Principles and Benefits of Chilled Ceilings

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Static cooling systems (chilled ceilings) have, over the past 40 years, proven themselves capable of delivering high levels of occupancy comfort at reduced running costs. Frenger designed, supplied and installed the "World's Largest Radiant Chilled Ceiling" system in 1962; the 175,000 square meter, 27 stories high, Shell Oil headquarters, situated on the river Thames in London. This building was also the first fully sealed air conditioned building in Europe and was revolutionary at its time as this Chilled Ceiling used the River Thames water to cool the building down. This was achieved by pumping in cool water from upstream to a secondary heat exchanger which in turn cooled (took heat out of the building by "Radiant" absorption) the building down, then depositing the warmer return water from the secondary heat exchanger downstream. This installation is still operating after nearly 50 years and is a testament to the integrity of the product and to Frenger's design capabilities.

Since this time the cooling requirements for a typical office environment have increased considerably; improved insulation, higher occupancy densities and a much higher usage of IT equipment have all fueled this increase. It became apparent in the mid 1990's that the cooling capacity of a traditional chilled ceiling was not sufficient to meet these increased heat-gains, and consequently higher-capacity passive chilled beam fin coil batteries were introduced into perimeter zones to offset the solar load generated at the building façade.

Although fin coil batteries provided the extra cooling at lower cost in £ / watt than the radiant ceiling, the perimeter aesthetics suffered due to the fin coil batteries requiring large size perforations and percentage open area to allow air ("convection") to circulate and this also reduced occupancy comfort, due to higher air velocities.

Frenger however saw the opportunity to take all the benefits from a traditional chilled ceiling for radiant cooling, and to develop a "hybrid" product solution that also has the cooling performance of passive beams coupled with "Radiant cooling" to yield the same aesthetics as associated with the traditional Radiant Chilled Ceilings and in turn low air velocities for compliance to ISO 7730 European Standard for "Indoor Air Comfort Conditions" given that Frenger's "hybrid" chilled beams have a 35% to 40% "Radiant" quotient.

Traditional Chilled Ceilings: First Generation

chilled ceilingThese attractive high quality ceiling systems provide the best in occupancy comfort given their high "Radiant" quotient. Approximately 70% of the total cooling is by radiant absorption and the remaining 30% by convection if the back of the tiles are insulated, and circa 55% Radiation and 45% Convection if the cooling tiles are uninsulated.

The cooling tiles are constructed from zinc coated steel which is polyester powdercoated to whatever the project colour requirements are.

Aluminium extruded heat exchange "pipeseats" are powdercoated black and are bonded to the back of the perforated metal tile. The tiles can be any size and as large as 1.35m x 1.35m, these are known as "Mega Tiles". The tiles are usually insulated with black tissue faced mineral wool pads with a class 'O' foil backing for increased "Radiant" component (70% Radiation / 30% Convection).

There is however a limiting factor of approximately 80w/m² of activated ceiling tiles if insulated. Up to 90w/m² of activated ceiling tiles is possible if ceiling tiles are uninsulated, however the Radiant component is approximately 55% and the rest of the cooling is by 45% Convection-element.

The above listed cooling effects are based on 8.5ºC difference between "mean water temperature" (MWT) and the "design room temperature", known as dt(K). See the cooling performance graphs on page 9 for insulated and uninsulated chilled tiles.

Please note however that not all of the ceiling can be activated with cooling coils, such as the ceiling grid.

In some instances, the ceiling grid (Tartan Grid as shown in the picture below can represent circa 20% of the overall ceiling if the grid was 1.5m x 1.5m and each plain grid was 150mm wide. An allowance of approximately 8% of the total ceiling area being taken up by light fittings should also be made, and as such the rule of thumb is that circa 72% of the total ceiling area (room size) is to be activated by cooling coils. As such 80watts/m² netts out at 57.6 watts/m² on the floor (insulated tiles) and 90 watts/m² netts out at 64.8 watts/m² on the floor (uninsulated tiles) at 8.5 (dtK).


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