Water offers a more energy efficient way of distributing energy in the form of heating and cooling around a building than ‘all air’ systems because of its high specific heat capacity and thermal conductivity.
This section is intended to give an overview of the following water-based systems:
- Chilled ceilings (including Ceilings and Rafts / Sails).
- Chilled beams (including Active and Passive).
- Other systems (including Multi-service Chilled Beams ‘MSCBs’ and four way discharge cassette chilled beams).
Chilled beams and chilled ceilings require a relatively modest cooling water temperature (14–17°C), which can be obtained using natural cold water storage or free cooling from outside air over periods of the year depending on climate.
Also, when mechanical cooling is used, a better energy performance can be achieved because of the higher chiller CoP (coefficient of performance). Where chilled beams are used for heating, the situation is similar in that it is possible to use low temperature heat sources or heat pumps with water flow temperatures of typically 30–45°C.
Radiant chilled ceilings
Radiant chilled ceilings usually incorporate a chilled water coil or element into the rear of the ceiling finish material. Typically, this means copper pipe matrix on the rear of metal ceiling tiles or panels. Insulation is usually applied on the upper surface of the chilled ceiling, as the useful cooling is required in the space below the ceiling.
As chilled water passes through the coil, it offers a cool ceiling surface that provides space cooling by both radiation and convection.
‘Radiant cooling’ involves the direct absorption of heat radiated from warm surfaces within the room, which occurs when there are cooler surfaces visible to the warmer surfaces. This type of system results in low air velocity with an even temperature distribution in the occupied zone, thus providing very good comfort levels.
Radiant chilled ceilings provide an architecturally acceptable surface, into which a range of services can be fitted. They can also usually be accommodated with shallow ceiling voids, so are suitable when vertical space is restricted. A separate ventilation system is required to supply fresh air to the space.
Figure below shows a lower surface perspective with a cut away showing the chilled ceiling elements bonded to the rear surface of the panel/tile.
Figure below shows how chilled water elements are interconnected and connected to the water flow and return distribution pipe-work. The same principle can apply to both “Bonded” and “Lay-in” panels/tiles.
Radiant chilled ceilings (plaster finish)
Small-bore diameter plastic capillary coils are secured to the ceiling or wall structure and completed with a plaster finish. A special thin plaster is required to minimise the effects of a lower thermal conductivity.
Radiant and convective chilled rafts / sails
Radiant and convective chilled rafts or sails incorporate a chilled water coil or element onto the rear of large flat panels which are suspended below the soffit or ceiling. There is no insulation fitted to the rear of the panel as the cooling device is within the room space and all cooling (radiation and convection) is useful cooling (see Figure below).
As chilled water flows through the coil, the lower surface of the raft or sail acts in precisely the same way as a radiant chilled ceiling with both radiant and convective cooling. The air above the raft or sail is also cooled and this provides additional convective output as it flows down over the edges of the rafts or sails.
The shape and size of rafts or sails can be varied to meet architectural requirements and services can easily be integrated. As the gap required above the raft or sails is small, they are suitable where vertical space is restricted. Sails can also be used for efficient radiant heating.
Convective chilled ceiling systems
These systems typically comprise a framework of angled fins (usually aluminium) with a chilled water pipe or water way (usually copper) integrated into the centre of each angled fin (see Figure below).
There is thermal transfer from the water to the copper to the aluminium, thus cooling the fins. As a result of this, a greater proportion of cooling is achieved by air convection through the angled fins rather than by direct radiation. This type of system can give higher cooling levels than a normal radiant system, but less than a Passive Chilled Beam.
Performance and characteristics
A summary of the characteristics of chilled ceiling systems can be found in the table below. The table is based on:
- A) BS.EN14240
- B) Temperature difference room to mean water temperature 8°K
- C) Water flow return temperature difference 2°K
- D) Room temperature 24°C
- E) Water mean temperature 16°C
- F) Active ceiling area as percentage of total ceiling area 80% except Rafts and Sails which are 67 per cent
Chilled beams
The basic thermal transfer component for chilled beams is a fin and tube heat exchanger, often referred to as coils. Rows of interconnected copper pipes are usually bonded to aluminium thermal conducting fins. This arrangement is then mounted in a sheet metal casing, which can either be:
- Freely suspended from a soffit, or
- Installed above a perforated metal ceiling (passive beam only), or
- Integrated flush into a suspended ceiling system.
Chilled beams work using convection rather than radiation. Because of the larger fin surface area, a higher thermal performance can be achieved with chilled beams as opposed to chilled ceilings. However, care needs to be taken in the selection process to ensure that high air velocities are not created in the occupied zone.
Summary
Chilled beams are usually formed as a bank of finned tubing, arranged in a square or rectangular profile. The tubing conveys chilled water and when encased and secured to the underside of a structural floor, the unit resembles a beam. An outer casing of sheet metal can be used to enclose the coiled pipes and this may be perforated to encourage convection through the bank of finned tubing. A passive cooling effect is by natural convection, but active cooling can be achieved by using a fan-driven primary air supply. To conceal the installation, the underside of the box may be finished flush with a perforated suspended ceiling.
Chilled ceilings were originally devised with chilled water pipes embedded within the underside of a concrete floor slab. The nominal increase in slab depth is justified by no visual intrusion of pipework. This form of radiant cooling has the disadvantage of creating a high thermal mass in the concrete slab, which is slow to respond to thermostatic control. These installations can also produce ‘indoor rain’ or condensation on the radiant underside of the slab. To prevent the ceiling from running wet, a suspended variation is preferred, with the option of an auxiliary or fan-driven primary air supply through perforations in the ceiling. These perforations will also increase the convective effect.
