Succeeding in the competitive commercial real estate market in suburban Philadelphia is no problem for One Tower Bridge, a landmark 15-story office building located in West Conshohocken, near the bustling Schuylkill Expressway. But to keep its edge, building owners needed to replace the nearly 30-year-old HVAC units that supplied the facility's floor-by-floor variable-air-volume (VAV) system.

Each unit supplies conditioned air to terminal units, small metal boxes located in the supply air duct just before the outlet diffuser. The terminal units in One Tower Bridge use pressure-independent sensors that control a damper, which regulates maximum and minimum volumes of conditioned air supplied to the space.

For comfort, the settings in each self-contained unit and each terminal can be adjusted to meet specific needs of tenants and the building owner. The problem was, transporting new, complete units to each floor was impossible because it wouldn’t fit in the freight elevators — and removing windows, walls, and ceilings would compromise the building’s elegant architecture.

Building engineers for One Tower Bridge turned to the VariCool EZ-Fit® self-contained modular unit by United CoolAir, created specifically for floor-by-floor VAV retrofits. The design uses three cooling stages with VZH variable-speed compressors in conjunction with variable-frequency drive fan speed control. The design enables a 4:1 turndown ratio for optimum capacity control. As a result, the unit efficiently handles mild weather requiring as little as 25 percent cooling capacity — but can ramp up to 100 percent capacity on the hottest days.

To optimize discharge air temperatures, the VariCool EZ-Fit’s variable capacity control and high turndown ratio employs advanced Danfoss inverter scroll compressors VZH. Danfoss also uses the CDS 303 VFD drive for VZH compressor motors. The drive enables fast acceleration from a standstill with a soft start ramp to managing minimum speed limits. The new systems resulted in 84 percent energy savings, reduced electricity consumption, and cut down on the noise caused by cycling on and off.