Design Process

In the past, Aprilaire relied primarily on instinct and experience in the early design stages to project how air would flow through the humidifier. Prospective designs were tested with prototypes made of plastic with the Water Panel inside. Engineers physically measured airflow through the device. If the measurements did not match the company’s specifications, design changes were made and another prototype constructed. Each design, prototype, and testing cycle took around three to five days and prototypes cost about $1000 each to produce.

In the last three years, Aprilaire design engineers have made their product development process much more efficient using CFdesign, the upfront computational fluid dynamics (CFD) software from Blue Ridge Numerics (Charlottesville, VA, U.S.; www.cfdesign.com).
A simplified design from SolidWorks is opened directly into CFdesign in its native model format. Volumes, void-filling, boundary conditions, and material properties are assigned automatically, and associativity of all geometry is maintained.

Automatic meshing in the CFD system simplifies and speeds the transition between CAD and CFD, according to Tom Anoszko, senior design engineer at Aprilaire.

“We have two design engineers and a designer using CFdesign, and none of us had previous experience with CFD,” says Anoszko. “Without experience, it is hard to tell where to keep the model simple and where to mesh with more detail. Automatic meshing helps us determine how much meshing is enough.”

Anoszko says Aprilaire doesn’t use CFD often enough to justify hiring a CFD specialist, so the software needs to be intuitive.

“Any of the three of us might go months without needing CFD,” says Anoszko. “Fortunately, it’s not difficult to get back up to speed with CFdesign after not using it for several months.”

Getting the Right Flow

In a typical simulation using the software, Aprilaire engineers set a 0.5-in. of pressure on one side of the Water Panel and zero pressure on the other. The simulation is launched with the press of a button. The resulting 3-D images clearly show the air velocity in different areas of the humidifier. The goal is to eliminate pockets of low and high velocity, maintaining a consistent flow throughout the system.

“It’s not a matter of the fastest airflow being the best,” says Anoszko. “Beyond a certain velocity, you end up blowing water off the evaporative pad. You want to hit the sweet spot where you are delivering air across the face of the Water Panel as efficiently as possible.”

After assessing these results, the humidifier model is revised in CAD and the simulation is run again. The cycle repeats until Aprilaire has the results it wants.

Doing More Faster

Each design and simulation iteration now takes about 8 hours. It could be faster, says Anoszko, if Aprilaire was able to dedicate more than one workstation to the task. He estimates that upfront CFD has reduced the typical design cycle for a new model of humidifier by a third, while quality has increased. “We are able to do many more iterations and use more creativity in our designs early in the design process,” he says. “Quality is improved because we have more time to do design iterations before committing. When we move forward with a design, we’re confident it is the right one. We have very few problems on the back end, where tooling becomes involved and it gets expensive to make changes.”

Thanks to Bob Cramblitt, principal of Cramblitt & Co. in Cary, NC, U.S., for providing this case study.


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