The LWF Blog

Fire Engineering for Healthcare Premises – Design Fires & Smoke – Part 34

May 10, 2021 11:05 am

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 33 of Fire Engineering for Healthcare Premises, LWF discussed fire growth. In part 34, we will talk about the use of design fire specifications.

A design fire specification is based on a particular scenario under consideration and depends upon the type of combustibles and where they are placed; the potential ignition sources; and ventilation conditions in the area.

The characterisation of a design fire will be expressed in terms of Heat Release Rate (HRR), fire area, and product and smoke production rates.

Data on HRR for many materials exists, but in practice, a fire scenario will include a mixture of materials and so the HRR data may be difficult to assign. This is especially the case where textiles and furnishings are fire-retarded to HTM 05-03 Part C.

It may be necessary to undertake statistical analysis of fires and the use of experimental fires under calorimeters to support the selection of a particular design fire.

A design fire may have a constant HRR – known as a steady state, or a time-varying HRR – known as transient.

The determination of design fires is a critical part of the role of the QDR team in a fire safety engineered design, as most fire safety engineering calculations follow from the specification and are dependent on it.

Smoke Behaviour and Control

In a healthcare building, as in other buildings, smoke from a fire will move from the immediate area of the fire to other parts of the building causing a danger to building occupants.

The time taken from the fire igniting to conditions within the building becoming life-threatening is the maximum time available to move occupants to a place of safety. This is known as the available safe egress time (ASET).

It is possible to implement strategies and systems to control the spread of smoke from a fire, but in order to plan and implement a smoke control system, it is necessary to calculate the movement of smoke in a given area.

The calculation of smoke movement can be achieved through a range of methods of varying complexity.

  • The buoyancy of smoke/fire gases
  • Turbulent mixing and entrainment (dilution)
  • Stack effect
  • Ventilation Systems in the building and consequential air movement
  • The potential effect of wind
  • Thermal radiation

In Part 35 of LWF’s blog series, LWF will look at what information can be gained from calculating smoke movement and how it can be used. 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 LWF on freephone 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.


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