Wednesday, 18 March 2009

Keeping the Rainforest Tropical

When one method posed problems, mechanical engineers turned to an alternate technology to make patrons and animals comfortable at the zoo.

a rainforest may not leap to mind when one thinks of Cleveland, but the fact that one can associate the two words is largely due to mechanical engineering and the flexibility the discipline often contains.

Without state-of-the-art HVAC technology, it's doubtful that this unique tropical habitat—located within the Cleveland Metroparks Zoo complex—could exist, and it continues to benefit from equipment improvements, such as advanced motor and drive design.

The RainForest at the zoo near Lake Erie contains two acres of plants and wildlife similar to those found in rainforests around the world. Each year, more than a million visitors come to this two-story, domed, simulated biosphere to experience what it's like walking through tropical regions of Central America, Africa, or Asia—and see some 600 animals in a natural setting, including birds, monkeys, reptiles, and colorful fish that ply lagoons, swamps, and warm rivers.

Despite wide swings in temperature and humidity on the Great Lakes, visitors and inhabitants of the RainForest enjoy a nearly constant 76°F and 76 percent humidity. This is due to the robust HVAC system that has evolved over the years.

Direct Air Systems Inc., with locations in Cleveland and Columbus, Ohio, working in conjunction with Zesco Inc., specialists in electrical-mechanical motion control and based in Cleveland, provide HVAC service to the RainForest.

The RainForest has two air handler units that are 100 percent outside air. To provide and maintain optimum environmental conditions for the facility, Direct Air Systems installed SEMCO energy wheel systems for the units, one of which has a throughput of 60,000 cfm used primarily for cooling. The other unit, rated at 40,000 cfm, has a preheater and humidifier rack. Both units have side-by-side, 10-foot-diameter, 1,000-pound dry desiccant heat wheels, which are necessary to conserve 18,000 pounds of water every day, transferring moisture from the RainForest's stale exhaust air and giving it to the dry outside airstream once every 2.5 hours. 

bringing in fresh air 

The term "desiccant" refers to material bonded to the surface of the heat wheels. The desiccant collects moisture and odors, which are exhausted out of the building from the upper portion of the wheels. 

The wheels rotate anywhere from seven to 18 times a minute, depending on the humidity level. Fresh air, referred to as "process air," is drawn in by the same wheels. 

The fresh air's temperature and humidity are moderated by the wheels' slow revolution and the fact that the wheels' mass and desiccant surface transfers a portion of the heat and moisture collected from the interior. Heaters, when necessary, warm the air before it passes to the RainForest's spacious interior, which has more than 60 temperature zones, including those for offices, a cafeteria, and gift shops.

Rather than being roof-mounted and exposed to the elements, as is commonly done with air handling units, the ones serving the RainForest are built into the facility to maintain unit efficiency that would otherwise be lost in Cleveland's warm summers and cold winters.

The desiccant process was selected for both efficiency and simplicity. It was concluded that boilers, Z-ducts, heat pipes, or other methods did not compare to the 85-percent efficiency the heat wheels provide. Additionally, heat wheels are fairly simple to operate. The thinking was that the simpler the fundamental mechanical equipment, the greater the reliability and ease of maintenance. That proved to be the case—up to a point. While the technology should have worked flawlessly, a nagging problem developed.

Each wheel rotates with a custom-fabricated, 31-foot-long belt and, when first installed, was equipped with a one-horsepower ac electric motor rated for 1,750 rpm, and a mechanical gearbox to provide a 5:1 gear reduction.

At the time of installation, this was a fairly common equipment configuration. However, it was discovered that mechanical gearboxes used for the RainForest were failing at an alarming rate. 

picking up the pace 

The difficulty was finally identified. It dealt with the revolutions per minute. The pace was too slow for the gearboxes' splash lubricating systems to properly engage. As a result, parts were not being properly oiled and were wearing out prematurely.

Direct Air Systems thought about its experiences with other HVAC applications and mentioned to the Cleveland Metroparks Zoo maintenance team an ac drive/ac motor solution that didn't require gearboxes. This was becoming an increasingly common arrangement and had a good track record. It was also state-of-the-art technology, moving away from the problems and complexities that moving-parts mechanisms presented.

"When we saw we weren't getting too far with the gearbox-lubrication issue, we turned to equipment that was available to us—now," Steve Snyder, president of Direct Air Systems, explained. "The direct torque approach we recommended was something that would be cost-effective, easy to maintain, and simple in its operation. Gear lubrication would not be an issue. Plus, direct torque is proven engineering. The Cleveland Metroparks sought bids for the project, as it is a public agency, and our bid was selected."

The retrofit involved ABB's Direct Torque Control solution, which uses the ac motor's torque as the primary control element.

The original one-hp ac motor and gearbox equipment in each of the energy wheel systems was removed and replaced with an ABB 5-hp induction motor/ac low-voltage drive combination. This arrangement allows the motor to be connected directly to the load without the need for a gearbox or pulse encoder. The ABB solution allows full motor torque down to zero speed.

Through the use of an algorithm, the ABB drives, in this case variable speed ACS models, can run without an encoder to provide speed feedback. The algorithm enables the drive to calculate the state of the motor's torque and flux 40,000 times per second. Elimination of the encoder further reduces maintenance and decreases downtime.

Although each energy wheel system is controlled by individual Johnson Controls systems, the status of the motors and drives is monitored by the RainForest's comprehensive Johnson Controls building management system.

In the event of a control failure, the ABB ACS drives are designed to go automatically to a preset rpm rate, to ensure that heat transfer is maintained. Spare motors are on hand at the RainForest and drives are kept at Direct Air Systems' office location, minutes away from the facility.

Since the installation of the ABB motor/drive combination more than four years ago, there has been no interruption in service. Direct Air Systems is seeing increasing use of direct torque control.

"It is definitely one of the approaches we recommend," Snyder explained. "Often there is more than one way to solve a problem. The Direct Torque Control method proved to be a good solution. It has three characteristics that we like: It's cost-effective, simple, and reliable."

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