Chilled Beams: An Overview
What are Chilled Beams?
Chilled Beams originated in 1986 with the first passive chilled beam being installed in Stockholm, Sweden. A chilled beam is an air recirculation device that uses water to transfer sensible heat to and from a room. It is made of convective coils that are placed in the ceiling of a thermal zone to provide sensible cooling and/or heating. Chilled Beams can be “two-pipe” (cooling only) or “four-pipe” (heating and cooling).
There are two types of chilled beams: passive and active. Passive Chilled Beams are cooling coils that are exposed to a space and rely on natural convection as the method of heat transfer. As the air in the space is heated it rises up in the space where it comes in contact with the chilled beam cooling coil. As the air is cooled it falls back into the occupied space, picks up heat, and repeats the cycle. Passive beams should only be used in cooling applications. If they were to be used in heating applications, the hot air would rise off the beam (stratify) and never reach the occupied space below.
Active Chilled Beams have a primary air connection that provides conditioned/dehumidified ventilation air. This primary air is pushed through the beam’s induction nozzles, which causes the beam to act as an air diffuser. Active beams may be used for both cooling and heating applications. The forced primary air can force the heated air down to the occupied space.
Chilled Beams are a more energy-efficient alternative to air-only conditioning systems as water has a higher heat transfer coefficient than air (1.00 calorie/gram °C vs. 0.24 calorie/gram °C). It is also more efficient to pump water than to move air. Liquid water is an incompressible substance, while air is compressible. Some fan energy is utilized during this compression process, which builds pressure, and forces the air through the duct distribution system. Since water is considered incompressible, the pump energy is put to almost immediate use of moving water through the piping system.
Certain models of Chilled Beams can also integrate other utilities/services into their housing which reduces the number of items located in a ceiling system (e.g. lighting fixtures and sprinkler heads).
How do Chilled Beams Work?
Chilled beams work by convective heat transfer, which occurs when the water is circulated through the coil of the beam. The beams convection methods can be natural or forced. Natural convection relies on the natural air circulation that is achieved by the temperature differential around the coil. Forced convection relies on supply air from an air handling unit (typically a dedicated outdoor air system (DOAS)) being connected to the beam and being forced across the coil using air nozzles inside the beam housing.
When to Apply Chill Beam Systems?
Chilled beams are suited for applications where space sensible loads are high relative to the ventilation and space latent cooling requirements. Typical applications include offices, science laboratories, hospital patient rooms, and more. Chilled Beam Systems make it possible to have higher ceilings as a significant amount of ductwork is removed from the design and only ventilation ductwork is required. A one-inch diameter water pipe can transport the same cooling energy as an 18-inch square air duct.
Chilled Beam Systems are quieter than forced-air systems (fan coil units, water source heat pumps, etc.) as they have no moving parts to them. Passive beams are virtually silent and active beams are as quiet or quieter than typical air diffusers.
When to Avoid Chilled Beam Systems?
You should avoid chilled beams in areas with exterior doors or operable windows when humidity/moisture control is not possible, and in spaces with high latent heat loads.
What are the Installation Requirements for Chill Beam Systems?
- Active chilled beams will require certain minimum airflow to achieve the desired cooling capacity for space. This minimum airflow should be compared to the ventilation airflow rate. If the beam required airflow is higher than the ventilation airflow rate, then some recirculation air may be required for the AHU/DOAS to save energy and provide only the ventilation air required by Code.
- Beams should be mounted no more than 20 feet above the occupied level.
- The building envelope should be adequately tight and the ventilation system should maintain the space with a slight positive pressure to prevent moisture infiltration through the building envelope.
- The ventilation AHU/DOAS must be designed to handle the outside air latent cooling and the space latent cooling. The AHU/DOAS must provide dry enough air to maintain the space dewpoint below the water supply temperature of the beam to prevent condensation from occurring.
How to Control Chilled Beam Systems?
Chilled beams can be controlled like air terminal units with space mounted thermostats in each zone.
What are the Safety Mechanisms?
Concern about chilled beams condensing and “sweating” is an issue, but not one that requires a multitude of controls to achieve. One low-cost solution is measuring (or calculating) the space dewpoint and provide a building automation system alarm when the calculated dewpoint rises above the chilled water supply temperature. If more safety is desired, this alarm could also close beam zone control valves. Yet another safety measure is to monitor the chilled water supply temperature. If the water supply temperature is lower than a user-defined setpoint, the circulation pump will shut off preventing the over-cooled water from being circulated through the chilled beams.
What are the Operations and Maintenance Costs of Chilled Beams?
O&M costs are greatly reduced as chilled beams do not have any moving parts to them. The coils operate in a dry/non-condensing method and therefore only require periodic vacuuming/cleaning of the coil and fins (typical applications every five (5) years).
What is the Service Life of Chilled Beams?
The service life of a chilled beam is essentially the same as a pressurized water coil, which is listed as 20 years according to the 2017 ASHRAE Fundamentals Handbook, Chapter 37, Table 4.
Article References:
2016 ASHRAE Fundamentals Handbook, Chapter 20, pages 20.9 and 20.10.
Trox Chilled Beam Design Guide, 2009.
Price Active and Passive Chilled Beams Engineering Guide, 2011.
Chris Kloes, PE, CEM, LEED AP, Senior Associate – Senior Mechanical Engineer
Chris is an experienced mechanical engineer with more than 20 years of experience in designing HVAC and plumbing systems His design experience includes various HVAC systems from laboratory/healthcare design to chilled beams. Chris’ plumbing design experience includes domestic water, sanitary, natural gas, compressed air, and roof drainage systems. Chris is a registered professional engineer, earned his LEED Accreditation in 2001, and is a Certified Energy Manager with energy auditing experience.