Selected Highlights of the Labs21 2007 Annual Conference

Sustainable Technologies for Old and New Buildings; Energy Reduction from Chilled Beams

Donald Haiges, P.E., SEi Companies

While chilled beam (CB) technology has been used in Europe for many years it is still relatively new in the United States. The CB concept, however, is not a new engineering principal but rather a new application. CB solutions are a variation of the chilled ceiling concept that was very popular in U.S. hospital design for patient rooms in the 1970s. The chilled ceiling provided uniform cooling without drafts for a generally light, sensible cooling load. “Chilled beam” is just a fancy term for an overhead room, chilled water cooling system. CBs can provide heating as well as cooling. Their use is becoming popular as a means to save energy, as well as to improve thermal comfort.

How They Work – CBs come in two general styles: passive and active. The passive CB is so named, as it is basically a static device. Looking like a horizontal finned car radiator, cooling is achieved by radiation and convection. The radiation effect works like this: heat from people and equipment radiates to and is absorbed by the CB. Also, the finned surface of the beam sets up natural air convection in the room. Since warm air naturally rises and cool air falls, warm air above the beam is cooled by the beam's finned element and then falls down into the room.

Photo of a passive chilled beam.

Passive CBs above slatted ceiling yet to be installed.

The active CB has greater capacity than a passive beam. The active beam is so named because its capacity is enhanced by a ducted supply-air connection to induce air across the CB coil via nozzles, producing greater cooling per square foot of beam surface area.  This principle is not unlike perimeter floor mounted induction units that were prevalent in high-rise building design in the 70's and 80's. The active CB is an induction unit.

Laboratory Application - To understand a proper application for the use of CBs in a laboratory application, we need to understand the HVAC laboratory dynamic and the derivation of the laboratory heat and air flow balance. Generally speaking, there are three types of laboratory scenarios: 1) Hood Driven Loads—where the air flow and air change rates are dictated by high hood densities such as a chemistry, research, or teaching laboratory; 2) Neutral Loads—where the minimum required air change rate or a light hood load is equivalent to the room sensible load. Such would be a biology laboratory with a single hood or biosafety cabinet and light equipment loading; and 3) Equipment Driven Loads—where the room air flow is determined by high internal sensible equipment heat gains, being greater than the hood makeup or minimum air change requirements. It is in this third scenario that the CB solution has the greatest opportunity for application.

In scenario three, the use of a CB can offer first cost and energy savings by reducing the primary total supply air compared to a conventional all air solution. Since most laboratories as a rule do not use recirculated air, all the air required to satisfy the room cooling load over and above the hood exhaust or a minimum air change rate is an added energy burden since it is all raw outdoor air (OA). If the added cooling load can be cooled directly by chilled water rather than OA, the energy to cool down or heat up the OA to room conditions is saved. The table below shows a simple benefit example of a three air change per hour (ACH) reduction in OA on an all OA system. These savings assume a 10 foot laboratory ceiling height, a summer design condition of 78° wet bulb, and a winter temperature of 0° F.

3  ACH  OA  REDUCTION  –  EXAMPLE SAVINGS

Square Feet

NSF

Air Reduction

CFM

Refrigeration

TONS

Heating

PPH

Horsepower
Sup & Exh
BHP

10,000

5,000

35

350

10

25,000

12,500

85

850

20

50,000

25,000

170

1,700

40

 

A fourth scenario within a laboratory building is the supporting laboratory offices, conference, teaching areas, and the like that are sensible load driven. Here the CB can be used to reduce the total room air flow otherwise required by an all air system. For example, assume a single office with a solar exposure requiring 250 cubic feet per minute (CFM) of air to meet the cooling load.  If the room requires only 50 CFM of ventilation and pressurization air, a per room savings of 200 CFM return per circulation air can be achieved. The supply and return CFM savings soon add up and, in turn, save fan horsepower.

Potential for Outdoor Air Reduction - Engineers know that for the research facility, the single most effective way to reduce mechanical and electrical system size, energy, and operational cost is to appropriately minimize the outdoor air requirement. From the above examples we can see how use of CBs can be used to achieve these reductions.  Such a reduction significantly and directly impacts the air handling systems and equipment; this in turn affects the refrigeration, heating, and electrical loads and equipment sizes. The idea looks like this:  Outdoor air = AHU equipment size = refrigeration = heating = electrical power = building square footage = floor to floor heights = building volume = efficiency = construction cost = long term owning and operating cost.  Hence we can see that if the CB is reducing air quantity, then we can see a corresponding reduction in supply, return, and exhaust ductwork and primary air handling equipment sizes. Since this air is most often 100 percent outdoor air, the resulting savings are even more dramatic with reductions in refrigeration and boiler sizes and capacity.

In the past, engineers have achieved OA reductions in laboratories through the use of unitary fan coil units (FCU). The FCU benefit in the equipment driven laboratory (scenario three) is apparent by providing high sensible cooling capacity without the added outdoor air load. The disadvantages have been the large fan coil equipment located within the laboratory along with the corresponding noise, maintenance of fans and filters, and the electrical connections. The CB is generally smaller in size than the FCU, can be designed to be very quiet, and has no unitary fans or filters for maintenance.

Case Study - When considering laboratory renovations in older facilities, a major obstacle is often adding the necessary utility infrastructure for the needs of today's more demanding research. From a design standpoint, the need for significantly more air, cooling, and power densities, and working in an occupied building all provide added challenges for the design team.  Such a design scenario may provide an opportunity for a CB option. Recently the Historic Main Building Group at the Massachusetts Institute of Technology (MIT) underwent a major renovation. The 250,000-square-foot (SF) first phase renovation and addition to this 1.1 million SF complex upgraded the century-old facility to position the building's infrastructure for 21st century research. The areas of research included physics, materials science, and spectroscopy, along with the typical office, classroom, and lecture components. The space types were very similar to the scenario types three and four noted previously.

The table below provides the comparison of three systems considered for the MIT modernization; all are variable air volume (VAV), air plus fan coils, and air plus CBs. Although the VAV system was considered for comparison, from a practical standpoint, the VAV scheme could not fit into the building due to the extensive ductwork required, the low floor-to-floor height, and the lack of available vertical chase space. In the table, note the significant reduction in the total required air CFM and its impact on air horsepower. The reduction in tonnage between the options is attributed to using an expanded comfort envelope for the CB design, i.e. 78°F, 60 percent relative humidity (RH) vs. the more normal 75°F, 50 percent RH. Some CB manufacturers contend that people feel at least as comfortable at 78°F with radiant cooling as they do at 75°F with overhead air systems. 

SYSTEMS COMPARISON  -  250,000 SF

System

OA  CFM

Total CFM

Tonnage

Air HP

Conventional VAV

110,000

325,000

1,100

550

Fan Coils

110,000

460,000

1,100

300

CBs

110,000

130,000

900

190

 

Condensation Control - From an engineering perspective a significant design element with the use of the CB is attention to room dehumidification control to assure condensation does not occur on the CB surfaces or the related chilled water piping. For this reason, use of CBs in the areas with high internal latent gains can be quite problematic. Room dehumidification is controlled by the room make up air requirement, with the supply air dehumidified below the room design dew point. This “dry air” is used to offset the internal room latent heat gain. Further, chilled water supplied to the CBs is not at the traditional 42°F cold water temperatures but more in the 58-60°F range, or 3°F above the room control dew point. This guarantees that condensation will not occur on the beam or piping. While active CBs can be provided with a drain pan, dehumidification is best controlled through the room supply air.

Piping Tips - In many CB applications, a separate CB piping system is provided due to the need for an increased supply water temperature, 58 to 68°F for example. In many laboratories, some level of process cooling may still be required or some use of fan coils are still necessary for cooling densities beyond the capacity of the CB. In one recent laboratory renovation, where a 42 to 56°F chilled water cooling loop already existed, this loop was used for the CBs incorporating a three-way blending valve and local zone fractional horsepower circulating pumps. Savings were achieved by not installing a separate CB central piping loop.

Saving Reheat – Previously, it has been discussed how energy savings can be achieved through reduced air flow and the reduction of OA with the corresponding heating and refrigeration reduction. In the laboratory, CBs can provide further savings through the reduction of reheat energy associated with all-air systems. In laboratories where supply air quantities are determined by peak equipment cooling loads, reheat is necessary to maintain room conditions when those loads do not materialize. When primary room supply air can be reduced through the use of the CB, or when winter supply air temperatures can be raised allowing room cooling by the CB, reheat energy can be significantly be reduced.

Summary -

The benefits of the CB application are many:

  • Energy savings through reduced air motor horsepower (HP) and air-side equipment sizes.
  • Reduced outdoor air loads in most laboratory applications reducing chiller and boiler sizes.
  • Energy savings by reducing the need for reheat.
  • Quiet operation due to reduced air volumes.
  • Improved and assured actual ventilation rates.
  • Reduced duct sizes, especially beneficial in renovation applications.
  • Improved air comfort through fewer drafts.

Yet the CB solution in not a “Silver Bullet!” As with any HVAC design, the full application must be evaluated with its corresponding limitations:

  • The CB becomes a ceiling design element requiring significant architectural coordination.
  • Design in high humidity areas requires control to prevent condensation on the coil surface.
  • The CB is not conducive to high latent cooling applications.
  • The CB installation is new to the building trades, suggesting an installation cost premium.
  • Design application will improve with honest case studies of new installations.
  • There are few manufacturers and many are located outside the United States.

We should see more acceptance of CB solutions as the design and construction community becomes familiar with the systems and as proven operational track records are established. CBs can provide a great alternative to fan coil and all air systems given the appropriate application.

View this entire presentation in PDF format (1.4 MB, 22 pp)

Biography

Donald Haiges is a Principal with the SEi Companies, a national mechanical and electrical engineering consulting firm. He is an architectural engineering graduate from The Pennsylvania State University with 30 years of engineering practice, responsible for the design of institutional and corporate research facilities. Don has been responsible for the engineering conceptualization, design, and start-up of more than 15 million square feet of such research facilities.  Representative clients in the academic community include MIT, Harvard, Columbia, Princeton, Vanderbilt, Brown, Tulane, Yale, and Middlebury. Clients in the pharmaceutical arena include Dupont, Lederle, Merck, Hoffmann-La Roche, Novartis, Pfizer, and Wyeth-Ayerst.