Questioning Active Chilled Beams
I read with some concern the enthusiastic article about active chilled beams (EBN). When I looked into these for a lab I was informally reviewing, they required outside ventilation air to deliver heating and cooling at unoccupied times in the building—when ventilation air often is not required at all, unless you have an air system that can be changed to use all inside air during unoccupied hours (if the entire portion of the building served by that air handler is unoccupied). The article compared these to VAV [variable air volume conditioned air delivery], which is a horrible system for a lot of reasons, so pretty much anything short of lousy rooftop units should look good by comparison!
A comparison that should be made—in my mind—is to a radiant-ceiling system, which offers many of the same advantages while reducing pressure required in the ventilation air system and not requiring outside air to be delivered whenever heating or cooling is required. The radiant system allows real separation of ventilation from heating and cooling delivery. Massive radiant (in slabs) might also make a good comparison. I suppose that if enough outside air is needed 24/7, the active beams may have more of a place, but I think those spaces might be rare. I also have been concerned about the lack of filtration of the air that is moved over the coils by induction. Would these coils get filthy over time?
The article also notes that passive chilled beams usually use underfloor ventilation. I am not sure why this might be true—air could just as well be ducted above, at lower cost and with less risk of issues associated with underfloor. It also says that passive beams require added ventilation—more than what? Ventilation air is separate and so can be controlled to the minimum needed for ventilation, not for inducing air flow over the coil. The article implies that ACBs use less energy because they move heating and cooling by water, not air, but, as noted above, they actually require MORE air than a passive radiant system.
As I said, I am certainly no expert on these systems, but am I missing something? The article sounded to me like it relied too much on manufacturer’s information.
Energy Balance, Inc.
As is typically the case in researching articles for EBN, we used a number of different sources, including manufacturers, technical publications (such as ASHRAE Journal), and experts in the design community. One of those experts we interviewed for the article, Peter Rumsey, P.E., of Rumsey Engineers in Oakland, California, offered the following response to Shapiro’s concerns:
1) ACBs do require ventilation air in order to produce needed heating or cooling. We have found in ACB buildings that the building can be kept at required unoccupied temperatures with a small amount of tempered ventilation air. The small amount of ventilation air will result in less induction and thus little heating and cooling from the coil in the ACB.
2) We design for no more than 0.5” of pressure drop through the chilled beam nozzles. This is similar to the pressure drop through a VAV box that is partially open. We also design low-pressure-drop ducting and low-face-velocity air handlers. Also, remember that we are pushing significantly less air through the building. Taken together, we are realizing 50% to 75% fan energy savings in ACB projects and only a small increase in pump energy.
Rumsey, who is one of North America’s leading innovators in the use of ACBs, appreciated Shapiro’s comments. “We look forward to more of the brightest engineers in the country taking a close look at ACB systems,” he told EBN.
Andy, S. (2010, May 1). Questioning Active Chilled Beams. Retrieved from https://www.buildinggreen.com/editorial/questioning-active-chilled-beams