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Liberty Mutual

Responding to Intensifying Building Use by Introducing an Evaporative Condensing HVAC Unit


When insurance company Liberty Mutual changed the use of a 350,000 s/f office building built in 1997 to that of a conference and training facility, it needed to house a significantly greater number of employees than the structure was originally designed to accommodate. With meeting rooms becoming uncomfortably warm due to the increased number of occupants, the owner quickly realized that the existing rooftop evaporative condensing units were not adequately managing the increased cooling load.


Trumbull Campbell Associates was tasked with designing an addition to the existing rooftop evaporative condensing system that would efficiently and cost-effectively deliver more ventilation to the building’s conference areas and workspaces to meet the building’s increased load. The owner also wanted to optimize the efficiency of any additional HVAC equipment through programmable controls that would safely and securely tie into the building’s existing automation system (BAS).


Trumbull Campbell recommended the application of a customized, large packaged 170-ton evaporative condensing rooftop unit (RTU) manufactured by AAON of Tulsa, OK. To tie the equipment into the BAS, Trumbull Campbell recommended the industry-leading Tridium digital controller, the next-generation technology of field- programmable HVAC sequences that optimizes the packaged RTU’s operating efficiency. The AAON RL series packaged RTU with an evaporative-cooled condenser presented a better solution than a traditional chilled water/cooling tower application, because it provided the most energy- and cost-efficient way to deliver cooled air to a facility of this size.

In evaporative-cooled condensing, water is sprayed over the condenser coil as the the condenser fans draw air across the coil to evaporate the spray and cool the refrigerant tubes. For Liberty Mutual, Trumbull Campbell recommended the addition of a patented airside de-superheater coil above the condenser’s wetted section to reduce the potential for scale formation and significantly cut water and chemical usage — two important factors for maintaining the building’s LEED Green status. The AAON de-superheat coil typically saves 22% of the makeup water required, as compared to traditional evaporative condensing systems available from the competition.

AAON was able to deliver the AAON unit in 12 weeks built to the owner’s precise specifications, including the tie-in of the Tridium controller to the BAS. The benefits for Liberty Mutual included lower total cost of ownership, lower energy usage and potential for energy rebates, reduced water consumption, decreased maintenance cost and reduced noise. Especially in New England, where energy costs are the highest in the nation, an evaporative-condenser system is saving Liberty Mutual more money over the long run than a traditional air-cooled packaged RTU.

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