The elegant One Tower Bridge is a landmark, 15-story office building located in West Conshohocken, Pennsylvania. To keep its competitive edge, it was clear the owners had to replace the nearly 30-year-old HVAC units supplying the facility. 

“We faced replacing 15 45-ton packaged HVAC units originally bought from Blazer Industries in 1988,” said John Sargent, building engineer for One Tower Bridge. “Transporting new, complete units to each floor was impossible. While the freight elevators are large, they certainly cannot transport a complete air-handling system. Removing windows or walls was also impossible. So, we were searching for a modular replacement unit that could fit our space restrictions and meet our particular performance requirements.”

Considering Self-Contained HVAC for VAV

The building offers 270,000 square feet of office space for tenants in all industries. To meet their comfort needs, the facility employs floor-by-floor, self-contained HVAC units for a 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 to regulate maximum and minimum volumes of conditioned air supplied to the space. The settings in each self-contained unit and each terminal can be individually adjusted.

That’s important because Philadelphia experiences wide swings in temperature and humidity. And like many buildings, One Tower Bridge is exposed to significant amounts of sunlight on two sides of the structure, creating an imbalance in solar heat gain between the north and south sides.

Mechanically Modulated Capacity Scroll Compressor Switchout

In 2015, One Tower Bridge replaced a couple of the original self-contained, water-cooled HVAC units that use reciprocal compressors with mechanically modulated capacity scroll compressors. Currently, each unit contains two mechanically modulated capacity scroll compressors and two conventional scroll compressors.

To handle varying loads, the units are configured to operate in two stages. Staging is split down the middle — the first stage runs two mechanically modulated capacity scrolls, one on each of the left and right sections; the second stage runs a conventional fixed-speed compressor located in each section.

However, this solution proved less than ideal. Only 60-ton units were available from the manufacturer instead of the required 45 tons. Consequently, during stage two, compressors were going off on alarm due to overcooling. As a workaround for the overcapacity, hot gas bypass was added to the stage-two compressor circuit, which was costly and reduced efficiency.

Another issue was noise. During stage-one operation, the mechanically modulated capacity scroll compressors emitted a clanging sound that couldn’t be blocked by acoustic blankets.

Variable-Speed Compressors 

“Initially, there were slim pickings to find a floor-by-floor solution that could meet all the requirements,” said Robert Miller of Coward Environmental Systems, the manufacturer’s representative providing One Tower Bridge’s mechanical systems. “But, in 2016, I became aware of another manufacturer offering modular units — United CoolAir. When John Sargent and I toured their plant, we immediately saw their new VariCool EZ-Fit® self-contained modular unit was a better solution.”

The VariCool EZ-Fit product line ranges from 12 to 90 tons, so a 45-ton selection was available. Additionally, the technology is engineered specifically for floor-by-floor VAV retrofit projects. It’s designed in three sections that easily mate to form a system module. Single modules are composed of a fan, an evaporator coil, and a condensing section. For this application, one module supplies the north side of the building, and another module supplies the south side.

The sections were completely assembled during manufacturing, charged with R-410a refrigerant, run tested, and then separated for shipment.

For this application, key custom components included electronically commutated motor (ECM) fans and Danfoss VZH variable-speed scroll compressors with VLT® compressor drive CDS 303 variable-frequency drives.

In November 2017, two VariCool EZ-Fit modules were installed on the third floor. Each module employs one 25-ton VZH compressor on one circuit and a conventional single-speed scroll compressor on the other three circuits, each cycling sequentially as more capacity is needed.

Another two VariCool EZ-Fit modules using a slightly different configuration were installed on the first floor. Each of these modules employs one 25-ton VZH scroll compressor on the first and third circuits and a fixed-speed, 20-ton scroll compressor on the second and fourth circuits. 

Stage-one cooling activates the two variable-speed compressors, stage-two cooling runs the regular scroll compressor, and then stage-three cooling runs all four compressors.

By adding a second variable-speed compressor, 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% cooling capacity — but can ramp up to 100% capacity on the hottest days.

Inside Advanced Compressor Capacity Control

There are significant differences in how compressors function. 

“Danfoss VZH compressors use a VFD to condition the current to ramp speed smoothly to match the load more precisely,” Miller said. “Danfoss also uses the CDS 303 VFD drive [also known as an inverter]. It enables fast acceleration from a standstill with soft start ramp to managing minimum speed limits. The result is optimum efficiency across the entire operating range.” 

Tweaking Comfort for Each Floor

“Danfoss came to our factory to ensure compressors, controls, expansion valves, and other components were properly integrated,” said Brad Dunn, national sales manager, United CoolAir.

The company provided full functional testing as well, including VS ramping and vibration/resonance analysis.

“Danfoss also helped us tweak the design to apply VZH compressors on two circuits instead of one to optimize balancing both sides of the building,” Dunn added. “As a result, we were able to build an efficient and flexible solution with significant capacity, size, and sound advantages.”

One Tower Bridge purchased additional dual variable-speed-compressor units for the seventh and 11th floors. The building now consists of the older units with reciprocating compressors, two Mammoth units with mechanically modulated capacity scrolls, and the four VariCool units with Danfoss VZH scroll compressors.

Monitored electricity usage from April 2018 through April 2019 was: 

• 22,127 kW for VariCool EZ-Fit units with Danfoss VZH compressors;
• 34,330 kW for units with mechanically modulated capacity scroll compressors; and 
• 38,292 kW for the older units with reciprocating compressors.

“The VariCool EZ-Fit design with dual VZH compressors is ideal for our building,” Sargent said. “This retrofit hits our temperature and humidity set points precisely. They reduce electricity consumption and noise, which our tenants value.”