Chilled beam

A chilled beam is a type of convection HVAC system designed to heat or cool large buildings.^[1] Pipes of water are passed through a "beam" (a heat exchanger) suspended a short distance from the ceiling of a room.^[2] As the beam chills the air around it, the air becomes denser and falls to the floor. It is replaced by warmer air moving up from below, causing a constant flow of convection and cooling the room.^[3][4] Heating works in much the same fashion, similar to a steam radiator. There are two types of chilled beams. Some passive types rely solely on convection whilst there is a "Radiant"/convective passive type which cools through a combination of radiant exchange (35%) and convection (65%) which can provide higher thermal comfort levels ^[5], while the active type (also called an "induction diffuser")^[6] uses ducts to push ("induce") air toward the unit (increasing its heating and cooling capacity).^[3]

The chilled beam is distinguishable from the chilled ceiling.^[2][6] The chilled ceiling uses water flowing through pipes like a chilled beam does; however, the pipes in a chilled ceiling lie behind metal ceiling plates, and the heated and cooled plates are the cause of convection and not the pipe unit itself.^[2] Chilled beams are about 85 percent more effective at convection than chilled ceilings.^[2]

Physics of the chilled beam

Water can carry significantly more energy than air. Although 1 cubic foot (0.028 m³) of air has a capacity to hold heat of 37 joules per kelvin (JK^-1), water has a heat capacity of 20,050 JK^-1.^[7] A metal pipe of water just 1 inch (2.5 cm) in diameter can carry as much energy as an 18-by-18-inch (46 by 46 cm) metal duct of air.^[7] This means that chilled beam HVAC systems require much less energy to provide the same heating and cooling effect as a traditional air HVAC system.

Chilled beam cooling systems require water to be treated by heating and cooling systems. Generally, water in a passive chilled beam system is cooled to about 16 to 19 °C (61 to 66 °F).^[8][9] In active chilled beam heating systems, water temperature is usually 40 to 50 °C (104 to 122 °F).^[10] (Chilled beam heating systems usually cannot rely solely on convection, however, and often require a fan-driven primary air circulation system to force the warmer air to the ground where most people sit and work.)^[10] There are effectiveness and cost differences between the two systems. Passive chilled beam systems can supply about 5.6 to 6.5 watts per foot (60 to 70 watts per metre) of cooling capacity.^[6] Active chilled beam systems are about twice as effective.^[6] In both cases, convection is so efficient that the ratio of incoming air to heated/cooled air can be as high as 6:1.^[11] However, studies of the energy cost-savings of active versus passive chilled beam systems remained inconclusive as of 2007, and appear to be highly dependent on the specific building.^[6]

The active chilled beam system employs fins to help heat and cool.^[6] Active chilled beam systems are effective to the point where outdoor air can be mixed with the indoor air without any traditional air conditioning (such as heating, cooling, humidifying, or dehumidifying), thus enabling a building to meet its "minimum outdoor air" air quality requirement.^[6]

Advantages and disadvantages

The primary advantage of the chilled beam system is its lower operating cost. For example, because the temperature of cooled water is higher than the temperature of cooled air but delivers the same cooling ability, the costs of the system are lower.^[9][12] Because cooling and heating of air are no longer linked to the delivery of air, buildings also save money by being able to run fewer air circulation fans and at lower speeds.^[6] One estimate places the amount of air handled at 25 to 50 percent less using chilled beam systems.^[11] By being able to target the delivery of clean outdoor air where it is needed (rather than injecting it into the entire system and heating or cooling it), there is a reduced need to treat large amounts of outdoor air (also saving money).^[6] In one case, the Genomic Science Building at the University of North Carolina at Chapel Hill lowered its HVAC costs by 20 percent with an active chilled beam system.^[13] This is a typical energy cost savings.^[6] Chilled beam systems also have some advantages in that they are almost noiseless,^[11] require little maintenance, and are highly efficient.^[14][15] Traditional fan-driven HVAC systems create somewhat higher air velocities,^[15] which some people find uncomfortable. Chilled beam HVAC systems also require less ceiling space than forced-air HVAC systems, which can lead to lower building heights and higher ceilings.^[9][12] Since they do not require high forced air flows, chilled beam systems also require reduced air distribution duct networks (which also helps to lower cost).^[9][12]

Chilled beam systems are not a panacea. Additional ductwork may be needed to meet minimum outdoor air requirements.^[6] Both types of chilled beam systems are less effective at heating than cooling, and supplementary heating systems are often needed.^[6] Chilled beam systems cannot be used alone in buildings where the ceilings are higher than 2.7 metres (8.9 ft), because the air will not properly circulate.^[10] A forced-air circulation system must be employed in such cases. If the water temperature is too low or humidity is high, condensation on the beam can occur—leading to a problem known as "internal rain."^[14][12][16] (In some cases, drier outside air can be mixed with the wetter inside air to reduced interior humidity levels while maintaining system performance.)^[11] Chilled beam systems are not recommended for areas with high humidity (such as theaters, gymnasiums, or cafeterias).^[12] Because they are less effective at cooling, passive chilled beam systems are generally ill-suited for semi-tropical and tropical climates.^[6] Hospitals generally cannot use chilled beam systems because of restrictions on using recirculated air.^[11] Chilled beam systems are also known to cause noticeable air circulation which can make some people uncomfortable.^[2] (Passive air deflection devices can help disrupt these air patterns, alleviating the problem.)^[17] Some designers have found that enlarging the ducts around active chilled beam systems to increase air circulation causes echoes in working areas and amplifies the sound of water moving through the pipes to noticeable levels.^[11]

Installation and adoption

The most common active chilled beam design is in the form of a capital letter "T", upside down and suspended from the ceiling.^[11] They are generally 1 to 2 feet (0.30 to 0.61 m) wide, and require less than 1 foot (0.30 m) of overhead space.^[11] A typical 2-foot (0.61 m) wide chilled beam system generally weighs about 15 pounds (6.8 kg) per 1 foot (0.30 m) length of the beam.^[11] Chilled beams are generally installed so that the center of each beam is no more than 3 metres (9.8 ft) from the center of the next beam.^[10] Some designers dislike the beams because they do not cover the entire ceiling so ducts, wiring, and other infrastructure can be seen.^[6] Some designers have installed one chilled beam system around the building perimeter (where temperature differences can be the greatest) and another in the interior of the building, to better control temperature throughout the structure.^[11] Higher system performance may be obtained by increasing the static pressure of the air in the building.^[11] The systems generally need little cleaning (vacuuming of dirt and dust from the fins every five years).^[11]

As of 2007, chilled beam HVAC systems were used more widely in Australia and Europe than in the United States.^[6] In Australia, the system was first used in 30 The Bond, Sydney which was the first building in Australia to achieve the rating of 5 stars ABGR ^[18][19]. Chilled beam HVAC systems have been used at London Heathrow Terminal 5^[20] and Constitution Center (the largest private office building in Washington, D.C.).^[21] The system has also received prominent use at Harvard Business School, Wellesley College, and the American headquarters of the pharmaceutical company AstraZeneca.^[21]

The multiservice chilled beam is a relatively new form of the chilled beam. Developed in 1996, it incorporates computer and electrical wiring, lighting, motion-detection sensors, and sprinklers into the chilled beam unit.^[22] The multiservice chilled beam was first installed at the Barclaycard building in Northampton, England, but has since been used at 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)^[23] and 101 New Cavendish Street (London).^[22][24]

Footnotes

1. ^ Oughton, Hodkinson, and Faber, 2008, p. 222-224.
2. ^ ^{a b c d e} Beggs, 2009, p. 271.
3. ^ ^{a b} Hamilton and Watkins, 2009, p. 158.
4. ^ Levermore, 2000, p. 407.
5. ^ Hill, C. "Chilled Beams". http://www.scribd.com/doc/28198219/HVAC-Handbook-Passive-Chilled-Beam. Retrieved 20 April 2011. 
6. ^ ^{a b c d e f g h i j k l m n o} Roth, Kurt; Dieckmann, John; Zogg, Robert; and Brodrick, James. "Chilled Beam Cooling." ASHRAE Journal. September 2007.
7. ^ ^{a b} Geary, 2010, p. 9.
8. ^ Hare and Fisher, 2000, p. 246.
9. ^ ^{a b c d} Sisle, Leonard, and Weiss, 2010, p. 152.
10. ^ ^{a b c d} Oughton, Hodkinson, and Faber, 2008, p. 223.
11. ^ ^{a b c d e f g h i j k l} Alexander, Darren and O'Rourke, Mike. "Design Considerations For Active Chilled Beams." ASHRAE Journal. September 1, 2008.
12. ^ ^{a b c d e} Gelfand and Freed, 2010, p. 146.
13. ^ Studt, Tim. "Active Chilled Beam Lowers Energy Use by 20%." Laboratory Equipment. August 1, 2008.
14. ^ ^{a b} Hundy, Trott, and Welch, 2008, p. 316.
15. ^ ^{a b} Mumovic and Santamouris, 2009, p. 251.
16. ^ Hall and Greeno, 2009, p. 240.
17. ^ Awbi, 2003, p. 87.
18. ^ Hill, C. "Chilled Beam". http://www.bovislendlease.com/llweb/llc/main.nsf/all/fp_thebond. Retrieved 20 April 2011. 
19. ^ Hill, C. "Chilled Beam". http://www.cundall.com/Default.asp?Page=292. Retrieved 20 April 2011. 
20. ^ "The Chilled Beams Now Arriving at Terminal 5." Modern Building Services. November 2007.
21. ^ ^{a b} Fruehling, Douglas. "Chilled Beam System Comes to D.C." Washington Business Journal. November 26, 2007.
22. ^ ^{a b} "Exploiting the Value of Multi-Service Chilled Beams." Modern Building Services. November 2004.
23. ^ Hill, C. "Chilled Beams". http://www.lrp.co.uk/uk/development.php?subsection=55%20Baker%20St%2C%20London%20W1. Retrieved 20 April 2011. 
24. ^ Hill, C. "Chilled Beam". http://101newcavendishstreet.com/index.html. Retrieved 20 April 2011. 

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