Ashrae Journal - November 2008 - B39

Hybrid Mode Controlling Indoor Dew Point To make further use of natural ventilation, the hybrid mode of natural ventilation is set to occur at higher outdoor air temperatures. In this mode windows are open and all fans are off as previously described, and the ceiling is chilled, providing radiant cooling. This mode is only allowed when the outdoor dew point is 5°F (2.8°C) below the chilled ceiling temperature. This mode can provide comfortable space temperatures up to 76°F (24°C) outdoor temperatures. Demand-Controlled Ventilation The large center areas are supplied ventilation through a ducted underfloor air-distribution system with high-induction swirl diffusers. The VAV boxes are controlled based on CO2 sensors with a temperature override. The temperature override was required for periods when the chilled ceiling could not meet the sensible cooling load. Main Air Handlers The dual path custom designed air handlers needed to incorporate multiple functions depending on the mode of operation. During the heating mode they function as dedicated outside air system (DOAS) with heat recovery. During the cooling mode, they provide a mixing of conditioned return air and conditioned outdoor air. The outdoor air path incorporates a runaround coil loop that enhances the latent heat removal. The return air path was required to supplement the sensible cooling capacity of the ceiling and to further dehumidify the indoor air. The mixture of these two airstreams (airflows are accurately measured with hot wire anemometers) is controlled by the highest CO2 zone reading. The final mode of operation that needed to be incorporated was a smoke evacuation mode in which windows and fans are strategically deployed to quickly evacuate the building of smoke in case of fire. Efficient Lighting/Daylight Harvesting Controlling indoor dew point was important from the standpoint of maintaining the chilled ceiling temperatures without risk of condensation. The indoor dew point was controlled based on the following system: • The outside ventilation air for the open areas is conditioned through a three-coil runaround system that significantly improves the dehumidification capacity of these systems. The warmer the outside air, the more effective this configuration is related to dehumidification. • There is a separate return air path on the open area air handlers (AHU-1 through 4) with staged coils that will provide dehumidification of the space. By staging the coils in the return air path, the air that passes through the active coil can be cooled to 55°F (13°C) for maximum moisture removal and mix with the room temperature air through the inactive coil to supply air to the underfloor system in the 63°F (17°C) range. • The classroom units are conventional VAV systems with cold air supplied at the ceiling. These systems will assist the dehumidification of the adjacent open area space. Control Sequence The design features include reduced lighting power densities of 1.3 W/ft2 (14 W/m2) in the open areas and 1.0 W/ft2 (11 W/m2) in the seminar/classroom areas. Active daylighting is modeled to maintain 50 footcandles (538 lx) in the classroom areas and 35 footcandles (377 lx) in the open areas. Daylighting control is continuously dimmed. Visible transmittance is varied by mechanically operated blinds and shades in the east and west glazing. Building Envelope The BAS control system must be robust to integrate all these systems into an intelligent smooth functioning system. A detailed control sequence was written to define the five modes of operation (heating, cooling, natural ventilation, hybrid, and smoke evacuation). Cooling was defined as the default mode with specific parameters defining the transition and time delays to other modes. A complete weather station was incorporated as part of the design to measure light levels, wind, dew point, dry-bulb temperature, and precipitation. For example, if precipitation is sensed, the windows close and the system switches out of natural or hybrid mode. If the fire alarm system sends a smoke evacuation signal to the BAS, the system commands the first floor windows to open, the east side shades to open, the interstitial windows closed, the awning windows open, blinds up, dampers at the air handlers switch position, and the exhaust fans rev up to high speed. BACnet Integration The design features upgraded insulation in the roof, wall, and glazing. Half of the third floor roof is vegetative. The west curtain wall consists of two skins with a minimum of 36 in (914 mm) interstitial space used for insulation or as ventilation chimney depending on the weather. A three-story winter garden on the west also buffers the interior. The winter garden is maintained at a minimum of 40°F (4°C) with the radiant floor heat in the winter and is ventilated in the summer. November 2008 Loyola University decided several years ago to use BACnet as its building controls communication protocol. The total quantity of BACnet I/O objects currently exceeds 26,000 physical points. The majority of the BACnet objects are provided by the vendor’s BACnet system. Working in collaboration with the University’s IT department, the facilities group created a virtual LAN (VLAN) for the building controls network. The VLAN is deployed across the campus IT network, and enables building controls to communicate using BACnet over Ethernet, reducing the complication of standard IP-based communications. Choosing a VLAN made it easier for the IT group to manage the building controls network traffic. As new controllers are added, the IT group provides a network B39 B A C n e t ® To d a y | A S u p p l e m e n t t o A S H R A E J o u r n a l

Ashrae Journal - November 2008

Table of Contents for the Digital Edition of Ashrae Journal - November 2008