Is the answer to better air quality under your feet?


19 October 2022
The FBA floor-mount swirl diffuser from TROX

Tim Tanner, product technical manager – air diffusers & attenuators, TROX UK, on when and where to use floor grilles.

In recent years, attention has turned to floor grilles/diffusers as a way of improving air quality and reducing energy consumption. But how do you decide whether underfloor ventilation will suit the application? This article will answer some of the key questions to be addressed when making this decision.

Q. How does the displacement air management approach used in underfloor systems work in this application?

Underfloor air distribution systems typically employ a displacement air movement strategy in which cool air is delivered at reduced velocity into the room from the floor void, through specially-designed floor grilles. As the supply air is always cooler than the room air, it moves slowly across the room. When the cooler air comes into contact with a heat load, such as a room occupant, it rises towards the ceiling, where the system will include equipment for extraction. See Figure 1 below. 

This differs from traditional air conditioning systems employing mixed air distribution approaches. In these, air is supplied at relatively high velocity from diffusers at ceiling level. The air moves along the ceiling, due to coanda effect, and reduces in velocity before entering the occupied zone. See Figure 2 below. 

Q. Which applications are best suited to underfloor ventilation/displacement air management?

The REHVA Guidebook on Displacement Ventilation advises that this air management approach is particularly well suited to applications in which contaminants are warmer and/or lighter than the surrounding air, and where the supply air is colder that the ambient air. It is particularly effective in applications requiring large air flow to be supplied into small rooms, and is ideal for tall rooms (where ceiling heights exceed 3 metres). 

In our experience, it is well worth considering displacement ventilation for sites such as theatres, cinemas, auditoria, sports halls, fitness rooms, restaurants, conference rooms, airport concourses, atria, shops, production/assembly halls, stores, and factories.

Q. What are the benefits of underfloor ventilation/displacement air management? And how are these improvements made possible? 

Improved air quality: As the air pools along the floor, rising when it meets a heat source (such as a person), it can take certain contaminates upwards, out of the occupied zone for ceiling level extract. There are, of course, a number of factors to consider in this regard. For example, floor diffusers and grilles coupled with displacement air strategies may not be suitable for applications where the contaminates will be heavier than air.

Energy efficiency: Depending on the application, this approach can also be a valuable way of reducing energy consumption. In mixed air distribution systems, air needs to be supplied into the room at higher velocities in order to achieve the necessary coanda effect. If the velocity is too low the air will enter the occupied zone too soon, creating problems such as ‘dumping’ or draughts. Higher velocity of supply air, of course, has an impact on energy consumption. 

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By contrast, when supplying air via floor grilles, the air velocities are lower, as there is no need to achieve coanda effect. This in turn can offer improved acoustics. In addition, the air supplied to the occupied zone does not have to be reduced to the lower temperatures necessary for mixed air distribution. In fact, for commercial premises, the temperature would typically be in the region of 19°C, just slightly cooler than the design temperature of the occupied zone. In addition, it may be possible to reduce cooling loads as only the occupied zone needs to be supplied with conditioned air. This can be particularly beneficial in rooms with high ceilings. These lower cooling loads reduce the demand placed on chillers and other energy-consuming components across the HVAC system. Plus, there is no longer a need for fans/motors within secondary terminal units such as fan coil units. There may also be increased opportunities for “free cooling” (using fresh air) for a large proportion of the year. An academic study in 2002 quantified potential energy savings of underfloor air distribution as being between 5% and 35%. 

Reduced duct work: Depending on the building design, a raised floor can allow the floor void to be pressurised and act like a plenum, with the air being balanced at each diffuser. This can mean that the requirement for ductwork is reduced.
Easier installation: As the equipment is installed into the floor and floor void, the requirement to work at height is removed, reducing health and safety risk for contractors and service engineers. 

Increased flexibility for building owners/occupants: As floor grilles are typically installed into floor tiles (or designed as replacement 600 x 600 tiles, in the case of the TROX AFG), reconfiguration of spaces is made easier. Since there is little or no ductwork involved, the floor tiles can be easily rearranged, and the tiles incorporating diffusers can simply be moved to different locations to suit each new configuration. Given the frequency and cost of ‘churn’, this increased flexibility is extremely valuable to building owners and occupiers throughout the lifecycle of the equipment.

Q. Are there applications in which it is inadvisable to use underfloor ventilation/displacement air management?

Displacement ventilation requires the supply air to be cooler than the room air. So, this approach is only suitable for cooling with a supply temperature range of -2K to -4K. As a general rule, heating is not recommended using displacement ventilation. The TROX FBA floor diffuser, however, incorporates a swirl unit, this causes high induction and allows for heating, as long as the temperature differential is kept to a minimum. In order to maximise the effectiveness of this heating, the FBAs should be evenly spread throughout the floor plate.  

The height of the room is an important factor as there can be a considerable amount of mixing in the region below the ceiling, due to the interaction between upward and downward moving buoyant air flows.  Generally, buoyancy-driven ventilation is less effective where ceiling heights are low, for example less than 2.5 m.

With regard to air quality, displacement air management is less preferable where the contaminants are colder/denser than the ambient air. Laboratories/science campuses, of course, will require specialist air management systems. TROX’s LabControl systems are purpose-designed for these applications. 

The AF linear bar grille

Q. Which floor grille/diffuser models are best suited to the application?

TROX UK manufactures three types of floor grille/diffuser: the AF (linear bar grille), the AFG (600 x 600 tile replacement grille), and the FBA (floor-mount swirl diffuser). 

As a general rule the AFG is the best option for data centres and other applications involving high cooling loads. It is designed for the supply of large volumes of air and is available with zinc-whisker free construction for added peace of mind in data centres. It is suitable for stringer widths of up to 40mm and pedestals of up to 90mm in diameter. 

The AF and FBA are ideal for sites such as offices, auditoria, airport concourses, theatres, cinemas, conference rooms, hotels, sports halls, fitness rooms, restaurants, shops and factories. The AF model is typically installed around the perimeter of the occupied space, to discharge cold air up the glazing to mitigate any solar gains.