Underfloor air distribution
Encyclopedia
Underfloor air distribution (UFAD) is an air distribution strategy for providing ventilation
and space conditioning in building
s as part of the design of an HVAC
system. UFAD systems use the air plenum
beneath a raised floor
to provide conditioned air through diffusers directly to the occupied zone.
s are desirable for cable management. UFAD is also common in command center
s, IT
data center
s and computer room
s that have large cooling loads from electronic equipment and requirements for routing power and data cables. The ASHRAE Underfloor Air Distribution Design Guide suggests that any building considering a raised floor
for cable distribution should consider UFAD.
to filter and condition air to the appropriate supply conditions so it can be delivered to the occupied zone. While overhead systems typically use ducts
to distribute the air, UFAD systems use the plenum formed by installation of a raised floor
. The plenum generally sits 0.3-0.48 m (12-18 in) above the structural concrete slab, although lower heights are possible. Specially designed floor diffusers are used as the supply outlets. 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. Other approaches may incorporate fan powered terminal units at the outlets, underfloor ducts, desktop vents or connections to Personal Environmental Control Systems.
. In a UFAD design, cold, fresh air stays in the lower, occupied part of the room, while heat sources such as occupants and equipment generate thermal plumes, which carry the warm air and pollutants towards the ceiling
where they are exhausted through the return air ducts. The temperature stratification created by the UFAD system has implication for space setpoints. Most of an occupant's body is in an area that is colder than the temperature at the thermostat height; therefore, current practice recommends raising thermostat setpoints compared to traditional overhead systems. The optimal ventilation strategy controls the supply outlets to limit the mixing of supply air with room air to just below the breathing height of the space. Above this height, stratified and more polluted air is allowed to occur. The air that the occupant breathes will have a lower concentration of contaminants compared to conventional uniformly mixed systems.
Many factors, including the ceiling height, diffuser characteristics, number of diffusers, supply air temperature, total flow rate, cooling load and conditioning mode affect the efficacy of the UFAD system. Swirl diffusers and perforated-floor-panel diffusers have been shown to create a low air velocity in the occupied zone, while linear diffusers created the highest velocity in the occupied zone, disturbing thermal stratification and posing a potential draft risk.Additionally, floor diffusers add an element of personal control within the reach of the occupant, as users can adjust the amount of air that is delivered by the diffuser though rotating the diffuser top.
-dependent and necessitates careful control of humidity to avoid condensation
. Critics, on the other hand, cite the shortage of rigorous research and testing to account for variations in climate, system design, thermal comfort and air quality to question whether UFAD is able to deliver improved energy efficiency in practice. Limited simulation tools, the shortage of design standards and relatively scarcity of exemplar projects compound these problems.
of warm air and the thermal plumes generated by heat sources as cooler air is delivered from lower elevations. While similar, UFAD tends to encourage more mixing within the occupied zone. The major practical differences are that in UFAD, air is supplied at a higher velocity through smaller-size supply outlets than in DV, and the supply outlets are usually controlled by the occupants.
Professional and Trade groups that provide research funding and publish standards or guides regarding UFAD systems include:
Ventilation (architecture)
Ventilating is the process of "changing" or replacing air in any space to provide high indoor air quality...
and space conditioning in building
Building
In architecture, construction, engineering, real estate development and technology the word building may refer to one of the following:...
s as part of the design of an HVAC
HVAC
HVAC refers to technology of indoor or automotive environmental comfort. HVAC system design is a major subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer...
system. UFAD systems use the air plenum
Plenum
Plenum may refer to:* Plenum chamber, a chamber intended to contain air, gas, or liquid at positive pressure* Plenism, or Horror vacui...
beneath a raised floor
Raised floor
A raised floor or access floor are types of floor that provide an elevated structural floor above a solid substrate to create a hidden void for the passage of mechanical and electrical services...
to provide conditioned air through diffusers directly to the occupied zone.
Applications
Underfloor air distribution is frequently used in office buildings, particularly highly-reconfigurable and open plan offices where raised floorRaised floor
A raised floor or access floor are types of floor that provide an elevated structural floor above a solid substrate to create a hidden void for the passage of mechanical and electrical services...
s are desirable for cable management. UFAD is also common in command center
Command center
A command center is any place that is used to provide centralized command for some purpose.While frequently considered to be a military facility, these can be used in many other cases by governments or businesses...
s, IT
Information technology
Information technology is the acquisition, processing, storage and dissemination of vocal, pictorial, textual and numerical information by a microelectronics-based combination of computing and telecommunications...
data center
Data center
A data center is a facility used to house computer systems and associated components, such as telecommunications and storage systems...
s and computer room
Computer room
A computer room can mean following:* Internet cafe, a public place where people can access Internet* Telecentre, a public place in developing countries where people can access Internet* Data center, a facility used to house computer systems...
s that have large cooling loads from electronic equipment and requirements for routing power and data cables. The ASHRAE Underfloor Air Distribution Design Guide suggests that any building considering a raised floor
Raised floor
A raised floor or access floor are types of floor that provide an elevated structural floor above a solid substrate to create a hidden void for the passage of mechanical and electrical services...
for cable distribution should consider UFAD.
System description
Like other HVAC systems, UFAD systems rely on air handling unitsAir handler
An air handler, or air handling unit , is a device used to condition and circulate air as part of a heating, ventilating, and air-conditioning system. An air handler is usually a large metal box containing a blower, heating or cooling elements, filter racks or chambers, sound attenuators, and...
to filter and condition air to the appropriate supply conditions so it can be delivered to the occupied zone. While overhead systems typically use ducts
Duct
A duct may refer to:* Duct , various ducts in anatomy and physiology* Duct , for transfer of air between spaces in a structure* Duct tape, a kind of adhesive tape* Ducted fan, motor for aircraft...
to distribute the air, UFAD systems use the plenum formed by installation of a raised floor
Raised floor
A raised floor or access floor are types of floor that provide an elevated structural floor above a solid substrate to create a hidden void for the passage of mechanical and electrical services...
. The plenum generally sits 0.3-0.48 m (12-18 in) above the structural concrete slab, although lower heights are possible. Specially designed floor diffusers are used as the supply outlets. 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. Other approaches may incorporate fan powered terminal units at the outlets, underfloor ducts, desktop vents or connections to Personal Environmental Control Systems.
UFAD air distribution and stratification
UFAD systems rely on the natural stratification that occurs when warm air rises due to thermal buoyancyStack effect
Stack effect is the movement of air into and out of buildings, chimneys, flue gas stacks, or other containers, and is driven by buoyancy. Buoyancy occurs due to a difference in indoor-to-outdoor air density resulting from temperature and moisture differences. The result is either a positive or...
. In a UFAD design, cold, fresh air stays in the lower, occupied part of the room, while heat sources such as occupants and equipment generate thermal plumes, which carry the warm air and pollutants towards the ceiling
Ceiling
A ceiling is an overhead interior surface that covers the upper limit of a room. It is generally not a structural element, but a finished surface concealing the underside of the floor or roof structure above....
where they are exhausted through the return air ducts. The temperature stratification created by the UFAD system has implication for space setpoints. Most of an occupant's body is in an area that is colder than the temperature at the thermostat height; therefore, current practice recommends raising thermostat setpoints compared to traditional overhead systems. The optimal ventilation strategy controls the supply outlets to limit the mixing of supply air with room air to just below the breathing height of the space. Above this height, stratified and more polluted air is allowed to occur. The air that the occupant breathes will have a lower concentration of contaminants compared to conventional uniformly mixed systems.
Many factors, including the ceiling height, diffuser characteristics, number of diffusers, supply air temperature, total flow rate, cooling load and conditioning mode affect the efficacy of the UFAD system. Swirl diffusers and perforated-floor-panel diffusers have been shown to create a low air velocity in the occupied zone, while linear diffusers created the highest velocity in the occupied zone, disturbing thermal stratification and posing a potential draft risk.Additionally, floor diffusers add an element of personal control within the reach of the occupant, as users can adjust the amount of air that is delivered by the diffuser though rotating the diffuser top.
UFAD cooling load considerations
Cooling load calculations for UFAD systems differ from overhead systems because of the effects of thermal stratification and contribution of supply and return plenum loads to the overall load. A modeling study based on EnergyPlus simulations showed that, generally, UFAD has a peak cooling load 19% higher than an overhead cooling load and 22% and 37% of the total zone UFAD cooling load goes to the supply plenum in the perimeter and interior, respectively. A simplified design load calculation tool for UFAD systems is freely available from the Center for the Built Environment.Thermal decay
Thermal decay is the temperature increase of the conditioned air due to convective heat gain as it travels through the underfloor supply plenum from the plenum inlet to the floor diffusers. This is caused by cool supply air coming into contact with the concrete slab and raised floor warmed by heat gains, for example from the floor below. According to a modeling study, thermal decay can be quite significant (up to 5 degrees K) and subsequently, compared to an idealized simulated UFAD case with no thermal decay, elevated diffuser air temperatures can lead to higher supply airflow rate and increased fan and chiller energy consumption. The same study found that thermal decay in summer is higher than in winter and it also depends on the climate. The ground floor with a slab on grade has less temperature rise compared to middle and top floors, and an increase of the supply air temperature causes a decrease in thermal decay. The temperature rise is not significantly affected by the perimeter zone orientation, the internal heat gain and the window-to-wall ratio. Thermal decay, thus, has large implications on the energy saving potential of UFAD systems and their ability to meet cooling requirements with supply temperatures above those of conventional overhead systems. Current research suggests that both energy and thermal performance can be improved in UFAD systems by ducting air to perimeter zones where loads tend to be the greatest. Critics suggest however that such underfloor ducting reduces the benefit of having a low-pressure plenum space, as well as adding design and installation complications when fitting ducts between floor tile pedestals.Air leakage in UFAD plenums
Leakage in UFAD supply plenums can be a major cause for inefficiency in a UFAD system. There are two types of leakage--leakage into the space and leakage into pathways that bypass the space. The first category of leakage does not result in an energy penalty because air is getting to the zone it is intended to cool. The second category of leakage increases fan energy in order to maintain a constant plenum pressure, resulting in increased energy use. Careful consideration needs to be paid in the construction phase of UFAD systems to ensure a well-sealed plenum.UFAD and energy
The energy efficiency of UFAD systems is a not fully solved issue, which has led to numerous research projects within the building science and mechanical engineering community. Proponents of UFAD point to the lower fan pressures required to deliver air in a building via the plenum as compared to through ducts. Typical plenum pressures are 25 Pa (0.1 inch of water column) or less. The improvements in cooling-system efficiency inherent in operation at higher temperatures save energy, and relatively higher supply air temperatures allow longer periods of economizer operation. However, an economizer strategy is highly climateClimate
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...
-dependent and necessitates careful control of humidity to avoid condensation
Condensation
Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....
. Critics, on the other hand, cite the shortage of rigorous research and testing to account for variations in climate, system design, thermal comfort and air quality to question whether UFAD is able to deliver improved energy efficiency in practice. Limited simulation tools, the shortage of design standards and relatively scarcity of exemplar projects compound these problems.
Overhead (mixing)
Conventional overhead mixing systems usually locate both the supply and return air ducts at the ceiling level. Supply air is supplied at velocities higher than typically acceptable for human comfort and the air temperature may be lower, higher, or the same as desired room temperature depending on the cooling/heating load. High-speed turbulent air jets mix incoming supply air with the room air.Displacement ventilation
Displacement Ventilation systems (DV) work on similar principals as UFAD systems. DV systems deliver cool air into the conditioned space at or near the floor level and return air at the ceiling level. This works by utilizing the natural buoyancyStack effect
Stack effect is the movement of air into and out of buildings, chimneys, flue gas stacks, or other containers, and is driven by buoyancy. Buoyancy occurs due to a difference in indoor-to-outdoor air density resulting from temperature and moisture differences. The result is either a positive or...
of warm air and the thermal plumes generated by heat sources as cooler air is delivered from lower elevations. While similar, UFAD tends to encourage more mixing within the occupied zone. The major practical differences are that in UFAD, air is supplied at a higher velocity through smaller-size supply outlets than in DV, and the supply outlets are usually controlled by the occupants.
External links
University-based research centers that currently conduct UFAD research:- The Center for the Built Environment (CBE), University of California, Berkeley. http://www.cbe.berkeley.edu/
- The International Centre for Indoor Environment and Energy (ICIEE), Technical University of Denmark. http://www.ie.dtu.dk/
Professional and Trade groups that provide research funding and publish standards or guides regarding UFAD systems include:
- American Society of Heating, Refrigerating and Air-Conditioning EngineersAmerican Society of Heating, Refrigerating and Air-Conditioning EngineersThe American Society of Heating, Refrigerating and Air Conditioning Engineers is an international technical society for all individuals and organizations interested in heating, ventilation, air-conditioning, and refrigeration...
, (ASHRAE) http://www.ashrae.org/ - Air-Conditioning and Refrigeration Technology Institute (ARTI)
- The Air-Conditioning, Heating, and Refrigeration Institute (AHRI) http://www.ahrinet.org/