Flexibility, Efficiency In San Antonio Arena

Transcription

1 2005, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ( Reprinted by permission from ASHRAE Journal, (Vol. 47, No. 9, September 2005). This article may not be copied nor distributed in either paper or digital form without ASHRAE s permission. Flexibility, Efficiency In San Antonio Arena By Jeffrey L. Ewens, P.E., Member ASHRAE, and Blake E. Ellis, P.E., Member ASHRAE T he state-of-the-art SBC Center in San Antonio is an 18,500- Tseat, multipurpose arena designed to host basketball games, rodeos, ice events, concerts, and a variety of other attractions. Key design components of the building s HVAC systems include fl exibility to address the building s multifunctionality and the ability to handle the harsh Texas heat and humidity. However, an equally diffi cult challenge was the design of the smoke control system to meet the requirements of the San Antonio Fire Department. The design team overcame these challenges with a unique HVAC design that accomplishes the goals of comfort, life safety and adaptability. Arena Bowl System Description A typical HVAC distribution configuration for an National Basketball Association (NBA) arena is to divide the bowl into four equal-sized quadrants. The SBC Center follows this fashion, and each quadrant is served by identical air-handling systems. Each quadrant contains three variable-air-volume (VAV) modular package air-handling units (AHUs). Two of the AHUs serve the seating bowl, producing 45,000 cfm ( L/s) of supply air each for a total of 90,000 cfm ( L/s) supplied to each quadrant of the bowl. The other AHU produces 27,500 cfm ( L/s) and serves the concourse spaces such as concessions, suites, toilets, public areas, and exterior envelope loads. Air is relieved from the building through four mixed flow fans (one per quadrant), each moving 72,500 cfm ( L/s) and configured for relief/exhaust (Figure 1 ). The distribution system for the seating bowl connects the two seating bowl AHUs before supplying air into three zones in each quadrant. The three zones are the upper seating deck, the lower seating deck and the arena ice floor. The ductwork system has motorized dampers to close off airflow to each of the zoned areas when they are not being used. The main concourse, upper concourse and suite level concourse are served by the four quadrant concourse AHUs with a mixture About the Authors Jeffrey L. Ewens, P.E., is a mechanical engineer with Henderson Engineers, in Lenexa, Kan. Blake E. Ellis, P.E., is a project manager with Burns & McDonnell, Kansas City, Mo. The authors received a 2005 ASHRAE Technology Award honorable mention for Public Assembly (New). 1 8 A S H R A E J o u r n a l a s h r a e. o r g S e p t e m b e r

2 The SBC Center in San Antonio is the home of the NBA s San Antonio Spurs and the San Antonio Livestock Expedition, as well as host for ice events, concerts, and more. of standard single duct VAV and fan-powered VAV terminal boxes providing temperature control. Ventilation Control Ventilation is a major component of any HVAC system design, but in a facility that can accommodate up to 18,500 spectators, its significance is amplified. While a large occupancy load drove the complexity of the system design, the use of the building also contributed to the challenge of proper ventilation. The SBC Center was designed as a multipurpose venue to host a variety of events, including ice shows. Most events can be held in a 75 F (24 C) space temperature environment. However, ice events require the indoor conditions to be maintained at 70 F (21 C) and 50% relative humidity (RH) or drier to prevent fogging near the ice surface. When the space humidity requirements for ice events are coupled with the high quantity of building occupants (18,500 people), the proper implementation of the applicable ventilation standard (ANSI/ASHRAE Standard , Ventilation for Acceptable Indoor Air Quality) was critical. Therefore, the HVAC design needed to provide acceptable indoor air quality and humidity control with widely varying occupancy levels and event durations while minimizing the amount of the hot and humid outside air brought into the facility that would require cooling and dehumidification. Many of the typical events such as sporting events have peak occupancies of less than three hours. Moreover, the arena seating bowl has an extremely large volume in comparison with the occupancy. These two conditions provided the opportunity for a more in-depth analysis of the outside air quantity required for a typical event. This analysis revealed that approximately 45 minutes of lag time was acceptable per Standard This lag time was determined using the 10 million ft 3 ( m 3 ) of air within the arena volume. However, instead of lagging the operation of the ventilation system behind occupancy, the outside air is provided to the space both prior to and during the time people occupy the space. The result of the calculation shown in Table 1 indicates that the ventilation rate effectively could be reduced to 7.33 cfm/person (3.46 L/s per person), although the design team reduced the rate quantity by only 50% (7.5 cfm/person [3.5 L/s per person]). 2 This value was used and resulted in a dramatic reduction (600 tons [2110 kw]) in the size of the building cooling system, thus offering significant first cost savings. (Note: using the current ventilation rate procedure described in Addendum n of Standard results in a ventilation rate of less than 6 cfm/person [2.8 L/s per person].) Most events held in the arena fall into the category of short duration peak occupancy. However, because the possibility exists that an event might last longer than three hours, and people (vs. other contaminant sources such as off-gassing of materials) by far are the main contributor to poor indoor air quality in this situation, the system design allows the outside air level in the seating bowl to be raised or lowered in response to multiple CO 2 sensors located in the seating bowl. This ventilation override ensures good air quality. Moisture Control The SBC Center has a 200 ft (61 m) long by 85 ft (26 m) wide ice floor to accommodate hockey and ice show events. Design- S e p t e m b e r A S H R A E J o u r n a l 1 9

3 ing the arena HVAC systems to control and maintain proper temperature and humidity levels was critically important due to the condensation issues resulting from the ice surface. Conditions in the seating bowl must be controlled to 70 F (21 C) and 50% relative humidity to prevent fogging. The major sources of moisture are infiltration through the building envelope, through open doors before an event, ventilation air, and latent heat generated by people. Determining effects of the first two is fairly straightforward, but predicting moisture generated by people is more challenging. For example, the seating bowl is to be maintained at 70 F (21 C), and the moisture generation figures in ASHRAE Handbook Fundamentals 3 are based on a space temperature of 75 F (24 C). Also, the activity levels of the people within the arena vary from the extremely active athlete to a seated spectator. To determine the appropriate moisture generation rate, a model occupancy group of seated, standing and active people was created. Using a chart that provides the moisture load from people at various space temperatures and activity levels 4 (i.e., seated, standing, light activity, and high activity), the latent heat generation of the model occupancy group was obtained for the 70 F (21 C) space temperature. Once the amount of moisture was calculated, different supply air temperatures were analyzed to determine the optimal way to achieve the 70 F (21 C), 50% RH design condition. Complicating this analysis was the fact that if a National Hockey League (NHL) team was to occupy the facility, the conditions would need to be maintained at a very cool and dry 65 F (18 C) and 45% to 50% RH. The study revealed that 320,000 cfm ( L/s) of 45 F (7 C) air would meet the 70 F (21 C), 50% RH requirement for a basketball game. However, at a 50 F (10 C) supply air temperature, nearly 1.1 million cfm ( L/s) of air was needed to meet the NHL hockey condition. To balance the two competing needs, 360,000 cfm ( L/s) of 45 F (7 C) air serves the seating bowl with heating water reheat to control the humidity levels to 50% to 60% RH for basketball games and 45% to 50% RH for ice events. To address future NHL needs, space is reserved in the mechanical rooms for four future active desiccant dehumidification units. These units will allow the HVAC system to meet the 65 F (18 C), 45% RH requirement without excessive airflow rates and without increasing the demand on the heating water reheat system. Smoke Control In a facility that accommodates 18,500 occupants, life safety is at the forefront of many design decisions. For this project, the need for a mechanical smoke control system arose from a desire to reduce exit widths by nearly 50% under a provision in the building code for smoke-protected seating assemblies. This 2 0 A S H R A E J o u r n a l a s h r a e. o r g S e p t e m b e r

4 decision allowed the same amount of seats to be installed in less area, thus providing two positive results: a lower first cost and a more intimate seating bowl. Due to the desire to minimize first costs, the smoke control system was integrated with the bowl and concourse HVAC systems. Coordination with the local fire officials resulted in a requirement for nine independent smoke control zones, more than three times a typical arena. The smoke-protected areas within these zones are the main concourse, the upper concourse, and the arena bowl. The end result of the analysis was a requirement for the four zones on the main concourse to exhaust a minimum of 20,762 cfm (9798 L/s) each, the four zones on the upper concourse to exhaust a minimum of 33,492 cfm ( L/s) each, and the single zone in the seating bowl to exhaust a minimum of 579,524 cfm ( L/s). To integrate the supply air system with the smoke exhaust system, the distribution shafts were strategically located to allow s Elec. Elev. Equip. s Relief/Exhaust Fans (In Vertical) system operation in two directions (see Figure 2 for a flow diagram). By opening and closing the dampers at the top of the shaft and at the concourse return Arena AHU Figure 1: Plan view of a quadrant mechanical room. Future Desiccant Connection 32/32 Concourse AHU air grilles, air can be directed to allow the relief fans to function as exhaust fans in both the concourse exhaust and bowl exhaust modes. The AHUs also provide the required makeup air in both concourse and bowl exhaust modes. It was a challenge, with the bowl and the concourse AHUs discharging only 470,000 cfm ( L/s) of the total exhaust flow rate of 580,000 cfm ( L/s) at design capacity. However, a desire to limit the differential pressure across the exterior walls and egress doors to 0.10 in. w.g. (25 Pa) (keeping the door opening force below 30 lb f /6.7 N) resulted in an infiltration of 60,000 cfm ( L/s) with the doors closed. That reduced the total makeup air requirement to 520,000 cfm ( L/s), but the HVAC systems were still 50,000 cfm ( L/s) short. To eliminate an additional makeup air fan, ductwork, and louvers, the bowl units operate at 111% of design flow rate and the concourse units operate at 109% of their design flow rate through variable speed drives (VSDs) in the smoke control modes. This made up the additional 50,000 cfm ( L/s), keeping the interior/exterior differential pressure below the 0.10 in. w.g. (25 Pa). Energy Efficiency In a building with a peak occupancy of only four to six hours per week, under- S e p t e m b e r A S H R A E J o u r n a l 2 1 s Arena AHU

5 standing how the building operates and designing for the HVAC systems to be shutoff when unoccupied is essential for energy conservation. In the seating bowl, this is accomplished on the supply air side by using three different zones where airflow can be shut-off completely. Using a 45 F (7 C) supply air temperature in the seating bowl instead of the more traditional 55 F (13 C) supply air temperature allows for a reduction in total airflow rates and reduces fan energy by 33% from a standard system. This concept was also used on the concourses with a 50 F (10 C) supply air temperature, which decreases airflow rates and fan energy by 20% over a traditional 55 F (13 C) supply air system. The seating bowl distribution system is served by dual VAV AHUs with VSDs. During low load conditions, such as when the upper seating bowl is unoccupied, this configuration requires only one unit to operate. Thus, fan energy consumption is reduced by only running one fan more efficiently during low load times. Building pressure is maintained at slightly positive pressure with respect to the exterior through the use of differential pressure sensors located at the peak of the roof where wind-tunnel results indicated no effect on the sensor reading based upon the wind direction. These sensors control relief fans equipped with VSDs for speed control. This allows the amount of relief from the building to adjust for the 24 concession stands with independently controlled exhaust (cumulative total of 60,960 cfm [ L/s] of kitchen exhaust) and the 40,000 cfm [ L/s] of toilet exhaust, all of which draw their makeup air from the seating bowl system and may or may not be operating during an event. Periphery components of the airside HVAC systems are the waterside systems that serve them. The heating water system uses a 30 F (17 C) temperature differential to reduce pumping Average Occupancy for Event Duration Outdoor Air Per Person (Required) Table 1: Seating bowl outdoor air requirements. 14,307 people 12.6 cfm/person Volume of Outdoor Air (Required) 40,559,400 ft 3 (Duration: 3 hrs., 45 min.) Acceptable Air in Arena (45 min. lag time) 10,489,500 ft 3 Arena Volume 10,069,000 ft 3 Minimum Value 10,069,000 ft 3 (min. of two above values) Amount of Outdoor Air to Add 30,490,400 ft 3 Flow Rate of Outdoor Air Outdoor Air at Peak Occupancy 135,513 cfm (Duration: 3 hrs., 45 min) 7.33 cfm/person energy in addition to the constant-primary, variable-secondary heating water pump arrangement. The chilled water system uses a constant-primary, variable-secondary pump arrangement with a 16 F (9 C) temperature differential to reduce system flow rates and pumping energy. The chilled water system also considered the environmental impact by incorporating HFC chillers with VSDs on the compressors to closely match the system load and increase chiller efficiency. In fact, the chillers for this building were selected based upon their efficiency in the September through May time frame while using reduced condenser water supply temperatures. This time frame was used because very few full-occupancy events are scheduled in arenas during the summer months. This technique allowed the most efficient chiller selection for how the building was going to realistically operate, not just 2 2 A S H R A E J o u r n a l a s h r a e. o r g S e p t e m b e r

6 at peak capacity or a more traditional part load factor, such as non-standard part-load value (NPLV). For the condenser water system, six cooling towers use twospeed fans for capacity control. With a total of 12 fans (two per cell), the airflow rate can be adjusted in 4% increments without the cost of adding VSDs. Roof Typical Long Throw Duct Typical Upper Seating Runout SP Typical Arena Lower Seating Arena Bowl Mechanical Room Arena Return Exhaust Plenum Conclusion Controlling temperature and humidity within a space with 18,500 people is difficult in the harsh south Texas environment. When the HVAC design also needs to address life-safety concerns with a mechanical smoke control system, the difficulty is compounded. Creating nine independent smoke control zones without adding any capital costs was challenging, but the HVAC systems in the Center meet the challenge with effective results. The systems were designed to address occupant comfort, to provide an acceptable indoor environment, to efficiently use construction funds, and to be energy efficient while having the ability to meet facility life-safety needs if called upon. References 1. ANSI/ASHRAE Standard , Ventilation for Acceptable Indoor Air Quality. Section , Figure ANSI/ASHRAE Standard , Ventilation for Acceptable Indoor Air Quality. par Mechanical Level Arena Seating Bowl Arena AHUs Suite/ Concourse AHU Upper Concourse Level Suite Level SP Main Concourse Level Figure 2: Quadrant airflow diagram ASHRAE Handbook Fundamentals, Chapter 28, Table Harriman, L.G. III, ed The Dehumidification Handbook 2 nd ed. Concord, NH: New Hampshire Bindery. p S e p t e m b e r A S H R A E J o u r n a l 2 3