TUTORIAL - UNDERFLOOR AIR-CONDITIONING

Now that raised access floors are a standard item in most offices, underfloor air-conditioning systems are becoming much more popular. Here, we look at the types of system available.

Air-conditioning is defined as the simultaneous control of temperature, humidity, air movement and the quality of the air in the space.
(source: The Trane Company)

The Ideal Conditions
For optimum comfort one should have warm feet and a cool head without noticeable draught. Air velocity in the workspace should not exceed 0.18-0.2 m/s and good air movement should be maintained to avoid stuffiness and dilute pollutants such as ozone and formaldehyde.

The FM Dilemma
Air-conditioning systems have tended to be either central station systems in a variety of configurations or smaller diversified plants such as fan coil systems. Facilities managers have been experiencing high costs associated with the maintenance and reconfiguration of systems above workstations and, on occasions, costly leaks of water have seriously damaged computer networks. In the drive to achieve economy, systems have tended towards higher volume/higher temperature differential solutions but this has led to increased complaints of draughts and noise. Many aspects of office design are now affected by legislation and operators have the task of adapting systems to meet changing legislation and office layouts.

The options
Building services engineers have been charged with addressing these issues. As a result, they have had to reconsider the options available, investigate new systems and use computers for simulation of air movement patterns, stratification etc. It has been found that air introduced from a low level offers improved conditions in the workspace while offering additional benefits in cost and time.

Early developments in Sweden led to the system called displacement ventilation. But as it is essentially a ventilation system with limited cooling (40-60 W/m 2 ), it does not fully meet the definition of air-conditioning. Displacement systems, because of the large areas they serve, often have a demand for heating and cooling within that large area. Additional equipment in the form of underfloor fan coils or electric trench heating is often applied at perimeters to reheat the cool air and introduce it into the space. However, because this warm air is naturally buoyant it rises to a high level and is exhausted before completely heating the space. The heating efficiency is impaired. Zones are usually large and therefore can provide only an average humidity which often leads to complaints from users.

Swirl Grills in the floor
Air is introduced approximately 3°C below room temperature to avoid complaints of draught, through "swirl" grilles or "perforated plate" outlets across the floor. This air picks up heat from the occupants and machinery, along with solar and fabric gains, and rises. Ceiling extract grilles remove this air, avoiding recirculation.

Chilled Ceilings
Displacement ventilation systems are of limited use in spaces with high heat loads because of the low cooling capacity available, so they are often teamed with a secondary system of chilled ceilings. This offers a total cooling capacity in the order of 120-150 W/m 2 but the cold ceiling may cause some "displaced" air to fall back into the room carrying with it some of the contamination.

Ceiling Condensation
Where there is a high latent load causing high humidity, the possibility of condensation occurring on cold surfaces of the metal ceiling needs to be checked. Controls can be incorporated to raise the surface temperature in such conditions, but this in turn reduces the cooling capacity often at the time it is most needed.

Central Station plants and ducting
Using large central station plants for the treatment of air supply requires floor voids of approximately 600 mm deep for the supply ductwork, room heights in the order of 2.7-3.2 m and a ceiling void of about 600 mm to accommodate the exhaust air and chilled ceiling equipment. Of course, the extra space needed to house the system means an increase in the cost of construction. And although users find that such systems are quiet in operation, they restrict flexibility, ceiling design and lighting design.

Lack of personal control
The environment created by displacement ventilation is reasonable but offers little or no personal control over conditions. A large number of swirl grilles are needed and care should be taken in their selection and location. This system has only limited application in offices and other spaces of low height as the desired stratification cannot be achieved.

Energy Costs
Displacement ventilation offers a high level of free cooling in mid season. But in mid winter, when the supply air requires tempering to about 18°C, it can be energy demanding. Restricting the volume to reduce the energy requirement further limits the cooling capacity.

Possible pollution levels
Mixing systems ensure good dilution whereas true displacement can have quite high levels of pollution carried in the ascending air as previously demonstrated by cigarette smoke trails in offices.