Presentation on theme: "Smoke shafts vs pressurisation"— Presentation transcript:
1Smoke shafts vs pressurisation Colt Technical SeminarPaul ComptonTechnical Director
2“I J O’Hea. Colt Founder” A brief history of ColtPrivate Company founded in 1931I J O’Hea OBE ( )2013 Group Turnover £152 millionManufactures in the Brazil, China, the Netherlands, Saudi Arabia, the UK and the USA“I J O’Hea. Colt Founder”
3Current UK Business markets Smoke ControlEnvironmental Comfort ControlNatural VentilationLouvreSolar ShadingBy investing in innovation, products, services and people, Colt International has established itself as an international leader in the fields of:Smoke ControlEnvironmental Comfort Control (HVAC)Natural VentilationLouvreSolar Shading
5Smoke shafts vs pressurisation Covering:What do these systems do?2. Legal basis and standards:Relative benefits, performance, space and cost4. Compensation for non-compliant layouts - Extended Travel Distances etc5. Which system should I choose?
6What do these systems do? Protect stairs from smoke ingressto aid safe evacuationto aid safe fire fighting accessReduce smoke ingress to lobbies or corridors adjoining stairsReduce risk of smoke spread via liftsProtect fire fighting or evacuation lifts from smoke ingressDesign basis is always a single fire, not located in the stair, lobby or lift.In residential buildings there is a need to protect people evacuating the building as they may do so at any time. Long term protection from smoke is therefore required for the stairs.In commercial buildings people are expected to evacuate immediately, before stairs are likely to be compromised. Long term protection is therefore only needed for fire fighter access and protection and therefore only fire fighting stairs have smoke control systems.There is generally not an expectation that the lobby or corridor on the fire floor will be kept smoke free. Smoke control systems in lobbies and corridors are generally intended to provide or enhance protection to stairs and lifts. The fact that they also improve conditions in the lobby or corridor is a very beneficial side effect.
7Legislation and standards In the UK the relevant legislation is Building Regulations and their associated guidance documents (Approved Document B in England and Wales and its equivalents in Scotland and Ireland).In ADB:Smoke control is recommended, directly or by reference to BS 5588 in:Residential escape stairsAll fire fighting stairsAOV, natural shafts and pressure differential systems are largely treated as equivalentsmechanical shafts are not discussed (too new)pressurisation can also be added to avoid discounting a stair or adding lobbies in commercial buildingsStrictly speaking a pressure differential system can be provided either by stair pressurisation or accommodation depressurisation. In practice stair pressurisation is invariably used.Although mechanical shafts are not mentioned in ADB, ADB does specifically allow alternative fire engineered solutions. Thus mechanical shafts are treated as a fire engineered alternative to the prescribed solutions. They have now become so commonplace as to be virtually treated as though they were a prescribed solution.A stair may be protected by a pressure differential system designed to EN as an alternative to discounting a stair or providing lobbies. This applies to escape stairs (but not fire fighting stairs) in buildings other than apartment blocks. Such stairs would not otherwise be required to have a smoke control system.The only time pressurisation is demanded by ADB is in fire fighting stairs serving deep basements >10m.
8Legislation and standards In the UK we have:BS 5588 (withdrawn)BS 9991BS 9999BS 9991 and BS 9999 recommend a pressure differential system if a building has a floor more than 30m above ground level.BS EN is the design standard for pressure differential systemsWe are currently in the odd position of ADB referring to a withdrawn standard. This will not change until a new edition of ADB is published, expected in 2016.Use of BS 9991 or BS 9999 are permitted as an alternative subject to Building Control approval.BS 9999 also recommends pressure differential systems: as an alternative to separating a dead end corridor with a fire door or to allow a single stair to continue to basement level without separation.There is no current design standard for mechanical shaft systems, although guidance is provided in a Smoke Control Association guide for smoke control in residential buildings.
9Closed Door Requirements Legislation and standardsBS ENSets requirements for 6 different classes of system.Only 2 are commonly used: A & B.Closed Door RequirementsOpen Door RequirementClassStair (Pa)Lift Shaft (Pa)Lobby (Pa)Velocity (m/s)Open DoorsA5050*45*0.75Stair door on fire floorB452.0Stair door on floor belowLobby door on fire floorLift door on floor belowFinal exit doorThe classes are based on building type and usage and take into account escape provisions and sleeping risks.Class A is the simplest and is used in Apartment blocks.Class B is used to protect fire fighting stairs.Thus the normal selection is class A for Apartment blocks and class B for all other buildings.There is a big difference between class A and class B systems. Class B is much more onerous and requires much larger plant, primarily due to the open door requirements.* = If pressurised
10Legislation and standards Do mechanical smoke shafts provide depressurisation to BS EN ?The standard was not written with this in mind50Pa?0.75m/s?100N?Standby fans?Conclusion: No, but it does match some requirements.EN is almost identical to BS , which was published in 1998, long before mechanical smoke shaft systems were developed.The depressurisation provisions are written around extraction from the room containing the fire with the intention of depressurising this room, not depressurisation of some intervening space.Mechanical shafts do not normally meet the requirement for a pressure differential between 45Pa and 55Pa. The pressure differential is a by product, not a design target, and is typically 20Pa to 30Pa. Depressurisation of a lobby to 50Pa is likely to cause problems with doors being dragged open and would increase smoke leakage into the lobby.If we take a typical door at 1.6m2 and atypical shaft extract rate of 3m3/s, then, assuming that the door to the stair and the fire room are both open, the average velocity through the doors would be 3 / ( ) = 0.94m/s. This exceeds the 0.75m/s requirement for a class A system, but again this is a by product rather than a design intent.EN sets a maximum door opening force of 100N when the pressure differential system is in operation. The 55Pa maximum pressure differential is set to achieve this with the majority of doors. Since a shaft system has a lower pressure differential, this requirement is invariably met.EN only requires standby fans for pressurisation in single stair buildings. But of course pressurisation fans only handle ambient air. Smoke shaft fans handle hot, smoky air and so it is good practice to always provide standby fans.My conclusion would be that EN does not cover mechanical shaft systems, but that such systems meet a number of the important functional requirements.
11Performance comparison Accommodation air releaseventilatorSmoke detectorPressure relief damperFIRE FLOORStairsGround floorFirst floor2 m/s average velocityLift door openDescribe systems briefly here.
12Performance comparison Mechanical smoke shaftPressurisationStair: Kept smoke freeLobby: Likely to be smoky for limited periodsLobby: Kept smoke free if pressurised, otherwise likely to be smoky for limited periodsLifts: Usually protected from smoke entry by light depressurisation of lobbyLifts: Kept smoke free if lift or lobbies pressurised. Otherwise not specifically protected
13Space requirements (shafts) A mechanical shaft system needs a single shaft, typically 0.5m2 to 1m2 cross section.A pressurisation system needs:A shaft for each area pressurised, typically 0.15m2 to 2.0m2Accommodation air release (another shaft?)LobbiesFor a mechanical shaft system in an ADB compliant building there is only ever a need for one shaft, serving the lobbies. The size will vary with the design air flow rate and the height of the building. A sensible aspect ratio is necessary to allow space for opening of the shaft ventilators and to ensure that the ventilators do not cause too much blockage to flow from below when closed.For a pressurisation system BS EN requires a separate shaft for each pressurised area. For residential building this might only be the stair, but for commercial buildings a class B system has to pressurise the stair, lift and lobbies. For residential buildings the stair shaft is often similar to or smaller than the lobby shaft for a mechanical shaft system. For commercial building s it is usually larger, often significantly so. The aspect ratio is less critical as there are no opening vents in the shafts, just grilles.If it is not practical to use automatic windows for accommodation air release (AAR) then another shaft (or shafts) is needed. For a mechanical AAR the shaft can be quite small, but if a natural shaft is used this will be at least 0.6m2 and often more.++StairLobbies?Lift?
14Space requirements (roof) It is sometimes possible to incorporate the fans for a mechanical shaft system in the shaft, so roof space is only required for a termination ventilator.More commonly the fans will be roof mounted, typically with a footprint around 4m x 1m.A pressurisation system needs a similar footprint for the fans, although the fans may well be larger, In addition space is needed for a cross duct to provide 2 sources of inlet, well separated (typically taken as being at least 5m apart).If pressure relief dampers are specified, rather than inverter fan pressure control, space also needs to be found for these.
15DesignThere is no published design method for mechanical shafts. Suppliers generally have a generic design with an extract rate in the order of 2m3/s to 4m3/s, proven by CFD analysis for a typical layout.Pressurisation flow rates are calculated specifically for each installation, using the calculations provided in EN These are not for the faint hearted. Supply air flow rates are typically between 2.5m3/s and 20m3/s, depending upon the building design and the class of pressurisation system. The higher flow rates generally occur for class B systems, protecting fire fighting stairs.
16negative Difficulties and issues – smoke shafts Staircase Excess depressurisationLarge doorsBasement stair air inletnegativeFire Fighting LobbyStaircaseThe primary issue with mechanical smoke shafts is ensuring that the lobby does not become too depressurised. Various methods have been successfully used.Although smoke shafts aren’t designed to EN , the guidance on maximum pressure differentials and door opening forces is followed.Large doors can make achieving a working design challenging: door opening forces are higher for a given pressure differential and there can be a tendency to drag inward opening doors open or for them not to reclose fully after opening.Ceiling height stair doors should be avoided, as should geometries with a door directly facing the stair door. Both increase the risk of smoke entering the stair.Too small a smoke shaft causes difficulties achieving a reasonable balance of extract rates between upper and lower storeys.Air inlet to basement stairs – door separating upper and lower stairs – FF to open?
17Difficulties and issues – pressurisation Large doorsQuality of building constructionAccommodation Air ReleaseLarge doors can significantly increase the required air flow and plant size. They also significantly increase the AAR size.Performance is very dependent upon quality of building of the stair core. Any problems are generally only found out late and tend to require urgent emergency action by the builder to overcome.Accommodation air release is often an issue, either because it’s been forgotten or because it is difficult to provide in the first place.Accommodation air release should not be provided from private residential apartments but from common areas. If in private areas, testing, maintenance and repairs can become a nightmare.
18Costs You get what you pay for. Pressurisation can provide the best protection but is the most expensive.A mechanical shaft is next best and next most expensive.A natural shaft is less expensive (but takes up more valuable space).AOV are low cost but provide the least good protection.Typical installed costs for a 10 storey residential building, excluding builders work, are:Pressurisation £60kMechanical shaft £50kNatural shaft £35kAOV £30k
19Non-ADB compliant buildings Extended travel distances in residential buildings Don’t even think about pressurisation as a compensating feature.Pressurisation of the corridor would require accommodation air release from every apartment.A mechanical smoke shaft system which continuously flushes through the corridor end to end in case of fire is always the best solution.
20Non-ADB compliant buildings Refurbishment / change of use It’s not uncommon for older buildings to be unable to comply with current layout requirements (number of stairs, lobbies, etc).Pressurisation?Alternatives?Building Control and the Fire Services often ask for pressurisation to EN to compensate. It’s often impossible and usually impractical to achieve this.The reality is that an engineered approach is usually needed to achieve the best practical solution, tailored to the specific building. If pressurisation is practical, use it.A common issue in London is conversion of relatively small, single stair offices to residential use. There is rarely space for common lobbies, or even shafts and no-one wants the structural risks of making openings for shafts anyway.The example shown is typical. It is to be a live & work unit with a single stair and no room for lobbies. Building Control asked for pressurisation. In this case it is expensive and complex for the size of project and could not sensibly fully comply with ENThe simplest solution is to forget pressurisation and shaft systems and put a simple flushing system in the stair.
21Which system do I choose? There’s no hard and fast answer, but this table might help guide you.Taller than 30m?If following BS 9991 or BS 9999, pressurisation is recommended.Space is tight?A mechanical shaft system is normally most space efficient (if AOVs not suitable).Budget is tight?Natural ventilation is the low cost option if practical.To avoid lobbies or discounting a stairPressurisation.For extended travel distancesAn enhanced mechanical shaft system is essential.