Specifications: EN , 2, 3, 7, 8 & Design Guidance: Great-Britain: BR – Design Methodologies for Smoke and Heat Exhaust Ventilation. European Standard EN has the status of a DIN Standard. A comma is used as the decimal marker. National foreword. EN – natural smoke and heat exhaust ventilators. SP Technical Research Institute of Sweden e are one of the leading bodies in the field of certification.
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EN May ICS. English Version. Smoke and heat control systems - Part 7: Smoke duct sections. Systèmes pour le contrôle des. EN E worldwide for CEN national Members. .. floors or even dwellings adjacent to the fire 10 EN (E) depressurization smoke control using pressure differentials where the air pressure in Download pdf. Design approaches for smoke control in atrium buildings. G 0 Hansell*, BSc, PhD , CEng, MCIBSE, AlFireE H P Morgan, BSc, CPhys, MlnstP.
The pressure differential system should be designed so as to limit the spread of smoke into the dedicated firefighting route under normal firefighting conditions.
The spread of smoke should be prevented from entering into sensitive areas such as those containing valuable equipment, data processing and other items that are particularly sensitive to smoke damage. It covers methods for calculating the parameters of pressure differential smoke control systems as part of the design procedure.
It gives test procedures for the systems used, as well as describing relevant, and critical, features of the installation and commissioning procedures needed to implement the calculated design in a building. It covers systems intended to protect means of escape such as stairwells, corridors and lobbies, as well as systems intended to provide a protected firefighting bridgehead for the Fire Services. The systems incorporate smoke control components in accordance with the relevant Parts of EN and kits comprising these and possibly other components see 3.
This document gives requirements and methods for the evaluation of conformity for such kits. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document including any amendments applies.
EN , Ventilation for buildings —Sheet metal air ducts and fittings with rectangular cross section — Dimensions EN , Ventilation for buildings —Sheet metal air ducts and fittings with circular cross section — Licensed copy: Smoke control ducts prEN , Smoke and heat control systems — Part 9: Power supplies prEN , Fire classification of construction products and building elements — Part 3: Classification using data from fire resistance tests on products and elements used in building service installations: Classification using data from fire resistance tests on components of smoke control systems EN ISO The kit needs to be placed on the market allowing a downloadr to download it in a single transaction from a single supplier.
The kit may include all, or only a subset of, the components necessary to form a complete pressure differential system 3. This space is materially enclosed by the bottom of the pit, the approximately vertical walls and the ceiling 3. A lobby connected to a lift well or other shaft is still a simple lobby if all such shafts are pressurized.
A simple lobby may be either unventilated or naturally ventilated 3. The design conditions have been placed in separate system classes which may be used to implement a design using pressure differentials for any given type of building. The classes of system are given in Table 1. Defend in place 4. Sleeping risk 4. The level of fire compartmentation is such that it is usually safe for occupants to remain within the building.
Class A system shall not be used in mixed use developments. The design requirements for a Class A system are shown in Figure 2. Figure 2 — Design conditions for Class A systems 4.
NOTE 1 The corresponding maximum pressure differential across the door can be determined using the procedure in Clause 15 and Annex A, as a function of the door configuration. NOTE 2 The force that can be exerted to open a door will be limited by the friction between the shoes and the floor and it may be necessary to avoid having slippery floor surfaces near doors opening into pressurized spaces, particularly in buildings in which there are very young, elderly or infirm persons.
During firefighting operations it will be necessary to open the door between the firefighting lobby and the accommodation to deal with a potentially fully developed fire.
In some fire situations it may be necessary to connect hoses to fire mains at a storey below the fire storey and trail these via the stair to the lobby on the fire storey. It is, therefore, often not possible to close the doors between these lobbies and the stair whilst firefighting operations are in progress. It is assumed that Licensed copy: It is, however, essential that the stair shaft be kept clear of serious smoke contamination.
In the later stages of fire development more than adequate leakage will generally be provided by breakage of external glazing. However, it cannot be assumed that windows will have failed before fire service arrival, and it is therefore necessary to ensure that sufficient leakage area is available via the external facade, the ventilation ductwork or specifically designed air release paths.
The system shall be designed so that the stairwell and lobby and, where provided, the lift shaft are kept clear of smoke. In the event of smoke entering the lobby, the pressure within the stair shall not drive smoke into the lift shaft or vice-versa. This shall be achieved by providing separate pressurization of the firefighting lift shaft, lobby and stair. The design requirements for a Class B system are shown in Figure 3.
If a door that has two leaves is assumed to be open for calculation purposes, one leaf may be assumed to be in the closed position for these calculations. The number of open doors assumed for design shall depend upon the location and type of firefighting facilities installed in the building, and in particular rising main outlets. Where the hose passes through a door, that door shall be considered to be fully open.
In the event of a simultaneous evacuation it is assumed that the stairways will be occupied for the nominal period of the evacuation, and thereafter will be clear of evacuees.
Consequently, the evacuation will occur during the early stages of fire development, and some smoke leakage onto the stairway can be tolerated. The airflow due to the pressurization system shall clear the stairway of this smoke. The occupants being evacuated are assumed to be alert and aware, and familiar with their surroundings, thus minimising the time they remain in the building.
The design conditions for Class C systems are shown in Figure 4. Door between accommodation area and the pressurized space on the fire storey is closed. All doors within the pressurized space that cross the escape route from the accommodation area to the final exit door are open All doors between the pressurized stair and the final exit door are open 10 Pa The final exit door is open The air release path from the accommodation area on the storey where the pressure difference is being measured is open A door to a floor other than the fire floor is open The doors between the accommodation area and the pressurized space are closed on all storeys Licensed copy: The design conditions for Class D systems are shown in Figure 5.
Figure 5 — Design conditions for Class D systems 4. The doors between the accommodation area and the pressurized space are open on two adjacent storeys All doors within the pressurized space on those two storeys that cross the escape route from the accommodation area to the final exit door are open 10 Pa All doors between the pressurized stair and the final exit door are open The final exit door is open The air release path from the accommodation area on the storey where the pressure difference being measured is open The doors between the accommodation area and the pressurized space on all storeys are closed All doors between the pressurized stair and the Licensed copy: The design conditions for Class E systems are shown in Figure 6.
Figure 6 — Design conditions for Class E systems 4. If the rising main outlets are only inside the corridor or the accommodation in front of the lobbies, the door between lobby and corridor or accommodation on the storey below the fire storey has additionally to be assumed to be open during firefighting operations. It is assumed that firefighting operations, such as the use of spray, contribute significantly to the holding back of hot smoky gases.
It is, however, essential that the staircase be kept clear of serious smoke contamination. The system shall be designed so that the stairwell and, where provided, the lift shaft are kept clear of smoke. This shall be achieved by providing separate pressurization of the firefighting lift shaft on one hand and the lobby and stair on the other hand. Table 6 — Minimum pressure differentials between specified areas for Class F systems when all doors Licensed copy: Requirement c above does not apply if there is a simple lobby between the staircase and the final exit door.
All doors of this lobby shall be self closing. Alternatively, the provisions of 4. Requirement b above does not apply if there is a simple lobby between the staircase and the final exit door.
NOTE 2 The force that can be exerted to open a door will be limited by the friction between the shoes and the floor and Licensed copy: The aim is to establish a pressure differential across any leakage paths that will ensure that smoke moves away from the protected space.
This is achieved by maintaining the protected space at a pressure higher than that of the fire zone. It is essential that adequate air release shall be provided from the accommodation to ensure that a pressure differential is maintained. See Figures 8 a and 8 b. In calculating the air supply needed for a pressurization system, assumptions have to be made about the leakage characteristics of the building, in particular between: If buildings contain spaces such as computer suites or medical facilities that are pressurized for reasons other than fire, consideration shall be given to protecting the pressurized escape routes from the effects of fire in these pressurized spaces.
See Clause 8 for more detailed information. It is essential that agreement shall be reached between the specifiers and the designers as to the installation and construction techniques that will be used in the building. Particular attention shall be paid to the construction of the shafts that will be pressurized and the building envelope. Unrealistic assumptions about the air tightness of these constructions are a common cause for pressurization systems failing to meet acceptance criteria.
In a single-stage pressurization system the pressurization is applied only when a fire occurs, and in a two- stage pressurization system a low level of air supply is maintained at all times, for example for ventilation, and is increased to the emergency level when a fire occurs. Either system is acceptable. Consideration shall be given to the siting and construction of the ductwork and fans to ensure that they are not compromised by a fire from within the unprotected space.
Additional over pressure relief shall be provided to ensure that the pressure build up when doors are closed does not make it difficult to open doors into the pressurized space. The use of pressurized and unpressurized stairwells serving the same storeys shall only be considered if either of the following conditions are met: Each pressurized escape route shall have its own independent air supply.
If doors are open near to the injection point, supply air can be lost through them and adequate pressurization may not be achieved at doors further from the injection point. This may be particularly true in the case of ground level injection systems where the exit door is likely to be open for substantial periods of time.
When a stair pressurization system is designed on the basis of an open door at final exit level, the vertical airflow in the shaft is likely to be high and consequently the pressure losses may be substantial.
The lobby shall have pressurizing air supplied through ductwork that is independent of that supplying the stair. The corridor shall have pressurizing air supplied from a duct that is separate from the lobby and the stairwell supply. It is important that provision be made on the fire storey for the air that has leaked into the unpressurized spaces to escape from the building.
This is essential in order to maintain the pressure differential between pressurized spaces and the accommodation. The required leakage rate will depend on the particular layout of the building and the application of the pressurization system. In the absence of such a study air release shall be provided by one of the following methods: Where the building is sealed special vents may need to be provided on all sides of the building see Clause 15 , b vertical shafts.
If venting the pressurizing air by building leakage or peripheral vents is not possible, vertical shafts may be used for this purpose see Clause 15 , Licensed copy: The release of the pressurizing air by mechanical extraction is a satisfactory method.
The mechanical extraction would be required to operate only during the period prior to window breakage see Clause If the venting is not evenly distributed around the external wall, the side with the largest area of venting shall be discounted for the calculation. When automatically controlled release venting is used, the venting shall take place on the fire storey only and the air release vents on all other storeys shall remain closed.
When the emergency pressurization system operates, the dampers closing the extract system shall open on the fire storeys only. In most circumstances the airflow requirement with doors open will be greater than with all doors closed.
If excessive pressures are allowed to develop in the protected space it may become difficult or impossible to open doors into the space see Clause To prevent the build up of excessive pressures it is necessary to provide overpressure relief vents. The pressure relief vent area may be closed by a counter-balanced flap valve so designed that it will only open when the pressure exceeds the design pressure.
Alternatively it is possible to utilise a system controlled by pressure sensors so that the air supply or exhaust can be continuously varied to produce the pressure or flow required. The overpressure relief from the pressurized space shall discharge either: A and Figure 9. This arrangement will carry the protection against smoke ingress right up to the door leading towards the accommodation area in which a fire might occur see Figure 10 and 5.
NOTE A lobby connected to a lift well or other shaft is still considered to be a simple lobby if all such shafts are pressurized independently. NOTE Where the situation arises of a pressurized lobby with two or more doors opening into the accommodation on a single storey, this situation should be subject to a fire engineered solution, in terms of air flow, and of air release, especially where the doors open into separate flow paths leading to different air release paths.
Encircled number denotes minimum design pressure differential, e. However, if the corridor has many doors or other leakage paths the air supply needed may be large.
The design aim shall be to ensure airflow from the stairwell, through the lobby, through the corridor to the external air, either directly or via the accommodation see Figure 11 a.
Only the corridor on the fire affected storey need be pressurized, or b all pressurized stairwells and only the lobbies and corridors on the fire incident floor shall be pressurized. By pressurizing the lift shaft it is possible to restrict the spread of smoke via the lift shaft to other storeys. The pressurization of the lift shaft may also be required for Class B systems see Figure 11 c. NOTE A lobby connected to a lift well or other shaft is still considered to be a simple lobby if all such shafts are Licensed copy: This arrangement will carry the protection against smoke right up to the door into the accommodation.
During firefighting operations it is necessary to open the door between the firefighting lobby and the accommodation to deal with a fully developed fire.
Where a stair is intended for firefighting purposes it is more appropriate to carry out the firefighting design procedure before that for means of escape. Information regarding air leakage areas for typical forms of construction is given in A. Guidance regarding the calculation of effective leakage areas for flow paths in series and in parallel is given in Clause In existing buildings the leakage areas will be highly dependent upon the quality of the workmanship and the nature of the structure, hence the actual leakage values may vary considerably from assumed design values.
Effective leakage areas shall, if possible, be evaluated by an on-site airflow measurement. To give the total required air supply rate, this value shall be multiplied by a factor of at least 1,5 to take account of uncertainties in identified leakage paths see Calculating the air release requirements is not necessary if the firefighting design procedure has previously been carried out.
In order to simplify the calculation procedure, it may be assumed that there is no interaction between the stairwell and the lift pressure differential systems this will tend to give an overestimate of the required total air supply rate to the stairwell because it does not take account of the additional air flow between the lift and the stairwell.
The following procedures are intended to establish the required air supply with the final exit door open, the stairwell and lobby doors on the fire floor open and adjacent storey door or doors open as identified in 4.
The anticipated leakage via all paths other than the open doors shall be multiplied by a factor of at least 1,5 to take account of uncertainties in identified leakage paths. During this initial period the potential for contamination of the protected routes is small. Before conditions on the fire storey become untenable the escape process from that storey ought to have been completed and the storey exit doors closed. Consequently, there is no need for the pressure differential system to hold back smoke from a fully developed fire at a door, as long as the air flow is sufficient to hold back smoke from the fire floor whilst persons are escaping.
Following evacuation of the fire-affected storey the fire may continue to develop with the potential to induce smoke flow into the stairwell via gaps around stairwell and lobby doors. It is therefore important to ensure that a positive pressure is maintained within the stairwell for the full duration of the evacuation process.
However, during this stage the final exit from the stairwell is likely to be in use, producing a loss of pressurizing air and hence tending to reduce the pressure in the stairwell, and it is necessary to take account of this when calculating the air supply. The design conditions for stairwell pressure differential systems are shown in Figures 9 a , 9 b , 10, 11 a , 11 b , 11 c , 12 a , 12 b , 13, 14, 15 and The force required to overcome the door closer will often not be known at a preliminary design stage and a maximum pressure differential of 60 Pa may be utilised for design purposes.
NOTE 2 The force that can be exerted to open a door will be limited by the friction between the shoes and the floor and it may be necessary to avoid having slippery floor surfaces near doors opening into pressurized spaces, particularly in buildings in which there are very young, elderly or infirmed persons. This protection is normally in the form of a specific room close to the means of escape staircase or forming part of a route to a storey exit, constructed of fire-resisting materials including fire-resisting doors with effective self-closing devices , in accordance with national provisions valid in the place of use of the system.
Refer to Figures 14 and 15 for typical plan layouts. If buildings contain spaces such as computer suites or medical facilities that are pressurized for reasons other than fire, consideration shall be given to protecting the pressurized escape route from the effects of fire in these pressurized spaces.
Refer to Figure 16 for a typical floor layout. It is important to note that there is no protection of any part of an escape route within the depressurized space itself, which may be entirely filled with smoke, or may even be fully involved in a fire. This constitutes a fundamental difference between depressurization and smoke exhaust ventilation.
However, in compartmented buildings it may be possible to depressurize individual spaces. See Figure 17 for the typical features of a depressurization system. The most appropriate use of depressurization systems is likely to be in basement spaces, see Figure 18 for layout. The design procedure will be the same for both systems, except that where the design is for fire fighting the exhaust volume flow rate will be increased to take account of the later stage of development of the fire.
Calculate the pressure difference across those same doors to maintain this airspeed. Ensure that the resistance of the leakage paths from the exterior of the building to the protected spaces is included in the calculation. NOTE If the final exit door is open, the flow resistances of these leakage paths will be sufficiently low to be ignored.
Usually this fan duty will be less than that identified in 9. If the compartment is open to the atrium, then it must have either a downstand barrier to create a reservoir within the compartment, or a high-powered exhaust slot at the boundary edge to achieve a similar effect Figure 8.
The minimum height of the smoke layer base in the room must be compatible with the openings on to the atrium, with the layer depth being no lower than the soffit of the opening Figure 9. Where no downstand exists and an exhaust slot is used instead, the exhaust capacity provided will need to be compatible with the layer depth Figure See the section on exhaust slots 'Slit extract' on page Recent work by Hanseldrawing on work by Zukowski et aand Quintiere et ahas shown that the rate of air entrainment into a plume of smoke rising above a fire M , may be obtained by using the equation: As the two values are approximately similar and the demarcation between them uncertain, then the value for all large-space rooms is taken to be 0.
Most small rooms will therefore take this value. There is no information available to show how Equation 1 or any current alternatives should be modified to allow for the effects of sprinkler spray interactions.
Consequently, it is used here unmodified. For other scenarios the following procedure may be adopted: Where the smoke flows beyond a downstand or lower ceiling level in the form of a plume of height Dd Figure 13 a arid 13 b , it has been shown23that the height of rise of the plume has an effect on the rate of flow of smoke leaving the opening. This effect can be expressed as a modification to the coefficient of discharge as follows: Height of smoke base m Figure 11 Rate of production of hot smoky gases sprinklered offices Where Dd 2 1.
For a plain opening with no downstand obstruction Figure 14 , Dd can be considered as the rise of the plume beyond the balcony edge.
The flowing layer depth 0, may be found from: Mw n - Mw n - 1 4- U a With downstand and projecting balcony x MW n This procedure usually converges after about five iterations and will therefore quickly yield M,, C, and D,. Figures 15 and 16 give t h e mass flow values in graphical form for various opening heights and widths, using t h e above procedure. A ceiling and projecting balcony 4 m above the floor have been assumed. I t should be noted that the shaded areas on the graphs represent t h e onset of flashover calculated using M, and Q, appropriate to the example illustrated and a layer temperature of approximately "C , and values of mass flow lying within this band should be regarded with caution.
I 7 b With high balcony 11 I. The narrower the room becomes, the less easily' the air can flow behind the plume. In this Report cellular offices are considered to be those in which the maximum room dimension is less than or equal to five times the diameter of the design fire size, and the incoming air can only enter from one direction Figure This demarcation dimension was chosen arbitrarily and has no theoretical derivation.
Research in this area is highly desirable. Width 5 x Df Restricted air flow to fire and plume ' positioned across the balcony to limit the size of this reservoir to ensure that the smoke retains its buoyancy.
Each reservoir should be limited to an area not exceeding m2, with a maximum length of 60 m by analogy with shopping maIlsI6. The screens around the balconies will, in general, be fairly close to potential fire compartments eg offices.
Being close, smoke issuing from such a compartment will deepen locally on meeting a transverse barrier. The depth of these screens should take into account local deepening see page Smoke removed from these lower level reservoirs should usually be ducted to outside the building but can be ducted into the ceiling reservoir of the atrium Figure The mass flow rate of smoke to be exhausted from the atrium roof will then be that calculated for the under-balcony condition Cellular room Early experiments with smoke flow in shopping malls29 and unpublished workI7 at FRS also N R Marshall, Fire Research Station; private communication, have shown that the smoke flowing from a room with a deep downstand and then under a balcony beyond the opening becomes turbulent with increasing mixing of air.
This subsequent evidence suggests that for the purpose of engineering design the mass flow rate of smoke entering the balcony reservoir MB can be taken to be approximately double the amount given by Equation 2, ie: Figure 18 Schematic section of an atrium with balconies Some atria are designed with balconies around the perimeter of the void, serving all the rooms at that level Figure Figure 19 illustrates in schematic form an atrium with floors two levels only are shown in both the figures which have balconies that leave a considerable area for pedestrians.
On each level there is a large area situated below each balcony. If screens activated by smoke detectors or as permanent features are hung down from the balcony edges, the region below each balcony can be turned into a ceiling reservoir.
This is similar to the procedure used in multi-storey shopping complexesI6. This balcony reservoir can then be provided with its own extraction system.
Other screens can be U Exhaust from balcony reservoir Figure 19 An under-balcony smoke reservoir 13 0 alternative escape routes, 0 shorter escape paths along the balcony, and the installation of sprinklers to cool the gases further.
Effects of sprinkler systems in smoke reservoirs Offices, shops, assembly, industrial and storage or other non-residential purpose groups are now expected to have sprinklers if they have a floor more than 30 metres above ground level.
Multi-storey buildings in the assembly, shop, industrial or storage purpose groups will also be fitted with sprinklers if individual uncompartmented floors exceed a given size. Sprinklers may also be required in other circumstances for insurance purposes.
I Figure 20 Under-balcony smoke reservoir venting into an atrium smoke reservoir Entrainment into smoke flows from compartments is being studied The purpose of this is to determine more accurately the influence of such factors as compartment opening geometry, the presence of a downstand fascia and balconyldownstand combinations. It follows that Equation 5 may be superseded in due course. Smoke layer temperature The mean temperature rise of the smoke layer above ambient 0 can be calculated from: Where the smoke layer is contained wholly within the room of origin by a smoke control system and has a large area, the sprinklers will cool the smoke layer further.
Similarly, where smoke is collected within a balcony reservoir adjacent to sprinklered offices, operation of sprinklers under the balconies will lead to increased heat loss reducing the buoyancy of smoke, which in turn can contribute to a progressive loss of visibility under the smoky layer. However, gases sufficiently hot enough to set off sprinklers will remain initially as a thermally buoyant layer under the balcony ceiling, and will not be pulled out of the layer by the sprinkler sprays.
When the fire occurs in an office, the operation of sprinklers under the balcony will not assist in controlling it. If too many sprinklers operated under the balcony, sprinklers in the office could become less effective as the available water supply approached its limits. The maximum smoke layer temperature which will allow safe evacuation without undue stress is in the order of "C.
If this gas temperature or lower cannot be achieved then consideration should be given to: Therefore if the extent of sprinkler coolirig is overestimated, the system could be underdesigned.
A system using natural ventilators depends on the buoyancy of the hot gases to expel smoke through the Table 1 Volume flow rate and temperature of gases from a 1 MW fire including cooling within room of origin Mass flow rate Mass rate of extraction kgs-9 Temperature of gases above ambient.
This radius is generally not known. In this case the system would be underdesigned if the sprinkler cooling were underestimated. In the absence of better information, it may be reasonable to assume that no more sprinklers will operate than are assumed when calculating the design of sprinkler systems and their water supply eg 18 heads for Ordinary Hazard Group 3.
The heat loss from smoky gases to sprinklers is currently the subject of research, although data suitable for design application are not yet available. Nevertheless, an approximate estimate can be obtained as follows: If the smoke passing a sprinkler is hotter than the sprinkler operating temperature, that sprinkler will eventually be set off and its spray will cool the smoke.
If the smoke is still hot enough the next sprinkler will operate, cooling the smoke further. A stage will be reached when the smoke temperature is insufficient to set off further sprinklers. The smoke layer temperature can thereafter be assumed to be approximately equal to the sprinkler operating For powered extract systems the cooling effect of sprinklers can be ignored in determining the volume extract rate required.
This will err on the side of safety. Alternatively, this further cooling and the consequent contraction of smoky gases can be approximately estimated on the basis of an average value between the sprinkler operating temperature and the calculated.
Where the fan exhaust openings are sufficiently well separated it can be assumed that one opening may be close to the fire, and Table 2 Volume flow rate and temperature of gases from a 6 MW fire including cooling within room of origin Mass flow rate Mass rate of extraction kgs-9 Temperature of gases above ambient "C Volume rate of extraction at maximum temperature s- ' 10 12 15 20 25 Note that the effect of sprinkler cooling is to reduce the heat flux Q, without significantly changing the mass flux.
It follows that once a new value of 0 has been estimated, the new heat flux can be found using Equation 6. Flowing layer depth Smoke entering a ceiling reservoir will flow from the point of entry towards the exhaust points.
This flow is driven by the buoyancy of the smoke. Even if there is a very large ventilation area downstream eg if the ceiling downstream were to be removed , this flowing layer would still have a depth related to the width available under the remaining ceiling which can now be considered a balcony , the temperature of the smoke and the mass flow rate of smoke.
Work by Morgan30 has shown that this depth can be calculated for unidirectional flow as follows: Where such structures exist and are an appreciable fraction of the overall layer depth, the depth below the obstacle should be found using Table 3 b rather than 3 a. Values of Cd will vary for differing flow geometries. However, for the purpose of engineering design c d can be taken to be 0. At the time of writing, values of Cd for intermediate depth downstands cannot be stated with confidence for the wide range of geometries to be found in practice.
It is suggested that either of the extreme values should be adopted in seeking a conservative design approach. The resulting values of layer depth for different balcony reservoir widths and mass flow rates of smoke are. Each depth shown in this table is the minimum possible regardless of the smoke extraction method employed downstream; consequently it represents the minimum depth for that reservoir.
These are typically installed around the edge of the voids to prevent smoke flowing up through the voids. If the void edge is close to the room this local deepening could cause smoke to underspill the smoke curtain and flow up through the void, possibly affecting escape from other storeys.
Clearly, the void edge screens must be deep enough to contain not only the established layer, but also the additional local deepening outside the room on fire.
The extent of local deepening can be found from Figure The depth of the established layer DB in Figure 21 under the balcony immediately downstream of the local deepening must first be found using the design procedure given in the preceding sections. Usually this means in the channel formed between the void edge screen and the room faqade.
It can be shown that the following scale-independent formula can be used to approximate Figure When ventilating compartments directly, if the faiade is normally sealed then facilities should be provided for the necessary quantity of replacement air to be supplied to the fire room automatically. This requirement often makes the provision of smoke ventilation to the room of origin prohibitive or.
The provision of replacement air to a system employing balcony reservoirs is far easier, provided the balconies are open to the atrium. If the area available for inlet becomes too restricted, incoming airflow through escape doors may be at too high a velocity for easy escape. Such air inflows through doors in public buildings could hinder Dg fm Figure 21 Local deepening at a transverse barrier Recent research32into the ability of people to move through an exit against an opposing airflow has shown that movement is not impeded for airspeeds below 5 ms-', and is not seriously impeded below 10 ms-' although some discomfort was reported at these higher airspeeds.
This suggests that inflow airspeeds should not usually exceed 5 ms-I. Other values may be appropriate for other circumstances. For example, in buildings where the population is largely familiar with the escape routes; where the incoming air is entering the fire room directly, or where in the instance of the inlet air being supplied via the atrium the major escape routes are away from the atrium; then a less onerous parameter can be applied.
Current advice regarding pressurisation system design33recommends a maximum pressure drop across a door of 60 Pa. This accords to a face velocity across a rectangular inlet opening of about 6 ms-I.
The pressure drop criterion may be increased if the population of the building is adult and physically fit, to perhaps Pa 8 ms-I. A fan-driven inlet air supply may be employed, but can give problems when mechanical extraction is used the building will usually be fairly well sealed in such circumstances. This is because the warmed air taken out will have a greater volume than the inlet air. As the fire grows and declines, the mismatch in volume between the inlet air and the extracted fire-warmed air will also change.
This can result in significant pressure differences appearing across any doors on the escape routes. For this reason simple 'push-pull' systems should be avoided. Minimum number of extraction points The number of extraction points within the reservoir is important since, for any specified layer depth, there is a maximum rate at which smoky gases can enter any individual extraction point.
Any further attempt to increase the rate of extraction through that point merely serves to draw air into the orifice from below the smoke layer. This is sometimes known as 'plugholing'. It follows that, for efficient extraction, the number of extraction points must be chosen to ensure that no air is drawn up in this way.
Table 4, which is subsequently modified based on experimental A J M Heselden, Fire Research Station; private communication, , lists the minimum numbers needed for different reservoir conditions and for a variety of mass flow rates being extracted from the ventilators in the reservoir. Table 4 strictly applies to ventilators which are small compared to the layer depth below the ventilators eg where the diameter is much less than the depth of the layer.
Where sprinklers are installed and additional cooling of the smoke layer needs to be accounted for, the number of extraction points required will differ from those shown in Table 4. The number can be determined by calculating the critical exhaust rate for 18 an opening A J M Heselden, Fire Research Station; private communication, , beyond which air will be drawn through the layer.
M N In both tables the first number of each pair denotes extraction points wcll away from the walls, and thc second is for those close to the walls. Where very large or physically extensive ventilators are used eg a long intake grill in the side of a horizontal duct an alternative method is possible.
In practice, for a given mass flow rate and layer depth, Table 3 a , 3 b or Equation 7 can be used to find the minimum value of accessible perimeter.
Such a system is likely to work best with further extraction distributed within the fire room, which for a sprinklered room may possibly be provided by the normal ventilation extraction system the normal ventilation input and recirculation of air being stopped or, for an unsprinklered room, by a partial smoke exhaust system. Roof exhaust system Single flap without wind deflectors Single flap with wind deflectors Dual single flap installed at an angle of Performance classes Testing the reliability Re Testing the snow load SL Testing the low ambient temperature Testing the wind load WL Testing the resistance to heat Testing the aerodynamic free opening area Initial inspection of factory production control and the quality plan The EC certificate of conformity Description of the CE type plate Declaration of conformity EN energy supply TR Dimensioning natural smoke and heat exhaust ventilation systems