# The LWF Blog

## Fire Engineering for Healthcare Premises – Fire Growth – Part 10

November 23, 2020 1:24 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 9 of Fire Engineering for Healthcare Premises, LWF considered fire growth in terms of the heat release rate and the laws of thermodynamics. In part 10, we will continue to look at the elements considered in calculating fire growth.

Fire engineering involves the use of mathematical equations to calculate various parameters in a fire engineered design. In determining thermal inertia, for instance, the properties indicating thermal conductivity, density and specific heating capacity are calculated to work out how quickly an object’s surface temperature will rise. The mathematical expression is pronounced kay-row-see and a low level would indicate the increased rate at which it heats in a fire situation.

The heat release rate of a fire is calculated using the heat of combustion and the heat of vaporisation. For some materials and elements, the properties are liable to change with time during a fire, as more volatile components tend to be burnt in the early stages of fire.

When considering fire severity, the correct choice of plume expression is important in the design of smoke-control systems to calculate the total amount of smoke and hot gases which will need to be exhausted.

The entrainment rate is dependent on the height of plume rise, the heat release rate of a fire, the fire area or perimeter and any deflection from vertical. The entrainment rate is important in determining the severity of vitiated fires.

The calculation of smoke properties is particularly important in areas of a hospital where dependent and/or very high dependency patients are present. In such areas, there should be no obscuration or concentration of smoke at head height.

There may be differing acceptable levels of smoke obscuration and concentration in different healthcare buildings, depending upon the usage. In atriums, for example, the smoke layer may be above head-height but still present a hazard due to heat radiation.

It should be noted that a smoke layer above dependent or very high dependency patients may be unacceptable due to the anxiety caused to the individuals by their inability to evacuate independently.