atria vEnTilaTion BE ParT of ThE 2016 TEchnical symPosium Be part of the 2016 Technical Symposium Andrew Acreds presented his poster on ventilating atria at the 2015 Technical Symposium. Papers, posters or case studys for the 2016 event at Heriot Watt University, Edinburgh must be submitted by 14 September 2015. Visit www.cibse.org/ symposium for more information. KcaB to BaSicS eor RmadE With some way to go before natural ventilation is completely accepted as a viable alternative to mechanical systems, Andrew Acred shares a simple model for implementing it effectively in atria buildings BE ParT of ThE 2016 TEchnical symPosium W e are still learning how to design and use natural ventilation. Historically, the need for a supply of fresh air at a comfortable temperature resulted in distinctive and eye-catching architectural elements, designed to harness the freely available forces of wind and thermal buoyancy. A classic example is the windcatcher towers (badgir) in Iran. Today, as we are squeezed for space on the ground and build upwards, more striking building features such as atria, lightwells and solar chimneys are finding their way into our architectural lexicon. As we recover from the sealed box designs of the 70s and 80s, we need to learn to use these building elements to deliver a healthy indoor environment, while minimising energy use. from the outside (either hotter or cooler) the greater the potential for natural ventilation. We still have some way to go, however, before natural ventilation is accepted as a viable alternative to mechanical systems, and we understand how to implement it most effectively, particularly in large buildings. By their nature, free running ventilation systems cannot deliver the desired internal environment at the push of a button, leading to the perception that natural ventilation systems are unreliable. Even worse, it can lead to air flowing in the wrong direction and an uncomfortable internal environment particularly on the top storey. Some examples of problematic flow patterns in a four-storey atrium building are shown in Figure 1. Overheating on the top storey either because of insufficient ventilation or recirculation of stuffy air from lower storeys (a reversed flow) is common. Exchange flows in which simultaneous inflow and outflow occur at a high-level vent can also reduce net ventilation flow rates, and contravene fire regulations because an inflow occurs at what should be an outlet vent.1 Reversed flow through top storey produces uncomfortable, stuffy internal environment Atrium vent is too small Insufficient ventilation and overheating on upper storeys Incorrect relative sizing of storey vents Exchange flow at atrium vent leads to reduced flow rates through occupied storeys the good and the bad Tall buildings offer opportunities for harnessing natural ventilation, not just from the wind but also from buoyancy. A tall atrium, linking multiple floors and filled with buoyant air, is a source of driving pressure for ventilation that, tapped into, can supply fresh air to occupants. The taller the atrium and the greater the temperature difference Atrium vent is too large Figure 1: Examples of problematic flow patterns in a four-storey atrium building Top-storey ventilation rate with atrium Atrium enhancement = Top-storey ventilation rate without atrium Atrium building Equivalent building without atrium Compare Figure 2: Definition of the atrium enhancement metric Our model does not capture all facets of a buildings design. It is a blueprint for an optimised design; a starting point for more detailed modelling using multizone software, CFD and other tools the need for simplicity These problems can be avoided and, indeed, have been avoided in a number of benchmark, naturally ventilated buildings. A notable example is Manitoba Hydro Place, in Winnipeg, Canada, a 22-storey building that uses three atria and a solar chimney for passive supply and extract of air. Its success lies not only in the building form, but also in the sophisticated BMS design with more than 25,000 sensors the commissioning of which would have been impossible without the clients willingness to engage in a fully integrated design process.2 We would all like clients to aim for outstanding building designs, but what about projects on tighter budgets, or with clients and tenants who are less willing to engage with the details of HVAC system design? The theme of this years CIBSE Technical Symposium was Simple buildings, better buildings? a question that was met with a resounding yes from participants across the board, from architects to BIM specialists. The take-home message, from my perspective, was that a simple design allows for better communication between all parties at all stages from design to construction to commissioning and operation maximising the chances of a successful outcome. an intuitive design approach My work with Professor Gary Hunt, at the University of Cambridge, focuses on Ancient windcatcher towers (bagdir) developing simple, back-of-the-envelope methods that can be used to provide intuitive guidance to designers and clients. Our method targets design at the conceptual stage, when the building form is fluid and high-level decisions on architectural elements, such as atria, are being made. We focus on the generic, multi-storey building form, with an atrium or solar chimney, or similar vertically spanning space that acts as an exhaust stack. The basis of our approach is a simple mathematical model that can be used to balance a number of core design parameters, including vent sizes, heat inputs, target air temperatures and building geometry. There are three notable features of the model. First, we focus only on buoyancy-driven ventilation, designing for the worst-case scenario in which wind is not available to assist ventilating flows. Second, all quantities are specified on a per-person basis including vent sizes and heat gains allowing for development of a demandbased design. Finally, we quantify the ventilation performance of the atrium using an atrium enhancement metric (see Figure 2 above) that compares the theoretical flow rate per person through the top storey with, and without, an atrium. Flows through the top storey are driven by the smallest stack height, making it the worst performing storey in terms of ventilation. Aiming for an atrium-enhanced flow on the top storey, therefore, ensures that the atrium is beneficial for driving flows through all storeys. optimising design: rules of thumb By running our simple model for a range of building operation scenarios which can be done rapidly because of the simplicity of the model we can optimise the building for natural ventilation and determine some rules of thumb for design. An effective design largely comes down to the correct relative sizing of vents. When the atrium outlet is too small, compared to the storey vents, reversed flows through the top storey are common. Conversely, exchange flows at the atrium outlet may occur when the atrium vent is too large, or when flows through the storeys are restricted. Modelling during the design of the Library of Birmingham led to balconies within its central atrium being aligned to allow air to pass unrestricted Figure 3: Ideal design bluepriWnt for an atrium building shown for a four-storey building but, in principle, applicable to any number of storeys For an atrium-optimised design, use equal per-person sizes for the top-storey vents and atrium vent Atrium enhances flows through all storeys: Enhancement > 1 Uni-directional outflow at atrium vent RELATIVE VENT SIZE PER PERSON FLOW RATE PER PERSON ATRIUM TEMPERATURE STOREY 4 STOREY 3 STOREY 2 STOREY 1 Vent size per person increases on ascending the building to compensate for the reduction in driving stack pressure from the atrium through the lower parts of the building, thereby avoiding exchange flows at the atrium outlet.3 The optimum design has equal perperson vent sizes at the atrium outlet and in the top storey (see Figure 3 above), providing our first rule of thumb. This shares control between all vents in the building, ensures a forward flow on all storeys, and minimises the likelihood of air flowing in the wrong direction. A second rule of thumb is that vent sizes should increase in higher storeys, to compensate for the reduction in driving stack pressure from the atrium, thereby avoiding overheating on the upper storeys. A final rule of thumb is that the atrium should extend at least one storey height above the top storey to ensure an enhanced flow through all storeys. This may not always be possible, as the height of the atrium may be limited by planning or budgetary constraints. In these cases, it may be beneficial to disconnect the top storey from the atrium and provide its ventilation using a separate system. This strategy has worked at the Lanchester Library at Coventry University,4 and is planned for the James Dyson building at the University of Cambridges engineering department. Forward flow on all storeys: fresh air supply at floor level, warm exhaust to the atrium at ceiling level Our approach is intended to provide simple and intuitive rules of thumb that are accessible to all from architects and engineers to clients and tenants and to encourage an inclusive approach to natural ventilation design Equal flow rate per person and equal air temperature on all storeys References 1 A Acred & G R Hunt (2013) Multiple flow regimes in stack ventilation of multi-storey atrium buildings. International Journal of Ventilation 12:1, 3140. 2 K Stormont (2014) Ken Dale Travel Bursary 2014: Natural Ventilation in High Rise and its application to the Middle East. CIBSE, London. 3 G McCutcheon (2011) Library of Birmingham: Environmental modelling. Buro Happold, Bath. Simple models in the design process Our model does not capture all facets of a buildings design, nor does it intend to. It is a blueprint for an optimised design; a starting point for more detailed modelling using multizone software, CFD and other tools. Our approach is intended to provide simple and intuitive rules of thumb that are accessible to all from architects and engineers to clients and tenants and to encourage an inclusive approach to natural ventilation design. For full details of the design approach, see the CIBSE Symposium conference paper5 and related publications by the author.1, 6 cJ 4 B Krausse, M Cook & K J Lomas (2007) Environmental performance f a naturally ventilated city centre library. Energy and Buildings 39, 792801. 5 A Acred & G R Hunt (2015) Optimising a multi-storey atrium building for stack ventilation. 2015 CIBSE Technical Symposium, London. 6 A Acred & G R Hunt (2014) Stack ventilation in multi-storey atrium buildings: a dimensionless design approach. Building and Environment 72, 4452. ANDREW ACRED is a research associate and Gary Hunt a professor at the University of Cambridge engineering department, in the fluid mechanics group