Case study: Airport expansion includes rooftop units

The HVAC system at Phoenix Sky Harbor International Airport required two new rooftop units

By Jon Silhol April 14, 2021

A concourse expansion project at Phoenix Sky Harbor International Airport used rooftop units. There were several new mechanical units that were part of the scope of work and two of the units were located on the roof. This was partially due to limited floor space available inside the building.

Locating two of the units on the roof enabled the design team to limit the length of the ductwork that would have be installed if the units were located inside. Determining appropriate locations for outside air intake locations are challenging at an airport. There are more exhaust, containment and security concerns than at most buildings. Locating two of the mechanical units on the roof solved the concern of keeping the outside air intakes far enough away from any pollutants.

The two rooftop units serve the hold rooms, retail and restaurant areas. These areas have a high occupancy and high ventilation rates. The high amount of required outside air along with Phoenix’s high ambient design temperatures, pushed the dry bulb entering air temperature to the cooling coil near 90°F. These units were supplied with a chilled water-cooling coil, which could accommodate the high entering air temperature. Special considerations would have had to be designed if the rooftop units were air-cooled.

The project incorporated many acoustical treatments. The rooftop units were located over back-of-house areas such as corridors and public restrooms. This was still not enough to meet the sound requirements of other areas. Duct liner was used in medium- and low-pressure ductwork in both the supply and return duct systems. There was not space on the roof to allow for horizontal supply and return ductwork connections, so vertical supply and return ductwork connections were used.

This required sound attenuators to be used near the connections to the rooftop units. The sound attenuators were straight duct type and varied from 3 to 5 feet in length. Custom vibration isolation curbs were also used to stop any vibration from the units to the building’s steel structure. These were provided with ¾-inch deflection spring isolators and coordinated with the internal isolation types of the rooftop unit fans.

The rooftop units needed to be an applied application because of the requirements of the project. There was a cost premium for these units when compared to package rooftop units. The mechanical engineering team worked with the general contractor and mechanical contractor throughout the design phase to keep the cost of the units within the project’s overall budget.

The supply fan, return fan and exhaust fan sections were each designed as fan arrays. This kept the size of any one motor to a minimum and also help reduce the sound power levels of the fan. Each fan section was designed to provide full design airflow in case of a fan failure in a N+1 arrangement. This allows for full conditioning to occur if there is a fan failure.

Portions of this expansion are going to be high-occupancy spaces that will require a large amount of outside air. Total enthalpy recovery wheels were used on the appropriate units to increase the energy savings and reduce the load on the existing central plant.  Also, a blank section in each unit was designed for an ionization system to be installed.

As mentioned previously, bringing in clean outside air can be challenging at an airport. This technology has been used in other parts of the airport and it maintains the proper indoor air quality levels for occupants of the airport. The energy recovery wheels, ionization system, reduced lighting loads and electrochromatic glass to adaptively change based on the outdoor conditions will be some of the key factors to help this building achieve the U.S. Green Building Council LEED Silver certification.

Using this technology allows for carbon filters not to be installed in each of the mechanical units. This is a significant cost savings for the energy use that the mechanical unit’s fans can operate without the pressure drop of carbon filters and maintenance cost savings with not having to replace the carbon filters on a routine schedule.

Author Bio: Jon Silhol is a mechanical engineer with SmithGroup. He is a member of ASHRAE and has 20 years of experience designing mechanical systems for various building types.