Active chilled beams - the open and closed types.
BRE pushes forth alternative solutions
Chilled beams as an alternative for air-conditioning systems are becoming increasingly popular all over the UK and Europe for their ability to offer potential energy savings. DAVID BUTLER of BRE, UK*, explains how this technology can be adapted in the Middle East.
01 April 2005
Driven partly by an interest in systems that offer potential energy and life cycle cost savings, and provide high levels of occupant comfort, the popularity of chilled beams for ventilation and cooling has grown steadily in northern Europe, including the UK, at the expense of more traditional systems such as fan coil systems and VAV systems.
Chilled beams are inherently simple devices with very low maintenance requirements and producing low noise levels and are often used in conjunction with displacement ventilation.
When they have to be supplied by chillers, especially in the hot climes of the Middle East, these should operate with a higher coefficient of system performance (COP) than with other types of cooling such as fan coil systems.
Chilled beams
The simplest type of chilled beam – the passive chilled beam – consists of a simple cooled element, which in most cases is a finned tube heat exchanger in a simple steel casing. Installed fully exposed below a ceiling, recessed within a ceiling or installed above a perforated ceiling, passive chilled beams provide cooling primarily through natural convection, although there is often some radiant cooling as well depending on where the chilled beam is located. Warm room air enters the top of the coil and is cooled as it moves downwards through the coil and then discharges from the bottom, as shown in Figure 1. There is no fan because the movement of air through the device is driven entirely by natural convection. Chilled water is normally supplied to the coil at no lower than 14 deg C in order to avoid condensation and to prevent excessively cold down draughts into the occupied space. Ventilation air is provided separately through ceiling or floor-mounted diffusers.
Active chilled beams have an integrated air supply in addition to the cooling element or coil. Primary air is introduced through nozzles along the length of the coil inducing room air to be drawn through the coil. A mixture of room air and primary air is supplied to the room, with induction ratios typically between 1:3 and 1:5. The air discharge is usually through longitudinal slots on both sides of the chilled beam, and depending on the velocity and slot profile may vary between horizontal and vertical discharge.
There are two sub-types of active chilled beam: active open chilled beam, and; active closed chilled beam. The open type of active chilled beam takes room air in at the top of the coil whereas the closed type induces room air through the coil from below the chilled beam and has an integral secondary return air path, as shown in Figure 2.
Integrated or multi-service chilled beams (MSCB) incorporate all ceiling-mounted services into one unit (Figure 3). These are typically based on an active chilled beam in a pressed steel casing and incorporating the lighting, sprinklers, PIR sensors, smoke detectors, PA systems and cabling. The advantage is that one factory-built unit, which integrates a range of services, significantly reduces installation time and cost. By requiring just one team of installers, it significantly reduces risk, and removes the difficulty of coordinating a range of trades. With these units, a suspended ceiling is not absolutely necessary, which can alleviate floor-to-ceiling height problems when refurbishing old buildings1, and allows more floors to be fitted into a given building height for new buildings (Figure 4). There is also a significant cost saving if a suspended ceiling is not required. Integrated chilled beams may incorporate both down and up lighting. A possible disadvantage is that the lifetime of the lighting units is likely to be less than for the chilled beam, which may cause replacement difficulties.
A proprietary variant of the active integrated chilled beam is also available as a ceiling cassette unit with built-in condensate tray and drain. However, chilled beams are normally designed to operate dry so that the room dew point temperature should normally be kept below the entering chilled water temperature.
Cooling performance
The cooling performance of chilled beams is usually quoted by manufacturers in terms of output per linear metre (W/m) of chilled beam. This has to be translated into W/sq m once the chilled beam layout has been determined. While the theoretical cooling capacity of a passive chilled beam (Figure 5) is proportional to the room-air and mean-water temperature difference, its practical cooling capacity is affected by the size and position of heat sources in the room. In extreme situations, a concentrated heat source immediately below a passive chilled beam may disrupt the flow of air through the coil and reduce its cooling performance. Chilled beams should ideally be located away from where occupants are normally present to avoid discomfort caused by direct cold draught. This is less of a problem with active beams because the air outlet can be arranged to be horizontal or near horizontal.
The values for maximum design cooling capacity for passive and active chilled beams as recommended by REHVA (Federation of European Heating and Air-conditioning Associations) are presented in Table 1. These values are based on maximum practicable performance and maximum performance for good thermal comfort, for people in office and other sedentary type occupancies. Higher cooling capacities are possible by using a greater number of chilled beams, or larger chilled beams, at the expense of thermal comfort.
The chilled water supply temperature is typically between 14 deg C and 18 deg C. To achieve a reasonable cooling capacity, the flow rate should be high enough to ensure turbulent flow in order to achieve good heat transfer (typically 0.03 l/s to 0.1 l/s for 15 mm tubes and 0.015 l/s to 0.04 l/s for 10 mm tubes). The entering and leaving water temperature difference is usually between 2 deg C and 3 deg C. This means that a higher overall flow rate is required compared with fan coil units to achieve the same cooling capacity. This will inevitably offset a small proportion of the cooling energy savings from having a higher chilled water supply temperature.
The temperature of the primary supply air for active chilled beams is usually between
18 deg C and 20 deg C, which is similar to the supply air temperature range for displacement ventilation systems.
In humid climates, where significant dehumidification is normally required, desiccant-based dehumidification or heat pipe technology may produce overall energy savings.
The cooling performance of passive chilled beams is critically dependent on having free air paths around the chilled beam. If a chilled beam is installed above a suspended ceiling then it is very important that the ceiling tiles in the vicinity of the chilled beam have large enough perforations to allow free passage of air into the ceiling void and out from the bottom of the beam into the room. Ideally passive chilled beams should be totally exposed to the conditioned space, and there should also be adequate clearance above a chilled beam for the entry of room air.
Displacement ventilation
Displacement ventilation, which is gaining popularity in the US and Scandinavian countries, is essentially a buoyancy-driven ‘displacement’ process. ‘Fresh’ ventilation air is introduced at low velocity and at a low level into an occupied zone at a temperature typically around 19 deg C, slightly cooler than the design room air temperature. Air is extracted at ceiling level. The supply air spreads out across the floor forming a reservoir of cool fresh air, which causes a vertical temperature gradient to develop (typically 5 to 6 deg C between supply and extract), resulting in higher temperatures at ceiling level than with standard mixed-flow ventilation systems.
Traditional Scandinavian design practice for office type buildings is to use displacement ventilation alone for cooling requirements up to around 40 W/sq m, and to use it in conjunction with cooled (‘radiant’) ceilings for higher cooling requirements. In industrial buildings much higher cooling capacities are achievable with displacement ventilation alone by lowering the supply air temperature and increasing air supply volumes. Research in the US3 and the UK4 suggest that higher cooling performance (from 50 W/sq m to as high as 120 W/sq m) could be acceptable in non-industrial buildings, but more research and case studies are required to confirm this.
Because chilled beams are convective cooling devices they are generally not used with displacement ventilation. Chilled (radiant) ceilings are often used instead. However, in perimeter zones, chilled beams are better able to meet the high solar heat gains than chilled ceilings and this is now quite a common approach taken in the UK.
Perimeter chilled beams
The concept of the perimeter-chilled beam (Figure 6) is very simple. Passive chilled beams are installed close to glazed façades or windows, and are designed to offset solar gains in the perimeter zone and minimise the depth of the zone of potential thermal discomfort. They minimise the disruptive effect that solar gains can have on air temperature and air circulation away from the perimeter, which is especially important where displacement ventilation and/or chilled ceilings are used.
A further advantage of locating chilled beams in the perimeter is that any warm plume rising from the window or blind enhances the temperature difference seen by the chilled beam raising its cooling performance without changing the size of the chilled beam or lowering the chilled water supply temperature. In some cases, a second row of chilled beams is also installed where the perimeter solar gains are too great for one row. In this way, solar heat gains around 300 W/sq m floor area can be effectively dealt with.
The performance of perimeter-chilled beams is highly sensitive to the design and configuration of the perimeter area including suspended ceilings and internal window blinds (where fitted). In practice this has often led to poor performance and conflict with architectural and aesthetic requirements, hence physical mock-up testing is often advised.
Maintenance
The lifetime of a chilled beam is expected to be at least 20 years, although with multi-service chilled beams some of the integrated services, for example light units, may need replacing more frequently than this. The only components local to the chilled beams are the controls, although the maintenance and service lifetime of these would be the same as for fan coil units.
Compared with chilled beams, fan coil units need frequent and regular maintenance including six or 12 monthly filter changes, cleaning of the condensate system two to three times a year and perhaps replacement of the fan motor every 10 years.
Chilled beams in humid areas
Chilled beams are normally operated dry because, apart from a small number of special proprietary chilled beams, they do not have condensate trays. Also maintaining dry operation reduces the requirement for either air filters or frequent coil cleaning. This means that the room dew point must be kept below the coldest part of the system. In practice, the primary air must be dehumidified and condensation detectors are also employed as a failsafe measure.
If condensation is detected then either the chilled water supply is stopped or the chilled water temperature is raised. Measuring room dry bulb temperature and relative humidity in order to determine the room dew point temperature is not a recommended method because of the need to regularly calibrate these sensors.
In hot humid climates, it is vital to control relative humidity of the space whenever chilled water is being supplied to the chilled beams. It is also important to minimise infiltration of outside air, which requires good attention to detailed building design and good workmanship. The air supply system should be started first in the morning to establish control of relative humidity in the space before the chilled water supply is started. Active chilled beams have been successfully used in a building in Singapore2 and there is no reason why they could not be used in humid Gulf coastal regions in well-sealed and controlled buildings.
There are no issues with the use of displacement ventilation in hot humid climates apart from meeting the required cooling capacity. In perimeter areas exposed to solar gains, ceiling void fan coil units or underfloor chilled water-cooling are all possible alternatives. For example, displacement ventilation plus perimeter underfloor cooling has been used at the new Bangkok International airport.
Energy efficiency benefits
Chilled beams typically operate with chilled water supplies of 14 deg C or higher instead of the 6 deg C usually required for traditional fan coil. Displacement ventilation systems usually use supply air at 19 deg C or higher. Both these technologies reduce cooling energy costs by allowing higher refrigeration efficiency (coefficient of performance) through reduced temperature lift.
For a typical vapour compression refrigeration system, a 1 deg C increase in evaporating temperature reduces energy use by 2 per cent to 4 per cent. The use of chilled beams may therefore result in between 15 and 30 per cent reduction in chiller energy consumption compared to the use of fan coil units. This is before any additional ‘free’ cooling benefits have been taken into account. However, there is a small pump energy penalty when using chilled beams due to the higher flow rates required to compensate for the lower chilled water flow and return temperature difference.
In humid climates, dehumidification loads are very high and the benefit of higher chilled water temperatures may not be realised unless the sensible and latent loads are served by separate refrigeration systems operating at different evaporating temperatures.
Conclusion
In the hot and humid regions there is less experience of using chilled beams, partly because it is often thought that their cooling capacity is too low, or that condensation would be too much of a problem. This paper has shown that neither needs to be the case. The condensation risk needs to be treated very carefully in humid regions but can be overcome in tightly sealed buildings with good control of the humidity of the ventilation supply air. Condensation detectors should stop the supply of chilled water to the chilled beams (or temporally raise its temperature to above the room dew point) if a failure of the dehumidification system occurs.
Obviously, chilled beams would not suit all buildings but they should be suitable for many commercial buildings with full air conditioning, good constructional standards and levels of maintenance and control.
References:
1 Hutchins G and N Pennell. Multi-service chilled beams (MSCBs) at the Empress State, Earls Court. CIBSE National Conference. Edinburgh. September 2003.
2 Virta M (ed). Chilled Beam Design. Draft Rehva design guidebook. Federation of European Heating and Air-conditioning Associations. Brussells. Draft dated September 2004.
3 Chen Q and L Glicksman. Design Guidelines for Displacement Ventilation. ASHRAE RP-949. American Society of Heating, refrigerating and Air-Conditioning Engineers. Atlanta. 2003.
4 Butler D, M Swainson and A Perry. Free cooling with displacement ventilation. BRE Information Paper IP 6/02. Building Research Establishment. Watford. 2002.
*This paper was presented at the Gulf Construction Conference Week.
