District Chilled Water Systems Design: A Tale Of Two Footprints
A large Midwestern university recently undertook growth plans for two major areas of development for its campus: a medical center and central academic areas. Providing utilities to these new developments on campus required infrastructure improvements, particularly for chilled water service.
To address this need and comply with the institution’s policies on energy efficiency, the university implemented a centralized chilled water plant approach. As the schedule of the two developments was very tight, two chilled water plants were constructed in a short time period. The design development and implementation of these two facilities involved operations and maintenance standards with a widely divergent facility-specific set of design constraints and application requirements, resulting in two distinctly different plants.
South Chiller Plant (SCP)
In 2008, as the university’s medical center was about to undertake a large construction project, the question of how to best provide utilities for the expansion came to the forefront. In addition to the new medical center expansion, the university had aging building chillers on its campus and was faced with the challenge of finding a new location for a chiller plant to support the existing heart hospital. The heart hospital had previously been provided with chilled water from a smaller chiller plant located in an adjacent building that was slated for demolition to make room for the new medical center expansion. The campus’ existing central chiller plant is remote from the medical center and could not support the required additional capacity. Therefore, a district plant approach was the only practical option.
The next step was to find a location that would support the required 30,000-ton capacity projection for this growing area of the campus. Several options were considered and eventually a corner parking lot, adjacent to a lab/classroom building and directly across from one of the main campus substations and a major highway, was selected as the site.
The result was the South Chiller Plant: a 96,000-sq-ft building that currently houses eight electric 2,500-ton chillers and ancillary systems, with space to add four more chillers in the future for a full build-out of 30,000 tons. Its electrical supply is provided from the campus medium-voltage distribution system directly off the three main busses at the university substation.
East Chiller Plant (ECP)
In 2010, the university decided to add additional residence halls, laboratories, research, and classroom buildings, and to modernize the HVAC systems in several existing buildings in the northeast portion of its campus. This expansion, along with the growing need for added cooling capacity to replace systems in existing buildings that had reached the end of their useful lives, was the driving force behind this second district cooling plant. The new plant was sited along the highly visible eastern boundary of campus, adjacent to a parking garage. While also constrained by site limitations, the new plant was planned for 15,000 tons of cooling capacity at full build-out, half the size of the SCP.
The East Chiller Plant is a 23,000-sq-ft building and currently houses five electric 2,500-ton chillers and support systems.
The SCP and ECP were constructed for the same university, during approximately the same time period, therefore it would be expected that the plants would be almost identical in design. However, the differences in physical location, loads served, and proximity to existing central chilled water facilities had very significant impacts on each facility’s design.
Both plants have the following common approaches and characteristics:
- A 2,500-ton chiller building block size. This increment was selected as an optimal size available from multiple manufacturers, providing an incremental capacity well suited to the university’s requirements for redundancy.
- Arc-resistant electrical switchgear to improve safety of operations personnel.
- Automated control systems at both plants designed for unmanned operation with monitoring from a remote campus plant.
- Bridge cranes on the operating floors to facilitate removal of the chiller compressors and other components.
- N+1 redundancy for all major mechanical and electrical systems.
- Variable primary pumping scheme.
- Steam basin heating for winter operations.
- BIM with 3-D modeling. This provided the university with the ability to virtually “walk through” the plant prior to construction and gave their operators an opportunity to comment on operation and maintenance issues, such as valve location and equipment removal.
Beyond those similarities, there are also quite a few noteworthy differences.
Plant Footprints. The available footprint for each building was the primary influence on specific layout for the mechanical and electrical systems at each location. The SCP was constructed in a vertically stacked arrangement due to the very small footprint. The full build out of 12 chillers is situated on two floors, each with a mezzanine area, for chilled water pumps, filters, and other ancillary systems. There is a third floor for condenser water pumps, and the cooling towers are on the roof immediately above the third floor. A basement serves as a distribution piping entry, an electric feeder entry, and a chilled water distribution header location.
The resulting height of the vertically oriented plant (cooling tower fan decks are 140 ft above grade) did offer some opportunities for space efficiency. Because the chillers are located between the condenser water system above and the chilled water distribution below, the large diameter headers (42-in chilled water and 30-in looped condenser water) and their branch connections to the chiller never have to occupy the same space. This minimized crossover allowed the floor-to-floor height to be reduced. In a similar fashion, the electrical spaces are vertically stacked on the north end of the plant, with conduits and cable trays branching horizontally at each floor to feed motor loads, again reducing crossovers (Figure 1).
The area required for cooling towers was another constraint on the overall footprint of the structure. In order to get the required heat exchange surface area and volume of air, it was necessary to utilize a customized, field erected counterflow tower, with all of the air drawn from one side of each cell. These cells were approximately twice the height of a conventional factory-packaged cooling tower cell. This added complexity to the project, as these towers needed to be built on-site and required additional planning and a site staging area. Due to the overall height of the building and the available materials for construction of the cooling towers, fire detection was provided in each cooling tower cell, rather than installing an expensive and difficult to maintain sprinkler system and fire pump. The SCP utilized a 190-ft by 80-ft footprint.
In contrast, the ECP site was not as constraining and was designed with a single-operating floor at grade, a partial mezzanine, and a full basement. The larger roof area (relative to cooling system capacity) also meant that factory-packaged cooling towers could be utilized for this plant. This saved considerable time and avoided the need for lay-down space. The towers were delivered and lifted onto the roof over the course of a few days, as opposed to the several months of assembly, which occurred at the SCP. The lower elevation of the roof also eliminated the need for fire detection or sprinklers in the cells per local building code. The ECP utilized a 120-ft by 80-ft footprint.
Plant Location. The SCP is adjacent to a river, which generated concerns about possible flooding. The entire basement was constructed to deal with a possible flood, and all critical equipment was installed above the basement level to reduce the likelihood of any interruption to chilled water production due to severe flooding and water damage. As a result, the basement is primarily used for the chilled water headered piping loop, chilled water filtration and strainers, and entries for domestic water services and other utilities. All of these functions are either non-critical or they could continue operation with full submergence during a catastrophic condition.
As mentioned earlier, the SCP location is adjacent to one of the campus’ main electrical substations. With a 30MVA plant demand load at full build out, this offered a huge advantage. Due to the plant’s proximity to the substation, it was practical to bring dedicated 13.8kV (campus distribution voltage) feeders directly into the plant that were powered directly off the substation main buses and then through multiple substation transformers to deliver the utilization voltages of 4.16kV and 575V. These feeders permitted the use of full voltage motor starters on the chillers, reducing the operating complexity of the plant.
With no concern for flooding on the ECP site, the chilled water and condenser water pumps were located in the basement. Having the chilled water and condenser water headers on the same building level increased the clear height needed in the basement to accommodate the large diameter pipe crossings; however, this arrangement also yielded aesthetic benefits. The amount of piping on the operating floor, which is fully glazed in a busy pedestrian streetscape, was greatly reduced.
Since the ECP was constructed at the end of the existing campus medium-voltage distribution, providing acceptable power was a challenge that required some innovation and analysis. The closest substation was 0.75 miles away and the 15MVA load exceeded the capacity of the distribution feeders in the area. Improvements were made to the campus distribution system, including a third backup feeder in the area, which increased the available feeder capacity. With these improvements, it was possible to supply the new plant and additional buildings in this area of the campus. Since the ECP was intended to co-exist with other buildings on a distribution feeder, motor starting analysis was required to evaluate the impact of starting the chillers on a loaded distribution circuit. Reduced voltage motor starters were installed to avoid voltage dips caused by chiller starts.
Redundancy & Critical Operations. The SCP was constructed to supply the Level 1 trauma medical center on the campus. In order to ensure that chilled water production is maintained during the most severe conditions and during planned additions and maintenance activities, several backups were designed into the plant. The SCP has looped chilled water headers, to allow sections to be isolated for repair while the remainder of the plant continues to operate.
The design also called for stand-by backup electrical generators for a portion of the plant load. In order to take advantage of the university’s existing fuel storage site, built to supply for campus boilers and additional emergency generation, a remote generator building was constructed approximately 0.75 miles from the chiller plant site. To reduce capital costs, reduce transformation costs, and allow the maximum amount of flexibility for the 15kV distribution in the plant, two 2,800 kW, 15kV generators were selected. Since the stand-by power enters the facility at the main distribution voltage, the system can be configured such that the generators can provide backup power to any of the chillers, pumps, and cooling towers at the SCP, providing the university with a high level of reliability.
Since the ECP was constructed to supply less critical loads, the same level of redundancy and flexibility in backup considerations was not included. This distribution piping system is also cross-connected with another district cooling system, which can meet the demand during cooler months. The ECP has a common header design that, while valved to accommodate equipment maintenance, is not looped, so work on the header itself would interrupt chilled water production. No backup power generation was included to maintain chilled water operations in the event of a power outage, since most buildings served by the ECP do not have emergency powered HVAC systems.
Both plants now supply chilled water to the campus. The SCP started supplying chilled water to the existing medical center buildings in August 2012. The project was fully completed in spring of 2013 and the loads for the plant have increased with the completion of a new major hospital in the summer of 2014. During peak load conditions the plant currently operates at approximately 0.74kW per ton inclusive of all support equipment. This is illustrated in the graph which trends plant efficiency (kW/ton) vs chilled water production (tons) for both plants from August 2013 through September 2014. The original design for the plant was to have a minimum cooling load of 3,000 tons which would correspond to a plant efficiency of 0.76 kW/ton. (Insert Figure 4)
The ECP started supplying a base load in May 2014. The anchor load for this plant is a new research laboratory and classroom building, which is still under construction. The plant is trending toward the same all-in operating efficiency of 0.74kW per ton at peak conditions and more load is anticipated to be added in the coming months.
While these plants were constructed around the same time and for the same university, it can be seen that they were very different designs. Varying site conditions, intended operation, and location had a significant impact on each design, which resulted in two very different projects. When considering district chilled water operations and design it is important to consider all aspects of the utility systems and the loads to be served.