Design Principles; Causes Of Smoke Movement; Stack Effect; Buoyancy - Honeywell AUTOMATIC CONTROL Engineering Manual

— Underwriters Laboratories (UL) Standards:
– UL 555, Standard for Fire Dampers and
Ceiling Dampers

DESIGN PRINCIPLES

CAUSES OF SMOKE MOVEMENT

The movement or flow of smoke in a building is caused by a
combination of stack effect, buoyancy, expansion, wind velocity,
and the HVAC system. See Figure 1. These items basically cause
pressure differences resulting in movement of the air and smoke
in a building.
REGISTER
BUILDING
SPACE
MECHANICAL
HVAC
SYSTEM
WIND
BUOYANT SMOKE
GAS
EXPANDS
Fig. 1. Factors Affecting the Movement of Smoke.
Before controls can be applied, it is necessary to first
understand the overall movement of smoke.

STACK EFFECT

Stack effect is caused by the indoor and outdoor air
temperature differences. The temperature difference causes a
difference in the density of the air inside and outside of the
building. This creates a pressure difference which can cause a
vertical movement of the air within the building. This
phenomenon is called stack effect. The air can move through
elevator shafts, stairwells, mechanical shafts, and other vertical
openings. The temperature-pressure difference is greater for
fire-heated air which may containing smoke than it is for normal
conditioned air. For further information on stack effect refer to
the Building Airflow System Control Applications section.
When it is colder outside than inside, there is a movement of
air upward within the building. This is called normal stack effect.
Stack effect is greater for a tall building than for a low building;
however, stack effect can exist in a one-story building. With
normal stack effect, air enters the building below the neutral
plane, approximately midheight, and exits above the neutral
STAIRCASE
CORRIDOR
BUILDING
STACK
M13022
SMOKE MANAGEMENT FUNDAMENTALS
– UL 555S, Standard for Leakage Rated Dampers
for Use In Smoke Control Systems
– UL 864, Standard for Control Units for Fire–
Protective Signaling Systems (UL Category UUKL)
plane. See Figure 2. Air neither enters nor exits at the neutral
plane, a level where the pressures are equal inside and outside
the building.
NORMAL STACK EFFECT
NOTE: ARROWS INDICATE DIRECTION OF AIR MOVEMENT
Fig. 2. Smoke Movement Caused by Normal or Reverse
Stack Effect.
When it is colder inside than outside, there is a movement of
air downward within the building. This is called reverse stack
effect. With reverse stack effect, air enters the building above
the neutral plane and exits below the neutral plane.
The pressure difference across the building's exterior wall
caused by temperature differences (normal or reverse stack
effect) according to Design of Smoke Management Systems
for Buildings published by ASHRAE is expressed as:
∆P = K s x
Where:
∆P = Pressure difference, in. wc
K s = Coefficient, 7.64
T o = Absolute temperature of outdoor air,
Rankine (R)
T i = Absolute temperature of air inside the shaft,
Rankine (R)
h = Distance from the neutral plane, ft

BUOYANCY

Buoyancy is the tendency of warm air or smoke to rise when
located in cool surrounding air. Buoyancy occurs because the
warmer air is less dense than the cooler air, resulting in pressure
differences. Large pressure differences are possible in tall fire
compartments.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
175
REVERSE STACK EFFECT
NEUTRAL
PLANE
C5153
3
( 1 1 )
– x h
T o T i