Essentials
SELELE MASHILO
Selele Mashilo has a mechanical engineering diploma from Tshwane University of Technology and
a refrigeration and air-conditioning diploma from Unisa. His experience includes over a decade in
government as deputy-director building services before rejoining the private sector in 1998 as HVAC&R
project engineer. He is the former chairperson of the Refrigeration and Air Conditioning Empowerment
Forum of SA (RAEFSA), the Air Conditioning and Refrigeration Industrial Council of SA (ACRICSA), and
Black Energy Services Companies (BESCO).
HVAC, FIRE AND SMOKE CONTROL
By Selele Mashilo
When there is a fire in a building, ducted ventilation systems can always
transport smoke around.
Smoke control is very important during the design stage of air
conditioning systems. During a fire in a building, lives may be
endangered by smoke carried by ducting in all areas served by air
conditioning.
Smoke can be controlled in various ways, either by switching off
the system by using smoke detectors or by applying smoke control
mechanisms which will alert tenants of the building.
During a fire in a building smoke movement to various areas is
caused by stack effect in a building, buoyancy, expansion, wind, and
ventilating and air conditioning systems.
Smoke is regarded is a killer during a fire in a building. Before
fire can even reach various remote areas of the building, smoke
might have already reached those points and also made it difficult to
evacuate tenants.
The response system on smoke control should have already
responded in time to avoid smoke reaching such remote areas
or the tenants must be alerted in time so that evacuation can be
effected.
Fire safety designers consider air conditioning systems as
potentially dangerous penetrators of building membranes that can
transport smoke and fire. Traditionally, HVAC systems must be shut
down during fire.
Though HVAC will be shut down, this does not necessarily
prevent smoke travelling through the ducting into various places
due to smoke buoyancy, stack effect and outside wind velocities
and pressures.
STACK EFFECT
A building may contain shafts for services, staircases, lifts or escalators.
When it is colder outside the building than inside the building, warm
and less dense air will always have an upward movement.
The opposite is true when the outside air is warmer than the
interior air and movement will be downward. The downward reverse
movement is called reverse stack effect. At standard atmospheric
pressure, the pressure difference due to either normal or reverse stack
effect is expressed as:
ASHRAE
∆p = K *(1/T0 - 1/Ti)*h
Where:
∆p = pressure difference in Pascals
T0 = absolute temperature of outside air
Ti = absolute temperature of air inside shafts
h = distance above neutral plane in metres
Figure 1. Pressure difference between a building shaft and the outside
due Normal Stack Effect.
The stack effect usually exists between a building and the outside of a
building. Figure 1 indicates the pressure difference between a building
shaft and the outside of the building. In the diagram a positive pressure
difference indicates that the shaft pressure is higher than the outside
pressure, and a negative pressure indicates the opposite.
Under normal stack effect the air movement will be upwards and
under reverse stack effect the air movement will be downwards. Now
one should imagine during fire in the building, the movement of smoke
and how it can be controlled by ventilation system.
Besides the air fuelling the fire, it is not always directed to the fire
and can affect smoke movement when certain areas are under a more
positive pressure than where the smoke originated. The shafts may be
pressurised to prevent smoke movement or used for extraction systems.
REFERENCE:
1. ASHRAE RACA
www.hvacronline.co.za RACA Journal I October 2020 45