Although it’s the second-oldest college in America, the College of William & Mary boasts the oldest college building in the United States, the Sir Christopher Wren Building, which was built in 1695 before the founding of nearby Williamsburg, VA. Now fast-forward to the 1960s, when the college expanded to accommodate Baby Boom students. That’s when Yates Hall was constructed, with 10 halls divided between three floors and a basement to house 266 freshmen. Today, the dormitory is also used for summer camps, which required the college to deal with an age-old problem: humidity.

“Humidity is a major issue in Williamsburg during the summer,” says Mark Ballman, mechanical project manager in the college’s Department of Facilities Management. “We’re located between the James and York rivers and just a few miles from the Chesapeake Bay. In Yates Hall, running the existing chilled water fancoil units didn’t deal with the high humidity. In fact, condensation and mold were beginning to appear on the ceilings.”

To solve the humidity problem, Ballman solicited bids for a 100% outside air makeup air-handling system during the summer and inter-seasonal periods when humidity is uncomfortable.

Excess humidity was eliminated at this college with a modulating hot gas reheat system.

“We proposed two packaged DX rooftop units in our X-DRY AC Series, which utilize Danfoss VSH variable-speed compressors,” says Anthony Palucci, director of sales for Annexair. “Other packaged units typically use fixed-speed compressors. Then they increase the cooling time to remove more water vapor from the supply air. But this design overcools the air.”

To take care of that problem, a humidistat operates a three-way valve to bypass hot refrigerant gas into a reheat coil placed after the cooling coil. The reheat coil raises the supply air temperature to the desired drybulb temperature. But cycling fixed-speed compressors on and off, in conjunction with hot gas bypass reheat, is very inefficient.

In contrast, the X-DRY design uses VSH variable-speed compressors and a process known as modulating hot gas reheat.

“It’s no secret that 100% makeup air systems using bypass hot gas reheat are costly to operate,” observes Palucci. “But with the X-DRY system, we use a modulating gas reheat valve to divert only the required amount of hot refrigerant gas to the reheat coil. We also employ variable-speed condenser fans to control head pressure on the refrigerant circuit. Finally, we take advantage of the speed control of Danfoss VSH compressors to deliver only the required cooling capacity.”

Annexair provided two X-DRY rooftop makeup air units (MAU). MAU-1 used two 23-ton VSH compressors, providing a maximum 46 tons of cooling and 5,000 cfm; MAU-2 employed four 23-ton compressors, providing 84 tons of cooling and 10,000 cfm.

The units only operate when the outside air temperature is above 55°F, supplying air at 70° drybulb and 50° dewpoint. The dry air is circulated from the two makeup air units into two plenums located in the hallways of each floor. Below 55° ambient, the system reverts back to the existing two-pipe fancoil units for heating.

“We selected VSH series compressors for our R-410A X-DRY systems, because they are a variable-speed scroll designed specifically for air conditioning and heat pumps,” says Palucci. “They come as a completely integrated package with all the features that make it easy to integrate into our systems.”

Electrically, the CDS302 VFD incorporates harmonic filters that practically eliminate harmonic distortion. A high power correction factor (over 0.90) avoids utility penalties. Plus, a patented oil injection system ensures operation at peak efficiency by maintaining sealing of the scroll sets at low speeds and cooling at high loads.

But by using two 23-ton compressors, the Annexair MAU-1 unit doubles the capacity range and turndown ratio, from a maximum of 46 tons down to seven tons for a 6:1 turndown ratio. And by using four 23-ton compressors, the MAU-2 can go from 84 tons to seven tons for a 12:1 turndown ratio.

Because about 90% of operating hours occur at part-load conditions, the turndown ratio allows compressor speed to be reduced to match lower capacity requirements. Consequently, variable-speed compressors can operate within +0.2° of setpoint. Fixed speed compressors, on the other hand, have to run at full speed and use either on/off cycling or other capacity-reduction method that wastes energy.

The high turndown ratio also helps meet the project’s number one goal: dehumidification.

“When a compressor can reduce capacity, it’s as if the size of the cooling coils becomes thermodynamically larger,” explains Palucci. “Anytime you expose the airstream to more coil surface area, you pull more water vapor out of the air.”