For example, packaged makeup air dehumidification units with reverse-cycle heat pump (RCHP) and enthalpy wheel options were specified recently for a retrofit at the Walker Upper Elementary School in Charlottesville. The packaged units were combined with a closed loop, water source heat pump concept the 33-year-old school employed to replace its existing two-pipe unit ventilator system.
A water-cooled, energy-recovery makeup air unit for air conditioning isn’t exactly a new concept in schools. However, adding a RCHP to eliminate a separate hot-water heating system, while also including an enthalpy wheel for energy recovery, including dehumidification (summer) and humidification (winter), is one of the first documented uses of it in a school application.
Consulting engineering firm Hanover Engineers, Mechanicsville, VA, specified the Dry-O-Tron® RK-Series energy-recovering makeup air dehumidifiers, from Dectron Internationale, Roswell, GA, including the custom additions.
The auditorium and gymnasium use two of these units each, and the cafeteria uses one for both heating and cooling as controlled by a building automation system from Novar Controls Corp., Barberton, OH.
“This (replacing unit ventilators with heat pumps and using makeup air dehumidifiers with RCHP and enthalpy wheels) appears to be a good, sound approach that has significantly improved the school’s indoor air quality as well as temperature control,” said Allen Lambert, vice president of mechanical engineering for Hanover.
Echoing Lambert is Marv Reese, the school district’s former facilities engineer who now advises the City of Charlottesville as a consultant for construction management firm SPN Inc., Rockville, MD.
“I feel this type of system is pretty efficient, and there’s a good chance the city will use it again in the retrofit of Charlottesville High School,” Reese added.
RCHP OPTION
The most significant savings in the $3.4 million retrofit, which was overseen by Charlottesville-based VMDO Architects PC, and managed by general contractor Martin Horn Inc., came from Lambert’s specification of the RCHP option in the makeup air units. The RCHP eliminated the need for new conventional boilers and other heating equipment for wintertime operation.Most of the school’s existing heating equipment was outdated and would have needed replacing if the RCHP option wasn’t specified. However, mechanical contractor L.A. Lacy, Charlottesville, did recondition a five-year-old boiler that now serves as the source of heat for the heat pump loop.
The outdoor air dehumidifier delivers dehumidified air to the space at a neutral dewpoint and temperature. This is accomplished by dehumidifying the air down to a 55¿ to 58¿F dewpoint temperature, and then using a hot gas reheat coil, reheat the air to 70¿ for classroom entry.
When air temperatures are below approximately 60¿F, the energy recovery wheel preheats the air and the Dectron units operate in reverse-cycle heat pump mode.
“Because of the relatively heavy density of people and the lighting load, schools often air condition when outdoor air temperatures drop to as low as 40 degrees,” explained Brian Cooper, a partner at the manufacturer’s representative firm Robert W. Hayes Co., Richmond, VA, which worked as a liaison between Lambert and Dectron’s custom engineering department.
“Between 40 and 70 degrees, classroom heat pumps are typically in the air conditioning mode, rejecting heat into the condenser loop. At the same time, the Dectron makeup air units operate in the heating mode, absorb heat from the condenser loop, and warm the outside air up to 70 degrees. This creates balance between the terminal heat pumps and the makeup air units, which is ideal for optimizing efficiency in a water source heat pump system.”
“The beauty of this type of system versus a simple unit ventilator system is each space has better temperature control,” added Fred Huckstep, a staff engineer with Lacy.
ENTHALPY WHEEL OPTION
Besides the RCHP, another cost-saving option is the enthalpy wheel, which eliminates the need for humidifiers in winter operation. The enthalpy wheel collects return-air humidity produced by room inhabitants and redistributes approximately 70% of it through the supply air to raise seasonably low relative humidity (rh) levels to American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) standards. In summertime operation, the enthalpy wheel preconditions high outdoor rh levels and reduces the mechanical dehumidification load.“The enthalpy wheel not only reduces the dehumidification load by over 50 percent, it also eliminates the need for humidifying equipment and does this while requiring less maintenance,” stated Cooper.
Most classrooms use seven unmodified RK-Series makeup air units that are combined with each space’s own heat pump, from The Trane Company, Tyler, TX, to provide heating and cooling. Hanover saved the school thousands of dollars by reusing the existing two-pipe system trunk and branch lines for classroom heat pumps, which were a mixture of direct replacement console units, some ceiling-mounted units, and a few large mechanical room air handlers.
Lambert’s design is said to save money in other ways, too. Water-cooled equipment is considered more efficient than its air-cooled counterpart because of the constant heat sink provided by a closed water loop. According to Cooper, the choice of water-cooled equipment also offers a longer life expectancy, because compressors operate at lower head pressures.
The units also offer a refrigerant subcooling coil, which precools liquid refrigerant with 58¿ air from the evaporator coil. This increases the refrigerant capacity, which subsequently improves the unit’s efficiency. In addition, Hanover saved money by renovating and reusing existing equipment such as the cooling tower by Marley Cooling Tower Co., Overland Park, KS.
The typical alternative to the chosen retrofit method is retaining the original unit ventilator strategy, but using new equipment that supports ASHRAE’s standard of 15 cfm per person of outside air. However, dehumidifying 15 cfm can be problematic for air conditioning equipment in hot, humid areas such as Virginia.
MAINTAINING AIR QUALITY
Using dehumidifiers combined with heat pumps is cost efficient and allowed Lambert to fulfill the school board’s request of bringing in 20 cfm per person to prevent indoor air quality problems associated with schools.“Indoor air quality … has many times been suspect in sick buildings, infectious virus spreading, and disease outbreaks, so the extra five cfm is a safeguard against that,” said Lambert.
Another reason other alternatives weren’t considered was because the existing building had little extra space to run ductwork from conventional air handlers, plus the original 33-year-old chiller needed replacement.
The school was completed in two phases. It is too soon to form opinions on the second phase. The first phase, which was completed a year ago, did include successful test classrooms that have led Lambert to the opinion that the overall strategy is a success.
The City of Charlottesville school district has approved a similar retrofit approach for its Buford Middle School. “The first phase that’s on-line at Walker right now seems to be working fine,” Lambert said. “This is a good concept that doesn’t fit every application, but it’s certainly a sound approach that might become more popular in future school hvac designs.”
Publication date: 10/22/2001
