All-air VAV system versus chilled-beam-system energy consumption at design and part-load conditions

At full design load and at 72% of design load, both systems must deliver the same amount of cooling to the space to maintain room temperature set point. Further reduction of load in the room results in all-air VAV systems delivering the same level of cooling and having to offset overcooling with parasitic reheat. Chilled-beam systems are able to reduce energy consumption by reducing cooling delivered to the space without the use of parasitic reheat.

12/12/2016


Learning objectives:

  • Inform consulting engineers on the benefits a chilled-beam system offers health care facilities.
  • Identify the best applications for chilled beams.
  • Learn why chilled beams contribute to energy savings as well as patient comfort and construction benefits.

Used in Europe for decades, chilled beams are gaining traction in health care facilities within the United States, both in new construction and retrofit applications. This energy-saving alternative to conventional HVAC systems uses a cooling coil to provide sensible cooling via circulated chilled water in the space, reducing air handler load and size.

Chilled beams come in both passive and active configurations. A passive chilled beam provides space cooling as warm air rises to the beam, is cooled, and then falls back toward the floor, creating convective air motion. An active chilled beam relies on air supplied from a remote air handler to induce room air across the chilled-water coil, increasing the sensible capacity as compared with passive chilled beams. In both cases, the beam uses water at a temperature that is higher than the temperature of water supplied to a conventional HVAC system, which significantly reduces chiller energy use. In addition, cooling is delivered via water with pumps rather than air with fans, resulting in additional energy savings.

At full design load and at 72% of design load, both systems must deliver the same amount of cooling to the space to maintain room temperature set point. Further reduction of load in the room results in all-air VAV systems delivering the same level of cooling and having to offset overcooling with parasitic reheat. Courtesy: Johnson ControlsChilled beams minimize primary air requirements

In general, chilled-beam systems can be installed wherever the ratio of sensible cooling to latent cooling is no lower than 0.7. In other words, 70% of the cooling load is sensible cooling and 30% is latent cooling. An application like this is ideal for active chilled beams because it minimizes the amount of primary air that must be brought to the beam. Anything lower requires additional primary air to dehumidify the space, which in turn, increases the size of the air handler and ductwork, requires more energy to move the air, and begins to compromise the benefits a chilled-beam system delivers.

The best applications for chilled beams are spaces that exhibit a significant difference between the sensible cooling load and the airflow required for ventilation. A good example is a hospital laboratory where processes or equipment generate a considerable amount of dry heat. A conventional (all-air) HVAC system brings in 12 to 18 air changes per hour to handle the high cooling load. With a chilled-beam system, the number of air changes could be as low as six, reducing airflow by up to 300% and resulting in considerable energy savings.

Health care facilities benefit from chilled beams

Patient rooms within health care facilities with few people and relatively large spaces represent another good application for chilled beams, because they have high prescriptive ventilation rates per code and low sensible loads. ANSI/AHSRAE/ASHE Standard 170-2013, Ventilation of Health Care Facilities, requires that a minimum of four total air changes per hour is supplied to the patient room. Two of those air changes must be outside air. Recent revisions to the standard allow induced or recirculated air to count toward the non-outside-air changes, opening the door for chilled-beam technology and its ability to provide the necessary space cooling with just the two outdoor-air changes.

A VAV system in a patient room requires that at least four air changes be maintained, regardless of the cooling load. If the system throttles back in partial-load conditions and reaches the point where it is only bringing in four air changes, any additional drop in the cooling load means the HVAC system will continue to supply the same volume of air to the room. To prevent overcooling and the resulting patient discomfort, the air being supplied to the space for the four air changes must be reheated, expending additional energy to maintain the temperature in the space.

A chilled-beam system operating under the same conditions simply reduces water flow to the  coil—or even shuts off the coil as the load decreases—and brings in only the two outdoor-air changes, using recirculated air for the other two required changes. Reheat is not necessary and fan energy is reduced. The result is a comfortable patient and substantial energy savings—energy associated with the water coil, the fan, and the absence of reheat.

Chilled beams save energy

According to the U.S. Energy Information Administration, HVAC energy makes up about 50% of all the energy consumed in a hospital, and half of that energy can be attributed to reheat functions, so reducing reheat significantly impacts a hospital’s energy consumption. Health care facilities also account for 9% of all the energy consumed by buildings in the United States, the result of high prescriptive airflow rates and 24/7 operations. Half of that number, or 4.5%, can be attributed to HVAC energy use. This suggests that installing chilled-beam systems in all patient-care facilities in the United States would result in significant savings, in large part because reheat would no longer be necessary.

Chilled beams enhance patient comfort

In addition to energy efficiency, a chilled-beam system contributes to patient comfort in part because the supply-air temperature in the space is much closer to the room temperature. Air is distributed evenly through the space, eliminating drafts and cold spots. Another benefit from a comfort perspective is quiet operation. Unlike VAV systems that ramp up and down to handle the cooling load, an active chilled-beam system provides consistent airflow at low volume, making it easier for patients to rest and sleep.

Additionally, a chilled-beam system contributes to improved indoor air quality, because the supply airflow is 100% outdoor air. In a health care space, air changes beyond the required outdoor-air changes are either recirculated from the air handler or are provided by room recirculating units, such as chilled beams. When using chilled beams and 100% outdoor-air systems, airborne pathogens generated by a patient do not return to a main air handler, this prevents pathogens returned to the air handler being distributed to all the spaces served by that air handler. Cross-contamination is avoided, and because there is no condensation on the coil, the likelihood of bacterial, fungal, or mold growth is substantially diminished.

Chilled beams also offer reduced maintenance costs, since there are no moving parts. Cleaning can be accomplished from the face of the unit. Lint, the result of frequent change-out of bedding in patient rooms, is less likely to accumulate on chilled beams than in HVAC ductwork because the coil surfaces remain dry. However, it is recommended that the beams be checked for lint buildup quarterly and vacuumed when necessary.

Low profile saves space and money

In addition to operational benefits, chilled beams deliver construction benefits. The systems can reduce the size/capacity of an air handler and ductwork because of lower volumes of supply air. Their low overall height can also save as much as 60% vertical space as compared with a conventional air system, which reduces slab-to-slab spacing and saves material costs throughout the entire floor. On the other hand, the low profile of chilled beams can allow for higher ceilings for a more visually appealing, open space.

It is worth noting that chilled beams can be customized to enhance a particular architectural style. For example, linear or cassette chilled beams come in 2- to 10-ft lengths and 12- or 24-in. widths. Others are designed to fit in a bulkhead or soffit or under windowsills. The latter work well in retrofit applications, where the beam system is replacing induction or fan-coil units located under windows.

When specifying chilled beams for new construction or renovation projects, engineers should be sure to have a control strategy in place to deal with condensation, in the event the building gets out of control from a humidity and temperature standpoint. However, it should not be a strategy that drives the cost of the project so high that it is no longer competitive with a VAV system. The goal is simply to ensure that the dew point in the space remains below the temperature water supplied to the beams. Of course, the tighter the building envelope, the less likely that humidity becomes a problem.

Everyone benefits

At a time when hospitals are being asked to do more with less—less money, less staff, less energy—chilled beams offer an attractive alternative to their more costly variable air volume counterparts. They have the potential to reduce equipment and material costs in new construction projects and fit nicely in retrofit applications. They offer significant energy savings, reduced maintenance requirements, and easy operation, which should please facility managers. Just as important, they contribute to patient satisfaction (and the reimbursement frequently tied to patient satisfaction) with the promise of consistent, quiet operation, good indoor air quality, and even cooling.


Jeff Scanes is director of air-distribution product sales at Johnson Controls. Jeff holds a Bachelor of Science degree in electronic engineering and has more than 25 years of HVAC sales engineering experience. Jeff is also a member of ASHRAE and ASHE. Edited by Joy Chang, digital project manager, CFE Media, jchang@cfemedia.com.



Anonymous , 01/05/17 01:11 AM:

A design in HC facility must have considerations to reduce infection to benefit operator and clients, then with CB reduction of Air return, how would Chilled beam increase this factor ?
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