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Corporate center chooses direct-fired absorption

May 9, 2000
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The project's consulting firm studied the option of tying the chilled-water systems of the two buildings together, thereby allowing the adjacent building to benefit from the economies of the absorption system.
Advanced absorption chillers have replaced 30-year-old machines at a Pasadena, CA office complex, improving efficiency and reliability at all load levels. In the process, the chiller replacement also extends the life of an existing centrifugal chiller plant in an adjacent building.

The new chillers can efficiently deliver chilled water at temperatures down to 40°F and at part-load levels down to 20% of capacity.

“Economy, flexibility, reliability — that’s what we wanted,” said Armand Alvarado, chief engineer of the Corporate Center Pasadena, describing the goals of the recent a/c plant project involving two of the four buildings in the facility. The building owner, Aetna Life Insurance Co., is completely renovating of the complex that was built in stages between 1965 and 1980.

In reviewing the 251 South Lake building during the acquisition process, the owner had identified a need for replacement chillers. This building dates to 1970 and was cooled by the original chillers, steam-fired absorption machines. Because the building was in a general renovation program, the time seemed appropriate to replace the chillers.

Involved in the chiller evaluation was Jon Haviland, director of engineering for Marx/Okubo Associates Ltd., a consulting firm that represented the owners in the plans for renovation of the building, including the hvac systems. Haviland said a wide range of chiller plant options was considered, including engine-driven chillers, ice storage, and electric machines.

Going with absorption

Ultimately the decision was made to install direct-fired absorption chillers. Haviland said, “The owners thought that absorption made the most sense for this building and offered an additional opportunity.”

One of the adjacent buildings was built in 1980 and had been served by three 225-ton single-stage, electric centrifugal machines. These were still in relatively good condition, but their operating costs were higher than those the owner expected with the new absorption equipment. This was, in part, because of the electric demand charge created by operation of the chillers at peak hours. According to Haviland, the team began studying the concept of tying the chilled-water systems for the two buildings together, thereby allowing the adjacent building to benefit from the economies of the absorption system. Thus, the idea was advanced to size the 251 building chillers somewhat larger than the requirement for that building alone.

The 251 building has a wide range in chilled-water demand, from a peak of 700 tons on hot workday summer afternoons, down to less than 70 tons during spring and fall evenings, when smaller portions of the building are used. The final decision was to install two 440-ton Trane Horizon® direct-fired absorption chillers.

By installing 880 tons of absorption capacity in the 251 building, and coupling the chilled-water systems of both buildings, the owners could reduce total operating costs, improve reliability, and react to changing energy prices. Further, this interconnection would reduce the load and runtime on the centrifugal machines in the adjacent building, thereby extending their lives.

Another individual involved in the Corporate Center project was David Ellner from Engineered Automation Systems, Inc. His firm did much of the detailed mechanical-electrical-plumbing engineering on the project. Ellner notes that previously the normal summer load in the 251 building could be carried by two centrifugal machines. Under the new arrangement, this load can be carried by one machine.

Ellner noted, “This increases system redundancy and allows the one operating centrifugal chiller to operate in the best part of its range. Use of the absorption chillers also reduces the electric demand charge for the facility.” Energy expenses are expected to decline as well. Another aspect is that the absorption machines can deliver chilled water at temperatures down to 40° — significantly lower than earlier-generation absorption equipment and lower than the optimum temperatures from the centrifugal plant in the other building. This lower chilled-water temperature improves the effectiveness of the existing airside equipment in both buildings.

The design challenges to the project included integrating the two central plants and redesigning the system to use a shared building automation system. This common system not only simplifies control but also allows shared dispatch of the chillers for energy cost optimization.

Buying a pony

In addition to the new absorption chillers in the 251 building, a 110-ton electric screw compressor “pony” chiller was installed to meet cooling requirements during the minimum use hours. The unit selected for this application was a Trane Model RTWA. This unit was installed in the penthouse near the absorption units.

Office spaces in the 251 building are being renovated. Part of this involves updating the air distribution system. The original low-pressure system is being converted from a constant volume to a vav system.

The new system includes digitally controlled air volume diffusers and improved building comfort monitoring and control. The availability of 40° water from the new chillers makes this system more responsive.

Because the absorption chillers include combustion sources, system designers needed to be aware of the ultra-low nitrogen oxide (NOx) emission requirements of the South Coast Air Quality Management District (SCAQMD). The chillers were able to meet the required NOx emission limit of 20 ppm.

Southern California Gas Co. served as the company’s agent in securing the necessary operating permits from the SCAQMD. The company had also participated in the feasibility and played an active role in the entire project.

Brett Gaviglio, sales engineer from the Trane Los Angeles commercial sales office, said that the original building was designed for absorption chillers. Therefore no major structural changes were needed. The original chillers were lifted out of their penthouse location by crane and the new chillers installed at the same location.

The cooling towers were sized for absorption chillers, so only minimum modifications were necessary. The principal changes were to refit the tower fans with high-efficiency motors and variable-speed drives.

The climate in Pasadena is mild, so building heating requirements are now met with a group of packaged boilers. In the winter months, these boilers are primarily used for warming up the building in the morning.

Greater efficiency

According to Haviland, the overall objective of the project involving the two buildings was to give the building engineer the opportunity to run the buildings in an optimized condition, whether the chilled-water systems were coupled or running separately.

“We wanted a system that provided flexibility with changing conditions of load, weather, and economics. We were fortunate to be working with an owner that took into consideration lifecycle costs, not just first costs.”

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