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Turn on, tune in, chill out

TECHNICAL & PRACTICE: The next generation of multi-service chilled beams should bring more benefits than just savings in time and money

'Chilled beams' are certainly a name to conjure with. After all, they're not necessarily chilly and they are not really beams, in the structural sense of the word. Perhaps there should be a campaign to rename them 'prefabricated service units', so that specifiers stop looking blank whenever the subject is mentioned.

In essence, chilled beams are simply service units designed to remove the excess heat loads commonly experienced in office buildings; those generated from solar gain, equipment loads (PCs, servers, copiers, etc), light fittings and body heat.

Traditionally, they take the form of beam-like units, usually hanging from, or set into, the suspended ceiling. Passive chilled beams work on the principle of warm air rising by natural convection and passing over water-filled cooling coils within the beam. These coils, supplied at about 15-18infinityC, reduce the temperature of the passing air, enabling cooler air to be reintroduced into the room and removing surplus heat. Active beams provide an additional cooling capacity by providing a primary fresh air supply, effectively wafting out cool air to assist circulation flows which, in turn, causes induced convection over the cooling elements.

Chilled beams offer many benefits over traditional fan coil air-conditioning units. They are quieter, the plenum feeds are smaller, they waft air rather than blast air into a room - thereby eliminating discomfort, and they can now be incorporated as a design element, instead of being consigned to the margins of the project as an HVAC afterthought.

Terry Farthing, sales director of chilled beam manufacturer Trox UK, says: 'Essentially, comfort is assessed by building users as a combination of room temperature and air velocity. It is the consequent turbulence intensity - the mixing and movement of air - that is the most important aspect.

'Standard fan-coil air conditioning systems may provide adequate velocities, but skidding air across the ceiling does not take account of the turbulence which may be caused at task level. The separate research by Fanger and Loveday [see references] shows that gently wafting air into a space, as is done by active chilled beams and displacement ventilation (and understanding its implications), is the best way to achieve suitable ventilation rates while ensuring satisfactory comfort levels.'

Cool design

Even though chilled beams have been around for quite some time (they have been regularly specified in Scandinavian countries for more than 20 years), their no-frills, box-like appearance has been off-putting for British architects and, consequently, they have not exactly caught on in the UK.

In a major development, Trox UK has integrated the mechanics of the chilled beam and general service elements within a sculptural unit called a multi-service chilled beam (MSCB). This new service layout has already begun to appeal to a number of leading architects.

Trox UK has completed schemes for Lloyd's Register of Shipping (AJ 17.1.02), Riverside House (home of FT. com) and the GLA building.Within these projects, each chilled beam unit has been manufactured to suit the architect's exact aesthetic requirements - from variations in grille design to the location of down or uplighters; from the dimensions of the unit to surface finish and moulding.

By making a design feature of the beams, suspended ceilings could possibly be eliminated, meaning that structural soffit heights could be reduced.After all, apart from the often dubious acoustic integrity of suspended ceilings, some people recognise that they are simply a covering layer to hide the services. The MSCB (see box) does the same job but in a more coordinated fashion which could, if correctly utilised, change the face of office design.

One we made earlier

As with all matters concerning prefabrication, to incorporate a chilled beam correctly service decisions will have to be given a higher priority in the design programme. If it is possible to make concerted choices about the location of service routes and points within the building, then the use of these exposed service units will come into their own. By locating all services (within reason) within a single prefabricated unit, manufacture and installation, and hence capital costs and maintenance, are simplified.

Although units vary, in general the chilled beams stand down about 300 mm from the structural soffit (active beams do not require any additional space for the primary source air supply, see diagram).

Therefore, a 3.2m floor-to-soffit height will have, say, MSCB downstands of 300mm at various points along the soffit, giving a head height at its lowest point of 2.9m. The overall effect in the room would possibly still be of a 3.2m ceiling since the occasional downstands would not compromise the perception of height. To achieve the same effect using a standard suspended ceiling specification, the floor-to-ceiling height of 3.2m would possibly require an additional 500mm void, at least, for services.

Although the MSCBs require more forethought in the design stage, their installation period should be quicker due to the straightforward nature of the plug-in fix system.

Units can be provided in modular lengths of up to 4m, although the MSCBs can be preassembled and delivered in units up to 24m in length if required (provided that adequate consideration has been given to health and safety and deliverability issues). Each MSCB unit is manufactured with all services installed and tested. Connections between modules (gaps of about 300mm) are made with proprietary cover plates masking the flexi-joint connections of water pipes and primary air ducts.

In normal practice, the MSCBs are set out in rows at 3m centres and fed from an edge plenum box.

The simple installation of MSCBs compares favourably with the number of competing trades working within the suspended ceiling void to install services on site. Problems with MSCBs only arise if the site conditions and the planned fixing location change after the manufacture of the units has begun. In terms of postcompletion maintenance, very little can go wrong with MSCBs. With clear access, hatches are easily accessible, comparing favourably with easily damaged and poorly replaced suspended ceiling panels.

Prefabrication is ideally suited to building modules and fit-out elements such as these. It is almost certain that as architects begin more successfully to coordinate their early designs with services engineers, they will learn to love the multi-service chilled beam - once, that is, they realise what it is.

References

Chilled Ceilings, Chilled Beams:

Technical Manual for Design Engineers . Trox UK, tel 01842 851288 l'Designing for Thermal Comfort in Combined Chilled Ceilings/ Displacement Ventilation Environments' .

Loveday, Parsons, Taki, Hodder and Jeal. trans ASHRAE .

1998.Vol 104, Pt 1B. pp 901-911 lModerate thermal environments - Determination of the PMV and PPD indices and specification of the conditions for thermal comfort .

BS EN ISO 7730:1995 (BSI) l'Comfort limits for asymmetric thermal radiation'.

Fanger, Ipsen, Langkilde, Olsen.

Christiansen and Tanable.Energy and Buildings ,8.

1985. pp225-236

BEAM DEFINITIONS

Chilled beams Ceiling-mounted service units, incorporating water-filled cooling coils.

Passive chilled beams Warm air passing through the beam is cooled and passes back into the room where it cools the room by natural convection.Excess heat is taken away by cooling coils to a heat exchanger. The use of perimeter chilled beams to offset solar gain reduces the need for full air conditioning and is potentially very energy efficient.

Active chilled beams As the passive variety but incorporating a primary air supply which introduces fresh cooled replacement air.As the warm air within the room rises, it passes through the beam, over cooling coils, and the supply air nozzles push the cooled room air and replacement air into the room at a low velocity.This in turn reinforces the natural convection patterns and circulates the air in the room.

Stale air is removed.

Multi-service chilled beams (MSCB) The beam casing is sufficiently sized to incorporate other service functions. The cooling element is set alongside downlighters and should be designed such that the heat from the light fittings assists the air flows. As a prefabricated service unit, the beam can incorporate coordinated service runs including fire detection, sprinklers, computer cabling, mains electrics, etc.

Displacement ventilation Fresh, cooled air is introduced to an occupied area at a low level, low velocity and at a temperature slightly lower than the room itself.

Spreading along the floor, the air forms a reservoir of cool, fresh air.

The heat sources in the room cause a rising current of air that is extracted and replaced with air drawn from the fresh air inlet.

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