Energy Upgrades at Offutt AFB
Constructing and installing a free cooling system at the home of U.S. Strategic Command was a multi-phase process complete with challenges, solutions and, ultimately, significant cost savings.
By Kyle J. Wilkinson, P.E., LEED AP, and Vishal G. Khanna, P.E., LEED AP
Systems upgrades recently were completed to increase energy efficiency at Offutt AFB, Neb., home of U.S. Strategic Command (STRATCOM).
Omaha, Neb.-based Advanced Engineering Systems (AES) has been utilized for multiple projects at the installation over a three-year period as part of a team that included the Omaha Public Power District and Bes-Tech, an electrical and mechanical engineering consulting firm specializing in industrial automation. Omaha Public Power District had the prime contract. Bes-Tech did the feasibility studies before and after construction. AES performed physical investigations, design work and construction administration.
The studies and work were done in four phases:
- An energy-saving study that included a survey of the steam traps in the STRATCOM building and adjacent buildings on the same system.
- Replacement of the traps that were recommended to be replaced along with other upgrades.
- A study on chilled water optimization in the STRATCOM building that analyzed the payback of adding free cooling with a water-to-water heat exchanger and the design to do so.
- The construction phase to incorporate a chilled water economizer.
REVIEW AND DOCUMENTATION
In the initial stage of the first phase, AES carried out a challenging, time-intensive examination of all of the existing steam traps. In the absence of documentation showing the locations of the traps, engineers worked with Offutt personnel to gather information that would indicate the general areas where traps might be found. Steam lines from the mechanical rooms also were followed above the ceilings and through all tunnels. Once the traps were located, some investigative work had to be done to determine what type of equipment was installed and associated systems served (since most traps did not have name plate data). An individual valve tag was given to each trap. Its location was recorded on a floor plan. Its physical data, operating condition, and the area it served were then entered into a spreadsheet.
The next step was to identify the traps that were failed open, failed closed, or leaking. The team utilized an Ultra Probe 100c (an ultrasonic inspection tool used to listen to the inner workings of the trap and the steam/condensate flowing through it), as well as a laser temperature gun and an infrared imaging camera. Engineers then estimated the amount of energy that was being lost and quantitatively determined how much energy could be saved by replacing the failed traps with new traps.
Ultimately, AES recommended replacing steam traps that were failed or leaking and called out specific models that would fit into existing space constraints. Additional recommendations included replacing the existing leaky valves, fittings, and pressure regulators that were found during the course of the investigation.
COORDINATION AND CONSTRUCTION
Phase 2 involved the replacement of the failed traps and leaky pipes/valves determined during the study. Construction had to be carefully coordinated to keep down time to minimal hours, and ample notification had to be given to users and occupants. Many of the traps/valves that had to be replaced were extremely difficult to access, so it was necessary to do extensive pre-planning and investigation in the field.
Phase 3 involved sizing and designing a free cooling system (water side economizer) to be integrated into the existing chilled water system. The existing chilled water system was comprised of three water cooled chillers (two were 1,800-T and the other 1,200-T), five cooling towers, condenser water pumps, and chilled water pumps. In order to calculate the capacity of new free cooling equipment, Bes-Tech recorded existing system temperatures and flows, then trended them over a complete heating season to estimate the maximum cooling load in the heating season. The data gathered showed that a 1,200-T free cooling heat exchanger would work. However, for future considerations, a 1,400-T unit was the final selection.
The study looked at the potential savings that could be realized by incorporating a free cooling heat exchanger in place of operating one of the chillers during the winter months. More specifically, the building would be switched to the economizer mode after October 15 whenever the forecast indicated outside air wet bulb temperature would be less than 45°F or less than 53°F for at least a week. After March 1st, the building would transition off the economizer and back to the chillers whenever outside air bulb temperatures would be greater than 48°F or 55°F for a week or more.
Based on 30-year average Omaha weather data, it was determined that incorporating a free cooling system could result in savings of approximately 5-million-T-hours annually—a yearly energy savings of about 3.3-million -kWh.
Once the decision was made to adopt the strategy, AES was brought in to design the new system and implement the changes. Several challenges were encountered along the way. Because it was dealing with the heart of STRATCOM’s cooling system, the firm was told to keep one chiller operational at all times. Since there could not be any shut downs, the system was designed by utilizing hot taps (to connect new piping to existing “live” piping), using existing valves to isolate a small part of the system, and distributing the load among the three chillers depending on the work being done and areas that needed to be isolated.
Part way through construction, engineers were informed that not only could there be no shut down, but redundancy needed to be maintained at all times as well. The obvious, but exorbitantly costly solution, was to bring in portable chillers on semi-trailers and hook up temporary power to keep them running. If one chiller went down, they could seamlessly switch to the temporary chiller. Instead, AES solved the dilemma by adding pipe in the chiller room to act as a secondary header so that one chilled water pump could pump into another chiller along with its normal chiller, thus providing the needed redundancy at minimal cost.
Added complications arose during design when it was discovered that there was no existing central water filtration or air removal system. The chilled water was so dirty and full of air that the system was experiencing air locks at the air handlers. To help clean the system as much as possible, a side stream air/dirt separator was added that will be used year round, not just during the winter months. This will not only save maintenance dollars but also create a more reliable system, reducing problems on downstream units and the risk of failures.
Tight space constraints presented yet another challenge. The building’s mechanical room was already packed with the chillers, existing pipes, and old electrical transformers among other items. Because shut downs were not an option, hot taps were utilized. However, hot tapping into large pipe sizes requires a fair amount of clearance. A solution was found by having the pipe tapped at a specific location at a specific angle between existing pipes. Since there was inadequate space for the physical placement of the additional equipment and pipes, AES requested and was granted permission to remove several items including old lockers and an abandoned electrical transformer to make room for a pump, filter, heat exchanger and piping.
The free cooling system at the STRATCOM building was installed during Phase 4 of the program over the spring and summer of 2013—with the system completed in the fall of 2013.
The system then was tested numerous times in spring 2014 and the facility staff was trained in an effort to prevent future issues and keep the chilled water flowing.