Chilled ceilings put under test

In recent years, chilled ceiling systems combined with displacement ventilation – sometimes referred to as static cooling or comfort cooling – have gained popularity. In the UK, traditional cooling strategies such as fan coil and variable air volume are now being challenged by these systems in the majority of new and refurbishment projects.

Laboratory tests have been carried out at the Building Research Establishment (BRE) to measure the performance of chilled ceiling products to be used in a new office building.

The tests involved a full-scale mock-up of an office scheme which was used to demonstrate the performance of the proposed system. The scheme, at 227 Bath Road, is situated on the main A4 trunk road two miles west of Slough Town Centre, with the site being developed by Barclays Property Investment Ltd.

The building provides approximately 2855 m2 net lettable of office space on its ground and two upper floor levels, as well as an undercroft car parking area. The three office floor plates are well proportioned, and of attractive letting size to be suitable for open-plan, cellular or a mixture of the two office floor types.

The office areas are mechanically ventilated using displacement ventilation. The system provides low velocity ventilation – at a temperature slightly cooler than design air temperature – into the occupied space via air diffusers.

The air moves along the floor in a radial pattern to create a cool layer near the floor. Convective plumes from internal heat sources (such as occupants and equipment) then entrain air from this layer into the occupied zone causing an upward movement, with the air increasing in temperature as it rises. The resulting warm, contaminated air is then exhausted at high-level.

For cooling loads up to about 80 W/m², displacement ventilation may be combined with chilled panels (ventilation 20 W/m² and chilled ceilings 60 W/m²). However, to provide 60 W/m² cooling, about 75-85% of the ceiling area needs to be covered by chilled panels.

For loads higher than 80 W/m², the application of chilled beams (which have a higher cooling capacity) becomes essential. In practice, the high heat gains in modern office spaces are served by a combination of chilled beams and a chilled ceiling. However, in a number of practical mock-ups the author found that the ceiling layout plays an important factor in the resulting room air movement and thermal field.

The 227 Bath Road project adopted this design strategy by incorporating radiant chilled panels and perimeter passive chilled beams – supplied by SAS International and Waterloo Air Management.

The office mock-up

To establish the performance of the above system, a programme of extensive tests was carried out at the BRE, prior to the system being installed.

A test room representing a typical perimeter office at Bath Road – with typical heat loads from occupants, computer equipment and solar gains – was used to simulate indoor climate conditions, and cooling performance was measured.

Stable boundary conditions were achieved by conducting the tests in the BRE's controlled environmental chamber. This facility allows the flexibility for creating a wide range of conditions, and the construction of a full-size office space.

The chamber provided insulated voids around the test room giving stable ambient temperatures, thus allowing closer control and measurement of the test boundary conditions.

Internal heat gains were specified to represent occupants (14 W/m²), equipment (30 W/m²), lighting (15 W/m²) and solar (165 W/m² of external facade).

Displacement ventilation air was introduced at the rate of 2·25 l/s/m² over the test area through floor diffusers at a dry bulb temperature of 19°C. Chilled water at 14°C was supplied to radiant chilled panels and passive perimeter chilled beams.

Cooling contributions from displacement ventilation, chilled panels and chilled beams were all measured. These were about 13%, 21% and 66% respectively for perimeter office tests.

For internal spaces where only displacement ventilation with chilled panels was used, the measured cooling splits were 28% and 72% respectively.

Variations in room air speeds and temperatures with height are shown in figures 1 and 2. This test was based on a maximum load of 136 W/m². The data in these figures are calculated by averaging for each variable at each horizontal plane of measurement. Apart from test three, the tests were for the perimeter space with one or two rows of perimeter chilled beams.

The environmental performance tests showed that the displacement ventilation and chilled ceiling devices gave a satisfactory thermal comfort environment for all the internal and perimeter tests. The thermal comfort environment within the space was well within the recommended ranges of both the ISO 7730 and ASHRAE 55-1992 standards.

The application of perimeter chilled beams with an opening along the top of the window and partitioning of the ceiling void has proven to be an effective method of isolating high solar heat gains.