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Fig. 1. A comparison of 6-ft beams from different manufacturers shows that, even with the same design conditions, cooling performance can differ greatly for a given supply air volume.
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Part 2: Design issues
Editor’s note: This three-part article is one of a series of Technical Bulletins and Best Practice Guides for laboratories, produced by Laboratories for the 21st Century (“Labs21”), a joint program of the U.S. Environmental Protection Agency and the U.S. Department of Energy. Geared toward architects, engineers, and facility managers, these publications provide information about technologies and practices to use in designing, constructing, and operating safe, sustainable high-performance laboratories. For more information about these free resources, see: www.Labs21century.gov/toolkit/bp_guide.htm.
Part one, published in August, discussed the basic characteristics and benefits of chilled beam systems for labs. Part two, below, covers design issues, including sizing a system, controls/integration, and the challenges of modeling. Part three, in the October edition, will discuss construction and commissioning, including system costs, how to hang the beams, code compliance, operations and maintenance. An appendix with a relevant case study of the Tahoe Center for Environmental Sciences, a Labs21 partner project, appeared in the August expanded edition at www.labdesignnews.com/august2009.
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Fig. 2. Example of a laboratory floor plan using chilled beams, air diffusers and fume hoods.
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This article discusses three areas of chilled beam system design: system sizing, controls and integration, and energy modeling. A chilled beam system designed for a laboratory with this information in mind can reduce building energy use and costs compared to a standard VAV reheat system.
The process for sizing a chilled beam system can be divided into four major steps:
- Select the type of beam, based on project performance and setpoints.
- Select a beam’s performance and manufacturer to match the required beam type.
- Determine the zone in which these beams will be placed and how their proximity to other equipment, such as fume hoods and lighting, will affect the ceiling arrangement and number of beams.
- Optimize the central system and the required airflow and temperature of the supply air and water.
Step 1. Select a Beam Type
Chilled beams vary in physical size, cooling capacity, airflow capacity and many other parameters, depending on the manufacturer. For a given laboratory, the beam type selected typically depends on the following design parameters: maximum allowable design pressure drop for both air and water sides, chilled water supply temperature, supply air temperature and allowable noise levels.
Air and water pressure dropPressure drops across both the water side and air side of a chilled beam play a large role in specifying a system. The pressure drops affect the optimal flow through a chilled beam and the cooling capacity potential. Typical water-side pressure drops can range from 10 to 15 ft of water column (ft w.c.) of head through the chilled beam coil.
On the air side, a chilled beam can be selected to have a pressure drop up to 1.5 in. However, we recommend designing for no more than 0.5 in. when selecting a beam. Compared with a VAV reheat system, chilled beams can have a small penalty of 0.25 to 0.5 in. of static pressure. But, this is insignificant compared to the total fan energy of a VAV system, which typically operates in the range of 3 to 8 in. of total static pressure.
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Fig. 3. Fume hood proximity to chilled beams and airflow patterns. Fume hoods require a uniform sash-face velocity of 100 fpm to maintain safe containment. Crossing airflow greater than 50 fpm can cause a loss of containment.
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According to Labs21 guidelines, for a low-pressure drop design, the supply system pressure drop should be between 2 to 3 in., so the chilled beam pressure drop can become more significant.
(See “A Design Guide for Energy-Efficient Research Laboratories” at www.labs21century. gov/toolkit/design_guide.htm.)
Chilled beam manufacturers will supply design tables for selecting the best beam based on these two pressure drop criteria. Establishing acceptable ranges for these pressure drops first can give guidance to the amount of air that can be supplied and the possible range of cooling capacities.
Chilled water supply temperature In a standard lab system, using 45°F chilled water runs the risk of condensing water on the chilled beam coil in the diffuser. To prevent such condensation, chilled water needs to be actively controlled to at least 3 or 4°F above the room air dew point. Because of this, most chilled beams use chilled water in the range of 55 to 62°F.
This elevated temperature can also lead to other benefits, such as the option to use water-side economizing or free cooling. In the right moderate climates, electric chillers can even be eliminated and chilled water can be produced directly from a cooling tower with a storage tank. In hot and humid climates, reducing the load and running a dedicated electric chiller that only serves the chilled beams can increase efficiency by 15 to 20%.
Air supply temperature Most chilled beam systems will supply ventilation air of 55 to 70°F at a dew point of 50 to 55°F. At 68 to 70°F, all the cooling is accomplished at the chilled beams and reheat energy can be eliminated. However, elevated air temperatures come at a cost. As the approach temperature between room air and chilled water decreases, the sensible cooling capacity of the room air also decreases. There is a tradeoff between the supply air temperature and the number of chilled beams required to meet the cooling load. As air temperature is increased, more chilled beams are required to meet the same load, which can increase costs and complicate ceiling arrangements.
Noise requirements Chilled beams vary in noise level, depending on their nozzle type and airflow rate. In general, chilled beam systems operate at or below standard laboratory system noise levels. For example, with one product, as the primary air static pressure increased by approximately 0.2 in w.c., the noise level increased by 7 to 10 decibel (dB). In a similar way, as airflow rates increased through the beam by roughly 100 ft3 per minute (cfm), noise levels jumped up by as much as 20 dB.
The whole point of noise requirements is that they should be considered when setting limits on pressure drops and airflow rates as a final check to ensure a reasonable range of sound on a case-by-case basis, depending on the project.
Step 2. Select beam performance
When selecting a chilled beam, it is important to note that not all beams are created equal. Some beams have a higher capacity for a given supply air volume. In addition, some beams include a choice of nozzle types, further differentiating their performance. Fig. 1 displays five different 6-ft beams, each with the same design conditions (Table 1). Beams come in all lengths, from 2 to 10 ft. Depending on the design requirements, one 6-ft beam can outperform a competitor’s 10-ft beam.
Fig. 1 shows the higher output of Manufacturer A’s beams compared with other comparably sized beams. This company builds more coils per linear ft into their beams to increase capacity and maintain a nominal beam length, leading to an increased weight per beam.
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Fig. 4. Due to the Coanda effect, air adheres to flush surfaces and will flow further out from a chilled beam with a dropped ceiling.
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Step 3. Determine zone layout
The number of chilled beams in a laboratory will depend on the load density expected, the square footage of the room, the number and location of fume hoods, and whether the ceiling is dropped or open. Most labs run in the range of 5 to 15 W/ft2 and can accommodate up to 25% of the ceiling space for chilled beams at higher load densities. Typically, 50% is a high limit to the amount of ceiling coverage by chilled beams. As coverage increases, installation and coordination of chilled beams and lighting can be cumbersome and installed cost can increase as well.
In general, a minimum of 6 ft on center will ensure a good coverage of the ceiling without causing too many coordination problems. Fig. 2 shows an example of a chilled beam floor plan. The room supplies air through four chilled beams to maintain ventilation requirements.
Proximity to fume hoods In laboratories, a uniform fume hood-sash airflow profile must be maintained to ensure safety. At the sash face, crossing airflows must not exceed 50 fpm or the fume hood containment may be compromised, triggering an alarm.
In many labs, fume hood placement will constrain chilled beam layouts. Chilled beams are ideally mounted perpendicular to the fume hood sash and 3 to 5 ft away from the hood (Fig. 3), so that the airflow supplied by the beam does not interfere with the smooth airflow to the hoods. If a laboratory requires that a chilled beam be mounted parallel to a fume hood, one-directional beams can be used, and some beams allow nozzles to be manually closed upon building startup.
Ceiling type Dropped ceilings can increase the throw of air off a chilled beam. Due to the Coanda effect, airflow will adhere to any flush surface at the outlet of the chilled beam and fall farther away from the beam (Fig. 4). This phenomenon can affect how a floor plan is arranged and where mixing might occur. With an open ceiling, chilled beams are hung freely and air will drop closer to the beams. Most beam manufacturers offer more details on incorporating this effect into the design.
Lighting needs and seismic supports can also physically limit the amount of chilled beams each zone can support. Chilled beams can be designed to incorporate lights or act as reflective surfaces to bounce light when needed.
Hydronic design considerations: Two- or four-pipe From a hydronic standpoint, there are two different types of beams: two-pipe and four-pipe. Both types can provide heating and cooling. A four-pipe beam has two separate coils: one for heating and one for cooling. A two-pipe beam has a single coil for either heating or cooling. Four-pipe beams weigh more, due to the increased mass of the additional coil, and can also cost more in building and support materials.
Depending on how a chilled beam is plumbed, a two- or four-pipe chilled beam can produce the same effects. For example, consider a case in which hot and cold water pipes (supply and return for both) are plumbed to a chilled beam in a room. That beam can either have two coils—one for heating and one for cooling (four-pipe)—or a single coil with switchover control valves (two-pipe) that switch between heating and cooling as needed.
Fig. 5 shows how a two-pipe beam can be plumbed to allow both heating and cooling at a zone level. The costs differ for these two approaches, depending on the application and how much piping is required.
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Fig. 5. Two-pipe hot-water/chilled-water (HW/CHW) switchover controls for chilled beams allows for both heating and cooling at a zone level.
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Step 4. Optimize the central system
If designed properly, a chilled beam system presents additional opportunities for saving energy and first cost at the central air handling, hot water, and chilled water systems. By using supply air ducts only for ventilation requirements, the size of ducts and central AHU can both be reduced, saving space and costs. By eliminating reheat, the overall hot water system can be reduced in size by reducing or removing zone heating coils and the necessary hot water pipe. And by using a higher chilled water supply temperature, smaller and more efficient chilled water systems can be specified.
For information on how to optimize the central system, as well as tips for controls and integration and comments on energy modeling challenges, see the expanded edition (www.labdesignnews.com/september2009).
Construction, commissioning and operations information will be covered in Part 3 in the next issue of Laboratory Design.
The authors of this guide were Peter Rumsey, PE, Neil Bulger, Joe Wenisch and Tyler Disney, all of Rumsey Engineers, Oakland, Calif. Contributors and reviewers were Mike Walters, Affiliated Engineers Inc., Madison, Wis.; Dan Amon, PE, U.S. Environmental Protection Agency, Washington, D.C.; William Lintner, PE, U.S. Dept. of Energy, Washington, D.C.; and Paul Mathew, Lawrence Berkeley National Laboratory, Berkeley, Calif. Technical editing and layout for the original Labs21 edition were provided by Julie Chao and Alice Ramirez in the Creative Services Office of the LBNL.
For more information on chilled beams in labs, contact Rumsey at prumsey@rumseyengineers.com. For general information on the Labs21 program, contact Amon (amon.dan@epa.gov) or Lintner (william.lintner@ee.doe.gov).
