Don’t Let Air Leaks Rain on the Chilled Beam ParadeBilly Pell
When engineers first began designing the new Bill and Melinda Gates Hall at Cornell University, they reviewed a variety of HVAC strategies. They knew that with the name Gates on the entranceway of its new computer science building, the structure had to perform flawlessly. After careful analysis they designed an HVAC system that incorporated an assortment of innovations headed by active (ACB) and passive (PCB) chilled beam technology.
Chilled beam systems use cold water circulating through pipes to cool the surrounding air. With the unit installed in or around the ceiling, the air it cools becomes dense and falls towards the floor. The vacuum created by the falling cool air is replaced by rising warmer air, which comes in contact with the chilled beams, and the process continues over and over, causing a constant airflow. Read more about chilled beam systems on Buildings.com.
“There are two basic types of chilled beam systems in use today; active and passive – the primary differentiator being that active systems introduce fresh outside air into the equation via ventilation ductwork connected to the unit,” said Aeroseal’s Neal Walsh.
“The outside air allows for additional cooling capabilities and eliminates many of the IAQ issues associated with using recirculating air. Through the use of dampers and other mechanisms, the introduction of outside air also allows for easier system adjustments.”
The Bill and Melinda Gates Hall incorporated multiple ACB and PCB units throughout the building with active chilled beams placed in classrooms, small conference rooms and computer labs, passive chilled beams used in perimeter and interior offices, and a combination of the two used in rooms with high cooling loads.
If designed and constructed properly, chilled beams can offer several advantages over traditional VAV systems, the primary one being energy efficiency.
In order for treated air in a VAV system to arrive at the proper temperature, it must first be over-cooled. This allows for any temperature dissipation while traveling through the ducts to the intended destinations. Since chilled beam systems treat the air in and around the location where it is being used, there is no need for delivery ducts, no temperature dissipation, no over-cooling – all of which adds up to reduced energy use.
Also, without the need for large fans to circulate the treated air, chilled beam systems require even less energy to operate. The amount of energy saved through minimizing fan power far exceeds any increased energy associated with pumping water though the system. And since the temperature of cooled water is higher than the temperature of cooled air, chilled beam systems are able to deliver the same cooling ability at a much lower cost.
Chilled Beams vs. Humidity
At the same time, chilled beam systems have their drawbacks, primary of which is their sensitivity to humid environments. To avoid condensation forming and dripping from the chilled beams, the surrounding air must sustain a proper low level of humidity. Maintaining the dew point of the indoor air below the surface temperature of the chilled beam is critical – and requires the careful calculation of various factors.
“It really is a balancing act,” said Rod Lord, managing director of SEED Engineering. “The temperature of the space, the number of occupants, air humidity and the amount of outside air being delivered to the system, all play a role in the proper functioning of a chilled beam system. Changing one of these calculations can make the difference between a highly effective HVAC system that provides a comfortable, high air quality environment, or one that creates an untenable mess.”
Unfortunately for the owners of a luxury high-rise in the heart of North Sydney, Australia’s business district, the latter was the case. The 23-story building’s VAV / chilled beam hybrid system never provided the energy efficiency it was designed to deliver. Tenants of the state-of-the-art building were never happy with the AC, and energy costs were well beyond initial design levels. Any adjustments to the system pushed it beyond safe dew point levels where condensation threatened to create a disastrous effect.
After conducting an analysis of the building, Lord and his team at SEED concluded that duct leakage was at the root of the problem. Leaks throughout the 20 vertical shafts and hundreds of square feet of horizontal ductwork prevented the required amount of air from reaching the chilled beam units.
“…leaks in the duct system were preventing sufficient air from reaching the beams…”
“Chilled beam systems are particularly sensitive to external factors, and since leaks in the duct system were preventing sufficient air from reaching the beams, they were unable to provide effective cooling and worse, there was condensation dripping from the various units,” said Lord. “The system would never function properly unless those leaks could be sealed.”
A similar issue arose during an elementary school renovation project in Northern Maryland. Engineers working with Prince George’s County Public Schools Department installed a state-of-the-art chilled beam system into the school, for energy-efficient heating and cooling. Despite detailed planning, design and analysis, the engineers couldn’t get the installed system to function correctly.
“The engineers suspected poor duct sealing was the root cause of the problem,” said Walsh, “and our testing verified that significant leakage did exist. Not only was duct leakage evident throughout the new trunks and branch lines, but the flex lines connecting the branches to the chilled beams were particularly leaky.”
Without the proper amount of fresh air reaching the chilled beams, the system was unable to provide sufficient amounts of treated air to the surrounding space. Excessive humidity caused condensation around the beams, and evidence of water dripping from the ceiling halted the project until the problem could be resolved.
“As with the Arthur Street project, once we confirmed that duct leakage was the main cause, fixing the problem was a straight forward operation for Aeroseal,” said Walsh. “The duct sealing process quickly located and sealed the leaks, and the computer-controlled system monitored results as they happened. Following setup, it was only a matter of hours before each section of ductwork was verified to be virtually leak-free. Once the sealing was completed and minor adjustments made, the chilled beam system worked like a charm.”
“…it was only a matter of hours before each section of ductwork was verified to be virtually leak-free.”
SVP, Commercial Sales and Strategy
While chilled beams are one of the most common HVAC systems in use today throughout Europe and Australia, their adoption here in the United States is just taking hold. The promise of quiet, low-impact energy efficient heating and cooling make chilled beams an attractive option in many commercial building applications.
“But as the use of chilled beam systems grow, the importance of leak-free ducting becomes more critical than ever before,” said Walsh. “As with any HVAC system, ensuring efficiency and high performance with chilled beams requires a whole-system approach to design, installation and maintenance. Leaks in this strategy can leave a contractor all wet.”