With the recent boom in construction and development on academic, institutional, medical, and research campuses, land for utility systems is becoming more difficult to find. Adjacency relationships among functional requirements on the campus are consolidating development into much smaller areas, leaving less space available for utility systems to be located close to the loads they serve.
To conserve useable land area, a growing trend on campuses is to combine increasing parking needs and chilled water production facilities in a common building. The fact that chiller plants and parking decks are both "utilitarian" facilities appears to make them a natural combination. While it is easy to locate these two facilities adjacent to each other on a common site, it is a bit more challenging to successfully combine their unique functional requirements into one structure.
Affiliated Engineers, Inc. (AEI) was chosen as a consultant in the recent design of three, large campus utility systems that followed this new trend. The first project involved a 2,000-ton chiller plant and a 236-space deck at Williams College (WC) in Williamstown, MA. The second project at the University of North Carolina at Greensboro (UNCG) in Greensboro, NC, included a 6,000-ton chiller plant and a 650-space deck. The third project at Johns Hopkins Hospital (JHH) in Baltimore, involved a 21,000-ton chiller plant and a 2,200-space deck.
Structural RequirementsThe most common structural system for a chiller plant consists of a steel-framed building, built on a slab-on-grade or a cast-in-place basement with a composite concrete first floor deck. The structural bays are relatively short, with intermediate framing and bracing to carry the heavy loads of the plant equipment and provide locations for pipe and conduit hanger connections.
In contrast, typical parking deck structures are based on "long span" construction. This type of construction is used to reduce the number of interior columns and optimize the design flexibility, functionality, and safety of the deck. Deck structural systems are typically cast-in-place, post-tensioned concrete or precast, pretensioned concrete. While this type of structure is well suited for a deck, it creates several challenges when trying to use it for a chiller plant.
First, the structural beam span is too long to support the larger concentrated loads induced by the plant equipment and piping. Second, the large deflections and movements normally allowed for in the design of deck structures are contrary to the fixed mechanical and electrical connections in a chiller plant. Therefore, it is easiest to separate the chiller plant structure, including any cooling tower platforms or yards, from the main deck structure using expansion joints to isolate the two distinct facilities.
Additionally, the ability to penetrate the post-tensioned or precast concrete systems with the piping required for the plant can require an extensive amount of coordination during design, and results in a system that is less "forgiving" in the future when trying to accommodate changes.
At WC, the plant and deck are located side by side in a common precast structure including a precast roof on the chiller plant space. The single-level plant and tower yard are located on grade starting at the lowest level of parking. In the plant, there is a structural restriction that conduits only 1 in. and smaller can be attached to the precast roof, therefore all major piping and equipment are supported from the slab-on-grade. This makes access and movement through the plant more difficult; however, with good planning the required maintenance access and clearances will be provided.
At UNCG, these issues were addressed by locating a single-level plant on grade below the lowest level of the deck. The towers are located in an enclosed yard on one end of the fourth level of the deck. The main deck structure is precast with cast-in-place columns.
A separate structural steel grid was installed and connected to the deck columns to support all the piping in the plant area so that no connections were made to the deck's precast members. This allowed the large movements of the deck structure to be isolated from the plant equipment and piping. The tower level was also constructed on a portion of the deck structure that was completely separated from the main deck structure by an expansion joint, allowing higher load capacity and a stiffer structure.
At JHH, the plant and deck was constructed with cast-in-place concrete. An expansion joint will separate the plant and the deck facilities. Because of their different needs, the structural grid changes from 30 ft on the deck side to 24 ft on the plant side. Additionally, the cooling towers will be supported by a structural steel frame that connects directly to the deck columns.
Floor-to-floor heightsThe ground level of a typical parking deck will have a floor-to-floor height of approximately 12 ft, 0 in. with a minimum clearance of 9 ft, 0 in. to accommodate accessible vehicle parking requirements, while other levels will have even lower clearances. This does not satisfy the higher height requirements of a plant, which usually ranges from 16 ft to 24 ft to accommodate the installation of the chillers and piping while providing reasonable equipment maintenance clearances. The low clearances in a deck will also restrict vehicle access to the plant.
By simply attaching the plant to the parking deck, it was not a major problem to accommodate the 17 ft clear height provided on the plant area at WC. However, because the primary maintenance access to the plant is through the deck, an exterior areaway has been provided to allow for delivery/removal of the major plant equipment.
A similar approach was used for the plant at JHH, which is a multilevel facility. The basement level, which houses the pump and pipe gallery, has an 18-ft clearance and the first floor chiller level has a 24-ft clear height. Again, the plant starts at the lowest level of the parking deck.
At UNCG, this issue was addressed by taking advantage of the ramping in the deck and the site topography to create a "high bay" clear space under the lowest level of the deck. Another feature of this design is that the plant space was created within the deck footprint with only a minor loss of deck area for usable parking spaces.
Equipment and service accessWhile all of the required service access to a parking deck can occur through the normal entrance and exit drives, a chiller plant has more specific needs. Provisions for major equipment installation/removal need to be made and are typically done via an overhead door or removable wall panel with grade-level access. In plants with multiple chillers, many times there are a series of doors or wall panels located in direct alignment with the chillers.
The cooling tower area also has specific access needs. Regardless of the type of tower used, there must be crane access to the tower area to allow for delivery and/or construction of the towers and service of major components. If the towers are on an upper level of the deck, a major constraint is that no vehicle taller than 7 ft, 0 in. will be able to access that level because of the low clearances in the deck. Therefore, positioning the towers in a location that is accessible by a crane both now and in the future is very important.
Primary service access to the relatively small equipment at the plant at WC is through interior double doors located on the lowest level of the parking deck and an exterior areaway to allow for delivery/removal of the major plant equipment.
At UNCG, the site topography allowed for grade-level access to the entire plant level from an existing service drive on the backside of the deck, and a series of overhead doors were provided at each chiller. The plant floor level was set at a pickup truck tailgate elevation to simplify the delivery of materials to the plant.
At JHH, the plant is configured with a main service aisle down the middle of the plant that aligns with a large overhead door located at one end. This door and aisle provide for all service and equipment access needs, including chiller tube pull areas, and is directly adjacent to a large loading dock in the parking deck.
Noise emissionsAt most institutions, controlling noise emissions from the chiller plant is critical, either because of the plant's proximity to the campus property line or the function of the surrounding facilities. While a deck can have intermittent periods with high levels of noise emissions, the primary concern is the more steady state noise that can be generated by the plant equipment, especially the cooling towers. The equipment inside the plant can be provided with attenuators, sound blankets, motor enclosures, etc., to quiet their operation. The fact that these pieces of equipment are enclosed in the building envelope will also normally contain most objectionable noises. Note that special attention should be given to the building envelope and any opening to make them as sound absorbing and limiting as possible.
Cooling towers have two major sound components: the fans and the falling water. Tower noise can also be generated by the gears and motors on the drive assembly, especially motors that are controlled by VFDs. Several approaches are available for quieting the operation of the towers, the simplest involving the siting of the towers. Elevating the towers and orientating the air inlets so they are directed away from the area to be protected is simple.
Another approach is to construct screen walls around the towers to act as a sound barrier. These walls can also be treated with acoustical absorbing materials to further reduce the noise generated by the towers. If the wall needs to be louvered for airflow, the use of special acoustical louvers installed in an inverted position will reduce noise and direct it upward.
Other approaches for reducing sound levels include the following: special low-speed fans; VFDs on fan motors that allow the fan to run at lower/quieter speeds whenever possible (VFDs also make changes in fan speed less detectable); enclosures to capture the whining sound produced by VFD motors; single-sided inlets instead of double-sided inlets to direct the water noise away from the protected areas; louvers at the tower inlets; basin mats that reduce basin splash noise; and sound attenuators at the tower inlets and/or outlets.
The towers at WC are packaged, counterflow, forced-draft towers that are installed in a large area well adjacent to the parking deck and plant. Because the deck is located directly adjacent to the campus property line that has a 40dBA sound level restriction, the towers have been equipped with inlet and outlet sound attenuators and VFD-controlled fans. The area well walls and tower casings will also be covered with special acoustical materials to absorb noise generated by the towers. Although this solution results in very low noise emissions, it is very energy intensive and will result in higher operating costs.
Both UNCG and JHH feature field-erected counter-flow towers with VFDs. Both projects located the towers on the top of the parking deck behind screen walls with louvers or screen material to allow for airflow on the sides, where noise emissions are not a concern and solid walls on the sides face adjacent property lines and buildings. The towers at UNCG are located within 50 ft of an adjacent high-rise dormitory on one end and within 150 ft of private residences on the other end. By aligning the four-cell cooling tower with the major axis of the dorm and perpendicular to the campus property line, there have been no reports of noise-related complaints.
Cooling tower design issuesAlong with noise issues, there are also design and operational issues related to the cooling towers in a combined chiller plant and parking deck facility. In addition to the significant physical area required by the towers, there are visual concerns from the tower appearance, to the fog (plume) that can form when the ambient air is cool, and to the water droplets (drift) of chemically treated water being exhausted from the towers.
To combat these issues there are again several techniques that can be utilized. Similar to the noise control approaches above, one of the simplest options is careful siting of the towers. By locating them on top of the deck or plant, the towers and their occasional plumes are out of "normal" sight lines.
The addition of screen walls can further hide the towers from view, but the design should be carefully reviewed with the cooling tower manufacturer so that they can make modifications to their standard tower selections for actual conditions to avoid recirculation and tower performance issues.
Locating the towers at a higher elevation with adequate "breathing space" allows the drift to disperse from the deck. This provides an opportunity for the water droplets to evaporate before precipitating on adjacent finished surfaces and parked cars.
A quick review of the prevailing winds at the project site should be used to reduce the tower drift concerns by locating the towers such that the normal wind patterns will blow the drift away from the parking deck. High-efficiency drift eliminators should be installed in the tower to minimize drift.
If tower plume is a major concern and the towers cannot be sited where plume is not objectionable, plume abatement systems can be added to the towers. These systems include installing heaters, heating coils, or precipitators in the tower's exhaust air outlet or spraying chemicals in the tower to reduce the moisture level in the airstream before it is exhausted and mixes with the cooler ambient air.
The big disadvantage of these systems is that they are expensive to install and operate for a problem that only occurs only under certain weather conditions. Through careful siting of the towers, plume was not considered to be a major concern on any of the projects completed by AEI.
Finally, one of the planning challenges with the towers is finding adequate space to locate them. Between their physical size and airflow clearances, they require a large footprint area. Generally, the footprint of the cooling towers and "breathing space" is larger than the area required for the chiller plant.
AestheticsBecause of both structures' physical mass, it can be challenging to disguise a chiller plant and parking deck while simultaneously providing for their functional needs. Many campuses have existing, and sometimes historical, standards for the architecture of new facilities. Combining the two facilities can introduce opportunities for some interesting architectural features and details that may not have existed if the facilities stood on their own, and this may actually allow the chiller plant to be "invisible" to most campus visitors.
Cost accounting and allocationSince separate campus entities are generally responsible for the parking and utility functions, it is important to develop a system that separately tracks and allocates the costs for each function. Some of the questions to be addressed are:
- What affect, if any (positive or negative), does combining the facilities have on the cost of the space (architecturally and structurally) for each facility? Who pays extra and who receives any savings?
- How are the landscaping and other site improvement costs around the facility divided?
- Who pays for parking stalls that may be lost due to the integration of the plant into the deck?
- How are the costs to extend site utilities to each building divided?
Although the design team generally does not get heavily involved in the actual allocation of costs, there are things it can do to help, such as:
- Provide detailed cost estimates that can be broken down and separated by the various user groups.
- Keep the user groups informed with project progress and details so that they can track their interests and budgets.
- Include bid packages or alternate bids that divide bid costs for portions of the project that may clearly belong to one specific group.
ConclusionWhile there are many challenges associated with integrating a chiller plant with a parking deck, there are equally as many opportunities for the design team to exercise their creativity and technical abilities. If this is done carefully and thoughtfully, chiller plants and parking decks can be a natural combination.ES
Special thanks to Johns Hopkins Hospital, the University of North Carolina at Greensboro, and Williams College for their permission to feature their projects in this article.