Best Practices For Underfloor Air Systems

Transcription

1 The following article was published in ASHRAE Journal, October Copyright 2004 American Society of ing, Refrigerating and Air-Conditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. Best Practices For Underfloor Air Systems By Mike Filler Between 1995 and 2002, the number of new office buildings with raised floors that use underfloor air distribution (UFAD) increased approximately 40%. 1 This rapid growth can be attributed to four potential benefits UFAD offers over traditional ceiling-based air distribution: 1. Lower churn costs allows for fast and inexpensive reconfiguration of office space, 2. Improved IAQ supplies conditioned air directly to the breathing zone, 3. Lower energy costs reduces fan and refrigeration energy, and 4. Potential to reduce floor-to-ceiling heights creates cost savings in structural and façade systems. Although UFAD has increased in popularity, knowledge of how to apply the technology is not widespread. This article addresses the design, construction and commissioning phases of the UFAD building project. Design Phase: Cooling-Airflow Calculations During the design phase, engineers carefully must determine load calculations for the building. These will differ from the calculations for a ceiling-based air-distribution system. One difference is the amount of a particular load that enters the occupied zone. For a ceiling-based system, the occupied zone is effectively floor-to-ceiling, because the space is well-mixed. For UFAD systems, the occupied zone is the vertical space from the floor to 6 ft (1.8 m) above the floor. This means that some loads never enter the occupied zone. Further, if return-air grilles are placed close to the building s perimeter wall, convective heat gain occurring there can be carried into the return air before it has an opportunity to enter the occupied zone. Both of these factors can reduce the heat load in the occupied zone. Ultimately, this reduction in space heat gain affects the calculation of cooling airflow in direct proportion (see Figure 1). About the Author Mike Filler is a product marketing engineer, Engineered Systems Group, YORK International, York, Pa. O c t o b e r A S H R A E J o u r n a l 3 9

2 Return Grill Return Airway 80 F Unoccupied Zone Ceiling Convective Energy 9 ft 6 ft 76 F Thermostat Occupied Zone Radiant Energy Raised Floor 0 ft 72 F Supply Airway Supply Airway 62 F Figure 1: Typical loads that do and do not enter the occupied zone. Another difference is the room-air temperature that is used for load calculations. With a ceiling-based system, the room-air temperature is considered to be uniform, floor to ceiling. With UFAD, there is temperature stratification within the space, with heat and pollutants concentrated in the upper levels. In fact, ANSI/ ASHRAE Standard allows a vertical temperature variation of 5.4 F (3 C) between head and ankle levels. (If an all-air UFAD system is designed for no stratification in the occupied zone, it will use more airflow, wasting fan energy [see Figure 2]). Because of stratification, it is recommended that engineers consider using 2 3 F (1 2 C) higher thermostatic temperature for UFAD systems, compared to overhead distribution. 2 This should have a direct impact on the cooling airflow calculations. The third airflow-calculation difference concerns the heat transfer from the warm, occupied zone above, and from the warm, return airway below in multistory buildings, into the cool underfloor airway 3 (see Figure 3). This heat transfer and the resulting thermal decay (loss of supply-air cooling ability) should be counted as part of the airflow requirement for each zone. The heat transferred from the occupied zone would be a credit (reduction) for cooling supply airflow quantities. As a rule of thumb, the supply-air temperature should not be allowed to increase more than 3 F (2 C) from the time it enters the underfloor airway until it is discharged into the conditioned space through the furthest terminal. If a greater thermal decay is possible because of a large floorplate, unlined stub ductwork should be used to carry the supply air further into the airway. Design Phase: Variable-Air-Volume Versus Constant-Air-Volume Thermal decay is another good reason to specify a variableair-volume (VAV) UFAD system. It has been found that the supply-air temperature is not always uniform in an underfloor airway, and may change throughout the day. A properly designed VAV/UFAD system is adept at accommodating these temperature variations. There is a belief that placing a manually adjustable diffuser Figure 2: Temperature stratification in UFAD-cooled space. near each occupant will result in greater temperature satisfaction. However, it has been found that most occupants do not understand that they can control airflow in their workspace, and that those who do understand typically do not make changes anyway. 4 As a result, engineers may find that automatic VAV diffusers provide better comfort than manual VAV diffusers. Design Phase: Ventilation In any VAV system, ensuring minimum ventilation is always a concern, because reduced loads could cause supply-air volumes to fall so far that insufficient ventilation is supplied to the room. To prevent inadequate ventilation in VAV/UFAD systems, some manufacturers offer optional collars to serve as a minimum stop for their dampers. Other manufacturers recommend using a series fan-powered VAV box that pulls supply air from the underfloor airway and heats the air as necessary. Minimum-ventilation requirements may also be met if the designer takes into account leakage from the underfloor airway into the conditioned space. Like the ducts in an overhead system, the underfloor airway experiences a small amount of air leakage through diffusers, raised-floor panels, and power-voice-data (PVD) boxes. This leakage may meet much, if not all, of the minimum-ventilation requirement. This will also affect the amount of diffuser airflow needed to satisfy the space heat gain. With precise CO 2 monitoring of the room, UFAD may also be able to reduce overall ventilation-air quantities more than overhead systems. Rather than having to ventilate the entire floor-to-ceiling space, a UFAD system only requires ventilation of the occupied zone. A CO 2 monitoring system will recognize when the occupied zone is adequately ventilated, regardless of the situation above 6 ft (1.8 m). Design Phase: Leakage Rates While a small amount of airway leakage is desirable to handle minimum ventilation requirements, excessive leakage through the floor panels into the space and other uncontrolled airflow 4 0 A S H R A E J o u r n a l a s h r a e. o r g O c t o b e r

3 can cause comfort problems and waste energy. For this reason, it is important to specify and detail how to ensure a leak-tight airway. Engineers need to pay especially close attention to the intersection of floor slabs with exterior walls and interior shafts (for elevators and risers) to ensure proper sealing. Also examine furred-in columns, stud walls penetrating the raised floor, and all penetrations for pipes, ducts, and cables. Because the slab will (hopefully) be tightly sealed, thought should be given to flood control. The structural engineer should determine whether a flood caused by a broken pipe or the sprinkler system would collapse the slab. If that possibility exists, floor drains with pressure valves should be incorporated in the design. Raised-floor leakage rates vary by manufacturer and floorcovering type and layout. During the design phase, specify an airway leak rate no more than the minimum VAV airflow, and specify a leak test to be performed as part of commissioning. A marginal UFAD system may leak as much as 20% to 25% of the system s total air volume, but a good system typically will leak no more than 10% to 15%. While even 10% to 15% leakage may seem excessive to some, it is important to remember that overhead duct systems leak an average of 20% of their airflow, and that leakage flows to the return-air airway and not to the occupied space. Figure 3: Thermal decay in the supply airway. Design Phase: Airway Height It is important to design enough room for and access to the equipment that needs to be located under the raised-access floor (RAF). Space becomes important when it is time to perform routine maintenance or make repairs or adjustments to the equipment. UFAD installations typically use 12- to 18-in. (305 to 457 mm) RAF. Laboratory testing has concluded that the minimum RAF height is 8 in. (203mm) for uniform air-distribution. 5 However, RAF height is often driven by the height of the underfloor fan terminals (if used) or the size of the floorplate. Underfloor fan terminals typically range in height from 8 in. (203 mm) to as much as 16 in. (406 mm) for large cooling-and-heating units. For larger floorplates, the size of the stub-ductwork (typically limited to 22 in. [559 mm] wide to fit between the RAF pedestals) may pose a height constraint. Design Phase: Ductwork and Air Handlers A UFAD system requires some special consideration of the ductwork. In the room, the return-air intake should be sized for a maximum of 500 fpm (2.5 m/s) and be located above the occupied zone. Air moving any faster will disrupt the stratification effect necessary for a properly designed UFAD system. Combination fire/smoke/control-zone dampers (FSDs) should be those specifically designed for modulating control. As with overhead systems, it is important to take into account the effect of air velocities on noise. It is recommended that air velocity entering the supply airway be limited to a maximum of 1,500 fpm (7.6 m/s) to reduce noise. On the other end of the noise spectrum, it should be noted that noise levels of UFAD systems are typically lower than overhead systems. For this reason, engineers may want to consider adding sound masking in low-occupancy areas for acoustic privacy. 6 The air-handling device will be responsible for providing some or all of the minimum ventilation requirement. So, the minimum airflow requirement for each device should be displayed on the mechanical schedule. The air-handling device also should be equipped with some sort of reheat capability, so it can handle the higher supplyair temperatures (SAT) about 60 F (16 C) versus 55 F (13 C) required by these systems without creating a humidity problem in the occupied spaces. In most cases, it is best to use one primary air-handling unit with a return-air bypass (see Figure 4). The typical face-and-bypass arrangement is not the same as the return-air bypass and will result in unacceptably high humidity levels. Other, more expensive reheat methods include modulating hot-gas, run-around heat pipes, or using as many as three fans (not including a return or relief fan) to supply air to perimeter zones. Design Phase: Control Systems Controls for an UFAD system are similar to those of an overhead VAV system, except the SAT is designed for a minimum of 60 F dry bulb 3, and the static pressure (SP) differential pressure between the airway and the occupied space is nominally designed for 0.05 in. w.g. (0.012 kpa). Some manufacturers may recommend higher airway SP (up to 0.10 in. w.g. [0.025 kpa]). However, this will increase uncontrolled leakage, theoretically by more than 40%. At light loads, the SAT is typically reset upward, and SP downward, to prevent overcooling and to maximize free-cooling (economizer) hours. Night set-back should be minimized to maintain proper building control, and morning warm-up will not work because of the thermal effects of the building slab. The control sequence also should have the ability to lower the off-coil temperature, based on the level of underfloor RH, in order to prevent condensation and maintain comfortable humidity levels in the space. Be aware that UFAD components use a number of different types of controls. By working with component manufacturers, design engineers can secure sequences of operation for the diffusers, actuators and fan-powered units necessary for proper zone control. O c t o b e r A S H R A E J o u r n a l 4 1

4 Construction Phase During the construction phase, contractors must be aware that the floor slab, ceiling, walls and column enclosures are the structure of the supply and return airways, and they must act to preserve the integrity of the airways. This means informing all trades of their responsibility with respect to sealing the airways. To ensure proper installation and operation of the underfloor system, here is a typical sequence of interior construction that takes place only after final design-layout drawings have been issued. Install and finish all fire-rated demising walls. Install ceilings and ceiling systems. Complete the floor slabs, sealing them as dictated by building codes and project specifications, and checking to be certain they are level. Mark the RAF pedestal-support-grid layout on the floor slab, being certain the grid lines match up with the main supply and branch ductwork. Install and prepare all fan-powered units, diffusers, and any associated ductwork before the RAF is installed. Install pressure-control dampers and FSDs, fan-terminal ductwork, and branch ductwork while the RAF is being installed to ensure proper room for the ductwork. Clean and vacuum the slab before cabling is installed. A clean airway prevents dust from being blown throughout the space during startup. Install all wiring systems on the slab and coordinate the systems with the floor-pedestal marks. Set up a workstation to install the UFAD terminals in the RAF panels, expediting the installation of floor pedestals, panels, and PVD boxes. Overlay the wiring (preferably plugand-play cabling) on the UFAD drawings. Unique tag numbers are often unnecessary for the UFAD drawings, because typically only a few types of diffusers and fan terminals are used. Install the RAF pedestals and panels, including those with UFAD terminals and PVD boxes. It is recommended that diffusers or control items not be installed along known partition lines or under furniture. Install diffusers in the correct orientation on floor panels and panels in the RAF to ensure airflow patterns match the drawings. In larger floorplates, i.e., when the diffusers are located within 15 ft (4.6 m) of the supply duct, install the diffusers with the inlet facing away from the air supply to avoid a velocity effect from the ductwork. (However, some diffusers have inlets on all sides and do not allow this.) Orient directional grilles to provide the desired airflow pattern. At the perimeter, airflow should be directed along and/or away from the exterior walls, while grilles in the interior should create an outwards circular pattern. An outward pattern allows for the greatest amount of induction and is recommended for initial installment, even if occupants change the pattern once the building is in use. Construct partition walls on the top of the RAF, covering Advertisement formerly in this space. 4 2 A S H R A E J o u r n a l a s h r a e. o r g O c t o b e r

5 Return Air High-Efficiency Filters Damper Outside Air Dampers Filters Cooling Coil Supply Fan Supply Air Figure 4: Air-handling unit with return-air bypass. the floor and any openings or air-distribution terminals to keep construction debris from entering the floor airway. Apply finishes to the walls. Remove the covering from the RAF and vacuum. Lay carpet on the RAF panels. Install furniture. Ensure all plug-and-play cable connections are properly connected and engaged in mating connectors. (This can be accomplished earlier, when the RAF is installed, depending on the sequence of the trades.) Vacuum all construction debris in the diffusers. (This is not necessary if diffuser protective sheets were used earlier.) Commission the mechanical systems. Advertisement formerly in this space. Commissioning Phase Following the construction phase, the UFAD system is commissioned, which should include an airway-leakage test and an airflow test. Begin by testing each primary supply fan to check for minimum airflow with all diffusers closed or at the minimum stop. If uncontrolled leakage (leakage not coming through the diffusers and not what is expected through the floor) is too high, the leakage needs to be found and corrected, and the primary supply fan must be retested. The leakage test itself should be repeated every few years or after any remodeling that includes reconfiguring the UFAD system. An airflow/sp test determines that the system can provide adequate air to all diffusers. Typically, test one diffuser per zone and verify that the zone controllers are functioning properly (based upon the predefined sequence of operation). Balancing the diffusers is necessary only on ducted diffusers (often used along the perimeter). Educate Building Occupants on UFAD After commissioning, building owners and occupants assume responsibility for the optimal operation of the UFAD system. The owner or facility manager should secure a complete set of as-built drawings, establishing the grid system and lo- O c t o b e r A S H R A E J o u r n a l 4 3

6 cating all UFAD components and controls. This grid system needs to be revised whenever changes are made to the UFAD system. It is also important for the owner to train maintenance people to operate the UFAD system. Begin by providing them with applicable service manuals that explain the operation and maintenance of the system, and review the manuals with them. Pay special attention to the impact of room-temperature stratification on comfort and control, as well as the control functions to reset the RH, supply-air temperature and floor SP as needed. If applicable, any control options should be explained, such as providing a pocket digital assistant as a maintenance tool. Finally, it is important to inform the building occupants about the UFAD system. Stress with them the importance of periodically vacuuming the underfloor airway as well as cleaning up spills. Explain the benefits of the system to them, including comfort, improved IAQ, flexibility, and energy efficiency. Point out the location of the diffusers and the availability of individual diffusers to meet special needs. Identify the location of thermostats and sensors for individual work areas or zones, their functions and how to use them, and provide contact information to register comfort complaints. Buildings with UFAD may require more thought during the design phase, especially when designers are working on their first building with UFAD. However, if best practices are carefully followed and the proper coordination occurs on the UFAD job, then the process can be smoother, shortening the construction cycle, ensuring a successful commissioning process, and ultimately securing the advantages in system flexibility, IAQ, and energy efficiency that are contributing to the rapid growth of UFAD. References 1. Bauman, F Underfloor Air Distribution Design Guide. American Society of ing, Refrigerating, and Air-Conditioning Engineers, Inc. 2. Webster, T., F. Bauman, J. Reese. 2002b. Underfloor air distribution: thermal stratification. ASHRAE Journal, May. 3. YORK International. 1999, YORK Modular Integrated Terminals: Convection Enhanced Ventilation Technical Manual. 4. Webster, T., R. Bannon, D. Lehrer Teledesic Broadband Center Field Study. Center for the Built Environment, University of California, Berkeley. April. 5. Bauman, F., P. Pecora, T. Webster How low can you go? Air flowperformance of low-height under-floor plenums. Center for the Built Environment, University of California, Berkeley. October. 6. Tate Raised Access Floors and Office Acoustics. Tate Technical Bulletin #235. Advertisement formerly in this space. 4 4 A S H R A E J o u r n a l a s h r a e. o r g O c t o b e r