Lawrence Webster Forrest (LWF), Fire Engineering and Fire Risk Management Consultants
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Lawrence Webster Forrest
Legion House
Lower Road
Kenley
Surrey
CR8 5NH

Tel: +44 (0)20 8668 8663 Fax: +44 (0)20 8668 8583
E-mail: fire@lwf.co.uk

Mechanical Smoke Ventilation Systems (MSVS)

Until recently, prescriptive documents such as Approved Document B (ADB) and British Standards gave guidance on smoke control systems primarily by natural means (i.e. external automatic opening vents (AOV’s) and natural smoke shafts) and pressure differential systems (namely pressurisation systems). However, the publication of BS 9991:2011 - Fire safety in the design, management and use of residential buildings – code of practice, provides guidance in the use of mechanical smoke ventilation systems (MSVS) for residential common corridors/lobbies and fire fighting shafts. Therefore, the aim of this is to bulletin is to document the advantages of an MSVS as an alternative to the smoke control systems outlined in the prescriptive document pre publication of the BS 9991:2011. It will look at how a MSVS can be incorporate into different buildings and specific building areas as part of a performance based design solution.

 Introduction

The incorporation of a smoke control system can significantly improve the life safety of the building. Smoke control systems come in many forms and range widely in complexity i.e. natural smoke venting, pressurisation systems and mechanical smoke ventilation systems. Regardless the system the basic objectives of each system is the same. The objectives of a smoke control system are:-

 ·         To keep escape routes free from smoke,
·         To contain or remove the smoke from the fire area to prevent it from migrating into other areas,
·         To facilitate fire fighting operations,
·         To protect life and minimise property loss.

Generic design guidance documents such as the ADB are used to comply with the building regulations. However, in building design today these guidance documents are not often suitable or cost effective for complex structures or existing buildings. Using a performance based design approach fire engineers are able to evaluate the smoke control  provisions within a building and deliver a more flexible, cost effective solution while not compromising the life safety of the building. Some of the factors that must be considered when using a performance based approach for smoke control include:

 ·         What are the objectives,
·         What are the risks to occupants and contents,
·         What type of fire load is present (i.e. residential, office, retail etc.),
·         What is the geometry of the building/floor etc,
·         Expected fire size.

How a MSVS System Works

The MSVS system effectively acts as a depressurisation system. It extracts smoke and heat from an area thus depressurising the space. As the surrounding areas (such as the staircase etc.) will have a higher pressure, air will be entrained from these areas towards the smoke shaft thus preventing smoke from flowing into the staircase or neighbouring areas. It is necessary to provided additional inlet air to the area being depressurised to prevent the area from becoming excessively depressurised. If the area becomes over depressurised, it can damage the smoke venting equipment and/or smoke will be actively pulled from the fire room. Pressure differentials across doors will also become too great making the door harder to open and/or pulled open between the depressurised area and fire area. The make up inlet air for the system may be achieved in numerous ways. This will depend on the building layouts. However, listed below are four ways in which make up air for the system can be achieved;

 ·        Natural inlet via AOV at the head of the staircase,

·         Natural inlet via external air,

·         Natural inlet via shaft,

·         Mechanical inlet via shaft.

Each of the above come with their own advantages and disadvantages.

MSVS in Residential Buildings (Flats and Maisonettes)

The basis for a smoke control system in a residential building is to prevent the smoke from flowing into the staircase so occupants escaping from a fire can do so unhindered and the fire service have a clear protected access to the seat of the fire.

ADB recommends that for a residential building with a floor over 11m (or containing more than 3 storeys above ground), every flat should be separated from the common stair by a protected lobby or common corridor. It also recommends that the common corridor/lobby should be ventilated via a 1.5m2 AOV direct to outside or, a 1.5m2 natural smoke shaft or, a pressure differential system. Considering that the lobby is ‘land locked’ and AOV’s to outside is not possible, the only viable solution based on the guidance documents is to provide a 1.5m2 natural smoke shaft or a pressure differential system. Instead of providing a natural smoke shaft or a pressure differential system an alternative to consider is an MSVS. In this day an age where cost is everything, a MSVS may be more cost effective than one might think.

As a MSVS actively ‘pulls’ heat and smoke from the common corridor/lobby and does not depend on the natural buoyancy of the smoke, the shaft area can be significantly reduced compared to a natural system. Of course the area of the smoke shaft for a MSVS depends on factors such as the size of corridor/lobby, fire loading in the accommodation, extract rates of the fans and size of the AOV opening into the shaft. All these factors are considered and through calculations from first principles or computation fluid dynamic (CFD) modelling the area of the shaft and extraction rates for the system can be determined. Generally, for a MSVS the shaft areas range from between 0.25m2 – 0.6m2 (Note: these are not definitive shaft areas, it is recommended to consult a fire engineer who can provide guidance as to what will be required). Comparing a natural smoke shaft area to a mechanical smoke shaft there is significant difference in size. Where space in a building is critical utilising a MSVS gives floor space back to the building on each level it serves. It is also important to note because the shaft will be passing through compartment floors it will need to be fire rated and as the mechanical smoke shaft is smaller it will require less fire rated construction therefore saving in construction costs.

A basic MSVS can be provided as an equivalent to a natural smoke vent system; however it is possible to enhance the performance of the system so that it can be used to justify extended travel distances in a common corridor / lobby. The MSVS is capable of extracting more smoke and heat from the corridor that will keep the corridor at tenable conditions so occupants can escape safely regardless of the travel distances. Using this principle and depending on the design of the building secondary staircases may be omitted. This would provide a massive cost saving to the client but also could have a financial gain to the client as the space could be used for additional accommodation etc.

If other residential buildings (purpose group 2b – hotels, student accommodation etc.) are deigned to a residential standard such as flats and maisonettes, then the same principles outlined above can be applied to these buildings also.

MSVS in Fire Fighting Shafts

Another situation where a MSVS can be employed to provide a more cost effective solution is to vent a fire fighting shaft. ADB and BS 9999:2008 detail provisions for when a fire fighting shaft should be incorporated into a building. The general recommendation is that a fire fighting shaft should be provided in buildings with a floor more than 18m above the fire a rescue vehicle access level, or a basement at more than 10m below fire and rescue vehicle access level. This generally applies to residential and office accommodation. The guidance documents also outline the provisions for fire fighting shafts purpose groups 4, 5 and 6 (shops, commercial, assembly & recreation and industrial buildings.

ADB recommends that where a fire fighting shaft serves residential accommodation i.e. flats and maisonettes the provision of smoke control for fire fighting shafts can be the same as to vent the common corridor/lobby as outlined previously i.e. 1.5m2 AOV direct to outside, 1.5m2 natural smoke shaft or the use of a pressure differential system. This is due to the high degree of compartmentation achieved in blocks of flats. Therefore, the same provisions outlined in the previous section (MSVS in residential buildings) can be applied to offer a more cost effective and safer solution for smoke control for fire fighting shafts in residential buildings.

ADB also gives recommendations for smoke control provisions for fire fighting shafts in buildings such as office, retail and industrial buildings. It recommends that the shaft should be constructed generally in accordance with BS 5588:2004 (now superseded by BS 9999:2008). Considering that a fire fighting shaft will not always have an external wall (to provide and AOV direct to outside), BS 9999:2008 recommends that a smoke shaft be in accordance with BRE Report 79204. The cross sectional area (geometric free area) of the smoke shaft should be at least 3m2, with the lobby ventilator should have a free area of at least 1.5m2. The area of the shaft is large due to the expected fire loads for this type of accommodation and the results of the BRE Report 79204. The BRE Report 79204 compared natural smoke venting of a fire fighting shaft through an external AOV and an AOV opening into a natural smoke shaft. The results found that a 3m2 shaft provided better results in the fire fighting shaft than that of an external AOV opening direct to outside. However, as explained previously a MSVS system is capable of extracting more heat and smoke from an area it may be possible through a performance based design approach to reduce the size of the ventilation shaft serving the fire fighting shaft for this type of accommodation and provide a more cost effective solution. Again this will be based on factors such as fire load with the accommodation, extraction rate of the fans and size of AOV’s opening into the shaft, but it’s not uncommon for a shaft to be engineered down to 0.6m2 in area. Comparing this to a 3m2 as required under the prescriptive documents it is clear that this approach could offer a significant cost saving.

Advantages and disadvantages of a MSVS

Below shows a list of advantages and disadvantages for a mechanical smoke venting system.

Advantages include:

·         Optimises space (more usable),
·         Cost saving (potential for removal of a staircase and reduced ventilation shaft areas),
·         More efficient than a code compliant solution (not effected by wind conditions),
·         Not as complex as a pressurisation system.

Disadvantages:

·         The system has to be proven to be approved (i.e. demonstrate the system works through calculations and/or CFD modelling,
·         Cost (not suitable for certain building i.e. in certain small buildings a code compliant smoke control system would be more cost effective),
·         Often not understood by non specialists.

Summary

This bulletin has provided an insight into mechanical smoke venting systems (MSVS) and how they can provide a cost effective solution when implemented into certain buildings. LWF can provide specialist fire safety advice on a number of alternative approaches to the prescriptive documents with regards to smoke control provisions in buildings. A performance based assessment of some of these issues can be advantageous through potential cost savings and an increase in useable floor area.

It should be noted however that all buildings are unique and have different geometries and fire loads. Hence, it is recommended to consult a fire engineer as early as possible in the design stage in order to highlight the areas where there is cost saving potential based on a performance based design.

References

1.           The Building Regulations 2000, Fire Safety, Approved Document B, Volume 2 – Buildings other than dwelling house (2006).
2.           British Standards Institution, BS 9999:2008 Code of practice for fire safety in the design and management and use of buildings.
3.           British Standards Institution, BS 9991:2011 Fire safety in design, management and use of residential buildings – Code of practice.
4.           British Research Establishment, BRE project report No. 79204 – Smoke shafts protecting fire fighting shafts: their performance and design.
5.           Smoke Control Association, Guidance on smoke control to common escape routes in apartment buildings (flats and maisonettes) (revision 1 – June 2012)
6.           British Standard Institution, BS EN 12101-6:2005, Smoke and heat control system, Part 6 – Specification for pressure differential system – Kits.

This bulletin was written by Ryan Harkin BSc (Hons)

Lawrence Webster Forrest Limited

LWF are fire engineering and fire risk management consultants with over twenty five years experience in the development of fire engineered technology and the application of fire safety standards including fire engineered techniques.

Should you wish to receive any further information on LWF and the services we provide please contact our Marketing Department.

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