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 over 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.”
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 upwards 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 temperatures to meet the requirements of the VAV boxes in the underflow circuits, but that also wastes energy.
“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 pressure-independent control valves. They replace the typical two-way balancing valve and control valve 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.
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 RPMs 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 part-load 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.
McGraw also found the chiller can handle less chilled-water flow 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 significant 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 compressor motors, changing the frequency of the current changes pump speed to match flow requirements. The RPMs of the pump must be variable to allow a slower speed when there is little demand for water and a higher speed when more water is needed. This allows the operator to match pump speed with water demand, which saves energy and reduces stress on components.
VFD technology is also used on fan motors. Each cooling-tower fan incorporates a VFD. The exhaust- and 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.