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The more challenging task, however, is The RainForest at Cleveland Metroparks Zoo. 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.
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.
BACKGROUNDThe 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 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.
Not only does The RainForest, a $30 million investment, envelop visitors in exotic surroundings, but it also serves as a reminder of what is being lost - unless more rainforest and jungle acreage can be protected from development.
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 summer days topping 90°F, visitors and inhabitants of The RainForest enjoy a nearly constant 76° and 76 percent 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.
The term “desiccant” refers to material bonded to the surface of the heat wheels that collects moisture, as well as odors, which are then exhausted out of the building via the upper portion of the wheels. According to Direct Air Systems, 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 on the bottom portion of the air handlers and filters through the wheels.
According to the service engineering team at Direct Air Systems, 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 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.
EFFICIENCY AND SIMPLICITY DISTINGUISH THE SYSTEMThe desiccant process was selected for both efficiency and simplicity, according to Direct Air Systems. It was concluded that boilers, z-ducts, heat pipes, or other methods did not compare to the 85 percent efficiency the heat wheels are designed to provide. Additionally, heat wheels are simple to operate, it said. The thinking from both Direct Air Systems and Zesco was that the more simple 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.
According to Direct Air Systems, each wheel rotates with a custom-fabricated 31-foot 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.
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. Once a year, one of the gearboxes had to be replaced. There was no discernable pattern pointing to a particular wheel-and-gear box arrangement. It was random. The only constant element in the problem was the routine failure of a gearbox.
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.
PATH TO A SOLUTIONSince the use of a gearbox was the common approach when the heat wheels were installed, the issue wasn’t initially seen as one of equipment selection, but viewed as a problem of application. According to Direct Air Systems, each heat wheel installation is custom-made and has its own set of dynamics to contend with.
Solutions to the problem included continuing the practice of simply replacing gearboxes as they failed; however, it soon became obvious that this was expensive and somewhat unpredictable.
Also considered was using gear boxes that lubricate differently, as well as making customized gear boxes specific to The RainForest application. As more thought went into devising ways to handle the lubrication problem, it was concluded that these possibilities were bordering on experimentation. The RainForest, with its 600 animal inhabitants, was not a good candidate to take such risks, especially when a more up-to-date solution was available.
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 said. 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 gear-box-lubrication issue, we turned to equipment that was available to us - now,” explained Steve Snyder, president of Direct Air Systems. “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 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 is designed to allow the motor to be connected directly to the motor/load without the need for a gearbox or pulse encoder, said the Cleveland firm. The ABB solution allows full motor torque down to zero speed, it said.
Through the use of an algorithm, the ABB drives, in this case variable-speed ACS models, are designed to run without an encoder so as to provide speed feedback. The algorithm is designed to enable the drive to calculate the state of the motor’s torque and flux 40,000 times per second, said Snyder. Elimination of the encoder is designed to further reduce maintenance and decrease 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. This is designed to ensure heat transfer is maintained, said Snyder. 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,” said Snyder. “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.”
For more information, visit www.abb.us/drives.com.
Publication date: 03/26/2007