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Variable-Speed Drives Critical in Preserving RainForest at Cleveland Metroparks Zoo

April 1, 2007
Part of the Cleveland Metroparks Zoo complex, The RainForest is a 2-acre, two-level exhibit showcasing more than 10,000 plants and 600 animals from the

Part of the Cleveland Metroparks Zoo complex, The RainForest is a 2-acre, two-level exhibit showcasing more than 10,000 plants and 600 animals from the tropics of Africa, Asia, and the Americas.

Despite wide swings in temperature and humidity on the Great Lakes, where Alberta Clipper storms swiftly can deliver below-freezing temperatures during winter and sun-drenched days topping 90°F during summer, visitors to and inhabitants of The RainForest enjoy nearly constant conditions of 76°F and 76-percent humidity. This can be attributed to a robust HVAC system that has evolved over the years to incorporate components that have improved the system's reliability by 100 percent.


The RainForest has two 100-percent-outside-air air-handling units (AHUs). One is rated at 60,000 cfm and used primarily for cooling, while the other is rated at 40,000 cfm and equipped with a pre-heater and humidifier rack. Both have side-by-side, 10-ft-diameter, 1,000-lb dry-desiccant heat wheels, which conserve 18,000 lb of water a day, transferring moisture from stale exhaust air to dry outside air once every 2.5 hr.

The wheels rotate seven to 18 times a minute, depending on the humidity level. Fresh air is drawn in near the bottom of the air handlers and filtered through the wheels.

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


Rather than being roof-mounted and exposed to the elements, as is common, the AHUs serving The RainForest are built into the facility to maintain efficiency that otherwise would be lost during Cleveland's warm summers and cold winters.


It was concluded that boilers, Z-ducts, heat pipes, and other methods did not compare to the 85-percent efficiency the heat wheels provide. Additionally, heat wheels are easy 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 — to a point, as a nagging problem developed.

Each wheel rotates with a custom-fabricated, 31-ft-long belt. When installed, the wheels were equipped with a 1-hp AC electric motor rated for 1,750 rpm and a mechanical gearbox providing a 5-to-1 gear reduction. While this was a fairly common equipment configuration at the time, gearboxes serving The RainForest were failing at an alarming rate. Once a year, one of the gearboxes had to be replaced. Complicating matters was that there was no discernable pattern pointing to a problem with a particular wheel-and-gearbox arrangement.

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


HVAC-service provider Direct Air Systems Inc. told the Cleveland Metroparks Zoo maintenance team about an AC-drive/AC-motor solution that does not require gearboxes: ABB's Direct Torque Control.

“The direct-torque approach we recommended was something that would be cost-effective, easy to maintain, and simple in its operation,” Steve Snyder, president of Direct Air Systems, explained. “Gear lubrication would not be an issue.”

The motor and gearbox equipment in each energy-wheel system was replaced with an ABB 5-hp induction-motor/AC-low-voltage-drive combination, which allows full motor torque down to zero speed.

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

The energy-wheel systems are controlled by individual Johnson Controls systems, while 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 ACS drives automatically go to a pre-set revolutions-per-minute rate to ensure heat transfer is maintained. Spare motors are inventoried at The RainForest, while drives are kept at Direct Air Systems' office, located minutes away.

Since the installation of the ABB motor/drives combination over four years ago, there has been no interruption in service.

“Often, there is more than one way to solve a problem,” Snyder said. “Based on the circumstances in this instance, the Direct Torque Control method proved to be a good solution. We have applied it on other projects as well. It has three characteristics we like. It's cost-effective, simple, and reliable.”

Information and photographs courtesy of ABB.
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