The Durst Organization, owner of the building, began looking at a replacement chiller plant, due in part to the widespread addition of heat-producing office computer equipment. Building cooling requirements had increased and the existing chiller plant was fully loaded. Also, the Durst staff had followed the development of newer, more energy efficient chiller equipment, and saw an opportunity to reduce both total energy usage and peak facility electrical demand. This could be achieved through thermal storage, or through the use of alternates to the use of electricity as a primary energy source. Finally, it was requested that the building be equipped with redundant cooling capacity so that business would not be affected by a possible chiller outage.
After studying the needs of the building and the available options, the Durst Group decided to go with electric centrifugal chillers and ice storage. The chillers selected were two Trane two-stage CenTraVacâ„¢ centrifugal chillers, Model CVHF, nominally rated at 1,000 and 800 tons. These machines were selected for their high operating efficiency. The 800-ton machine was selected for ice storage and uses a 15 percent glycol solution to operate at ice making temperatures. The project also included a Calmac ice storage system and related equipment.
One of the major project design challenges was finding space for the optimum number of ice storage tanks. The solution was to install the tanks as close together as possible consistent with service needs, and to create an elevated "mezzanine level" grid above the mechanical room floor to allow the tanks to be double decked. While spaces were being evaluated, a storage room adjacent to the mechanical room was identified that could be partially dedicated to an additional eight ice tanks. Ultimately, the project involved 20 Calmac Model 1908 tanks and eight model 1190 tanks, for a total of 3,280 ton-hours of storage. The engineering firm recommended a series-flow design for the chiller water, with the 800-ton machine used for ice making, and the 1000-ton machine for standard chiller water operation.
The ice for the storage tanks is normally created from 6 p.m., when most of the tenants have left, to 1 a.m. In ice making, the 800-ton chiller uses a glycol return temperature of 27 degrees F. For daytime building cooling, ice is used to pre-chill water going to the standard chiller during the hours of 8 a.m. to 6 p.m., or even later if available. The system is designed for 44 degree F leaving chiller water and 59 degree F return. If necessary, the two chillers can also be operated simultaneously; however, this is normally avoided during daytime hours to minimize electric demand and energy usage.
Publication date: 10/27/2003