KENT COLLEGE | EARTH LABYRINTH VENTILATION Modelling BSP modelled the scheme using dynamic simulation based on a week in July, when the weather is warm and the school will use the hall for a performance every day. We decided that we would aim for a target dry-resultant temperature of 28C at the back of the circle, which is the location of the highest audience seat, says Gearoid Donnelly, associate at BSP. The system worked; modelling showed that the temperature in this location would peak at 26.5C, even on the hottest day, which Staniland says the team were more than happy with (see Performance week graph, opposite). The same earth duct/roof vent system is used to purge the building of heat overnight. In late evening, the dampers in the earth ducts and mixed-mode ventilation (MMV) units open fully. Cool night air, at about 18C, will pass through the building, removing heat from the intake ducts concrete walls and from the internal surfaces of the hall, including the exposed thermal mass of the halls fair-faced brickwork walls. On summer mornings, school assembly will take place in the cooled hall. At this time of day, the system will operate under control of CO2 sensors, simultaneously modulating the dampers in the earth ducts and roof units to control the rate of airflow through the space. The system will operate in this mode throughout the day, remaining closed to keep the space cool and opening only when the auditorium is occupied and CO2 levels rise. The performance of the system was modelled using computational fluid dynamics at RIBA Stage 4, to show the spacetemperature distribution and prove the effectiveness of the design. The earth ducts are not used in winter; instead, outdoor air is supplied to the space using the MMV units alone. In winter, the MMV Winter mode 6 Labyrinth shut Supply and extract through MMV units 7 MMV unit: mixed-mode ventilation unit 1 2 3 Four MMV units penetrate the roof units operate in one of two modes depending on the temperature of the outside air. When it is relatively mild, with an outside air temperature of 12C or above, the fans in the MMV units will remain off. One half of the split duct will continue to operate as a chimney to exhaust warm stale air from the hall, while the other half of the duct will act as an outdoor air supply duct. On entering the hall, the denser, cooler outdoor air will sink towards the floor. As it does so, it will mix with the warmed room air at a temperature of 35C or so, trapped beneath the halls roof. Through natural mixing, you are entraining the hot air in with the supply air, which is sinking under the pull of gravity, so you are making use of heated air that would otherwise have been wasted heating the roof, says Staniland. By the time the cool air has reached the neck of someone sitting in the gallery, it will have entrained a significant amount of the surrounding air, so that its temperature will 5 4 1 Mixing fans in the MMV unit mix intake air with hot air under roof, for delivery to space by gravity 2 Motorised damper shut 3M ixed air, cooler than the room air, falls by gravity. Entrained surrounding room air raises the supply air temperature to comfortable levels before it reaches audience 4 Temperature/CO2 wall sensors 5 Heating by natural convectors concealed inside labyrinth 6 Exhaust air occurs through exhaust split duct in MMV unit 7 Cold outside air enters through inlet side of split duct in MMV unit In winter, the earth ducts are not used, and outdoor air is supplied to the space using the MMV units alone The modest boiler plant is evidence of significant fabric efficiency 26 July 2020 www.cibsejournal.com CIBSE July 2020 p24-27 Kent college.indd 26 19/06/2020 15:53