Basic principles that apply to installation of SHEVS Flashcards
Describe the basic design principal of SHEVS
SHEVS should be designed to control the smoke layer to a maximum depth
below ceiling level by venting the hot smoke and gases to open air and before
they cool and fall to contaminate the upper walkways of the mall. The design
should also take into account the possibility of the rising smoke plume swirling
back as it passes upper balconies or walkways being used by people escaping
Describe the design principle of a smoke reservoir
Smoke reservoirs should form smoke zones. There may be more than one
reservoir in any compartment but no reservoir should serve more than one
compartment. Each reservoir should be large enough to retain the volume of
smoke that will enter it in the design fire scenario.
What is the maximum length of a smoke reservoir ?
A reservoir should not be more than 60m long so as to avoid people having to
move below the smoke layer for extended periods, so causing them concern.
List the factors regarding the removal of smoke and gases from reservoirs.
Hot smoke and gases can be removed from the reservoirs either by natural or
powered ventilation
The rate of exhaust must be approximately equal to the
probable rate at which hot smoke and gases will enter the reservoir from a fully
developed fire.
Fresh air must enter the ventilated area at a rate at least equal to the rate of
extraction and low enough not to prematurely mix with and thus cool the hot
smoke and gases.
The siting of the exit points where the hot smoke and gases
leave the building also needs consideration to avoid creating a hazard elsewhere
Where there is likelihood that external wind forces on sloping roofs of more
than 30° will adversely affect natural ventilation, either windshields should be
provided to protect the ventilator; or additional ventilators provided which are
under the control of a wind sensor.
Regarding fire safety enforcement applicants must prove SHEVS performance by ?
The formula for calculating SHEVS requirements are
complex but applicants should provide the necessary information to demonstrate
the viability of a proposal.
SHEVS are designed around a “design fire”. In life safety applications, SHEVS
should
maintain a clear air layer above escape routes high enough for people to
use them without breathing smoke contaminated air.
The temperature of the gases
in the buoyant layer under which people may have to pass should not exceed
200°C.
recommended minimum height of clear air
Using a design fire for SHEVS what are the recommended heights for clear air ?
Public buildings ( eg single storey malls, exhibition halls) 3m min
Non public buildings ( eg prisons apartments and offices PAO ! ) 2.5m min
Car parks 2.5m min or 0.8 of floor
height (whichever is
smaller)
NB:
Where the temperature of the hot smoke and gases is likely to be less than 50°C
above ambient temperature, the heights quoted above must be increased by 0.5m.
The designer should state whether the SHEVS will be either by natural
ventilation or powered ventilation and what else re replacement air ?
- an adequate supply of
replacement air to be effective. - The entry points for replacement air should be
specified or shown on plan.
3.inlet air should always enter the building at low
level, below the smoke layer.
4.Where the replacement air is admitted through doors or passages used for means
of escape purposes, the air speeds when the system is operating to capacity
should not exceed 5m/sec
5.Doors used as air inlets should open
automatically when the system is activated, even when the premises are closed
and the door is locked against intruders.
Smoke and heat exhaust ventilation systems, with a reservoir, are not suitable where ?
the predicted height from the base of the fire (usually the floor) to the base of the smoke layer
in a reservoir is less than 10% of the height from the floor to the ceiling.
Smoke and heat exhaust ventilation systems are not suitable where
the height from the base of the fire (the floor) to the base of the smoke layer is more than
90% of the height from the base of the fire to the ceiling.
Whats the basic design principle of SHEVS ?
for smoke and hot gases from a fire to rise by
natural buoyancy into a smoke reservoir. From there ventilators will exhaust the
smoke and gas to open air.
What 2 things must a designer of SHEVs smoke reservoir calculate from a design fire ?
mass flow rate and the temperature
of hot smoke and gases from a design fire collecting in a reservoir.
What determines the mass flow rate
the conductive heat flux generated by
the fire. A typical conductive heat flux flowing from a fire is:
