Behind the scenes, a lot of mechanical engineering goes into creating and maintaining a tropical rainforest near Lake Erie, just 40 miles from the Canadian border. The RainForest at Cleveland Metroparks Zoo contains two acres of plants and wildlife similar to that found in rainforests around the world. Each year, over one million visitors come to this two-story, domed, simulated biosphere to experience what it is like to walk 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.
Without state-of-the-art HVAC technology, it is doubtful 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.
A Fragile Environment in a Rugged Climate
Despite wide swings in temperature and humidity on the Great Lakes, ranging from below-freezing temperatures in winter to sun-drenched summer days that top 90°F, RainForest visitors and inhabitants enjoy a nearly constant 76°F, 76% humidity. This is due to a robust HVAC system that has evolved over the years to incorporate components that have improved the system's reliability by 100%. Most recently, a retrofit involving ABB's Direct Torque Control solution, which uses the ac motor's torque as the primary control element, has kept the system working perfectly.
Direct Air Systems, Inc, with locations in Cleveland and Columbus, OH, working in conjunction with Cleveland-based Zesco Inc, specialists in electrical-mechanical motion control, provide HVAC service to the RainForest. Two air handler units are 100% outside air. To provide and maintain optimum environmental conditions, Direct Air Systems installed SEMCO energy wheel systems for the units, one of which has a throughout of 60,000 cfm used primarily for cooling; while the other unit, rated at 40,000 cfm, is equipped with a preheater and humidifier rack. Both units have side-by-side,10 ft diameter, 1,000 lb dry desiccant heat wheels, which are necessary to conserve 18,000 lb 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.
Bonded to the surface of the heat wheels, the desiccant collects moisture and odors, which are then exhausted out of the building via the upper portion of the wheels.
The wheels rotate anywhere from 7-18 times a minute, depending on the humidity level. Fresh air, referred to as "process air," is drawn in on the bottom portion of the air handlers and filters through the 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 over 60 temperature zones, including those for offices, cafeteria, and gift shops.
Rather than being roof-mounted and exposed to the elements, as is common with air handling units, those 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.
An Efficient, Simple System
The desiccant process was selected for both efficiency and simplicity. Extremely simple to operate, the heat wheels deliver 85% efficiency. But while the technology should have worked flawlessly, a nagging problem developed.
Each wheel rotates with a custom fabricated 31 ft long belt and, when first installed, was equipped with a 1 hp ac electric motor rated for 1,750 rpm, and a mechanical gearbox to provide a 5:1 gear reduction. However, mechanical gearboxes used for the RainForest were failing at an alarming rate. Once a year, one of the gearboxes had to be replaced.
Finally, it became clear that the system's revolutions per minute were 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.
With Drives, No Gearboxes Required
Direct Air Systems thought about their 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 retrofit involved ABB's Direct Torque Control solution, which uses the ac motor's torque as the primary control element.
The original 1 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 motor/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 heat transfer is maintained. Spare motors are inventoried 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/drives combination over 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 explains. "Often there is more than one way to solve a problem. 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."
