Handbook of air conditioning and refrigeration / Shan K

Effective Area and Flow Rates
In several airflow passages connected in parallel, the effective area A e, in ft 2 (m 2 ), can be calculated as
Ae � A1 � A2 � � � � � An (22.8)
where A1, A2, ��� , An � airflow areas for path 1, 2, ���, n, ft2 (m2 )
In airflow paths connected in series, each with a flow area A1, A2, ���, An, in ft2 (m2 ), the effective
area for these airflow paths connected in series Ae can be calculated as
where A � flow area or effective area, ft 2 (m 2 )
�p � pressure difference across flow path or opening, in. WC (Pa)
22.10 EFFECT OF AUTOMATIC SPRINKLER
ON FIRE PROTECTION
AIR SYSTEMS: FAN COMBINATION AND SMOKE CONTROL 22.27
�1 / 2 1
�� � ��
A2� n
(22.9)
If the flow coefficient is taken as 0.65 and air density �a � 0.075 lb/ft 3 (1.2 kg/m 3 Ae ��
), the air volume
flow rate V˙ , in cfm, flowing through a crack, gap, or opening can be calculated as
1
2 A1 �
1
2 A2 1 / 2
V˙ � 2610A(�p)
(22.10)
Automatic sprinkler systems are effective and reliable fire protection systems and should be
installed in buildings to provide fire protection for the occupants during a building fire. Based on
experimental results, Mawhinney and Tamura (1994) summarized the effect of automatic sprinklers
on fire protection as follows:
● For unshielded fires, automatic fire sprinklers reduced both smoke and fire hazard to negligible
levels. Unshielded fire is a fire exposed to the water sprays from activated sprinklers.
● For shielded fires with activated water sprays from the sprinklers, fire continues to burn at a reduced
rate and to produce smoke until the fuel inside the shielded area has all burned. Concentrations
of CO 2 and CO in the smoke were dangerously high. Shield fires represent fires that are
shielded from the water sprays delivered by the sprinklers above because of a desk, table, or other
furniture.
According to experiments, shielded crib fires produced CO concentrations as high as 8 and 9
percent and CO concentrations between 1.0 and 1.5 percent (10,000 and 15,000 ppm) in a multistory
building. Peacock et al. (1989) indicated that a concentration of 15,000 ppm CO would
present a severe life-safety hazard to any one trapped on the fire floor.
● Sprinklers reduced the temperature and radiant heat from shielded fires to nonthreatening levels
within the boundary of the area covered by the sprinkler system. For a sprinkler system design
that meets NFPA Standard 13 requirements, a moderate increase in ventilation to the fire floor
during a building fire should not significantly increase the fire temperatures.
● Buoyancy forces for shielded, sprinklered fire were almost negligible. The recommendation for
the pressure difference between different smoke control zones (fire zone and adjacent zones)
�p � 0.05 in. WC (12.5 Pa) in NFPA (1988) for zone smoke control design in sprinklered buildings
(ceiling height � 9 ft or 2.7 m) is more than sufficient to prevent smoke movement, provided
that the door to the fire floor remains closed.
● Recommended practice for zone smoke control design should allow for some airflow into the
stairwell to prevent the spread of smoke into the stairwell when the door to the fire floor is
opened.