Innovative Energy Engineering

Chilled Beams - CB

Chilled Beams induce a secondary flow of space air through a chilled water cooling coil. This reduces the need for air supplied by the AHU and can reduce fan energy and duct sizes somewhat. Limitations include the lack of air-flow regulation and the lack of dehumidification. CB in most cases are constant-volume systems. When misapplied, space humidity can form condensation at the coils and it "rains" in the room. Controls preventing the condensation, could stop the systems in humid areas. In general CB systems are not appropriate for office or other applications with high latent loads and the opportunity to vary ventilation.

Efficiency in Office Applications and Real World Problems

Often CB systems are sold as the new energy saving system. But when misapplied, energy consumption will be higher than VAV systems at a higher up-front cost. Trane and Taylor Engineering studied CB applications in offices and found an energy disadvantage. Many of the zone and AHU control strategies of VAV systems can't be implemented, decreasing efficiency even further.

In case of malfunction of controls condensate can form inside the space and cause damage and discomfort to occupants. Areas with high infiltration(i.e. entrances, windows) can cause condensate build-up and "rain". The coils receive dirty air and will require more frequent cleaning. Aesthetics can be an issue since the beams are visible. Cost is higher. In order to take advantage of the option to use higher chilled water temperature, a second chiller is required. This is because one chiller still needs to chill the water to low temperatures for DOAS dehumidification. This makes the system only practical for large systems.

We have investigated CB systems in varying applications in Wisconsin and Minnesota. Mechanics, operators, designers and commissioning agents were interviewed The systems were of varying levels of sophistication and installed at very high levels of detail and commissioning. The common theme is that they can operate reliably if the AHU is operated 24/7, which eliminates any energy savings compared to VAV. When operated intermittently, humidity or "rain" problems can occur easily. In addition trouble-shooting and control optimization require much higher qualified personnel than regular systems. Controls require humidity sensors that need to be calibrated frequently. Even if in theory an energy savings could be achieved, practical problems will force operators to operate in a "safe" manner. unlike VAV systems, CB systems are not forgiving. A failure in the DOAS, or less capacity of a chiller, or a sensor failure can shut down the system or cause humidity and rain problems.

We recommend to use CB systems only in applications with high ventilation requirements and high sensible cooling loads, chilled beam systems can be more efficient than VAV or other systems. One Example is hospital patient room conditioning where code requires constant high ventilation rates and medical equipment generates a lot of heat. Climate zones with very low humidity also may be suitable for CB systems.

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