AIR CONDITIONING, COOLING & VENTILATION
Underfloor ventilation – the best suited applications
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Delivering air into the occupied space at floor level can provide valuable advantages, improving air quality at the same time as reducing air conditioning costs and energy consumption. That’s according to Tim Tanner, product technical manager – ventilation technology, TROX UK
he advantages of underfloor ventilation systems are largely a result of the air displacement strategies involved. In these systems, cool air is delivered 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. Depending on the application, the displacement ventilation strategy employed in underfloor ventilation systems can be a valuable way of reducing energy consumption compared to traditional air conditioning approaches. In mixed air distribution systems, for example, 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. By contrast, when supplying air via floor grilles, the air velocities are lower, as there is no need to achieve coanda effect. 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. It may also 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. This in turn can offer improved acoustics. 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%.
Air quality, installation and flexibility
Figure 1 displacement-air-ventilation
Displacement ventilation approaches can also deliver air quality benefits. As the air rises when it meets a heat source, it can take certain
contaminates upwards, out of the occupied zone for ceiling level extract. Further benefits arise from the potential for the floor void to be pressurised to act like a plenum, reducing the requirement for ductwork. There are also practical advantages for installation and maintenance, as the requirement to work at height is removed, reducing health and safety risk for contractors and service engineers. Furthermore, 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.
Application dos and don’ts
Importantly, the approach described above, employing floor grilles and displacement air management, is not generally recommended for heating of spaces. It is common with traditional displacement systems that the room is served by a few large units with the air entering the room in laminar flow. If warm air is supplied at floor level in laminar flow into a cold room, the fresh air will rise and be extracted at ceiling level, in effect short circuiting the occupied zone and failing to achieve the desired effect. The REHVA Guidebook on Displacement
Ventilation guidance is that this approach has proved to be superior to mixing ventilation in
8 BUILDING SERVICES & ENVIRONMENTAL ENGINEER APRIL 2023
applications such as restaurants, meeting rooms [offices] and classrooms.
Displacement ventilation is usually preferable in the following cases:
• Where the contaminants are warmer and/or lighter than the surrounding air
• Where the supply air is colder than the ambient air
• In tall rooms, e.g. where the room heights are more than 3 metres
• Where large air flows shall be supplied in small rooms
Displacement ventilation may be less preferable than mixing ventilation in the following cases:
• Where surplus heat is the main problem, and not air quality
• Where ceiling heights are lower than approximately 2 – 3 metres
• Where disturbances to air flows are strong • Where the contaminants are colder/ denser than the ambient air
Added to this, applications such as laboratories/science campuses and so on require specialist air management systems. TROX’s LabControl systems are purpose- designed for these applications.
An additional important point to consider is the final usage of the space. It is vital to load test the floor grilles/diffusers to ensure that they can support the weight of any person or object that may be moved across them after installation. TROX provides data on load testing and the standardised ratings of floor grilles to assist you in product selection. The TROX AFG floor grille, for example, has been designed specifically for extra heavy duty applications.
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