Last edited 17 Aug 2017

Underfloor air distribution UFAD

Contents

[edit] Introduction

Underfloor air distribution (UFAD) is a strategy for providing ventilation and space conditioning in buildings as part of an HVAC system. UFAD systems use the underfloor plenum beneath a raised floor to provide conditioned air through floor diffusers directly to the occupied zone.

Once the air enters through the floor diffusers, it spreads across the floor forming a reservoir of fresh cool air. Any sources of heat (such as people or computers) generate a thermal plume lifting contaminated air to high level for removal from the space.

Since air is supplied in much closer proximity to the occupants than in conventional overhead systems, supply air temperatures must be higher. Usually, a task/ambient conditioning (TAC) system is used to allow individuals to control thermal conditions in small and localised zones.

Under floor air distribution is frequently used in office buildings, particularly highly-reconfigurable and open plan offices with raised floors. UFAD is also common in command centres, IT data centres and server rooms that have large cooling loads from electronic equipment and requirements for routing power and data cables. The ASHRAE Under floor Air Distribution Design Guide suggests that any building considering a raised floor for cable distribution should consider UFAD.

Currently, three types of UFAD systems are available:

  • Supply air delivered via passive floor registers and/or fan-powered terminal boxes supplied by a pressurised underfloor plenum and central air handler.
  • Supply air delivered via active, locally-controlled, fan-powered registers (in the floor or workstations), supplied by a very low-pressure underfloor plenum and central air handler.
  • Supply air delivered via under floor ducts to terminal devices or supply outlets.

[edit] Benefits

There are a number of general benefits to UFAD systems.

[edit] Barriers to use

While there are many advantages of UFAD systems, there are still some barriers to adoption:

[edit] UFAD System Design Process

[edit] Plenum design

There are several approaches to address zones with significantly different thermal loads:

  • Plenum partitioning with ducted VAV (Variable Air Volume) devices supplying air to each zone.
  • Plenum partitioning with fan-powered terminal devices supplying air to each zone.
  • Thermostatically controlled VAV diffusers may be used in both partitioned and open plenums.
  • Local fan-driven supply outlets may be used in both partitioned and open plenums.
  • Open plenums with mixing boxes and ducted outlets.

[edit] Supply air temperature and flow rate

Because the air is supplied directly into the occupied zone, supply air temperatures must be warmer than for conventional overhead system design. For cooling applications, supply air temperatures at the diffusers should be maintained no lower than 17 – 20°C (63 – 68°F) to avoid overcooling nearby occupants. This supply temperature can be even higher under partial load conditions.

Mixed air temperature after the cooling coil, or plenum inlet temperature must be determined by taking into account temperature increase (or decrease, depending on the slab temperature) as the air flows through the underfloor plenum. Current estimates for typical air flow rates in an underfloor plenum with a slab that is 3°C (5°F) warmer than the plenum inlet air temperature call for a 1°C (2°F) increase for every 10 m (33 ft) of distance travelled through the plenum.

In temperate climates, where high humidity is not a problem, these warmer supply air temperatures increase the potential for economiser use, and allow higher cooling coil temperatures to be set, if desired.

Cooling air quantities for UFAD systems should be carefully determined. Higher supply air temperatures would suggest that higher supply air volumes are required, but the higher return temperatures created by stratification reduce the required increase in volume. A calculation of the portion of heat sources that bypass the occupied zone in the space allows cooling air quantities to be further reduced. The net effect is that for most designs, controlled stratification in the space allows cooling air quantities for UFAD systems to be equal to or less than those required under the same conditions using overhead air distribution.

Control strategies for temperature and flow rate will vary depending on the magnitude and variability of loads in each control zone, as well as other system design issues.

With UFAD systems, conditioned air from the air handling unit (AHU) is ducted into the underfloor plenum where it typically flows freely to the supply outlets. Underfloor systems are generally configured to have a relatively large number of smaller supply outlets, many in close proximity to the building occupants, as compared to a conventional overhead system.

[edit] Diffusers

The flexibility of mounting supply diffusers in movable raised access floor panels is a major advantage of UFAD systems. The inherent ability to easily move diffusers to more closely match the distribution of loads in the space makes the placement of diffusers a much easier task. In fact, initial layouts can be quite crude. Final placement can take place after the location of furniture and loads and when the preferences of individual occupants have been more accurately determined.

Passive diffusers are defined as air supply outlets that rely on a pressurised underfloor plenum to deliver air from the plenum through the diffuser into the conditioned space of the building.

Active diffusers are defined as air supply outlets that rely on a local fan to deliver air from the plenum through the diffuser into the conditioned space of the building.

Passive diffusers can generally be converted to an active diffuser by simply attaching a fan-powered outlet box to the underside of the diffuser or grill.

[edit] Construction Phase Guidelines

This text was extracted from a more detailed PDF report that can be accessed in full here File:Underfloor air distibuiton technology.pdf.

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