Laboratories are not known for their energy efficiency. These spaces can consume up to 10 times more energy than office buildings, leading facility managers and engineers to prioritize finding ways to reduce energy consumption and operating costs – without sacrificing efficiency. One way to do that is by using chilled beam systems, which have grown increasingly popular in the U.S. in the last decade. Chilled beams have proven to be viable alternatives to traditional Variable Air Volume (VAV) systems, demonstrating energy savings upwards of 20% in laboratories compared to VAV Reheat.
To better understand how and why chilled beams are effective in laboratories, let’s examine the usage of these systems, strategies for effective design and operation in laboratory environments.
How Chilled Beams Work
There are two types of chilled beams, passive and active.
Passive chilled beams cool spaces using natural convective forces and include a heat exchange coil in an enclosure that is suspended from the underside of the building structure. Chilled water flows through the coil and cools the surrounding warm air; the denser cool air falls back into the space. Passive beams require separate air diffusers to carry dehumidified ventilation air into the space, usually at the floor level. For this reason, they are rarely, if ever, used in laboratories.
Active chilled beams rely on pretreated primary air delivered from central air-handling units (AHU’s) to pressurize a series of small induction nozzles within the chilled beam unit. These nozzles create jets of air causing room air to be induced across a coil where flowing water heats or cools this (secondary) air.
Both passive and active beams are designed to provide sensible cooling only with the latent cooling (i.e. space dehumidification) being accomplished by the central AHU’s. Depending on the lab use and loads, the primary air is delivered to the active chilled beams as constant or variable volume, with the cooling/heating output being controlled by either two position or modulating control valves to vary the water flow through the integral coils. Chilled beams can be integrated into suspended ceiling systems or hung from the structural slab for exposed use.
Because chilled beams provide most of a space’s sensible cooling, the central air handling system can be much smaller than usual since its primary purpose is to provide the ventilation air and latent cooling to the space. This effectively decouples the sensible cooling from the ventilation requirements. And since chilled beams have no moving parts, maintenance is limited to infrequent cleaning of the coils.
Laboratory HVAC Energy Use
The ventilation requirements for laboratories are different from the needs of a typical office building. Minimum ventilation rates are dictated by safety requirements rather than cooling or heating loads, while maximum rates are determined by either the make-up air requirements for the fume hoods or the sensible cooling requirements of the space (if the equipment cooling load is high). There are special requirements for laboratories where chemicals or gases as present as well. They cannot use recirculating AHU’s, so the ventilation air must be 100% outdoor air at all times.
Overall, these parameters can result in an air system sized for ventilation air changes rates from 6-to-12 or more, depending on the lab use and equipment loads.
A traditional "all-air" system will typically deliver cool air to the building at around 55°F when there is demand for OA dehumidification, or to satisfy the sensible cooling requirements of the highest load lab in the building. This often results in a mismatch of ventilation air and cooling requirements, forcing the zone VAV boxes to reheat the cooled air to prevent over-cooling the space when sensible loads are low. Even more reheating occurs when ventilation air is increased to provide make-up air for the fume hoods, resulting in lower efficiency.
And decreased efficiency hurts the bottom line: Energy studies have shown that cooling and reheating air can account for as much as 20% of the total HVAC energy costs in laboratories.
Eliminating Reheat and Saving Energy
Active chilled beams can help boost energy efficiency in a number of ways.One is by eliminating most of the reheat energy resulting from decoupling ventilation and cooling demands. With active chilled beams the ventilation air can be delivered at a warmer temperature through a 100% outside air AHU, commonly known as a dedicated outdoor air system (DOAS). With the DOAS primary air set to around 65-70°F space overcooling is far less likely to occur, even in labs calling for high volumes of make-up air for the fume hoods. The water coils within the chilled beams provide cooling or heating capacity on a zone-by-zone basis. During OA dehumidification hours, the DOAS unit removes moisture by cooling the air to 55°F or below and is reheated with energy recovered from the exhaust air using enthalpy wheels, heat pipes or run around coils.
Another is through using water to transport heat, resulting in an air system size reduction of 60 percent compared to VAV Reheat. This feature reduces overall fan energy consumption and is ideal for laboratories with high sensible loads and low fume hood densities.
The increased efficiencies associated with chilled beams also help laboratories maximize their space. The reduced reheat and boosted transport efficiencies of water mean the main plant items (chillers, boilers and AHU’s) can be smaller than with a traditional system’s. The duct distribution system is also more compact, which reduces service congestion in the ceiling interstitial. Finally, a smaller system can translate into lower first costs for an HVAC system compared to VAV Reheat.
There is a challenge with using chilled beams in labs, however: the need for dual chilled water temperatures. Specifically, a low temperature circuit (LTCHW, 40-45°F) for the DOAS and medium temperature circuit (MTCHW, 56-58°F) for the active chilled beams. The most common strategy is to design a closed secondary loop separated by a plate and frame heat exchanger, ensuring that the LTCHW cannot accidentlly find its way into the active chilled beams. That could potentially cause condensation on the coils.
Building Humidity Control
The best chilled beam system designs equip the building with a small number of room dew point sensors. This allows the building management system to monitor the humidity across the building and reset the DOAS air dew point or reschedule the MTCHW loop temperature if the space dew point rises above a preset temperature. Facilities engineers can use the room dew point sensors to precisely control the amount of OA latent cooling at the DOAS unit to further reduce energy costs, an operational strategy that is more difficult to accomplish with a VAV Reheat system. Despite being used in early system designs; pipe mounted condensation sensors are rarely used today since room dew point monitoring provides enough advance warning of potential condensation.
Chilled Beams Misconceptions
Despite growing in popularity over the last 10+ years, there are still a number of persisting myths and misconceptions about chilled beams. For instance, despite there being several successful installations in the likes of Florida, Hawaii and even the Caribbean, there is still hesitancy among designers and owners to use the system in humid climates because of condensation concerns. The reality is the system can be used in any building where the space humidity can be controlled; however, energy savings will not be realized in applications where the internal latent gains are high, such as wet labs. In other cases, engineers may be reluctant to consider active chilled beams because they are simply unfamiliar with the design of these systems.
Cost is also a concern. Mechanical contractors unfamiliar with chilled beams will be wary of underpricing a system they have never previously installed, but several case studies have shown chilled beams have been installed cost competitively with traditional systems.
Conclusion
It didn’t happen overnight, but a larger number of engineers and facility managers have realized – and are realizing -- the advantages chilled beam systems can offer to laboratory applications, specifically in terms of energy and space savings. Debunking misconceptions and educating laboratory owners on the construction of and design using chilled beams is the first hurdle to overcoming barriers to adoption. Then it’s about showing what the technology can do. Those who have installed these systems have realized greater energy efficiency, substantial cost savings and improved performance.
For information on this topic, please contact Nick Searle at nick.searle@titus-hvac.com or Titus Communications at communications@titus-hvac.com.
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