# The LWF Blog

## Fire Engineering for Healthcare Premises – Smoke spread and control – Part 13

December 14, 2020 12:42 pm

In LWF’s blog series for healthcare professionals, our aim is to give information on best practice of fire safety in hospitals and other healthcare premises. In part 12 of Fire Engineering for Healthcare Premises, LWF considered the effects of suppression, compartment fire modelling and cone calorimetry. In part 13, we discuss the pertinent elements of smoke spread and control.

It is ideal for smoke to be buoyant so that there is a clear layer below the smoke layer. If the gas temperatures are low, other air movement, such as wind, can disturb a smoke layer and the clear layer may become contaminated.

Available safe egress time (ASET) and required safe egress time (RSET) are terms used when discussing the time for building occupants to evacuate a building or area to a place of safety in a fire situation, in a safe manner. Each element is not a single deterministic value, but has probability distributions associated with them. The analysis undertaken should consider the safety factor and not simply the principle that ASET should be greater than RSET.

Either ASET or RSET should account for the time taken from fire ignition to it being detected and the alarm raised.

A ceiling jet is a relatively thin layer of flowing hot gases that develops under a horizontal surface (e.g., ceiling) as a result of plume impingement and the flowing gas being forced to move horizontally. Characterisation of the ceiling jet is important for the calculation of operation times of detectors and sprinklers.

Spill plumes can occur in atria, shopping malls and geometrically complex buildings. In order to calculate ASET, spill plumes must be identified and calculated for. The calculations are also necessary for the sizing of extracts in smoke and heat exhaust ventilation systems.

Computational fluid dynamics (CFD) simulations may be used to calculate spill plumes where the geometry is complex and verification can be achieved through hot-smoke tests.

The stack effect or chimney effect is the movement of air into and out of buildings, chimneys, flue-gas stacks, or other containers, resulting from air buoyancy. The presence of the stack effect and location of the neural pressure plane must be considered when designing smoke control systems in tall hospital buildings. The effect has been responsible for the rapid spread of fire upwards in some tall buildings.