Original Sins
A project in the mid-1990s had provided two sets of escalators at the main entryway for the facility. This project had a design flaw that helps to illustrate the challenges associated with the design of HVAC systems for large spaces. The main entry to the airport faces west toward the prevailing winds, and it is open to the outdoors at both the baggage and ticketing levels as well as at two parking levels. During the winter, a large fire protection pipe had burst and flooded the lower level of the entry. Quartz infrared heaters were then installed to keep these pipes from freezing in the future and to provide a higher level of comfort.The airport did not feel that the heaters were an aesthetically pleasing solution. The problem with this design was that each of the two escalator areas was served by a packaged rooftop unit that housed a small hot water heating coil providing a mere 20 degrees F air temperature rise. This design might have worked south of the Mason-Dixon Line, but not in Ohio.
To make matters worse, each rooftop unit had a thermostat that was situated midway up the height of the facility and was never able to sense and react to the coldest air down near the baggage level doors. When the baggage handling doors were opened up to the deplaning areas at the opposite side of the facility, cold air was able to rush through the facility virtually unimpeded.
Redemption
URS Corporation was instructed to alleviate this problem when designing the new entry for the facility several years later. The mechanical department worked closely with the architectural design department on this issue, making sure vestibules of sufficient size were created on each of the four levels that opened up to the outdoors. Every entry point was designed to require patrons to pass through two sets of automatic doors before entering the atrium. Failure of the HVAC system to keep the cold prevailing winds at bay was not going to happen this time around.In addition to vestibules, the airport required a good method of delivering heat. Variable volume AHUs serving fan-assisted terminal boxes were selected. Parallel boxes were used to deliver air to interior spaces, while series boxes were employed at the vestibules.
We chose series boxes to keep a high level of airflow moving constantly through linear diffusers at the doors, creating an effect similar to an air curtain but without the associated noise. The return air paths from the vestibules leading to the AHUs were purposely "undersized." This was done to keep the vestibules positively pressurized, minimizing the infiltration of outdoor air.
The terminal boxes in the vestibules were then selected with two-row hot water heating coils, instead of the typical one-row coil, to yield a discharge air temperature in excess of 120 degrees.
A special sequence of operation was added for the series terminal boxes to prevent backward rotation and subsequent damage to the box fan. The series boxes are indexed on prior to starting its respective AHU. Even with this sequence, there can be problems if the installing contractor does not fully understand the operation of this type of system.
An example of this pitfall occurred when a contractor was running a system through preliminary life safety testing and was getting the series fan-assisted boxes to shut down on smoke detection one at a time. It happened to be for a large restaurant that had multiple dinning areas and kitchens that were served from one large AHU. The control sequences required that the main supply fan continue operating even if one of the fan-assisted boxes sensed smoke. This was to prevent kitchen-related nuisance trips from shutting down ventilation and the source of heat to the entire glass-dominated facility.
During testing, a box fan was shut down upon smoke detection and the wheel came to a stop; it then proceeded to spin backwards and pick up momentum due to the pressure of the AHU through the main air valve. This valve does not close automatically on box shutdown.
When the smoke condition was cleared at the box and the box fan was restarted, the action of the motor attempting to put the wheel back in forward rotation was too much on the assembly, and it was promptly destroyed. Witnesses described loud noises, but this did not stop the testing team from destroying four more of the 14 terminal boxes.
On to the Atrium
The architectural design of the new atrium entry is stunning with the top of the vault nearly 100 ft above the lowest level. This atrium divides the new entry longitudinally and requires that two separate HVAC systems serve the space from either side. Because of this, the mechanical department requested that the mechanical rooms be created with separate boiler and chiller plants to serve each of two 42,000-cfm AHUs.The two mechanical rooms housing the AHUs were dictated and defined by the geometry of the atrium and ended up being somewhat small and wedge-shaped. Because of this, the client requested that the mechanical rooms be created as two-story spaces to accommodate the large quantity of equipment and ductwork depicted in Figure 1.
To keep noise and vibration to a minimum in the adjacent atrium, the supply and return fans for each AHU were isolated with concrete inertia bases. Return air is drawn through the atrium partway up, allowing the air above this height to stratify. As a result, the cooling load was reduced to account for the effect of this feature.
At one time, a fountain was considered for the atrium. If this feature had not been eliminated early in the design, it would have required that air motion be considered at the top of the atrium. The potential for warm, moist air condensing on the barrel-vaulted glass would be high.
The way to avoid this type of situation is to keep air moving across the glass and surrounding steel, causing these potentially cold surfaces to assume a temperature closer to that of the indoors. The latent load attributed to fountains is related to the evaporation rate and is a function of water surface area, water temperature, temperature and humidity of the air, and of the relative velocity between the air and water.
The cooling loads imposed by the automated people movers and escalators were large, and unlike elevators, they are continuous. This cooling load did not allow the discharge air temperature to be reset above 60 degrees in the winter.
Not only are the vestibules positively pressurized, but also the entire atrium is pressurized as well by delivering, at a minimum, 16,000 cfm of outdoor air 24 hours a day. This is achieved by creating a set differential between the supply and return airflows. The flow rates are measured in both airstreams, and the return air fan speed is modulated to maintain a set differential of 8,000 cfm at each of the two AHUs. The difference in this airflow is made up through the outside air path. It is this quantity of outside air that creates positive pressure in the space thereby keeping the prevailing winds at bay.
Smoke Management
Building codes are in place to provide for life safety and for the safeguarding of property. One method of accomplishing life safety is to ensure that all buildings are easily evacuated during a fire. This can be achieved through strategies such as dictating the size and quantity of egress openings, the width of stairways and corridors, and the fire-resistive rating of building components.Code requirements will increase right along with the size and complexity of a building design. Bigger facilities will usually have more occupants as well, which will normally add to the statutory demands. Atriums present a challenge to the code authority as they are often designed into structures where many people may congregate such as malls, large office complexes, and as in this case, airports, and other transportation concourses.
In addition, the atrium is usually in the path where many of the building occupants will attempt to exit in an emergency. Unfortunately, there is no good way to maintain a fire rating around the atrium without affecting aesthetics. Occupants that scramble out of the stair towers and into the atrium need to be able to identify a clear path of exit in order to escape safely. This is where the atrium smoke management system comes into play. Its purpose is not to keep occupants alive indefinitely, but rather to keep the smoke from descending and obscuring the vision of the exiting occupants during a fire.
The volumetric rate of smoke production was calculated using an equation that makes it proportional to the height of the smoke interface level to the 5/3 power. With the atrium at nearly 100 ft, the smoke production could have become a big number. Fortunately, the highest level of exit access was considerably less than 100 ft. The smoke management system as a result, required a total of only 243,000 cfm of exhaust (Table 1).
This system was designed under the BOCA National Building Code, where the design fire is 2,000 Btu per second. This jurisdiction is now under the International Building Code/2000 that sets the design fire much higher, at 5,000 Btu per second.
However, the code goes on to state that the magnitude of the design fire may be reduced if "a rational analysis is performed by the registered design professional and approved by the building official." This can have a big impact on the design of atriums, such as this one, that are very high but have a relatively small footprint. A sympathetic code official who is well-versed in these systems can review and approve of alternative designs that do not compromise the safety of the occupants.
The smoke management system for this facility is composed of six exhaust fans, the two AHUs, plus additional makeup air entering through automatic sliding doors at the lowest two levels.
These automatic doors are interlocked to open upon initiation of the smoke control system and are fed from the emergency power system. It is important for the makeup air to be introduced low in the atrium and at a low velocity. This allows the exhaust fans to draw the products of combustion upward in an orderly manner, minimizing turbulence and the associated mixing.
The intakes to the six smoke control fans are located high in the atrium and are hidden behind six architectural panels that are roughly 20 ft per side. The panels stand off of the large north and south walls with just enough of a gap (about 6 in.) to permit the required airflow at a reasonable velocity (Figure 2).
The smoke exhaust fans are of a special design for smoke control applications and are tested to operate for 4 hours at 500 degrees and for 15 minutes at 1,000 degrees.
The two AHUs, each rated at 42,000 cfm, are indexed to 100% OA and attempt to maintain the discharge air temperature setpoint, which varies between 55 degrees and 60 degrees depending on the season.
The primary air valves on the terminal boxes are driven open and the VFDs on the AHUs are ramped up to maximum speed. The return fans for these air handlers are indexed off. In another arrangement, the return fans could have been used to assist the smoke evacuation, however their discharge location was not appropriate to allow for this.
Both the heating and cooling systems are filled with 40% ethylene glycol, so that if the smoke control system is initiated in very cold weather, the coils will be protected. The heating and cooling systems were not designed to fully temper the air under these emergency conditions on a design day.
In the event of an emergency, water would be flowing in the sprinkler system long before the temperature of the massive atrium would drop to the point where the fire protection pipe would be in jeopardy of freeze damage.
The fire alarm control panel includes a contact closure that initiates the smoke control sequence when any of the following events occur: smoke is sensed in either AHU; smoke is sensed by the area-wide atrium smoke detection system; water flow is detected in the sprinkler zone serving the atrium area. Next to the fire alarm panel is a firefighter's control panel that has toggle switches allowing the fire department to control the equipment that make up the smoke management system. There are hand-off-auto switches for the six smoke exhaust fans and the two AHUs, plus open-close-auto switches for the automatic vestibule doors at the lower levels. ES
EDITOR'S NOTE: Some images associated with this article do not translate to the Internet. To view the images, please refer to the print version of this month's issue.
