A Rainforest Grows in Cleveland
Behind the scenes, a lot of control engineering goes into creating and maintaining a tropical rainforest near Lake Erie, just 40 miles from the Canadian border. It is a feat accomplished by Cleveland (Ohio) Metroparks, a recreational authority that provides an “emerald necklace” of woodlands, golf courses, hiking trails, and other attractions surrounding this Midwest city—know...
Behind the scenes, a lot of control engineering goes into creating and maintaining a tropical rainforest near Lake Erie, just 40 miles from the Canadian border. It is a feat accomplished by Cleveland (Ohio) Metroparks, a recreational authority that provides an “emerald necklace” of woodlands, golf courses, hiking trails, and other attractions surrounding this Midwest city—known as much for its sports teams as its world-class orchestra and Rock ’n Roll Hall of Fame. Without state-of-the-art HVAC technology, it is doubtful this unique tropical habitat, located within the Cleveland Metroparks Zoo complex, could exist...not for long, anyway!
The RainForest at Cleveland Metroparks Zoo contains two acres of plants and wildlife similar to those found in rainforests around the world. Each year, more than 1 million visitors come to this two-story, domed, simulated biosphere to experience what it is to walk through tropical regions of Central America, Africa, or Asia. Visitors get to see some 600 animals, including birds, monkeys, reptiles, and colorful fish that ply lagoons, swamps, and warm rivers, in a natural setting.
Fragile inside, rugged outside
Despite wide swings in temperature and humidity on the Great Lakes, where “Alberta Clipper” storms can swiftly deliver below-freezing temperatures in winter and sun-drenched days top 90 °F in summer, visitors and inhabitants of The RainForest enjoy a nearly constant 76 °F and 76% humidity. The robust HVAC system has evolved to incorporate components that have improved system reliability by 100%, protecting the $30 million Cleveland rainforest that also serves as a reminder of what is being lost without protection from overdevelopment.
Direct Air Systems, with locations in Cleveland and Columbus, OH, working in conjunction with Zesco, a Cleveland-based specialist in electrical-mechanical motion control, provide HVAC service to The RainForest. The RainForest has two air handler units that carry 100% 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 throughput of 60,000 cfm and is used primarily for cooling; while the other unit, rated at 40,000 cfm, is equipped with a pre-heater and humidifier rack. Both units have side-by-side 10-ft diameter, 1,000-lb dry-desiccant heat wheels. Conserving 18,000 lb of water every day, these units transfer moisture from The RainForest’s stale exhaust air to the dry incoming air stream once every 2.5 hours.
Desiccant material (a drying agent) bonded to the surface of the heat wheels collects moisture and odors from air passing over their lower portions. The air is then exhausted from the building via the wheels’ upper portions. The wheels rotate from seven to 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.
Built-in air handling units
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.
The desiccant process was selected for efficiency and simplicity. It was concluded that boilers, z-ducts, heat pipes or other methods did not compare to the 85% efficiency the heat wheels provide. Additionally, heat wheels operate simply, adding 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.
Originally, each wheel was driven by a custom-fabricated 31-ft-long belt and powered by a 1-hp 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 configuration. However, The RainForest’s mechanical gearboxes began failing at an alarming rate. Once a year, one of the gearboxes had to be replaced. There was no discernable pattern pointing to a particular wheel-and-gearbox arrangement.
The difficulty was finally identified: the rotational speed 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.
Path to a solution
Since gearboxes were typical when heat wheels were installed, the issue wasn’t initially seen as one of equipment selection, but viewed as an application problem. Each heat wheel installation is custom-made and has its own set of dynamics.
Possible solutions included continuing the practice of replacing gearboxes as they failed (expensive and unpredictable); using gearboxes that lubricate differently; and making customized gearboxes for The RainForest. These choices were deemed as too risky for 600 exotic animal inhabitants.
Direct Air Systems suggested that using ac-drives/ac-motors without gearboxes (direct torque) was becoming an increasingly common arrangement and had a good track record, without the problems and complexities that moving-parts mechanisms presented.
“When we saw we weren’t getting too far with the gear-box-lubrication issue, we turned to equipment that was available to us now,” Steve Snyder, president of Direct Air Systems, explains. Direct torque is proven and would be cost-effective, easy to maintain, and simple. Gear lubrication would not be an issue. “Cleveland Metroparks, as it is a public agency, solicited bids for the project and finally selected our bid.”
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 energy wheel system 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 need for a gearbox or pulse encoder. The ABB solution allows full motor torque down to zero speed.
With 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 reduced maintenance and decreased downtime.
Although each energy wheel system is controlled by 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 pre-set 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 from the facility. Since installation of the ABB motor/drive combinations 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. The method “has three characteristics we like. It’s cost-effective, simple, and reliable.”
Through contributions by visitors to The RainForest, and the efforts of other organizations and governments, limited amounts of scarce acreage of rainforests and jungles now are protected from overdevelopment. Continuing support will set aside even more of this valuable land that serves as a wildlife refuge for native plants and animals, and improves the earth’s environment by taking carbon dioxide out of the atmosphere. Additionally, these protected areas are proving to be important sources for tomorrow’s miracle medicines.
Featured on Animal Planet, The RainForest is one of several attractions at the Cleveland Metroparks Zoo, which covers 168 acres and features the largest collection of primate species in North America.
C.G. Masi is a senior editor at Control Engineering , firstname.lastname@example.org