ACR Journal

October 2022 | November 2022 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. 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. VENTILATION 26 Tim Tanner, product technical manager – air diffusers & attenuators, TROX UK, on when and where to use floor grilles. Is the answer to better air quality under your feet? 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 Volume 8 No.6 Tim Tanner Figure 1 Figure 2