Next-Generation Cooling Is Looking Up
May 2, 2007
No one likes being wrong, and I suppose that consulting engineers like myself may hold this sentiment more strongly than others. However, I will take this risk by making a prediction that VAV systems will lose their place as the system of choice for large commercial applications at some point during the course of my career. It should be realized that the VAV system, which has served us well by providing comfort and working to ensure the health of building occupants, is unfortunately an energy abuser. This may seem an unfair characterization, since VAV rescued us from the constant volume reheat era that existed prior to the energy crisis of the 1970s. However, this system relies on electric-powered fans pushing large volumes of air through a distribution network consisting of bent sheet metal, screws, and lots of adhesive. If the comfort of the building occupant is threatened, a control system will reduce the amount of air delivered and, as a last resort, the air will then be reheated.
HVAC systems consume enough of the world’s energy that others outside of our industry are beginning to take an interest in what we do for a living and are watching and commenting on how we conduct our business. You may have noticed that the political and social climate is changing and that this could significantly influence the way we design building services very soon. So what does all of this have to do with chilled beams? Well, this technology is one of several that promises to offer relief from the excessive use of fan energy as well as from the reliance on reheat to achieve comfort.
Individuals in our midst are on a quest to uncover an HVAC system that will reduce energy consumption while not compromising the level of comfort to which our clients have grown accustomed. Not only can chilled beams decrease energy use, but they can also increase building efficiency by reducing the floor space required for mechanical rooms and by decreasing the floor-to-floor height required for ductwork.
Many technologies have the potential to positively alter the built environment, while others merely possess a unique approach and have been subsequently exploited by marketing departments.
Chilled beam technology should not be confused with radiant cooling, although there are some similarities. Chilled beams are mounted overhead, whereas radiant cooling can be in the form of piping in the floor slab, capillary mats integrated into walls and ceilings, as well as tubing mounted to the back of ceiling panels. Radiant cooling systems do not employ fans, while certain chilled beam approaches do. Both radiant cooling and chilled beams utilize warmer chilled water temperatures and require a dry atmosphere to prevent condensation.
However, an important difference is that chilled beams rely mainly on convection, as opposed to radiation, to transport heat from the space they serve. Chilled beams have been manufactured and used in Europe and Australia for about 15 years, but they are now getting noticed in the U.S. due to the efforts of individuals and organizations interested in tackling the perceived problem of building energy use.
Chilled BeamsThe term “chilled beam” should not be construed to imply that the building structural elements are utilized or incorporated into the construction of these devices, as they are simply standalone components that are typically hung at the ceiling level from the building structure. Certain radiant cooling systems are, however, integrated into the building either in the slab or plastered into the walls or ceiling. A chilled beam is constructed of copper tubing mechanically bonded to aluminum fins, all housed into a sheet metal enclosure for aesthetics.
There are two categories of chilled beams. The first is the static or passive beam that relies on natural convection to accelerate warm air at the top of the room past the cool finned elements of the beam. The passive chilled beam can also be manufactured in a “hybrid” manner to incorporate a radiant ceiling panel feature to augment the convective heat transfer.
The second type of device is termed a ventilated or active beam and uses conditioned outdoor air ducted from a central system to pass over the cooling element. The active beam can also be designed so that the ducted ventilation air is delivered at a high velocity to aid the convection process by inducing three to five times the amount of room air past the cooling element. Active beams can be of either the open or closed type with the latter designed so that only room air is induced over the heat exchanger, keeping the air located above the ceiling plenum out of the process. These active beams are similar to the high-pressure induction units that can still be found in many U.S. high-rise buildings constructed years ago.
A chilled beam can be specified with many useful features in what is termed an integrated service beam. Service beams can include direct and indirect lighting, occupancy and daylight sensors, smoke detectors, public announcement speakers, openings for sprinklers, as well as cabling and pathways for voice, data, and power.
These beams can be specified with isolation and control valves as well as the attendant piping specialties, which would be housed in an extension of the beam and reached through integral access doors. Service beams can be mounted exposed, can be incorporated into a suspended ceiling, and in fact, can be designed to include the ceiling system as one complete assembly. The advantages to this approach are a single source of responsibility for many building service components as well as minimizing coordination and perhaps overall installed cost.
ApplicationThere is an art to the positioning of chilled beams to maximize performance and to enhance occupant comfort. For instance, passive chilled beams can be placed parallel to exterior perimeter walls to counteract the effects of solar gain. In fact, the rising warm air near windows that will naturally come in contact with the cooling element will increase its effectiveness as a heat exchanger. There should be sufficient space between the top of the chilled beam and the underside of the structure to ensure that the naturally buoyant warm air can rise up past the beam, make its turn, and then accelerate past the cool fins of the heat exchanger. The velocity is highest directly below the passive beam and therefore, these beams should not be placed directly above workstations. The use of perforated ceilings with at least a 50% free area can reduce the risk of potentially uncomfortable drafts below the beam.
Likewise, placing a passive beam directly above a high heat load such as a copy machine can reduce the capacity of the beam, as the rising warm air will counteract the falling current of cool air attempting to develop off the beam.
Unlike passive beams, active beams can be placed directly over workstations and high convective loads, as these types of beams induce warmer room air up through their center and then disperse the air laterally across the top of the room. This is similar to the Coanda effect generated by high induction diffusers in the cooling mode. However, the upward convective flow of internal heat loads located further away can affect the performance of an active beam by disturbing the cool jet of air leaving the beam.
The recommendations appear to be mixed on the positioning of active beams, as it is most economical to maintain long runs, which usually means positioning them perpendicular to the exterior walls, even in shallow buildings. This approach is reserved for large open office areas and in the case of enclosed offices at the perimeter, the beams are often positioned parallel to the outside wall. However, in cold climates this can work to cause drafts along the windows unless a unidirectional beam is selected.
The literature on active and passive beams seems to point to an inherent sensitivity in these devices that may warrant attention by the designer in applying these products. There is mention of constructing full-scale mock-ups as well as the use of computational fluid dynamics, which are not normal design procedures in the U.S. However, manufacturers can provide software and assistance in the application of their chilled beam products. Beams have been used outside of the U.S. for about 15 years, so there is sufficient information available, but it is predominantly in metric units. The cooling capacity of an active chilled beam is roughly 80 Btuh/sq ft (250 W/m2) and can be stretched to 110 Btuh/sq ft (350 W/m2). Passive beams have a lower output at about 50 Btuh/sq ft (150 W/m2) and peaking at the lower end of the active beam range of 80 Btuh/sq ft (250 W/m2).
This being said, there are recommended cooling design values for the application of chilled beams, which are lower that the catalogued output. The recommended “optimum” range is between 20 Btuh/sq ft (60 W/m2) and 25 Btuh/sq ft (80 W/m2) or roughly 500 to 600 sq ft/ton. This capacity is more than adequate for many commercial applications considering that the ventilation load would be handled by another system.
The limits of active and passive beams can be increased to accommodate even severe cooling conditions imposed by a high solar gain and thermal conductance through the building façade. Even so, it should be stressed that all new construction, whether using chilled beams or conventional technologies, should proceed with the commitment for a tight building and a high-performance envelope that would include pressure testing, enhanced insulation, and spectrally selective glazing.
Because chilled beams are mainly a convective technology and are mounted at the ceiling level, their effectiveness in the heating mode is limited and should not be expected to produce more than about 8 Btuh/sq ft (25 W/m2) to 16 Btuh/sq ft (50 W/m2). This upper end should be considered adequate for many applications even in cold climates, as the often-significant ventilation load is taken care of elsewhere.
Nonetheless, it is prudent to consider using a perimeter heating system, especially beneath windows. The concept of heating with active beams as opposed to passive beams makes more sense as the active beams have the ducted ventilation air as a motive force. For this reason, the ventilation system needs to be operating for the beam system to meaningfully contribute to the heating of the building. Passive beams rely indirectly on currents developed from cold glass and ventilation air from nearby diffusers, but you do not want placement of diffusers to interfere with passive chilled beam operation in the cooling mode.
Distribution And Control SystemsA critical element in the successful application of chilled beam systems is condensation control where one must ensure that the beam and associated piping specialties do not drop below the ambient dewpoint for sustained periods. Elevating the entering water temperature can work to a point, unless there are high sensible loads that would make maintaining the drybulb setpoint impossible. Typical entering chilled water temperatures range between 57°F and 64° (14°to 18°C) and the more daring practitioner may aim for a room dewpoint temperature equal to this value and therefore may specify insulation on the valves and piping to the chilled beam.
A more conservative approach is to keep the room dewpoint temperature a couple of degrees below the entering supply water temperature, which means that after accounting for latent loads in the space, that the introduction of ventilation air would result in dewpoint temperatures between roughly 56° and 62° (13° to 17°C ).
With regard to piping and insulation, a vapor barrier need not be included on the piping systems in many applications and some designers choose to use an uninsulated, thick-wall plastic piping system, as the thermal resistance of the plastic keeps the surface temperature warmer than metallic pipe.
The chilled beam concept does not preclude the use of natural ventilation or operable windows, as there are installations that use devices such as dewpoint sensors and window position switches to automatically raise chilled water temperatures or to shut off flow to beams when the situation warrants.
Interesting central plant concepts can develop from the use of chilled beams, with one being that during warm weather chilled water generated at a traditional 44° (7°C) can first be run through a dedicated outside air unit and the return water from this unit can then be delivered to the chilled beams. The temperature drop across the beam can be expected to be about 7° (4°C), which would yield return water temperatures in the mid 60°s to perhaps 70° (21°C). This could result in an efficient multistage central chilled water plant.
Low outdoor air dewpoint temperatures during cool weather allow the ventilation system to operate without the need for chilled water and the chilled water central plant can then distribute directly to the chilled beam system and with the warmer temperature requirements.
A better approach would be to allow chilled water to pass through the ventilation system cooling coils where it can do double duty by pre-heating the outdoor air as well as obtain some free cooling for the chilled beam loop. With warmer loop temperatures there can be extended periods where free cooling can be obtained from a cooling tower-driven waterside economizer or through a ground coupled heat exchanger. In fact, well water or deep surface water may be able to provide a year-round source of cooling by supplying roughly 60° water without the need for a refrigeration system.
As chilled beams operate to cool the building, the building ventilation system need only be sized to account for the amount of fresh air needed, which can drastically reduce the size of the system in many applications. The fresh air can also be controlled and directed in a more accurate manner by having it delivered based upon space requirements. Ventilation systems can be constant or variable volume, high or low pressure, and air can be delivered overhead through diffusers or active beams.
Air can also be delivered low, in a displacement ventilation configuration or can be supplied in an underfloor application. A benefit to the designer is that there would be no need for the laborious “XYZ equation” that plan examiners have become fond of scrutinizing.
With lower airflow rates, reheat energy can be eliminated along with the capital expenditures associated with this wasteful practice. The ventilation system equipment can be configured with heat recovery to extract energy from the air before being relieved from the building and with no return airstream to contend with, a portion of the heat generated by the lighting system leaves the building before it has a chance to reach a cooling coil and contribute to the load.
A system that relies mainly on passive, as opposed to active, beams may have large areas that may be able to be designed without the use of room thermostats as these beams can be somewhat self-regulating when the design chilled water temperatures happen to be close to the room temperature. As load begins to be imposed upon the room, the room temperature will rise, thereby increasing the temperature difference between the space and the beam, which will consequently increase its output.
It seems natural that when designing the cooling system around chilled beams, choosing a radiant heating method may compliment the cooling system. Radiant heating applications often require lower water temperatures than other types of heating systems, which can lead to a more energy-efficient heating plant if condensing boilers are used since the return water temperature is cooler and can therefore extract more heat from the flue gases.
ConclusionsSocial awareness and the resulting political pressure to reduce energy consumption are such that even the most stubborn of engineers must sense this changing environment. Consequently, it is our responsibility to explore concepts that have the potential to accommodate society’s desire for positive change in the way we design, build, and operate buildings. The aspect that makes chilled beams such a promising technology is its emancipation from reheat as well as from large fan systems and the resulting reliance upon obtrusive ductwork.
Besides the benefit of conserving energy, I believe that it would be nice to live in a world where mechanical engineers do not have to wrestle with architects over issues such as floor-to-floor heights and the size of rooms for central ventilation systems. None of this information presented here is new; however, I have become a convert of what I envision as perhaps the next big HVAC trend, which is to minimize both fan and reheat energy. What VAV systems were to the last three decades, chilled beams or similar concepts could be to the next three. ES