Under-floor air distribution (UFAD) is an air distribution strategy to provide ventilation and space conditioning in buildings as part of the design of HVAC systems. The UFAD system uses a plenary under-floor supply plane located between the structural concrete plates and an elevated floor system to supply air-conditioning through the floor diffusers directly into the occupied zone of the building. Thermal stratification is one of the characteristics of the UFAD system feature, which allows the higher thermostat setpoint compared to traditional overhead (OH) systems. The UFAD cooling load profile differs from traditional OH systems because of the impact of elevated floors, in particular UFAD may have a higher peak cooling load than the OH system. This is because heat is obtained from building penetration and gaps within the structure itself.
UFAD has several potential advantages over traditional overhead systems, including layout flexibility, improved thermal comfort, better ventilation efficiency, improved energy efficiency in an appropriate climate and reduced lifecycle costs. UFAD is often used in office buildings, especially the highly reconfigurable and open plan offices where raised floors are desirable for cable management. UFAD is appropriate for a number of different types of buildings including advertising, schools, churches, airports, museums, libraries etc. Famous buildings that use the UFAD system in North America include the New York Times Building, the Bank of America Tower and the San Francisco Federal Building. Careful consideration needs to be made in the construction phase of the UFAD system to ensure a sealed plenary to avoid air leaks in the UFAD supply plenum.
Video Underfloor air distribution
Deskripsi sistem
The UFAD system relies on an air handling unit to filter and conditioned air to the appropriate supply conditions so it can be delivered to the occupied zone. While overhead systems typically use channels to distribute air, UFAD systems use an underfloor pleno formed by elevated floor mounting. The plenary generally sits 0.3 and 0.46 meters (12 and 18 in) above the structural concrete plate, although a lower altitude is possible. Specially designed floor diffusers are used as supply outlets. The most common UFAD configuration consists of a central air handling unit that drains air through a pressurized plenum and into the space through the floor diffusers. Another approach may incorporate fan-powered terminal units at outlets, under-ground channels, desktop ventilation or connections to Personal Environmental Control Systems.
Maps Underfloor air distribution
UFAD distribution and air stratification
Thermal stratification is the result of a process that coats the internal air according to the relative density. The resulting air layer is a high density vertical gradient and cooler air below and a lower density and warmer air above. Due to the convective movement of natural air, stratification is used mainly in cooling conditions.
The UFAD system makes use of the natural stratification that occurs when warm air rises due to thermal buoyancy. In UFAD design, air-conditioned remains at the bottom of the occupied room, while heat sources such as occupants and appliances produce hot clumps, which bring warm air and heat sources to produce pollutants to the ceiling where they run out through returns. air duct. The temperature stratification made by the UFAD system has implications for the space setpoint. Most of the occupant's body is in a cooler area than the temperature at the height of the thermostat; Therefore, current practice recommends increasing setpoint thermostats compared to traditional overhead systems. The optimal ventilation strategy controls the supply outlet to limit the mixing of air supply with air space to just below the space breath height. Above this height, stratified and more air pollution is allowed to occur. The occupied air will have lower concentrations of contaminants compared to conventional uniform mix systems.
Many factors, including ceiling height, diffuser characteristics, number of diffusers, air supply supply, total flow rate, cooling load and conditioning modes affect the efficiency of UFAD system ventilation. Screen drilling and hole panels have been shown to create low air velocities in occupied zones, while linear diffusers create the highest speeds in occupied zones, disrupting thermal stratification and posing potential design risks. In addition, floor diffusers add personal control elements within the reach of hosts, as users can adjust the amount of air sent by the diffuser even though it rotates the top of the diffuser.
Application Characteristics
UFAD cooling load
Cooling load profiles for UFAD systems and overhead systems are different, mainly due to the thermal storage effect of lighter and heavier floor panels compared to heavier floor plate masses. The presence of an elevated floor reduces the slab's ability to store heat, resulting in a system with a higher floor-topping cooling load than a raised floorless system. In the OH system, especially in the perimeter zone, part of the acquisition of incoming solar heat is stored on the floor plate during the day, thereby reducing the peak zone cooling load, and is released at night when the system dies. In the UFAD system, the presence of raised floors transforming the sun absorbs large floor plates into lighter weight materials, leading to a relatively higher peak-zone cooling load. A modeling study based on the EnergyPlus simulation shows that, in general, UFAD has a 19% higher peak cooling load than the above cooling load and 22% and 37% of the total UFAD cooling load zone leading to plenum supply in the perimeter and interior, respectively.
The Center for the Artificial Environment developed a new index of UFAD cooling load ratio (UCLR), defined by the calculated peak load peak ratio for UFAD to calculated peak cooling load for well-mixed systems, for calculating UFAD cooling loads for each zone with peak cooling loads traditional overhead system (mixed wells). UCLR is determined by zone type, floor level and zone orientation. Plenary Fraction Fraction (SPF), Zone Fraction (ZF) and Plenum Return Fraction (RPF) were developed similarly to calculate plenary supply, zones and plenum cooling loads.
UFAD design tools for zone airflow requirements
There are two design tools available to determine the air flow rate requirements for UFAD systems, developed at Purdue University as part of the ASHRAE Research Project (RP-1522). The others were developed at the Center for the Artificial Environment (CBE) at the University of California Berkeley.
The ASHRAE Research Project (RP-1522) developed a simplified tool that predicts vertical temperature differences between the head and ankle occupants, the level of supply airflow for a plenary zone, the number of diffusers and the effectiveness of the air distribution. This tool requires the user to determine the zone cooling load and the fraction of the cooling load provided to the lower floor plenary. It also requires the user to enter the air supply temperature either on the diffuser or in the channel but with the plenary discharge flow ratio to the zonal supply flow. This tool allows users to choose from three types of diffusers and applies to seven types of buildings, including offices, classrooms, workshops, restaurants, retail stores, conference rooms and auditoriums.
The UFAD CBE design tool based on extensive research is able to predict cooling loads for UFAD systems with inputs from the design cooling loads calculated for the same building as the overhead system. It also predicts air flow rate, room temperature stratification, and plenum temperature gain for interior and perimeter zones of typical high rise office buildings using UFAD systems. The CBE tool allows users to choose from four different plenum configurations (series, reverse series, independent and common) and three floor-diffusers (swirl, square and linear bar grill). An online version of the design tool is publicly available in the Built Environment Center.
Plenum temperature rise
Plenum, an increase in air supply, is an increase in conditioned air due to convective heat reinforcement as it flows through the plenary supply below the floor from the inlet plenum to the floor diffusers. This phenomenon is also called thermal decay. The rise in plenum air temperature is caused by cold supply air in contact with concrete floors warmer than concrete and floor. According to a modeling study, the rise in air temperature can be very significant (as much as 5 Â ° C or 9 Â ° F) and then, compared to the ideal UFAD simulation case without rising air temperature, high diffuser air temperature may cause the level of supply air flow higher energy consumption and increased fan and chiller. The same study found that the rise in summer temperatures is higher than in winter and it also depends on climate. Ground floor with floor plate has a lower temperature rise compared to middle and upper floors, and an increase in air supply temperature causes a decrease in temperature rise. The temperature rise is not significantly affected by the orientation of the perimeter zone, the increase in internal heat and the window to wall ratio. Supply of plenum air rise, therefore, implies the potential energy savings of UFAD systems and its ability to meet cooling requirements with a supply temperature over a conventional overhead system. Current research shows that energy and heat performance can be improved in the UFAD system by draining air into the perimeter zone where the load tends to be the largest. Critics suggest that ducting beneath the floor is like reducing the benefits of low pressure plenum chamber, as well as adding design and installation of complications when installing the channel between tile flooring.
Air leak at UFAD plenums
Leaks in UFAD plenary supplies can be a major cause of inefficiencies in UFAD systems. There are two types of leaks - leaks into space and leaks into lines that cut space. The first leak category did not produce an energy penalty because the air into the zone was intended to cool. The second leak category increases the fan energy to keep the plenum pressure fixed, thus increasing energy use. Careful consideration should be paid in the construction phase of the UFAD system to ensure a well sealed plenary.
UFAD and energy
The UFAD system's energy rating is a problem that is not completely solved, which has led to many research projects in building science and engineering engineering communities. The UFAD proponents point to the lower fan pressure required to send air into the building through the plenary than through the channel. The typical plenum pressure is 25 pascal (0.0036 psi) (0.1 inches of water column) or less. Improvements in the efficiency of the cooling system inherent in operation at higher temperatures save energy, and relatively higher air supply temperatures allow for longer economizer operations. However, the economizer strategy relies heavily on climate and requires careful moisture control to avoid condensation. Critics, on the other hand, cite the lack of rigorous research and testing to account for variations in climate, system design, thermal comfort and air quality to question whether UFAD is capable of delivering energy efficiency improvements in practice. Limited simulation tools, lack of design standards and the relative scarcity of pilot projects mix these issues.
Apps
Under-floor air distribution is often used in office buildings, especially highly reconfigurable and open offices where raised floors are desirable for cable management. UFAD is also common in command centers, IT data centers and Server rooms that have large cooling loads from electronic equipment and requirements for power routing and data cabling. The ASHRAE Underfloor Air Distribution Design Guide suggests that any building considering elevated floors for cable distribution should consider UFAD.
Special space considerations should be taken when using the UFAD system in the laboratory because of the critical space pressure requirement and the potential for chemical migration into the access floor plenum due to spills. The UFAD system is not recommended in certain facilities or spaces, such as non-residential small buildings, wet rooms such as toilets and pool areas, kitchens and dining rooms and gymnasiums, because UFAD can produce very difficult or expensive designs. UFAD systems can also be used with other HVAC systems, such as displacement vents, upward air distribution systems, beamed ceilings or cold beam systems to get better performance.
UFAD compared to other distribution systems
Overhead
The conventional overhead counter system usually finds the supply and returns the air duct at the ceiling level. Supply air is supplied at a higher rate than normally received for human comfort and air temperature may be lower, higher, or equal to the desired room temperature depending on the cooling/heating load. The air jet of high-speed turbulence enters the air supply mixture with room air.
Well-designed UFAD systems have several potential advantages over traditional overhead systems, such as layout flexibility, improved thermal comfort, improved ventilation efficiency and indoor air quality, improved energy efficiency in an appropriate climate and reduced lifecycle costs.
Moving vents
Displacement Ventilation systems (DV) work on the same principles as the UFAD system. The DV system delivers cool air into the conditioned spaces on or near the floor and returns air at the ceiling level. It works by harnessing the natural buoyancy of warm air and the hot blob produced by a heat source because the cooler air is sent from a lower altitude. Although similar, UFAD tends to encourage more mixing within the occupation zone. The main practical difference is that in UFAD, air is supplied at higher speeds through a smaller sized supply outlet than in DV, and the supply outlet is usually controlled by the occupants.
List of famous buildings using UFAD system
References
External links
- Slide from Center for Artificial Environment Workshop on UFAD.
- Artificial Center for the Environment (CBE), University of California, Berkeley. http://www.cbe.berkeley.edu/
Professional and Trade Groups that provide research funding and publish standards or guidance on UFAD systems include:
- The American Society of Heating, Refrigerating and Air-Conditioning Engineers, (ASHRAE) http://www.ashrae.org/
- Institute of Air Conditioning and Air Conditioning Technology (ARTI)
- The Air-Conditioning, Heating, and Refrigeration Institute (AHRI) http://www.ahrinet.org/
Source of the article : Wikipedia
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