Īir stratification capitalizes on thermal buoyancy to layer high quality supply air at occupant level and leave unoccupied air unconditioned. Due to the naturally convective movement of air, stratification is used predominantly in cooling conditions. The resulting air stratum is a vertical gradient with high-density and cooler air below and low-density and warmer air above. Thermal stratification is the result of processes which layer the internal air in accordance with relative density. UFAD air distribution and stratification Other approaches may incorporate fan powered terminal units at the outlets, underfloor ducts, desktop vents or connections to Personal Environmental Control Systems. The most common UFAD configuration consists of a central air handling unit delivering air through a pressurized plenum and into the space through floor diffusers. Specially designed floor diffusers are used as the supply outlets. The plenum generally sits 0.3 and 0.46 metres (12 and 18 in) above the structural concrete slab, although lower heights are possible. While overhead systems typically use ducts to distribute the air, UFAD systems use the underfloor plenum formed by installation of a raised floor. UFAD systems rely on air handling units to filter and condition air to the appropriate supply conditions so it can be delivered to the occupied zone. 6 List of notable buildings using UFAD systems.5 UFAD compared to other distribution systems.3.2 UFAD design tools for zone airflow requirements.2 UFAD air distribution and stratification.Careful considerations need to be made in the construction phase of UFAD systems to ensure a well-sealed plenum to avoid air leakage in UFAD supply plenums. Notable buildings using UFAD system in North America include The New York Times Building, Bank of America Tower and San Francisco Federal Building. UFAD is appropriate for a number of different building types including commercials, schools, churches, airports, museums, libraries etc. UFAD is often used in office buildings, particularly highly-reconfigurable and open plan offices where raised floors are desirable for cable management. UFAD has several potential advantages over traditional overhead systems, including layout flexibility, improved thermal comfort and ventilation efficiency, reduced energy use in suitable climates and life-cycle costs. This is because heat is gained from building penetrations and gaps within the structure itself. UFAD cooling load profile is different from a traditional OH system due to the impact of raised floor, particularly UFAD may have a higher peak cooling load than that of OH systems. Thermal stratification is one of the featured characteristics of UFAD systems, which allows higher thermostat setpoints compared to the traditional overhead systems (OH). Key differences include the use of an underfloor air supply plenum, warmer supply air temperatures, localized air distribution (with or without individual control) and thermal stratification. UFAD systems are similar to conventional overhead systems (OH) in terms of the types of equipment used at the cooling and heating plants and primary air-handling units (AHU). UFAD systems use an underfloor supply plenum located between the structural concrete slab and a raised floor system to supply conditioned air through floor diffusers directly into the occupied zone of the building. Underfloor air distribution (UFAD) is an air distribution strategy for providing ventilation and space conditioning in buildings as part of the design of a HVAC system.
Diagram of air movement in an underfloor air distribution system