Air-Handling System Details in a Nutshell

By David Linamen, P.E., Principal of Engineering, Burt Hill Kosar Rittelmann, Butler, Pa. -- Consulting-Specifying Engineer, 7/1/2004

When it came to mechanically ventilating the convention center, it was the team's goal to reduce as much energy-consuming equipment as possible.

The load characteristics to consider were as follows:

• The main hall would be subject to a variety of loads.
• Industrial shows would provide the highest sensible loads (connected capacity of 20 watts per sq. ft., plus one person per 40 to 50 sq. ft.; these loads are not addressed in design because they are very infrequent).
• Adequate design accommodates 10 watts per sq. ft. in addition to lighting.
• Ability to address significant latent loads when main exhibit hall is used for large gatherings.

Considerations for system applicability were as follows:

• The main exhibit hall normally should be served by one or more all-air systems.
• System should be capable of operating on 100% outdoor air for setup.
• Main exhibit space might have equipment that produces an unusual amount of fumes or odors, such as restaurants or printing displays.
• Smaller meeting rooms should have independent systems to address irregular occupancies.
• Offices and restaurants will often operate for more hours than meeting rooms, so they should be on separate system.
  Based on these considerations, we determined that systems should:
• Be modular in fashion so that they could be energized and de-energized as the spaces they serve were occupied and unoccupied.
• Be adjustable so as to vary outdoor air volume requirements as the occupant densities varied.
• Provide maximum flexibility for arrangement and setup of displays.
• Utilize low-temperature air in spaces where it would be most beneficial to reduce supply-air volume requirements and ultimately, fan horsepower requirements, for spaces with intense internal heat gains (this strategy would also work well in spaces with high latent loads).

As spelled out in previous installments of Project Journal, a natural ventilation scheme was implemented for part-time use in the exhibit halls when temperatures are between 45°F and 65°F, as well as for nighttime flushing. For the other various venues in the building, as well as the halls, when natural ventilation is not in use, a total of 47 separate air systems handle ventilation duties. Low-temperature air systems, specifically, deliver air at 41°F. Early investigations into low-temperature air strategies indicated that air could be supplied at this temperature without utilization of brine solutions or other exotic fluids for the transport mechanism in the chilled-water system.

It was also determined that air at this temperature could be distributed by relatively conventional means without much risk of creating cold drafts.

A total of 14 units serve the exhibit halls; 19 serve the prefunction areas. The use of low-temperature air resulted in a 33% savings in total air supply, with a corresponding savings in fan horsepower.

During the programming phase of the project, it was determined that there would never be 100% simultaneous occupancy of the meeting rooms and the auditorium. A system was therefore developed that provided primary supply of low-temperature air from four central airhanding units. This air was ducted to 19 separate secondary AHUs that mixed primary air with return air to achieve the proper temperature and quality of air to satisfy the load in the meeting rooms and auditorium. The secondary AHUs also had the ability to shift air supply between meeting rooms and prefunction areas, recognizing that occupancy also shifts between the prefunction spaces and the meeting rooms. The primary AHUs have a total capacity of 121,600 cfm. The secondary AHUs have a total capacity of 177,400 cfm. Other, more conventional AHUs serve the ballroom, kitchen, administrative offices and locker rooms.

Additionally, carbon dioxide sensors are used to control ventilation air volumes for all areas of the building.

The exhibit halls were not so conventional. The architect expressed a desire to develop ducted air distribution for the exhibit halls that would complement the structure, which mimicks the curve of the city's cable-stayed bridges. He requested that the scale of the ductwork be kept as small as possible so as not to verpower the aesthetic of the cable truss. After a brief discussion, we pro osed that the ductwork extend across the exhibit halls parallel to the cable trusses and that the ductwork follow the same arc as the bottom cord of the cabletrusses. It was further proposed that center-to-center spacing on the ductwork be identical to that of the cable trusses, and it was discussed that by separating each duct section into two ducts—one on either side of each cable truss—the scale of the ductwork could be kept smaller. It was further discussed that by supplying the air from both ends of each length of duct as it crosses the exhibit hall, the size could remain even smaller. It was noted that the utilization of low-temperature air would also contribute to minimizing the size of the ductwork. To do so, the ductwork would have to:

• Be very flexible in order to move with the cable structure.
• Be as light as possible to minimize the added load imparted to the structure.
• Be well insulated to avoid condensation resulting from the low-temperature air passing through the ducts.
• Have special diffusers to distribute the low temperature drafts evenly at all locations along the duct without objectionable drafts.

Fortunately, a member of the design team with experience in sheet metal fabrication suggested the use of fabric duct. Unquestionably, fabric duct is light and flexible, but more importantly, it has a porosity that allows air to "bleed" through it. This creates a microclimate around the duct that prevents condensation from forming on the duct surface.

Further analysis also determined that the manufacturer could vary the position, size and spacing of the holes in the side of the duct where the air discharges into the conditioned space. This achieves a desired uniform distribution of air along the entire length of each duct, even as the ducts arc across the hall floor.

Cooling is derived from an independently operated 6,000-ton chiller plant, designed and constructed by a third-party energy service company. It consists of four 1,500-ton electric centrifugal chillers. Steam, also independently generated from a district system, provides heat. The steam is reduced from approximately 100 lbs. at the entrance to the building to 15 lbs. for distribution in the building.

This installment of "Project Journal" provides specific details on the David L. Lawrence Convention Center's key air-handling and HVAC systems. For a more complete perspective on the project from David Linamen, the project's lead mechanical engineer, visit the HVAC and Building Automation community at www.csemag.com.