# The LWF Blog

## Fire Safety Engineering for Design – Cross-ventilation Systems – Part 200

September 9, 2024 10:12 amLWF’s Fire Safety Engineering blog series is written for Architects, building designers and others in the construction industry to highlight and promote discussion on all topics around fire engineering. In part 199, LWF talked about impulse jet ventilation. In part 200, we will continue exploring cross-ventilation systems by discussing opposed air flow systems.

Limiting average air velocity is calculated as follows;

‘Average air velocity’ is determined by dividing the extract rate by the area of all the openings, including those which do not open into the communicating space. An additional allowance should be made for unknown leakage paths, typically this is 15% and it is usually adequate where a more exact figure is not known.

The above calculation is where *v*_{e} is the limiting average air velocity (m^{.}s^{-1}), *g* is the acceleration due to gravity (9.81 m^{.}s^{-2}), *H* is the height of the opening measured from the bottom of the opening (m), *T*_{f} is the temperature of the heated smoke (K) and *T*_{0} is the temperature of the ambient air (K).

Smoke may be prevented from flowing into a small communicating space from a larger one, provided the small communicating space is located within the smoke layer. It is done by supplying air to the small space (and can be determined by using the equation above).

Where opposed air flow is used to prevent smoke spread from a large space into an adjoining smaller space, below the smoke layer interface, it is done by providing input air into the space the designer wishes to remain smoke free. The air is supplied at the ‘limiting average velocity’, calculated here:

Where *v*_{e} is the limiting average air velocity (m · s^{–1}), *Q* is the heat release rate of the fire (kW) and *z* is the distance above the base of the fire to the bottom of the opening (m). This equation is only valid where the limiting average air velocity is not greater than 1.02 m·s^{–1}, or where *z* is less than 3 m.

Equation no.2 cannot be used applying it to corridor fires or situations where smoke enters a corridor via an open door from an adjoining room, where it is proposed to prevent further spread into the corridor by supplying inlet air into the corridor area. In such situations, the following equation should be used instead, however it’s not applicable to sprinkler-controlled fires, as the minimum velocity would be too small.

Where *v*_{k }is the limiting average air velocity to prevent smoke flowing upstream (m · s^{–1}), *Q* is the heat release rate of the fire (kW), *w* is the corridor width (m), *p* is the density of upstream air (kg · m^{–3}), *c* is the specific heat of downstream gases, *T* is the temperature of the downstream mixture of air and smoke (K), *K* = 1 (constant) and *g* is the acceleration due to gravity (9.81 m · s^{–2}).

(Source: CIBSE Guide E)

In part 201 of LWF’s series on fire engineering we will begin to discuss pressure differential systems, starting with depressurisation systems. 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 since 1986 to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact the LWF office on 0800 410 1130.*

*While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.*