ASHRAE Journal - July 2009 - 32

UFAD systems supply air closer to the occupants, so velocities must be lower and temperatures higher to maintain comfort. Increasing supply velocities Architect Engineer to meet high cooling loads may break down the temperature stratification that provides the energy Loads Envelope, Interior Design HVAC Design performance benefits described previously.1 Another option to increase cooling capacity with UFAD would be increasing the number of supply diffusers (i.e., increasing the airflow by increasing supply area rather than velocity). However, coverage of floor surface Architect Engineer by furnishings and occupants allows fewer diffuser Energy Analysis/Envelope placement locations than with overhead systems. Building Concept/First Draft Assessment Keeping cooling loads in perimeter zones within the Concept Revised. “Good” Envelope, Final Occupancy range suitable for UFAD application depends on the Energy Saving Density, Low Equipment Loads Strategies architects’ envelope design, particularly the window and LPD (Coordinate With 11 Interior loads (lights, Tenant and Interior Design) area exposed to direct gain. Reduced Loads Opportunity for UFAD people, and equipment) depend on the occupant-tofloor-area ratio and equipment use, as well as the Opportunity to Install Other Raised Floor lighting design (e.g., task versus ambient). Services in the Underfloor Size Underfloor Oversizing air supply systems is especially probEquipment lematic with constant air volume (CAV) UFAD be(Ventilation and Services) cause it may jeopardize the stratification that is key Include Raised Floor in the Building Design to achieving the potential performance benefits.9 A 6 revealed that Location of the Diffusers post-occupancy study by Fisk, et al., Airflow Calculation, (Coordinate With Tenant Number of Diffusers “low internal thermal loads and moderate supply airand Interior Design) flow rates” contributed to lower than expected thermal Air Distribution Design stratification and ventilation effectiveness. Therefore, (Ductwork)/Plenum Pressure particular attention should be paid to “normal” loads, as well as peak design loads of CAV.12 (Oversized Fan Size Reduction variable air volume [VAV] systems would reduce airflow rates to meet the actual cooling loads, which would increase stratification.) Often, these matters Figure 1a (top): Information flow. Linear design approach. Figure 1b (bottom): must be resolved in a compressed time frame, so the Information flow for integrated design and UFAD. interactions that are a desirable part of integrated design may be curtailed. If the interior is to be fitted out by Although overhead air-distribution system components tenants, information on loads may be vague. require coordination with other elements such as partitions, Figures 1a and 1b contrast the information flow between structure, and piping, UFAD installation must address furarchitects and engineers with linear versus integrated design niture placement to a much greater degree. Obstruction of approaches. The potential benefits of UFAD are less likely to diffusers by furniture may cause distortion of desired airflow be achieved in the linear approach. patterns. In addition, the proximity of diffusers to occupants may drastically reduce air temperature at foot level, increase Implementation: The Importance of Details the vertical temperature difference between head and foot, and While integrated design is necessary to achieve the po- cause discomfort. tential benefits of UFAD, installation and operation are at For these reasons, achieving the potential performance least equally important. Sealing joints in the raised floor benefits of UFAD is affected by a larger number of design (especially around floor penetrations such as columns) is disciplines and trades than with overhead systems. Design key to avoid air leakage with open plenum UFAD systems.9 teams should work closely with contractors and facility On the other hand, a ducted UFAD system designed with managers to inform them of their substantial influence on features such as extra flex duct length to diffusers to ease system performance and the details requiring particular reconfiguration may be handicapped by unnecessary seal- attention. ing of floor tiles and/or intentional misalignment of carpet and floor tiles to reduce plenum leakage. Location of heavy System Setting and Operation furniture and interior partitions above equipment that may In addition to more comprehensive design and construction require access for regular maintenance also reduces the practices, the satisfactory performance of UFAD depends on advantages of UFAD. system operations and, ultimately, on the user. Operators, facil- Linear Design Approach Integrated Design Approach 32 ASHRAE Journal Design Concept ashrae.org July 2009
ASHRAE Journal - July 2009

Table of Contents for the Digital Edition of ASHRAE Journal - July 2009
