oPinion When cFd is noT The model AnsWer There can be too much reliance on fire engineering when specifying natural smoke shafts, argues SCSs Allan Meek, who says standardised products are often the most appropriate solution Allan Meek, is managing director at SCS Group, a specialist building engineering services company focusing on BEMS, smoke control and passive ventilation projects www.groupscs.co.uk @groupscs Twitter: There is a general assumption that computational fluid dynamics (CFD) is an integral part of any smoke shaft system design, but I question whether it should be. Across construction, standardisation and offsite manufacture lead to greater efficiencies and cost savings without any negative impact on the success of projects or their long-term safety and sustainability. But smoke control has long been shrouded in a degree of mystery, which makes it hard for clients or main contractors to have real confidence when specifying a solution. Take a mechanically ventilated smoke shaft; if the subcontractor or consultant tells the client they need a bespoke, designed solution, they sign on the dotted line, with the term life safety foremost in their minds. Natural smoke shafts as found in Approved Document B were formally conceived more than a decade ago, in BRE79204, by modelling a typical building with vent size as the key variable and many specific conditions built in. Consequently, the proposed 3m2 shaft solution could never fit every building configuration, but it is universally considered good enough; in mechanical smoke shafts which, because of their reduced footprint, are todays default solution extract flow rate is the key variable. A mechanical smoke shaft is designed using CFD models, which are created with assumptions built in for example, the time taken for people to escape a burning building and the area of inlet from shattered windows. The models are used to calculate flow rates and determine a desired fan duty. Having worked on hundreds of projects, and commissioned hundreds of CFDs on designed mechanical smoke ventilation shafts, I have seen particularly within certain parameters (for example, less than 20-storey buildings and single-shaft systems) that the outcomes are generally very similar in terms of flow rate and desired extract fan duty. Why not standardise? So my question is this: in the same way that we specify standard 1.5m2 automatic opening vents (AOVs) or standard smoke detectors or fire doors why dont we stop pretending that each smoke shaft/stairwell arrangement is bespoke and start to specify standard fans and controls? With so many assumptions and good enough standards appearing in accepted documentation and practice, why the emphasis on case-by-case design to such precise margins where extract flow rate is concerned? I realise the words life safety system can be intimidating, but the generally accepted guidelines ADB, EN12101 and the SCA guidance are easy enough to follow, and the systems are far from complicated. However, we need to divorce product from design; any smoke system is just a collection of products (roof fans, AOVs, smoke detectors and so on) linked by a main control system, which should be talking to the BEMS and other building automation services. Less focus should be placed on individual system design concepts and more on the products, to ensure they are fit for purpose and kept properly operational during a buildings lifetime. These products are always situated in the same way, and operate to the same cause and effect protocol. The design aspect is really just about how many of each product you need, based on building size, and that can be done with a simple spreadsheet. This isnt just talk or theory I have sample CFD models to back up the experience. These are taken for short and tall buildings, with short and long corridors, and the results show that the same fan and control arrangement is appropriate to maintain the desired flow rate. Smoke control has been shrouded in a degree of mystery, claims Meek If a client is told they need a bespoke solution, they sign on the dotted line, with life safety in their minds Keep it simple While there is a case for design in extreme cases where an unusually high flow rate is needed we must consider the potential risks of each solution being individually designed. Over-engineering can lead to added complexity, which may hinder firefighting services, operating override controls; under-specifying can lead to systems that dont operate adequately in real-world situations. Mechanical shaft systems are the most common way to protect high-rise buildings surely standardisation is the lowest-risk way to go? We need to empower installers and consultants to make their own selections and be rid of this unnecessary reliance on fire engineering. cJ how a smoke shaft system works 1 zone panel 3 zone panel Accommodation / Click/tap here to enlarge image Principle An extract fan is placed on top of a shaft that runs the full height of the building. If fire breaks out, smoke is allowed to enter the shaft via an automatically opening vent (vents on other floors remain closed to prevent contamination) and is drawn away by the fan, to be expelled into the atmosphere. Clean inlet air is provided from the stairwell, allowing the safe escape of occupants and easier access for firefighters. The system is operated automatically via smoke detectors, or by integration with the fire-alarm system. Setup A builders work shaft is fitted with a mechanical extract fan at its head, while smoke dampers/AOVs are placed in the lobby of each floor. These, plus smoke detectors and local zone control panels, are linked to a central control unit governed by a touchscreen. At the head of the stairs, a rooftop AOV is installed; this can open to allow in fresh air. Operation 1 The damper/AOV into the shaft serving the lobby where smoke is detected opens 2. A signal is sent to stop other dampers opening, so smoke cant spread to other floors 3. The duty extract fan starts 4. The AOV at the head of the stairs opens, providing a fresh-air inlet 5. The door to the fire lobby opens mechanically (if pressure sensors are installed they can provide information to control the fan speed), to prevent negative pressure forming in the lobby if the door to the stairwell is closed.