Part 1: Characteristics & Benefits
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.
This article discusses the basic characteristics and benefits of chilled beam systems for labs. Part two, to be published in September, will cover 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, appears in this month's expanded edition at www.labdesignnews.com/august2009.
The Labs21 website also provides full information about the agency’s 2009 annual conference, to be held in Indianapolis, Sept. 22-24. The co-meeting is sponsored by I2SL, the International Institute for Sustainable Laboratories.
Laboratories commonly use far more energy than typical office buildings, primarily due to the intensive ventilation required to address environmental, health, and safety concerns. As a result, facility designers and engineers are constantly seeking new ways to reduce energy consumption while maintaining performance. Active chilled beam systems are gaining in popularity among laboratory designers because these systems allow ventilation requirements to be decoupled from sensible heating and cooling loads. This decoupling eliminates the need for reheat coils for temperature control and reduces the fan energy required to maintain comfort.
Chilled beam systems are prevalent in European commercial office buildings but have not yet been widely applied in the U.S. Such systems offer many compelling benefits, including high cooling capacities, excellent performance, and dramatic energy savings for little or no additional costs over conventional systems. This guide presents best practice strategies for designing, constructing, operating and maintaining chilled beam systems in laboratories.
How chilled beams work
Chilled beams (also called induction diffusers) are fundamentally different from the all-air diffusers used throughout most U.S. buildings. There are two categories of commonly used chilled beams: active and passive. Active chilled beams rely on air handlers supplying outside air to condition a space and a cold water piping system that circulates water through integral cooling coils.
The primary airflow from the air handling unit (AHU) to the zone is introduced through small air jets, which typically induce three to five times the amount of room airflow through the beam’s coil (Fig. 1, top middle). The induction process provides local recirculation of room air.
Passive chilled beams rely simply on the natural convection in a room and have no direct air supply. As heat is transferred from the room air to the beam’s coil, the air is cooled and falls into the occupied zone. As this occurs, warm room air up by the ceiling is drawn down through the passive beam coils (Fig. 2, top right). Passive beams are best suited to applications with high heat loads and low ventilation air requirements, and therefore have limited application in most laboratories. This guide focuses only on active chilled beams, referred to from this point on simply as chilled beams.
Chilled beams can accommodate sensible and latent loads. However, in properly designed laboratory environments, chilled beams only provide the sensible cooling, while the central air handling system provides the latent cooling. This design avoids the additional costs of running condensate drain piping to each beam in the building.
When designing with chilled beams, there are two critical considerations: chilled water temperature and humidity level in the conditioned space. If standard chilled water (45°F) is used in the chilled beam, there is a risk of condensing water on the coil. To prevent such condensation, the chilled beam water temperature must be actively maintained above the room air dew point. Both of these design criteria will be discussed in the next issue (under “system sizing” in Part 2 of this article).
Benefits of chilled beams in labs
Chilled beams, while not appropriate for every laboratory, can offer many benefits compared with the variable air volume (VAV) reheat scheme commonly used in most standard lab systems (Fig. 3). In the VAV scheme, boxes with reheat coils, control dampers and airflow measurement devices are placed in each zone. While this system meets building requirements, it uses significant amounts of fan and reheat energy.
The following typical laboratory cases demonstrate how chilled beams can reduce reheat energy, accurately meet outside air requirements, and reduce building-wide systems requirements, compared to VAV systems. The three cases differ in the amount of air required for ventilation, safety, cooling, and fume hoods.
Case 1. Ventilation-driven airflow
All laboratories require a fixed amount of ventilation air to maintain safety. This case refers to laboratories where this “general exhaust” requirement (typically six air changes per hour) drives the airflow, as distinct from a fume-hood–driven airflow covered in Case 3.
Laboratories are dynamic buildings with a variety of rooms, each with their own general exhaust requirements. In a typical laboratory HVAC system with VAV reheat, the room with the highest heating load dictates the air temperature supplied by the central AHU. Each lab space then reheats the air, as needed. Reheating such high volumes of air for each room presents a huge potential energy loss. A chilled beam design avoids this energy loss by supplying a higher temperature to each zone and dynamically cooling each space individually. With a fixed amount of ventilation air, chilled beams control the individual laboratory temperature by adjusting the flow of chilled or hot water across the beams to match any changing loads. In this case, using chilled beams eliminates reheat energy and minimizes outside air conditioning.
Case 2. Cooling-load-driven airflow
When cooling loads in a lab drive the design airflow rates, the use of chilled beams (which decouple the air and cooling requirements) can dramatically reduce the size of air systems.
In a typical VAV reheat system, each space meets its own cooling load by increasing the volume of cold air supplied. This situation creates a dependent relationship between the airflow and the cooling capacity. In a chilled beam system, cooling is accomplished with pumped chilled water rather than blown cold air. Water has a volumetric heat capacity 3,500 times that of air, which translates to a reduction in fan energy by a factor of seven in typical pump and fan arrangements.
On an annual basis, the coil in the chilled beam accomplishes at least half of the cooling with the remaining load handled by the primary air. Furthermore, the ramp-up of air typical in VAV reheat systems no longer occurs in labs with high heat loads. In many detailed energy analyses of labs, cooling air and then reheating it can easily account for 20% of annual HVAC energy costs.1
When chilled beam systems are used, ducting can be downsized and the air handler central system reduced to handle less than half of the air needed by a typical system.1 The savings realized can be used to pay for the added piping and chilled beam capital costs. If modest reductions in floor-to-floor height due to smaller ducting are taken into account, using a chilled beam system can translate into an overall savings in construction costs and significantly reduced operation costs as well.
Case 3. Fume-hood-driven airflow
The benefits of chilled beams are minimal for labs with a high density of fume hoods or other process exhaust. In these labs, higher airflow rates are required for safety; ducts are sized for these higher airflows; and savings from reducing ducting and the central system are not possible.
If a building has only a few labs with a high density of fume hoods, chilled beams can still be a solution in those areas of the building with a low density of hoods (a maximum of two hoods per laboratory module). Small VAV boxes with a heating coil can supply additional air in the labs with a high density of hoods, while the remaining labs use chilled beams. In cases like this, careful life-cycle cost analysis will determine the viability of chilled beam systems.
Design and construction information will be covered in parts 2 and 3 in the next two issues of Laboratory Design. For a detailed case history of a lab where chilled beams have been used successfully, see this month’s expanded edition at www.labdesignnews.com/august2009.
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 firstname.lastname@example.org. For general information on the Labs21 program, contact Amon (email@example.com) or Lintner (firstname.lastname@example.org).
1. Rumsey, P., and J. Weale. “Chilled Beams in Labs: Eliminating Reheat & Saving Energy on a Budget.” ASHRAE Journal, January 2007, pp 18–23, 25.
Published in Laboratory Design News, Volume 14, No. 8, August, 2009, p. 1.