Bus garage puts the brakes on poor IAQ with ERV, air curtains
Although it’s only 25% complete, the Central Ohio Transit Authority’s (COTA) ongoing $75 million conversion to compressed natural gas (CNG) fueled buses is already saving more than 2.5 million gallons of diesel fuel and emitting 100 tons less bus emission contaminants annually in the greater Columbus, Ohio, area.
The bus conversion project is accompanying an HVAC retrofit of the 400,000-sq-ft CNG bus maintenance and garage areas into an energy-saving showcase that will preserve the environment and help COTA reach its future LEED silver goal.
The recent HVAC retrofit of the McKinley Avenue facility, now ranked as Ohio’s largest CNG fueling station, includes heat recovery air handlers, condensing boilers, and 10 environmental-separating air curtains for bus entrances, thanks to the ingenuity of project HVAC engineers and contractors. Key players in the project were architect, R.L. Bowen Associates in Cleveland; consulting engineers, Dynamix Engineering Ltd. in Columbus; mechanical contractor Kirk Williams Co. from Grove City, OH; and Jon Hancock, a former COTA transportation facility manager, who oversaw much of the design.
The Phase II mechanical design portion of the three-phase project was particularly challenging in that specification for all airstream ventilation equipment complied with National Fire Protection Association (NFPA) governing standards for compressed natural gases and vehicle fuels such as NFPA-30A, 52, 70, 88A, plus state and local fire marshal compliances. Consequently, Dynamix Engineering’s equipment specifications mandated combinations of energy efficiency and life/safety measures. For example, seven 14- by 16-ft doorways complement Dynamix Engineering’s energy efficiency model with the installation of air curtains that retain up to 80% of the building’s heat during perpetual door cycles as buses pass in and out of the maintenance and garage areas. Three additional air curtains separate environments at one large interior common doorway shared by the maintenance and garage areas.
“Using air curtains on those large doorways has proven to be a huge benefit in energy savings and indoor air comfort,” said Hancock.
They’re not ordinary air curtains, however. Each 1,600-cfm air curtain from the Hazardous Location Construction (HLC) Series manufactured by Berner International is constructed of nonferrous metals and features three- and five-hp explosion-proof motors as per Class 1—Group D—Division 1.
The air curtains also add to IAQ with optional permanent washable aluminum mesh filters that help remove airborne particulate emission contaminants from the remaining diesel fleet. The air curtains also include 708,140-Btuh hot water coils to provide air comfort to employees working near opening doors.
Also complementary to the LEED goal is Dynamix’s replacement of three gas-fired grade-mounted and several hot water indoor air handling systems that were beyond their serviceable life. Replacements consist of 10 exterior grade-mounted and 12 mezzanine-level semi-custom hot water and heat recovery air handlers manufactured by Innovent Air. The units feature 2-in-thick insulated double wall construction and fully water-down capabilities. While considerably larger to accommodate up to six air changes per hour versus the previous system’s single hourly air change, the energy use is similar. The energy recovering air-to-air heat exchanger is a cross-flow, flat plate type. It’s critical to efficiently maintaining comparatively better 55? to 60?F and 65? to 70°F employee wintertime indoor air comfort for the bus garage and maintenance areas, respectively.
The HVAC system is controlled by a BAS by Honeywell, but also overridden by a 16-zone network of more than 200 combustible gas and carbon monoxide detection sensors manufactured by Sierra Monitor Corp. The system perpetually monitors combustible gas lower explosive limit (LEL) levels and signals the air handlers for additional outdoor air when contaminant set points are exceeded. Some of the air handlers have VFD to provide additional outdoor air until gas levels fall below LEL level set points. Persistently high levels eventually open all doors automatically, activate exhaust fans, and switch mechanical systems to 100% outdoor air to purge the spaces for a complete air exchange in 10 to 12 minutes depending on the space.
Dynamix Engineering’s design also includes 21 exhaust fans ranging from 3,500 to 21,000 cfm that help evacuate detected gases as well as help control building pressurization.
Adding to the overall energy savings are two high-efficiency condensing and six near-condensing 3-million Btuh boilers by the Hydrotherm and RBI boiler divisions, respectively, of Mestek. The boilers supply air curtain and air handler heating coils for wintertime heating.
The project’s IAQ improvement via increased air changes has not overtly spiked HVAC operational costs due to the combined conservation methods of energy recovery and air curtains. Air comfort was also successfully maintained during last year’s frigid Ohio winter near doorways and elsewhere throughout the facility.
COTA has converted 75 buses to CNG for a savings of more than $1 million in fuel costs and a projected $4 million annually once the remaining 255 diesel buses are converted in approximately eight years. ES