Elementary” HVAC

High-efficiency HVAC technology tames temperature and humidity at Florida school.

Built in 1921, A.K. Suter Elementary School, Pensacola, FL, used a retrofitted patchwork of packaged and split-system air-conditioning equipment. To keep students comfortable in a warm, humid climate, the Escambia County School

District (ECSD) decided to rebuild the school from scratch using new, high-efficiency

HVAC technology.

“The walls are constructed with insulating concrete forms (ICF) and a vapor barrier, so the building envelope is well insulated” said Roger McGraw, P.E., mechanical engineer for ECSD facility planning. “The new HVAC system is state of the art. Two ultra-efficient variable-speed centrifugal chillers with Danfoss (Tallahassee, FL) Turbocor oil-free compressors ensure efficient chilled-water production. The chilled water is supplied to several air-handling units (AHUs) and more than 100 variable-air-volume (VAV) boxes, each using Danfoss AB-QM pressure-independent balancing and control valves to optimize flow. This combination of technologies saves energy and handles our major comfort challenge—humidity.”

AIR YOU CAN WEAR “We’ve got ‘air you can wear’ from April through October,” said Jeremy Oksanen, the project’s system designer and mechanical engineer with Premier Engineering Group, Pensacola, FL.

According to McGraw, “For elementary schools, our dry-bulb setpoints are 72 F cooling and 70 F heating. But our biggest challenge is always how to deal with humidity in an efficient manner. We can get into situations where a room is 72 F, but everybody thinks it’s hot and clammy because it’s just so humid in there.”

“In the old school, we had split-system heat pumps and some packaged rooftop units. However, they were all very old, so we did not have that much control. Due to the humidity, the equipment was in all-cooling mode all the time. That’s one reason the energy bills were high,” he said.

The second biggest problem McGraw hoped to solve was zoning. “Zoning is critical for comfort,” he observed. “When you try to create separate zones for different areas, it seems somebody, somewhere, is uncomfortable most of the time.”

Chilled water is supplied to several air-handling units (AHUs) and more than 100 variable-air-volume (VAV) boxes, each using Danfoss AB-QM pressure-independent balancing and control valves to optimize flow, save energy, and handle the major comfort challenge—humidity.

Inset. The Escambia County School District decided to rebuild the A.K. Suter Elementary School in Pensacola, FL, from scratch using new, high-efficiency HVAC technology.

DESIGNING A VAV SYSTEM “We put a lot of thought into how to design a system that combines efficiency and comfort,” said Oksanen. “To deliver conditioned air to classrooms, this design is based on using single-duct VAV boxes.” Each single-duct VAV box used for this project is ducted to an air handler that contains a cooling coil with a 51 F to 53 F setpoint—cold enough for sufficient water vapor to condense on the coil for dehumidification, and then a reheat coil at the VAV box with a variable setpoint to raise the air temperature to avoid overcooling. The supply air temperature is reset upward, based on humidity levels and valve position to reduce reheat.

The cooling coils and reheat coils in the VAV boxes are connected to a four-pipe, variable primary hydronic system. Variable-speed pumps drive the chilled or hot water through the chiller or boiler loop to supply the coils inside the VAV boxes or AHUs. One pair of pipes circulates chilled water to coiling coils; another pair circulates hot water to reheat coils.

Oksanen points out that if the system is unbalanced, VAV boxes on some circuits will receive more water than required. In effect, those circuits “steal” flow from other circuits. The first circuits are in overflow, which creates underflow in other circuits. The underflow VAV boxes do not get sufficient chilled or hot water to meet the cooling and heating requirements.

Pushing pumps to the maximum is one way to solve the underflow problem. However, this simply increases pressure to increase total flow, which increases pump energy consumption and stresses pumps and valves. Another tactic is to adjust supply water

FIRST IMPRESSIONS ARE LASTING IMPRESSIONS

“BEST IN CLASS” Higher Education

Kent State University CAED Building

The Best Impressions are Made with Brick

Choosing brick over other alternatives is the foundation for a smart, long-term investment in durability and beauty. Brick gives you the highest quality, energy effi cient, environmentally friendly product over other structural building materials. It has a life expectancy of hundreds of years. Belden Brick is the unparalleled choice above all other material options. Don’t settle for less. Ask for the best. Ask for the industry leader delivering the largest selection of more than 500 colors, 20 sizes, 13 textures and unlimited shapes. Belden Brick can meet all your building needs with the time-honored quality and experience the Belden name represents.

An ISO 9001 Compliant Quality Management System. An ISO 14001 Compliant Environmental Management System. beldenbrick.com

temperatures to meet the requirements of the VAV boxes in the underflow circuits, but that also wastes energy.

OVERFLOW/UNDERFLOW BALANCE “When you have variable primary systems, we can never depend on manual balancing valves,” McGraw observed. “That’s because the system rarely operates in the same conditions to allow manually balancing the system.”

“Instead, we like using Danfoss AB-QM pressureindependent control valves. They replace the typical two-way balancing valve and controlvalve pair that usually get installed on the return flow side of VAV cooling and heating coils,” he said.

To balance the system, the valves incorporate an integrated differential pressure controller that enables stable control with 100% “authority”—a term that means the AB-QM valve has complete control of the pressure drop in the system. As a result, at partial loads, there is no overflow because the valve will always limit the flow to exactly meet requirements. From a system design standpoint, installing the valves divides the entire system into completely independent control loops.

Segmenting the hydronic distribution piping into independent loops or “modules” ensures the design flow—which, in this case, is typically 6 gpm—is available at all the VAV terminal units at design-temperature setpoints. Thus, the AB-QM valve is the key that unlocks the three requirements for optimal system balancing by: supplying design flow to all terminal units at design conditions; minimizing variation in the differential pressure (pressure drop) across the control valve; and ensuring the water flow is compatible components and interfaces.

Another benefit is the energy efficiency resulting from proper valve seating. If hot water is leaking through the valve into the reheat coil, the air is being heated needlessly. Consequently, the damper on the VAV box opens to supply more and more air for cooling, so people are comfortable, but, it’s wasting a ton of energy. In contrast, the AB-QM valve automatically knows how to seat itself to a fully closed position. PART-LOAD CONDITIONS To generate chilled water at the 44 F setpoint, two water-cooled centrifugal chillers are used, each using two Danfoss Turbocor TT400 variable-speed, magnetic-bearing compressors. Each pair of compressors delivers 250 tons of nominal cooling capacity per chiller.

Together, the two chillers provide what is known as “N+1 redundancy,” meaning that one chiller is available as a standby or backup, as well as being available to provide additional capacity in the event of school expansion.

“Part-load efficiency is really important in this application,” said McGraw. “The ICF building envelope is very effective, so ambient conditions outdoors have little effect on the cooling load.”

McGraw calculates that the lead chiller runs at 60% capacity 85% of the time, and below 20% capacity nearly a third of the time. It goes above 80% capacity only 3.3% of the total operating hours. “A constant-speed centrifugal chiller is spinning the centrifugal impeller at maximum rpm regardless of outdoor conditions,” said Oksanen. “When full cooling capacity isn’t required, mechanical throttling vanes or valves can be used to reduce the chiller’s capacity. However, the motor is still running at full rpms, which wastes energy.”

With the Turbocor compressors, the shaft/impeller speed is reduced and—in combination with the inlet guide vane assembly—capacity can be turned down to match the cooling load required. To reduce speed quickly and reliably, the compressor uses a synchronous permanent-magnet brushless motor. Each motor is integrated with a variable-frequency drive (VFD) that controls the voltage and amperage. VFD technology makes it easy to change speed by reducing the frequency of the current supplied to the motor. The drive varies frequency between 300 and 800 Hz, which provides a compressor-speed range from 9,000 to 29,000 rpm without using a gear set.

Efficiency is further enhanced by the oil-free magnetic bearings, which eliminate the friction associated with using traditional contact bearings. The absence of oil lubrication eliminates the efficiency losses that can occur when oil fouls a chiller’s heat exchanger tubes. Tube fouling decreases heat transfer. One study shows that as little as 3.5% oil content in a refrigerant charge can reduce efficiency as much as 8%.

Because 80% and higher capacity is required for only about 3% of the chiller’s operating hours, the high partload efficiency means the chiller is saving energy during 97% of annual operating hours. For the A.K. Suter application, the Turbocor-enabled chillers are consuming only about .45 kW/ton—nearly 50% less than a constant-speed chiller.

VARIABLE FLOW McGraw also found the chiller can handle less chilled-water fl ow than other chiller designs. “The chiller brand we went with doesn’t require as much water and, therefore, uses less pressure (lower pressure drop) to move water through the evaporator. That becomes signifi cant over time. After 20 years of pumping, if you can reduce head pressure across the chiller by 10 ft., you’ll save a lot of pump energy.” Energy is also saved by using VFDs on all eight pumps used in the system (one VFD per pump). Similar to how a VFD is used on com

pressor motors, changing the frequency of the current changes pump speed to match flow

requirements. The pump rpms must be variable to allow a slower speed when there is little demand for water and a higher speed when more

Two variable-speed centrifugal chillers with Danfoss Turbocor oil-free compressors ensure effi cient chilled-water production. water is needed. This allows the operator to match pump speed with water demand, which saves en

ergy and reduces stress on components. VFD technology is also used on fan motors. Each cooling-tower fan incorporates a VFD. The exhaustand outside-air fans associated with the energy recovery use VFD to ensure proper airflows due to filter loading and other conditions. All AHU fans are multi-zone variable-volume or single-zone variable-volume designs using direct VFD motors.

In the end, McGraw was delighted to see new A.K. Suter Elementary School finish at the top of the class from an energy and comfort standpoint. “The yearly energy use for old elementary schools in ECSD is more than 0.080 MBTU/sq. ft. I was hoping the new school would drop at least 50% to below the elementary school average of 0.045 MBTU. The performance is really stellar—just 0.024 MBTU/sq. ft. per, far below any public elementary or high school in Escambia County. CA

Want more information from Danfoss (danfoss.us)? Circle 6 on the Reader Service Card. Sources listed below are linked at

commercialarchitecturemagazine.com/1901danfoss.

Escambia County School District, ecsd-fl .schoolloop.com Premier Engineering Group, linkedin.com/company/premier-engineering-group-llc

Good chemistry.

Better chemistry. Better stone. Calcium silicate’s internal chemistry is di erent from that of concrete and far superior. Our unique process delivers the most durable, strongest product on the market.

EDITORS’ CHOICE

Combination, heat-only boilers

AquaBalance Series 2:

Wall-mount combination/heat-only boilers Titanium-infused heat exchanger technology Retrofits or new construction

AquaBalance Series 2 wall-mount combination and heat-only boilers have an energy-saving AFUE rating of 95%. The updated units combine the warmth of high-efficiency space heating with the convenience of on-demand domestic hot water (combi version) in a reliable, space-saving, and reportedly easy-to-install package. The units use stainless-steel, titanium-infused heat exchanger technology, with applications for retrofits or new construction in apartments or condominiums. The boilers are available in 80-, 120-, and 155-MBH input, in combi or heat-only versions. The 155-size combi-boiler produces a constant supply of hot water of 4 GPM at a 70-deg. F temperature rise. The unit has the power to supply hydronic heat for as much as a 2,500 sq.-ft. area.

Weil-McLain, Burr Ridge, IL Circle 58 weil-mclain.com

Pre-treatment for evaporative equipment Water Saver is a capacitive deionization pre-treatment system to improve the efficiency of heat transfer and water efficiency for evaporative cooling systems. The capacitive deionization technology reduces dissolved ion concentrations to lower makeup water conductivity prior to use in evaporative cooling systems. Dissolved ions are removed from make-up water as they are attracted to the charged capacitors. The system improves water efficiency by increasing operating cycles of concentration, reducing blowdown and the use of treatment chemicals.

Evapco Inc., Westminster, MD Circle 59 evapco.com

Faucet collection Offering design flexibility, the Atrio bath faucet collection provides a choice of cross or lever handles, a variety of configurations, and a complete selection of accessories. The collection is available in StarLight chrome and brushed nickel Infinity finish. The EPA WaterSense-certified lavatory faucets have a water-efficient flow rate of 1.2 gpm and are compliant with CEC regulations in California.

Grohe America, Roselle, IL Circle 60 grohe.us

Extended-capacity chiller The York YZ magnetic-bearing centrifugal chiller includes chillers beyond 1,000 tons (3,515 kW) to 1,350 tons (4,747 kW) of refrigeration. The extended capacity, AHRI-certified chiller uses an integral, variable-speed drive and advanced magnetic-bearing technology that features a single moving assembly suspended in a magnetic field that does not require lubrication. This technology requires 80% fewer moving parts than traditional oil or refrigerant-lubricated drivelines.

Johnson Controls, Milwaukee Circle 61 johnsoncontrols.com

HVLS ceiling fans Model DS direct drive, high-volume, low-speed ceiling fans have an optimized airfoil design for maximum efficiency and coverage area. Available in eight sizes from 8 to 24 ft. in diameter, with performance to 243,000 cfm, the fans include a 3 1/2-ft. drop tube with universal ceiling mount.

Greenheck, Schofield, WI Circle 62 greenheck.com

Basin designs Three basin designs are additions to the Sundara handwashing system. Tide, Surf, and Inlet include single, double, and triple configurations and join the established Reef and Drift models. The system is available in seven basin colors across the five designs. Sensor faucet and soap dispensers are available in multiple finishes. The system can be specified with an in-wall carrier system.

Zurn Industries, Milwaukee Circle 63 zurn.com

Circular-flow ceiling cassettes The RGLX series Halcyon circular-flow ceiling cassette system uses a DC twin rotary compressor, DC fan motor, and refrigerant cycle control. The units reported achieve as much as 21.4 SEER. Using 360-deg. louvers, the airflow design allows conditioned air to reach every corner of an area. Each louver can be individually programmed to customize airflow for maximum comfort and space layout. A four-speed fan control provides sound levels in Quiet Mode as low as 28 dB.

Fujitsu General America, Fairfield, NJ Circle 64 fujitsugeneral.com

Fabric duct, diffuser systems A highly customizable line of fabric air duct and dispersion systems provides facilities with precise control of indoor environments. The systems are useful in industries that require strict temperature and humidity control, such as airports, sports facilities, and other commercial spaces. The entire run of a fabric system acts as a diffuser providing uniform airflow along the length of the system. Various methods achieve this uniformity, including air-porous fabrics, linear vents, nozzles, and orifices. Fabric systems are customized for each application and are designed for precision airflow and targeted air speeds.

DuctSox Corp., Dubuque, IA Circle 65 ductsox.com

Vertical-blade dampers Part of the TED50 line, the TED50V and TED50VXT are low-leak, thermally efficient dampers with insulated extruded-aluminum vertical blades. Designed for reliable performance in cold temperatures, each blade has a thermal break strategically placed between twin-blade edge seals. This creates a neutral zone that eliminates thermal transfer and reduces the potential for condensation. The TED50V uses Ruskiprene blade and jamb seal material. The TED50VXT is designed to protect against extreme cold temperature conditions by using mechanically fastened extruded silicone on the blade seals and ribbed-extruded silicone on the jamb seals.

Ruskin, Grandview, MO Circle 66 ruskin.com

Domestic hot-water recirculation Hot-LinkPlus-e offers high-efficiency, intelligent domestic hot-water recirculation. A dedicated return line is not required. The unit combines a 006e3 ECM hot-water circulator with a Hot-Link valve and Smart Plug Instant Hot Water Control in one package. The HotLink valve, installed at a building’s furthest water fixture, sends cooled water back to the water heater so hot water lines remain hot.

Taco Comfort Solutions, Cranston, RI Circle 67 tacocomfort.com