ASHRAE Journal- September 2008

walls and other full height obstructions. Shading of the exterior of the building also helps minimize the radiant effect from the floor and curtain wall, and contribute to higher occupant comfort within the space.4 Installation Hanging a beam from the structure is as easy as mounting a light fixture. However, due to their typical operating weight of approximately 15 lb/ft (6.8 kg/m) for 2 ft (600 mm) wide, double deflection units, ACBs cannot be supported by a T-bar ceiling as is possible with fluorescent fixture. ACBs provide four adjustable hanging brackets that can be fastened to the concrete underfloor via threaded rod or aircraft cable. Seismic locations may require additional bracing. To reduce on-site brazing work, quick-connect flexible hoses and duct connections can be considered, decreasing the overall labor component associated with each unit, and providing a level of flexibility in unit positioning. The piping system should always be thoroughly flushed prior to the final connection of the ACBs to the distribution network, to prevent coil tube contamination. Water-Side Control As the relative humidity within a building envelope could fluctuate as a result of only a certain number of floors using the operable windows, dew-point sensors within the building should be used to protect the envelope from a loss of dew-point control. There is a level of sophistication required by building owners using mixed mode ventilation, as educating the occupants in the risks of opening the windows with this type of system can create problems for the building environment. However, given the advantages of free cooling afforded in certain climates, and in an effort to achieve LEED compliance, perhaps motorized operable windows could provide a control solution that offers the best of both worlds? Air-Side Control Chilled water temperatures for ACB loops range between 55°F to 61°F (12.7°C to 16.1°C) although the space design conditions and the minimum ventilation rates dictate that the beams receive 59°F to 61°F (15°C to 16.1°C) The separation between the secondary chilled water supply temperature, and both the dew point of the primary air and dew-point design of the space, is the technique used to prevent condensation on the beam-mounted cooling coil. Overventilating slightly or increasing the temperature separation also adds a margin of safety to preventing the formation of condensate. Two position on-off control valves are typically used to provide zone control. Modulating valves may result in creating laminar flow within the ACB cooling coils. As individual space temperature control can be achieved with two-position on-off valves, and yield an acceptable proportional control band, this method of capacity control is favored over modulating water control from a first-cost perspective. LEED® compliance is satisfied for individual room temperature controls, with relatively modest expense. Additionally, ventilation and the dehumidification it offers remains, even with the majority of the cooling effect disabled via the two position on-off control valve. Three-way mixing valves, injection pump systems, and other strategies can be used to manage loop water temperatures by floor plate or zone. This would be of particular interest for buildings with mixed mode ventilation. Dew-point sensors within zones with operable windows should be used to reset or inhibit the chilled water servicing that zone, thereby affecting dew-point control, without losing control of the secondary water loop servicing the balance of a building. Once the windows are closed and dew-point control regained, the chilled water temperature could be lowered at the mixing station servicing each floor plate or building zone. 56 ASHRAE Journal Discharge air from the DOAS is used to dehumidify the space. It is often quite common to service the makeup air unit with 45°F (7.2°C) chilled water. As such, 51°F to 52°F (10.6°C to 11.1°C) dew-point temperatures supplied to the occupied zone are relatively easy to achieve in many areas of the country by slightly oversizing the evaporator coils. Coil face velocities under 400 fpm (2 m/s) increases the coil residence time and lowers the overall fan static pressure to further decrease system operating costs. Energy wheels and wraparound-style heat pipes also can be used to enhance the dehumidification effect in more humid climates. Please note, in a draw-through configuration, it would be common to experience a 2°F to 3°F (1.1°C to 1.7°C) temperature rise as a result of fan and motor heat pickup within the DOAS unit, resulting in a duct discharge temperature of 54°F to 55°F (12.2°C to 12.7°C). Although not always necessary, primary air tempering is best provided with an energy wheel, as boiler reheat is considered wasteful, and inappropriate, given this superior option. Additionally, for areas where the primary air requirements are high, there is a risk of over-cooling the space under conditions of low load. In such cases, there is a valid argument to consider using some type of tempering device to reclaim a portion of the sensible building heat that is exhausted, or consider a VAV control strategy as it relates to zones of highly variable latent loads. Tempered primary air could be delivered to the space in a more neutral condition such as 65°F to 68°F (18.3°C to 20°C) or reset to be cooler at design conditions. As the duct distribution network is common to other beams in other zones, it is not practical to control the duct static pressure to affect control of each beam or beam zone without the use of VAV. Static pressure control is nonlinear and gives poor control resolution. VAV control allows one to deal with highly variable latent loads such as boardrooms, classrooms, cafeterias, lunchrooms, etc., and also provides tight temperature control for variable sensible loads. Since ACBs are driven by the ventilation air, further savings can be realized at the DOAS, by minimizing airflow to spaces that do not constantly require the ventilation rate at off-peak operating conditions, assuming that the building diversity can maintain the overall envelope in terms of dew-point control. ashrae.org September 2008 http://ashrae.org

Table of Contents Feed for the Digital Edition of ASHRAE Journal- September 2008

ASHRAE Journal- September 2008 Section: Contents Contents Section: Commentary Options for Sustainability Section: Industry News The Silk Route for Energy Solar Thermal Is Unrealized Opportunity Industry Groups Sue City of Albuquerque Section: Letters Letters Section: Meetings and Shows Meetings and Shows Section: Feature Articles Article-Radiant Floor Cooling Systems Article-HVAC Design for Sustainable Lab Article-Mixed Mode Ventilation Article-Single- Design Considerations For Active Chilled Beams Article-Acoustic Design In Green Buildings Article-Teams, Contracts & BIM Section: Building Sciences Some Old Lessons Distilled Section: Washington Report Energy in Federal Buildings Section: Products HVAC&R Product Showplace Section: Emerging Technologies Toplighting & Lighting Controls For Commercial Buildings Section: Special Products Fans & Blowers Section: Classified Ads Classified Ads Section: Advertising Index Advertising Index

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