The LWF Blog

Fire Engineering Design and Risk Assessment – Fire and Smoke Ventilation – Part 5

February 15, 2017 11:16 am

In this Fire Engineering Design and Risk Assessment blog series for Architects and others who work in construction, we have been looking at the types of smoke control system which may be used as part of an active fire protection package. In today’s blog, we’re going to talk about smoke control systems which are designed to dilute the amount of smoke in the air in order to achieve a tenable (safe) condition.

 

In order to achieve/maintain tenable conditions, a smoke control system can be designed specifically to dilute smoke in a given space. The parameters of the design may be an assurance that the conditions created do not exceed design criteria in terms of breathability or the compartment temperature. Such systems can both involve the removal of smoke and simple dilution.

 

While such a system may achieve such design parameters, it is unlikely that this will be achieved over an extended period of time, unless the space in question is very large or the fire is very small, it is likely that the tenable conditions will not be able to be maintained for long while a fire is occurring.

 

In cases where smoke must be cleared from a space after a fire has been extinguished, dilution through a smoke control system is often used. This means that the cold smoke can be extracted via a rate of six air changes per hour, which is the commonly used rate. The rate used in atria in England and Wales is reduced to four air changes per hour, in cases where sprinklers are provided.

It is possible to calculate the time taken to improve visibility within a space to a pre-determined level:

 

M_s / M_s0 = exp ( –n_vt_d )

where m_s is the concentration at time t_d, m_s0 is the initial
concentration, n_v is the air changes per hour (h−1) and t_d is the dilution time (h).

 

In an atrium up to 18m in height, it is possible to remove smoke to delay the drop of the smoke layer base by mechanical means, but it is also achievable by using natural venting and the stack effect, taking advantage of internal environment flows.

 

Natural vents can be affected by outside wind and in order to provide a prediction of its effects, the situation can be measured with a wind tunnel test or CFD modelling.

 

The number of air changes can be calculated using conventional techniques. Since there is no need to maintain a clear layer, replacement air may can usually be from high and low levels.

 

As a means of smoke dilution or dispersal, cross-ventilation is widely used especially for firefighting operations. It’s based historically on vent areas of 2.5% or 5% of the total floor area of the building, at least half of which must be equally distributed on two sides of a space. Although theory behind this arrangement is scant, it is reasonable to assume that the relatively large vent areas which result in large spaces are such that in the majority of cases, this would be an effective means of removal of both heat and smoke.

 

In Part 6 of this series, we’ll talk about systems to protect car parks. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.

 

Lawrence Webster Forrest has been working with their clients for over 25 years to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact Peter Gyere on 020 8668 8663.