Underfloor air distribution system (UFAD) are commonly used for movie theatres, performance art theatres, large open offices, computer room etc.
UFAD systems utilize the space under an access floor as an air plenum. Properly designed UFAD systems take advantage of thermal stratification. ASHRAE recommend that, for comfort, the temperature in the occupied zone be between 73° and 77°F, relative humidity be between 25 - 60%, and the maximum velocity in occupied zone be 50 fpm in cooling or 30 fpm in heating.
The key to successful access floor systems is the ability of the access floor diffuser to rapidly mix room air into the supply air at low velocities. Because supply air is introduced directly into the occupied zone, it is important that the supply air reach the ASHRAE recommended temperature and velocity, mixing of the supply air into the space should happen rapidly.
The typical application for a UFAD system is the open plan office. Floor space is at a premium in a cubicle so a smaller clear area around the diffuser will allow more usable space in the cubicle. The UFAD diffuser manufacturer defines the required clear area that their diffuser needs to achieve the ASHRAE recommended temperature and velocity.
Originally UFAD systems were for computer rooms. The design intent was to cool computer equipment and not to provide comfort. The computer room design concept typically provides too cold of a space for comfort. The growing interest in UFAD systems is primarily due to companies’ need to easily rearrange office layouts, information and communications based offices, economics of ownership, and green building programs such as LEED.
The pressurized plenum (the area between the slab and the raised floor) is essentially a large duct maintained at a constant pressure differential to the room above; typically between 0.05 and 0.10 in. pressure (w.g.). This pressure is maintained through the supply of conditioned air from a number of supply duct terminations. The spacing and location of these ducts are dependent on the air supply requirement and the plenum depth, with shallow plenums and / or high air quantities requiring more air supply duct outlets under the floor. UFAD diffusers are specially designed grilles with a user adjustable damper to regulate flow. Plenum heights typically range from 12” to 24”. The plenum height is usually determined by the height requirements of other equipment that will be located under the floor. The number of inlets required to supply the plenum with sufficient air to run the diffusers is dependent upon the plenum size and the number of diffusers, which in turn is determined by the load of the space. As a general rule, the longest distance from the supply air outlet in the plenum and the farthest diffuser should not exceed 50-75 feet. Distances longer then this are subject to thermal losses and the discharge temperature of the diffuser may be too high.
The perimeter is often handling much larger loads and require the most equipment. Although the concept of UFAD systems is to be modular, the function of handling perimeter loads is not modular. Those loads come with the building envelope, which is always a line of some sort.
Perimeter heating cannot be accomplished with the same system as the interior load cooling system. Separate ducting of hot or reheated air, hydronic systems, or perimeter fan powered systems are often used to condition the skin load on the building.
Due to the upward air flow, returns should be located at the ceiling or on a high side wall. This allows the heat from ceiling lights to be returned before it is able to mix with the conditioned air in the occupied zone. There will also be a small amount of “free cooling” due to the natural buoyancy of hot air. If the system must use 55°F supply air for humidity reasons, some of the return air can be recirculated from the ceiling to the underfloor plenum to raise the temperature of the air to 63°F to 68°F.
A potential problem with the higher supply temperatures used in underfloor supply systems is the higher potential moisture content of the warmer supply air used in these systems. The supply system must reduce relative humidifies to less than 60% to meet Indoor Air Quality (IAQ) concerns, and this requires dew points less than 65°F. This implies either reheat or blending of air to achieve a 65°F supply, 55°F dew point condition.