ABSTRACT
The classical representation of a fire in a room or building represents the structure with an opening, such as a door or a window, to the ambient surroundings or to an adjacent compartment. We shall call such an opening a
vent
. The flow of gases in and out through a vent is controlled by the pressure difference across the vent. In order to estimate any of the environmental consequences of a fire in an enclosure, it is necessary to quantify the mass of hot gases exiting and the mass of colder gases (air) entering the enclosure. This gives information on the mass and energy balance and therefore allows calculation of such environmental consequences as hot gas temperature and smoke layer height. This chapter derives engineering equations used to calculate pressure differences across vents, as well as equations for calculating the mass flow of gases in and out through vents, for several common enclosure fire scenarios. Vent flows for nearly closed compartments or those with very small vents and leakages, where the pressure build-up is mainly due to dynamic pressures, will be dealt with in Chapter 8.
