Diffusers star in Five Points Plaza's pursuit of more efficiency in tight spaces
When the 17-story, 129,000-sq-ft building was built in 1962, it was a prestigious office building in the heart of downtown Atlanta, and it possessed a state-of-the-art hvac system for that time. Floors 1 through 17 were served by two centrifugal chillers; two hot water boilers; dual pumps for chilled, hot, and condensed water; a central station AHU; and a central return air fan - all located in a penthouse equipment room.
A three-pipe, hot and chilled water piping system fed heating and cooling radiant ceiling panels zoned with 14 three-way control valves on each floor. The AHU supplied a constant volume of treated air through a vertical supply air duct with takeoffs at each floor where it was distributed through light troffers to the occupied space. Return air was run through the vertical shaft containing the supply air duct.
Keeping up with the competitionBy 1996 the building faced stiff competition for occupants. New and renovated buildings were taking business away and Five Points' inefficient lighting and high maintenance costs associated with the deteriorating radiant panel heating and cooling system mandated a complete upgrade of the mechanical and electrical systems.
The consulting engineers of Morris E. Harrison & Associates Inc. (MEH&A, Norcross, GA) were retained to determine how it could be done. The firm's president, Morris Harrison Sr., says the system was in dire need of replacement not just because of its age, but also because parts of the system had been bypassed over the years. "Since the ceiling cavity space was very limited in the building, there weren't a whole lot of options," he recalls.
MEH&A found that all of the central station equipment was still serviceable, but that the radiant ceiling panel system would have to be replaced. There could be enough static pressure in the last branch for a low-pressure vav terminal using the existing air ducts if the AHU and return air fans could be upgraded to provide an additional 15% capacity.
The trunk ducts on each floor passed through sleeved openings in structural concrete beams extending to within 6 in. of the ceiling and could not be enlarged. A duct analysis showed that by increasing the static pressure in the main supply air riser, providing a pressure-independence station (Acutherm static pressure controller and damper) at the duct takeoff to each floor, and enlarging some of the diffuser branch ducts, the duct system would be able to provide design air in the last branch, but only at a low pressure.
At the same time, existing fan performance curves showed that fan capacities could be increased 15% by installing a larger, premium-efficiency motor. Fan static pressure control could be maintained by the addition of vfd's.
Low-pressure Therma-Fuser(tm) thermally powered modular vav diffusers offered a workable solution. They also provided individual temperature control, adaptability for future tenant layouts, and low maintenance - all prime design objectives.
Existing radiant panels, valves, piping, and light troffers were all removed. Fan motors were replaced and vsd's added. Therma-Fuser 320 Model TB-C (cooling only) modules were installed in interior spaces and 352 Model TF-HC (vav heating/vav cooling) Therma-Fuser modules with minimum flow stops were used in all perimeter spaces. Runouts to TF-HC units were provided with an electric duct heater controlled by a wall thermostat set approximately 4 degrees F below the TF-HC cooling setpoint.
Results: Three years laterThe system has been in operation for almost three years following a smooth installation (in the nearly empty building) by Richard Bowers and Co., an affiliate of the company that bought the building, RB Development and Property Services (Atlanta). According to Daniel Brawley, vice president of operations for RB Management Services, Inc., the building's hvac performance before the retrofit and extensive renovations and its operation after are incomparable.
The building's occupancy, which was at about 30% before, is now 100%, and so many other things changed in the building that a before-and-after comparison is truly an apples-to-oranges situation. However, Brawley does allow that "assuming the way things were previously, we have surely saved a lot of money." "Also," he adds, "the number of complaints in that building are well within the scope of what we expect in a Class 'A' office building."
The building, which was seemingly a short step from obsolescence, is now occupied by hundreds of government workers who don't lodge many "hot or cold calls," the most direct measure of occupant satisfaction.ES