under-floor air-conditioning system

Introduction
Under-floor air-conditioning systems have been installed in other parts of the world for nearly 20 years. In Singapore, one form of under-floor air-conditioning systems is commonly found in the computer room air-conditioning where the cool air is supplied from the floor.

Concept Of Underfloor Air-Conditioning System
In general, under-floor air-conditioning system (UAS) uses the same air-conditioning equipment, namely, chillers, pumps, cooling towers and AHUs as in conventional air-conditioning system. The main difference between the two is the manner that air is being distributed. Conventional system supplies air from the ceiling level while UAS supplies air from floor level and returns to the air-handling unit from the ceiling.

UAS by virtue of its design has the advantage of moving air in the same direction as the thermal lift in the room. Heat generated from the occupied zone normally moves upwards. This upward movement helps the UAS in removing heat from the occupied zone. It is important to note that the conventional system supplies air from the top and continuously fights against the thermal lift. The cool supply air mixes with the rising heat from the occupied zone and it will require a colder supply air to provide the same space condition.

Temperature Distribution In Occupied Zone
The supply air temperature in the UAS should be around 17-18 °C. This is to ensure that the room temperature near the floor level is 0.5 to 1° C lower than that at a level of 1.8 m above the floor. This means that with the room temperature in the occupied zone around 22-24 °C, the return air temperature could be around 25-28 °C depending on the specific room cooling load.

Normally, the supply air is mixed with about 10 to 20% "fresh" air from outside before entering the cooling coil. In Singapore's context, it is advisable to have this outside air pre-cooled prior to being mixed with the return air. This is to ensure that both the temperature and humidity of the air mixture are better controlled. Furthermore, since a supply air temperature of 17-18 °C is required, the air-handling unit should be equipped with a modulating face and by-pass damper so that the desired supply temperature can easily be achieved by mixing the conditioned off-coil air with unconditioned re-circulated air.

Reduced Energy Consumption
Energy consumption in a UAS is lower than the conventional overhead system in many ways:

(a) For commercial office space, CP13:1999 prescribes that the minimum outside air quantity shall be 2.3 m3/h per m2 floor area. This ventilation requirement is based on a theoretical ventilation effectiveness, Ev = 1.0, However, in conventional system where both the supply air and return air grilles are fixed on the suspended ceiling, it is common for the supply air to be short-circuiting into return air or disruption of supply air by partition walls before reaching the occupied zone. As a result, Ev is always less than 1.0. In fact various studies show that an Ev of as low as 0.65 for conventional overhead system is normal. With Ev = 0.65, actual outside air quantity to be introduced into the air-conditioned space would need to be increased to 3.5 m3/h floor area. This represents an increase in equipment cooling capacity of 12.5 Watt per m2 or 3.54 RT for a 1,000 m2 office space which is nearly 10% of the total cooling load. In the case of UAS, 100% of the outside air passes directly through the occupied zone towards the ceiling return system. Hence, Ev of 1.0 is achievable and the minimum prescriptive outside air quantity is adequate.

(b) UAS with minimal ductwork uses lower fan pressure. To maintain a plenum pressure of 30 to 40 Pa, the typical UAS total system pressure is only 200 to 300 Pa. This is significantly lower when compared with a system pressure of over 600 pa for a conventional overhead system. From the Fan Law, where fan electrical power is proportional to the cube of the square root of system pressure, the energy saving is obvious for the UAS. This works out to be 3 to 5 times more fan energy in the case of conventional system.

(c) UAS supplies air in the same direction as the thermal lift (rising heat) and thus consumes less energy (fan) to cool the building. Furthermore the unoccupied zone is not cooled to comfort temperature unlike the conventional system. This stratified zone can easily constitute 40% of room volume.

(d) Higher supply air temperature of 18°C for UAS as compared with 13°C for the conventional system requires lower refrigeration energy (compressor). Chiller savings result from using higher chiller leaving water temperatures due to the higher supply air temperatures. However, these cooling energy savings can be substantially reduced or eliminated because of the need for moisture control in humid climates such as Singapore will require pre-cooling of fresh air.

Capital Cost
With the drastic reduction in ductworks, the capital cost of a UAS shall theoretically be lower. Any ductwork required will be minimal and is of low pressure type. However, there is possible increase in cost if several smaller AHUs are used in lieu of a big one. The twist floor outlets are more costly that ceiling grilles. Additional insulation will be required for the concrete floor. Hence, the capital cost for air-conditioning installation will be only marginally lower.

There is possible saving in the installation of the second tier of automatic sprinkler system inside the false ceiling space if its height can be reduced to less than 800mm. However, if the height of the under-floor space is more than 300mm, we have to install a smoke detection system in the under-floor space thereby increasing the cost. Hence, the capital cost for fire protection/detection installation may also be only marginally lower.

Under-floor air-conditioning can offer great saving in overall cost but it requires a coordinated team approach from the outset of the project so as to achieve the optimal result. From an article on this subject in the January 2002 issue of Journal of the Chartered Institution of Building Services Engineers, it mentioned Swedish National Pension Fund has reported savings in overall cost of in the region of 5% to 7% of total construction cost.

More Flexibility For Changes
The UAS offers full flexibility in changes to office layout. Changes to an under-floor air-conditioning system require few if any costs for rewiring and re-balancing. The pressurised plenum is self-balancing. The UAS floor outlets can be re-located quite easily to suit any new workstation layout, thus facilitating re-organization of office layouts as well as savings in renovation/refurbishing costs and time. With conventional system, space air circulation is often severely hindered by the siting of partitions and workstations. This will reduce subsequent renovation cost.

Conclusion
Despite the advantages of under-floor systems, there are barriers to widespread adoption of UAS technology. There is a perceived higher risk to designers and building owners due to a lack of objective information and standardised design guidelines, perceived higher costs, problems with standards and codes, and a lack of documented case studies with whole-building performance and cost-savings data.