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Montreal, May 1, 2007 - Dectron Internationale (TSX: DTL):, a leader in the heating, ventilation and air conditioning, indoor air security and water generation markets, is pleased to announce its financial results for the fiscal year ended January 31, 2007 (in thousands of Canadian dollars) ...
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| Dectron DRY-O-TRON |
August 16, 2001 |
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School Saves Equipment, Operating and Maintenance Costs with Make-Up Air Unit's Reverse Cycle Heat Pump, Enthalpy Wheel.
Consulting engineer's high tech design eliminates need for conventional heating and humidifying equipment.
Charlottesville, Va.- HVAC engineers are starting to realize that nearly anything can be specified into a custom make-up air unit.
For example, packaged make-up air dehumidification units with reverse cycle heat pump (RCHP) and enthalpy wheel options were specified recently for a HVAC retrofit at the Walker Upper Elementary School, Charlottesville, Va. 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 make-up 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 unique and one of the HVAC industry's first documented use of it in a school application.
Consulting engineering firm, Hanover Engineers, Mechanicsville, Va., specified the Dectron Internationale, Roswell, Ga., Dry-O-TronÒ RK-Series of energy-recovering make-up air dehumidifiers with the custom additions. The auditorium and gymnasium use two units each, and the cafeteria uses one for both heating and cooling as controlled by a Novar Controls Corp., Barberton, Ohio, building automation system.
"This (replacing unit ventilators with heat pumps and using make-up 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, Hanover Engineers, a 24-year-old mechanical, electrical, structural and data communications engineering firm that has experience in more than 100 school HVAC designs.
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 upcoming retrofit of Charlottesville High School," Reese added.
The most significant savings in the $3.4 million HVAC retrofit, which was overseen by Charlottesville, Va.-based VMDO Architects PC, and managed by general contractor, Martin Horn Inc., came from Lambert's specification of Dectron's RCHP option in the make-up 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, Va., 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 dew point and temperature. This is accomplished by dehumidifying the air down to a 55°-58°F dew point temperature and then using a hot gas reheat coil to reheat the air to 70°F for classroom entry.
When air temperatures are below approximately 60°F, the energy recovery wheel pre-heats the air and the Dectron units operate in a reverse cycle, heat pump mode. "Because of the relatively heavy density of people and lighting load, schools often air condition when outdoor air temperatures drop to as low as 40°F," explained Brian Cooper, a partner at the manufacturer's representative firm, Robert W. Hayes Co., Richmond, Virginia, who worked as a liaison between Lambert's specifications and Dectron's custom engineering department. "Between 40°F and 70°F classroom heat pumps are typically in the air conditioning mode, rejecting heat into the condenser loop. At the same time, the Dectron make-up air units operate in the heating mode, absorb heat from the condenser loop and warm the outside air up to 70°F. This creates a degree of balance between the terminal heat pumps and the make-up 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 at the 80-year-old Lacy.
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 percent 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," added Cooper.
Most classrooms use seven unmodified RK-Series make-up air units that are combined with each space's own Trane Co., Tyler, Texas, heat pump 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 saves money in other ways, too. Water-cooled equipment is more efficient than its air-cooled counterpart because of the constant heat sink provided by a closed water loop. Air-cooled equipment's efficiency however, drops as fluctuating outdoor temperatures drop. The choice of water-cooled equipment also offers a longer life expectancy, because compressors operate at lower head pressures, according to Cooper.
The units also offer a refrigerant sub-cooling coil, which pre-cools liquid refrigerant with 58°F air from the evaporator coil. This increases the refrigerant capacity, which subsequently improves the unit's efficiency.
Hanover also saved the project additional money by renovating and reusing existing equipment such as the on-site cooling tower by Marley Cooling Tower Co., Overland Park, Kan.
The alternative to Lambert's chosen retrofit method is retaining the original unit ventilator strategy, but using new equipment that supports the ASHRAE's recommended 15 cfm per person of outside air standard for schools. However, dehumidifying 15 cfm can be problematic for simple air conditioning equipment in hot, humid areas such as Virginia. Using dehumidifiers combined with heat pumps is more cost efficient and allowed Lambert to fulfill the school board's request of bringing in 20 cfm per person as a guarantee against indoor air quality problems associated with schools. "Indoor air quality in schools across the country has many times been suspect in sick buildings, infectious virus spreading, or disease outbreaks, so the extra 5 cfm is a safeguard against that," said Lambert.
Another reason the 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 Walker School was completed in two phases. The soon-to-be-completed second phase hasn't been on line long enough to form opinions. The first phase, which was completed a year ago, did include successful test classrooms that leads Lambert to the opinion that the overall HVAC strategy is a success. The City of Charlottesville school district has already approved a similar HVAC retrofit approach for its Buford Middle School. "The first phase that's on line at Walker right now seems to be working fine," added Lambert. "This is good concept that doesn't fit every application, but it's certainly a good sound approach that might become more popular in future school HVAC design."
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