LinkLine: Spring 2014
By David P. Rebhuhn, PE (GA PE#32928), NCEES (#18079), CSI Member
Chilled beams are generally viewed as an alternative to a traditional, fan-driven, HVAC system. They are a convective cooling and/or heating device (2-pipe or 4-pipe) constructed of copper tubing and bonded to aluminum fins, housed in a sheet metal enclosure, and usually installed at ceiling level. There are 3 variations:
- Passive type that relies totally on natural convection or the radiant/convective passive type that cools through a combination of radiant and natural convection. Warm air rises and is cooled by the chilled beam. Once it's cooled, the air falls back to the floor. It is replaced by warmer air moving up from below, causing a constant and cyclic flow of convection and cooling the room. Heating works pretty much in the same fashion, similar to a steam radiator. Fresh air must be introduced from a separate source to meet ventilation requirements.
- Active type (sometimes referred to as an induction diffuser) that is connected to ducted supply air containing a percentage of fresh air that induces room air toward the units which mixes with the ducted supply air, and discharged through linear diffusers.
- Integrated/multi-service type that incorporates computer and electrical wiring, cable pathways, lighting, occupancy sensors, speakers, smoke detectors, and sprinklers into the chilled beam unit.
The application of chilled beam technology is relatively new in the United States, even though it has been applied in Europe and Australia for over 20 years. Today, chilled beams are one of the most common HVAC systems installed in Europe. It has only been within the last decade the technology has seen some application in North America. As of 2007, chilled beam HVAC systems were more widely used in Australia and Europe than in the United States. Acceptance in the USA has steadily been increasing since 2007.
Noteworthy Project Locations
In Australia, the system was first used in "30 THE BOND", Sydney, which received a 5 Star Australian Building Greenhouse Rating benchmark, which is the approximate equivalent of a Gold LEED greenhouse certification level. Chilled beam HVAC systems have been used at London Heathrow Terminal 5 in England and the Constitution Center which is the largest private office building in Washington, D.C. The system has also been installed at Harvard Business School, Wellesley College, and the American headquarters of the pharmaceutical company AstraZeneca. The multiservice beam was first installed at the Barclaycard building in Northampton, England, and has since been installed in the headquarters of Lloyd's Register (London), Airbus UK (Bristol), and the Greater London Authority; Riverside House (London); Empress State Building (London); 55 Baker Street (London); and 101 New Cavendish Street (London).
- The temperature of chilled water supplied to a passive chilled beam is approximately 61 - 66 F (16 - 19 C) as compared to a nominal 45 F (7 C) for a traditional HVAC system and the total amount of air handled in a building can be 25-30 percent less using chilled beams. In other words, a 1 inch (2.5 cm) dia. pipe can carry as much energy as a 18 x 18 inch (46 x 46 cm) sheet metal air distribution duct meaning that that chilled beam HVAC systems require significantly less energy to provide the same cooling effect as a traditional HVAC system. The increased heat transfer properties of water allow the air transport system to be reduced in size and cost.
- Supplying chilled beam systems with 100% outdoor air (based on ventilation requirements) could be directly exhausted, eliminating pollutants that would otherwise be transported via return air ductwork or between air distribution zones resulting in an improved IAQ.
- Chilled beams do not require fans resulting in noise levels of 40 NC or less.
- Chilled beam systems require less ceiling space than forced-air systems because chilled beam systems do not require mechanical equipment rooms or sizeable ductwork. This means that chilled beams would be an excellent application where floor-to-slab height is minimal. Lower building heights can also translate to aesthetically pleasing higher ceilings as well as a reduction in cost due to less building materials.
- A building with chilled beams will allow for smaller mechanical (or complete elimination) through less floor space requirements due to downsized air handling equipment.
- Integrated/multi-service beams incorporating other space services as pointed out at the beginning of this article, reduce coordination efforts and installation time.
- Additional ductwork may be required to meet fresh air requirements.
- Both passive and active systems are less effective at heating than cooling, sometimes requiring supplementary heating systems. In active chilled beam heating systems, water temperatures are usually 104 to 122 F (40 to 50 C) which is similar to a traditional, air-moving, heating system. Natural air convection cannot be relied on solely so a fan-driven primary air circulation system may be necessary to deliver the heated air to the lower levels where people sit and work.
- Chilled beam systems cannot be used alone in buildings where the ceilings are higher than 8.9 ft (2.7 meters) because the air will not properly circulate. A forced-air circulation system must be used in those situations.
- Positioning of the beams is not easy. Adequate space between the top of the passive beam and the bottom of the structure needs to be provided to make sure warm air can rise, turn, and go past the heat exchanger's cooling fins.
- Passive beams should be kept away from copy machines, printers, etc. because the warm air from these machines offsets the cool air from the beam.
- Chilled beam systems are not recommended for areas with high internal humidity such as theaters, gymnasiums, or cafeterias.
- Hospitals generally cannot use chilled beam systems because of restrictions on using re-circulated air.
- Chilled beam systems could cause noticeable air circulation which can make some people object to. Passive air deflection devices disrupting the air patterns can help alleviate the problem.
- Enlarging the ducts around active chilled beam systems in order to increase air circulation may cause echoes in working areas as well as elevating the sound of water moving through the pipes to unacceptable levels.
These systems are better suited for projects where the HVAC system is primarily based on heating and sensible cooling space loads rather than ventilation loads. Buildings with a high heat gain from equipment, solar radiation, etc. are excellent candidates. If the ventilation system is so large that it could simultaneously handle the heating and cooling loads, then chilled beams probably are not a viable application. In spaces where thermal comfort is important, chilled beams can work well. Entrance lobbies would not be a good application. Specific examples of suitable installations include K-12 and post-secondary educational facilities, office buildings, LEED and Green buildings, data centers, television studios, and load driven laboratories.
A discussion of chilled beams would not be complete without mentioning the opportunity for their use in the retrofit of older, existing induction unit installations. Back in the 1950's and 1960's, induction units were used in large buildings where space was at a premium and the small primary air ductwork used with these systems was advantageous in reducing floor heights and mechanical space requirements. These units were usually mounted on the floor against an exterior wall, concealed under custom built enclosures. Induction units became very unpopular during the energy crisis of the 1970's because of their high inlet static pressure requirement which elevated the fan horsepower requirement. These units exhibited higher noise levels along with compromised air distribution patterns. Unit controls were based on pneumatics requiring a high level of maintenance while unable to produce acceptable levels of comfort and efficiencies as compared to present day DDC systems. A wholesale renovation of the existing HVAC system in these older buildings to a more traditional, fan-driven HVAC system may be cost prohibitive due to the existing low floor-to-floor heights, HVAC piping, and small, high velocity fresh air risers. It could be more cost effective to abandon the existing building in favor of new construction. Retrofitting the building with chilled beam technology rather than replacing the old induction units could be the solution to budget and improved comfort concerns. The existing infrastructure such as ductwork, HVAC piping, etc. could be reused in support of the retrofit to chilled beams.
- The most common active chilled beam design is in the form of an inverted "T", suspended from the ceiling. The width of each beam is generally 1 to 2 feet (0.30 to 0.61 m) wide, requiring less than 1 foot (0.30 m) of space overhead. The weight of a typical 2 foot (0.61 m) wide chilled beam is approximately 15 pounds (6.8 kg) per 1 foot (0.30 m) of beam length.
- Chilled beams are usually installed so that the center to center distance of multiple parallel beams is no more than 9.8 feet (3.0 m).
- Since the beams do not cover the entire ceiling, exposed ducts, wiring, and other infrastructure can become aesthetically challenging.
- Locating the beams around the perimeter of the building addresses locations where the temperature differences usually are the greatest.
- Locating the beams in the interior space seems to result in better temperature control throughout the building.
- Increasing the static air pressure of the building interior can go a long way is controlling infiltration resulting in improved system performance.
- Chilled beams do not have moving parts which translates to lower maintenance costs when compared to traditional, fan-driven systems.
- Air filters are not required and the vacuuming of dust and dirt from the fins typically is required every 5 years.
- Since chilled beams are sensible cooling devices, the absence of cooling condensate will avoid any bacterial or mold growth issues. Drain pans are not required which require cleaning even though some manufacturers offer a drip tray as an option.
Applications in Hot Humid Climates
Passive chilled beams have limited application in hot and humid climates, although they can be used when needed to supplement the load requirement where an active beam falls short. Humidity becomes an issue if the surface temperature of any cooling coil or unit panel falls below the dew point of the surrounding air resulting in the formation of condensate. This is can be prevented by making sure the primary air supplied to the beam is dry and adequate to handle the space latent loads and will limit the indoor dew point temperature to below 55F. How dry this air must be will depend on the CFM and the space load. Chilled water supplied to the beam to handle space sensible loads, should be provided at temperatures above the local dew point to avoid the formation of condensate. Chilled water temperatures delivered at 58-60 F should keep the beam surface temperature above the local dew point. The physical shape of the beam may provide relief in situations where the surface may momentarily dip below the local dew point temperature. Moisture will begin to accumulate on the surface of the coil or panel. This is a very slow process and due to the size of the beam, there is a limited amount of condensate that can form. This can be prevented by sound design and implementation of the correct control strategy as follows:
Self-contained air volume limiter when fed from a VAV air handling unit.
Control space temperature with modulating chilled water supply valve or varying EWT to the chilled beam coil in response to a zone thermostat signal with further reduction in cooling capability by reducing the primary air supply. Modulation of the chilled water flow rate or temperature should be the primary method since it has little or no effect on space ventilation and/or dehumidification. Only after the chilled water flow has been discontinued should the primary airflow rate be reduced.
As long as space dew point temperature is maintained within a range of plus/minus 2 F. and the chilled water supply is at or above this value, condensation on the chilled beam surfaces can be avoided.
Should there be periods when room humidity conditions drift or rise above design, and a dew point sensor detects condensate formation (or the potential), typical control action would be to modulate flow to the beam or reset the chilled water supply (higher) in order to reduce beam capacity an increase surface temperatures. An alternative to this is to simply shut off the CHWS to the zone and allow the conditioned primary air (if sized with excess capacity) to assist in returning the space to its proper humidity level. This is not recommended in humid climates as thermal control may be lost resulting in a space that can't be occupied comfortably.
Monitoring of the space temperature, dew point, moisture on CHWS piping, and outdoor conditions can be accomplished with traditional sensing devices available from major controls manufacturers. A variety of high-performance sensors are available when more demanding requirements are presented. These include impedance dew point sensors, chilled mirror sensors, and instrumentation utilizing dark spot optical hydrocarbon dew point detection.