Mechanical and HVAC design goes underfloor
This article focuses on measures the multi-discipline design-build team took to ensure the UFAD project at a federal facility would be a success.
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During the design and construction of a 1.5-million-sq-ft facility for the new U.S. Census Bureau Headquarters in Suitland, Md., for the General Services Administration (GSA), Southland Industries was presented with several MEP challenges. The project included 1-million sq ft of underfloor air distribution (UFAD), and Southland Industries, Dulles, Va., had become familiar with the mechanical system layout, controls strategies, construction, quality control, and integrated team design requirements to achieve a successful underfloor air project. This experience was refined during the recently completed mechanical design for the new 1,000,000-sq-ft headquarters complex for the Defense Information Systems Agency (DISA), including 700,000 sq ft of UFAD.
This office and computer/telecommunications lab complex allows the agency to relocate its employees from a number of older buildings scattered throughout Virginia to one consolidated facility in Fort Meade, Md. The project was awarded under the 2005 Base Relocation and Closure Act and will be completed by Sept. 15, 2011.
The design-build team's decision to use a raised access floor for the DISA project and Southland's decision to use an underfloor air mechanical system design, were based primarily on the tenant's desire for a flexible open office layout, the intensity of the information technology power, and the cabling infrastructure.
Recently, there has been publicity on the challenges and underperformance of this type of mechanical system. Having gained a total of 1.7-million-sq ft of underfloor air experience during the Census Bureau and DISA projects, the team will share some of the lessons learned. This article focuses on measures the multi-discipline design-build team took to ensure the project's successful outcome. Southland is complemented on the project by the design-build team of Hensel Phelps Construction Co ., Greeley, Colo.; RTKL , Baltimore; and MC Dean , Dulles, Va.
Raised access floor and UFAD decision
The project was procured for DISA by the U.S. Army Corps of Engineers using a performance-based RFP (request for proposal) in a design-build competition. Although the RFP did not emphasize a raised access floor (it requested only a 4-in.-high raised access floor as an add alternate for cable management purposes), the team made a critical decision early in the design process. The team decided to use an 18-in.-high raised access floor for the office building portions of this facility on the basis of the agency's stated need for total flexibility in making future changes to its telecommunications infrastructure. During the design-build competition, the team took the risk of straying from the most rigid interpretation of the RFP and was rewarded by being awarded the project.
The telecommunications and network cabling in most commercial facilities, whether with a raised access floor or in a ceiling plenum, is allowed to be routed between components by the shortest route. This results in a tangle of cabling and makes fast changes in the future much more challenging than when the cabling is routed and tagged in a more formal manner. For this facility, the RFP required that the cabling be routed in cable trays, clearly marked and segregated by security level (Figure 1).
The telecommunications engineers therefore played a pivotal role in coordination of the underfloor layout for the project. After the coordination effort is completed, however, the building will benefit throughout its life from having an uncluttered supply air plenum compared to free running cables.
Using the raised access floor as a pressurized plenum for the conditioned supply air to the space in lieu of an overhead ducted system results provides considerable cost savings. This is in spite of the fact that the UFAD system does use a certain amount of ducting under the floor. In this case, the general contractor was convinced that the overall savings were sufficient to defray the additional cost of the raised access floor itself. Once this was established during early budgeting in the pre-proposal phase, the general contractor instructed his team to go ahead with this design.
The RFP specified that the design had to meet Energy Policy Act 2005 , which involves saving 30% in overall building energy use compared to the baseline building specified in ASHRAE Standard 90.1-2004, and achieve a U.S. Green Building Council LEED Silver rating. These requirements had a major impact on the building design in general and particularly the HVAC design. The design team decided that it was important to have an air distribution system that contributed to these goals. While not the most important energy strategies to achieve the 30% savings, the UFAD system does contribute in two ways:
The UFAD system is a low-pressure air distribution system requiring less fan static pressure to distribute the supply air than conventional variable air volume (VAV) systems, resulting in less overall fan energy.
By using large built-up air handling units at rooftop level to serve the building, an airside economizer cycle could be incorporated with the use of a higher supply air temperature (average 65 F in lieu of conventional 55 F), which results in extended economizer use. In this climate an enthalpy economizer is necessary, but the hours are still extended.
The higher supply air temperature does not result in higher overall airflow because the stratification of room air temperature allows the cooling loads to be discounted to keep the total supply airflow approximately the same as that of a conventional overhead system. In fact, stratification is improved by ensuring that the system is not overdesigned, and therefore not over-aired.
Other items that contributed to the energy goal include low lighting watt density (0.7 W/ft overall); lighting controls for occupancy and daylighting; a highly efficient central-chilled water plant serving the entire facility; envelope improvements involving glazing performance; and wall-and-roof-U factors.
At the time this article was written, the energy model for the project is not complete; however, the results will be published at a later date when available.
Other benefits of the UFAD system are as follows:
An increased number of temperature control zones: Adjustable swirl diffusers per cubicle or private office.
UFAD systems are much quieter than overhead VAV systems, which can be an issue in open office environments due to crosstalk nuisance.
Improved ventilation effectiveness: Supply air is introduced at low level.
Improved IAQ: Only the first 4 ft of the occupied zone is completely mixed and room air is allowed to stratify above.
The UFAD system
The UFAD system chosen for this project (Figure 2) was selected and designed with certain specifications in mind. Low-pressure underfloor ducts distribute air from the main supply shafts to limit the travel of the supply air across the plenum to less than 50 ft to any outlet. Adjustable swirl diffusers in the carpeted-floor tiles located in each cubicle and private office provide supply air from the pressurized plenum.
The perimeter was treated as a “skin” system, meaning a narrow (approximately 1-ft wide) exterior zone within which only the exterior envelope heat gains and losses are handled. Using perimeter underfloor fan terminals (UFTs), the system draws air from the pressurized plenum. Efficient (70% to 80%) electronically commutated motors in the UFTs allowed variable speed operation of the fans. Insulated flexible supply air duct from the UFTs connects to linear bar type and vertical discharge floor diffusers under the windows. A central low-temperature hot-water heating system distributes to hot water coils in each UFT, providing heating. A moisture detector located on the base floor near each UFT allows the BAS to alarm and pinpoint the location if a leak occurs in the underfloor piping system.
Each exposure of the “skin” system is controlled separately to meet the perimeter envelope heating and cooling loads. The loads are met by using solar compensated outdoor air temperature to vary the speed of the UFT fans and modulate the hot water heating coil valves in sequence. By controlling the perimeter as a skin system, a large cooling-only interior zone is created for the rest of the air handling zone. The “skin” system arrangement is a major contributing factor toward a flexible UFAD design in an open-plan office environment.
Each office floor is divided into three air handling zones served by risers located in the core of the office buildings. Sheet-metal zone dividers below the floor allow the maintenance of a constant underfloor plenum pressure for each zone, controlling dampers in each distribution duct. The dampers are controlled in unison when supplying a common zone.
Conference rooms (and other major variable load spaces) are provided with a UFT (with hot-water heating coil), and fan speed and hot-water coil valves are controlled in sequence by space temperature and/or space CO2 concentration. Underfloor partitions isolate each conference room from the main underfloor plenum.
A number of swirl diffusers, appropriate to the increased airflow, supply the air to each conference room. Transfer air openings in the above ceiling partitions connect to the general return air plenum.
For each building, three roof-level custom air handling units (AHUs) supply air to the risers in the central core shafts at a constant temperature range of 62 to 68 F. The chilled water (CHW) cooling coils (fed from a central cooling plant with 42 to 60 F CHW) provide dehumidification by delivering a constant supply air temperature of 50 F in summer. The air is then blended with return air via coil bypass dampers to maintain the required supply air temperature to the UFAD system. Each AHU includes a heated and lighted corridor, accessed from the roof, so that the unit can be serviced away from weather conditions, and provides a return/relief air path from the shaft to the unit. Propeller-type relief fans and louvers are located in the service corridor, controlled by building space pressure.
The UFAD system is essentially a constant air volume system, but there are variations in airflow at the AHU in response to the variable air volume UFTs. The supply fan in each AHU has a variable speed drive controlled from downstream duct pressure. Air is returned via perforated return air openings in the ceiling, then back to the return air shaft conveying return air to the AHUs.
UFAD best practices
The key to a successful UFAD project is to follow a number of best practices that have been distilled from experience gained on this type of project. They involve many disciplines and can be placed into three broad categories: design and operation, underfloor plenum integrity, and commissioning.
Design and operation
Integrate design of underfloor systems by all disciplines on the design and build team.
Dehumidify all outdoor air entering the building to a fixed dew point temperature at the central AHUs.
Use the thermal mass of the underfloor slab and allow for its heat transfer in the design.
Automatically control underfloor pressure in response to space temperature changes.
Automatically control the supply air temperature for optimum comfort and energy use, and to eliminate any possibility of condensation.
Coordinate the layout of UFAD fan terminals under the floor with the interior design team for optimum accessibility for future maintenance.
Include moisture sensors adjacent to each UFT to give early warning of underfloor water leakage.
Seal the underfloor concrete slab to eliminate dusting and ensure cleanliness of the underfloor plenum.
Underfloor plenum integrity
Follow a rigorous quality control process, which comprises a program of plenum sealing at all key locations:
Perimeter seam along slab and exterior wall
Enclosed column seam at slab line
Drywall partition condition
Base of stairs on building slab
Top of stair interface with concrete deck
Elevator shaft below access floor line
Expansion joint in concrete deck.
HVAC and Plumbing
Opening in plenum walls for ducts
Pipe penetration through plenum walls.
Electrical and Data
Conduit through plenum walls
Open ends of conduit in the plenum space
Cable penetrations through plenum walls with cable sleeve
Cable penetrations through access floor with grommets and seals.
Perimeter seal at wall
In addition to other commissioning duties, the commissioning agent must follow UFAD-specific items such as the following:
Design: Review project documents to ensure all necessary underfloor details are incorporated.
Preconstruction: Participate in training of trades and supervisory personnel to ensure that proper procedures are followed.
During construction: Perform leakage testing for Category 1 leakage (beneficial leakage to the occupied space) and Category 2 leakage (non-beneficial leakage through the external structure or to return air paths). Ensure that leakage is within predetermined limits, set during design, for each individual UFAD pressure zone in all office buildings.
Construction: Perform regular quality inspections of the ongoing construction process and report any plenum integrity issues to the project management and design team.
The biggest lessons learned from the Census Bureau project, which the team is taking into account during the DISA project, are an integrated team design of the underfloor systems; implementation of the detailed reset control sequences for the underfloor plenum supply temperature and floor pressure; and educating everyone involved to become the “leak police”—not just the commissioning and quality control teams, but the mechanical, electrical, and drywall trades. Following all the processes and procedures outlined here will go a long way to ensuring a successful UFAD project
Peters is retiring from Southland Industries as senior vice president, engineering after 23 years with the company. Previously, he was with Carrier Corp. in Syracuse, N.Y., and with an architectura/engineering firm in Pittsburgh.
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