A Decision Is Made

Fortunately, Brian Miller, a design engineer with Lee Com-pany in Nashville, came up with a solution: using many smaller rooftop units, which wouldn’t be too heavy for the roof. The end result was the installation of 14 5-ton units and one 3-ton unit, a load the roof could handle.

“We ended up using Lennox ‘L Series’ rooftop units with concentric duct packages for distribution. We liked the units’ serviceability, their Network Control Panel (NCP), pricing, and factory-mounted option,” says Miller. Without the concentric ductwork, the facility might have needed extra steel support structures, which would have added quite a bit to the cost of the job, says Miller.

The NCP was also important to Donelson Foods, as the company desired a centralized control sequence. The NCP is a direct digital controller (ddc) system that allows the rooftop units to communicate to a central location. This panel can communicate with 31 L-Series units tied together with a communication bus.

“The nice thing about the NCP is that in the very near future it will be able to communicate with other Lennox units and competitive units by adding a module and sensors. This will create a lot of opportunities for us to help building owners with a cost-effective ddc solution,” Chris Saxon, commercial territory manager, Lennox Industries, Nashville.

The NCP was used to control 10 of the rooftop units on the Donelson Foods project. The owners liked the fact that if a problem develops with the unit, the network will actually notify the proper personnel and tell them what the error code is and what the problem is.

Each rooftop unit has an Integrated Modular Control (IMC), which communicates with the NCP. The IMC instantly diagnoses any problems, then relays that problem as a numeric code to the NCP. A complete diagnostic history, including the time and date the problem occurred, also appears on the central control screen. The pushbutton screen makes it easy for operators to use.

Another feature required on this particular job was a Heresite coating on the evaporator coils, which was applied at the factory by the manufacturer. “They needed this coating on their evaporator coils to inhibit the deterioration of the aluminum fins. It will definitely help maintain the integrity of the coils,” says Saxon, who notes that the coatings should last the lifetimes of the units.

Project Was Challenging

Based on the requirements listed earlier, it’s obvious that this design was a bit of a challenge. Miller says that’s especially true because no dehumidification equipment was used.

“It was difficult to try and control humidity without having humidity restraints in the space and still being able to bring in the outside air requirements to meet ASHRAE.”

To compensate, Miller says, they slowed down some of the fans, basically derating the units a little bit. “Fortunately, the coil sizes on the L Series have a lot more surface area than your typical units, so we were able to pull a little more latent load out of the air,” adds Miller.

Donelson Foods is now looking to add more extensive filtration to the facility. They will possibly retrofit the concentric diffuser packages with pleated-type filters on the return air side. Miller is glad about that, as he would have liked to have put additional filtration in the system before start-up.

But overall, the project went well. The customer is happy with the system, as it is dependable, service friendly, and provides them with an environment that helps them keep their production at the required levels. The remote test stations for the smoke detectors, and the hinged access doors on the rooftop units were also definite benefits.

“We like these types of projects,” says Miller. “They’re challenging, but they’re not so complex that you expose yourself to unnecessary risks. They’re nice because they force you to think more than a typical commercial building.”

Publication date: 06/25/2001