VENTILATION | MEASURING EFFECTIVENESS Figure 3: The example laboratory where the rst case has two extract grilles and the second has three. (Courtesy of AnSight, Ann Arbor, MI, USA). Exactly the same lab, exactly the same air changes of six per hour, just adding one more extract grille. left visualises the volume of the entire space, whereas the figure on the right is a slice that relates just to the breathing zone. The Spread Index can be calculated for the entire space or just for a critical zone and so can be developed at the breathing level by taking the volume of the dirty blob in the breathing zone and dividing it by the volume of the breathing zone. The Spread Index can be calculated for the entire space or just for a critical zone. The goal of a ventilation systems is not only to reduce the spread of contaminants, but also to reduce the concentration to an acceptable level as quickly as possible. The associated metric is the Purge Time, which is how long it will take that red blob to disappear from the space. Once the contaminants are released, the goal is to minimise the Spread Index to zero as fast as possible. Khankari provided a practical example of how this can assist in the ventilation system design for a laboratory by considering two cases. In one case it has two exhaust grilles, and in the other, three as shown in Figure 3. Figure 4 shows the visualisation at the breathing level while maintaining the same air change rate. With three extract grilles, that red dirty zone has reduced Titon Ultimate dMEV The Titon Ultimate dMEV decentralised mechanical extract fan meets all of the new requirements. It provides extremely low noise levels, as low as 10dB(A) and gives you that extra reassurance and peace of mind. Easily meets new 20Pa Key Features Low specific fan power down to 0.11 W/l/s Meets stringent 20 Pa back pressure requirement (5 metre of duct with 4 90 bends) Simple commissioning and set up Quiet running, only 10 dB(A) at 3m, low speed Extremely low running costs Low profile aesthetic circular design. 4 button and LED display to allow for simple control 3 configurable speed options (Trickle, Boost and High Boost) Constant flow to guarantee ventilation performance As seen on the back pressure requirements PCDB Unique high performance impeller and guide vane design CIBSE accredited CPD courses available Your ventilation experts t: 01206 713800 e: marketing@titon.co.uk The industry leader Visit our dedicated website at: titon.com/ultimate CIBSE Journal (Titon) September 2022.indd 1 18/08/2022 14:51:33 86 September 2022 www.cibsejournal.com CIBSE Sept 22 pp84-87 CFD Ventilation.indd 86 26/08/2022 17:04