12 AIR INLETS/REPLACEMENT AIR
12.1 For any scheme to be effective, sufficient replacement
air must be provided in order that extract ventilators
operate properly (see Appendix 2 Reference 6).
12.2 Doors and windows, as well as ventilators, can be used
for replacement air. However, if intended for use in
this way the'y should be fitted with automatic
controls compatible with the system. The designer
should state the ratio of extract to inlet area used
in his calculations.
12.3 To avoid disruption of the smoke layer in any single
compartment, the base of the smoke layer should be at
least 1-2m above any door/opening which might either
by design or chance be a source for replacement air,
unless the incoming air velocity is less than lm/s, in
which instance it should be no less than 500mm above
the opening.
12.4 The design inlet velocity shall not exceed 5.0m/s.
12.5 Ideally, replacement air should be introduced at
low level from all directions. However, natural
ventilators in the non-fire compartments can be used
if smoke curtains/screens are fitted to prevent short
circuiting.
12.6 Powered ventilators can be used for replacement air.
Care should be taken as they may cause turbulence
which can increase smoke/fire spread.
12.7 Where security bars, "insect" mesh or bird screens are
fitted, the effect of these should be considered when
determining the ventilator discharge coefficient.