No More Shake, Rattle and Roll

Herring knows from personal experience why he needed a low-load machine. "I live on the concourse level," he said. "Even though I was four floors above it, I could tell when the old machine went to low loads; it rattled and vibrated and the condenser tubes were shaken up so badly we were concerned about losing them. We were lucky we didn't have a catastrophic failure, because we were due anytime."

To achieve efficient low-load operation, the McQuay dual compressor centrifugal chiller uses the same evaporator and condenser heat transfer surface as a single compressor chiller; but instead of one, it is equipped with two compressors. The two compressors can be individually balanced for optimum operation at 60% of capacity with one compressor fully loaded, designed to provide true full-load efficiency at part-load operation.

Redundancy Carries the Day-and the Load

Although the building's 742 units are individually heated and cooled with fancoil units, Herring is responsible for cooling the commercial space: 55 floors of corridors, plus a concourse, lobby, shops, and party rooms. That's over 235,000 sq ft in all, and Herring couldn't afford to lose service due to a maintenance shutdown or failure. Installing a dual compressor chiller saved significantly in installation costs and gave him the backup he needed when a bearing needed to be unexpectedly replaced.

"This happened in July, and again I was surprised that one compressor could do what it did. I was able to keep the corridors and rooms comfortable and dry, and I had no complaints. I never would have tested this in July, but I was more than happy with the results."

Herring was able to maintain normal operating conditions because the chiller is equipped with two compressors, two lubrication systems, two control systems, and two starters, all connected to a common evaporator and condenser. Should a component fail, it can be removed or repaired while the second compressor continues to operate, providing 60% of the chiller design capacity available.

The chiller's positive pressure design offers greater sustainable performance over its lifecycle. Positive pressure eliminates the intrusion of noncondensable gases that can "rob" efficiency. These foreign gases compete with refrigerant for heat exchange surface and can reduce efficiency by as much as 14% at full load. Environmentally friendly HFC-134a refrigerant operates above atmospheric pressure in the entire refrigerant circuit.

Though the chiller performance has exceeded Herring's expectations, he is still most impressed with the day it was delivered and installed. "The mechanical room was designed for a bigger, clunkier chiller, but we had no trouble getting the new one in," said Herring. "In fact, the millwrights didn't have to back up once, even though they had to turn several corners and negotiate their way around the boilers."

From his 10 years working in the building, Herring knows firsthand the importance of keeping equipment up and running. "It's a high-maintenance building, but that's because of our extensive preventive maintenance program," he said. "We have 60 McQuay air handlers, most of which are still in good condition even though they are over 25 years old. Plus we do our own boiler waterside and all the plumbing and electrical maintenance in the units. With a crew of five, we're busy all the time."

And now with a new chiller installed, Herring expects that routine, scheduled maintenance will keep the chiller up and running to his expectations. The machine's redundancy, plus its quiet operation, allow him to sleep better at night.ES