SCS Series Manual —
P/N 15712:L 7/18/16
21
General System Components
Concepts of Smoke Control
fans can be of single thickness or airfoil design. Tubeaxial fans are used to provide low- to medium-pressures for HVAC applications.
Vaneaxial fans are capable of operating at higher pressures than tubeaxial fans. Blades of vaneaxial fans can be of single thickness or airfoil
design. Vaneaxial fans are used to provide low- to high-pressures for HVAC applications.
Unlike centrifugal fans, the backward rotation of axial fans normally results in a reverse airflow. This reverse airflow is at a reduced rate.
Dampers
In HVAC systems, dampers are used to balance airflow, control airflow, relieve excess pressure, resist the passage of fire, or resist the pas-
sage of smoke. Dampers used to resist the passage of fire are called fire dampers. Dampers used to control the passage of smoke are called
smoke dampers. Fire dampers and smoke dampers can be of multi-blade or curtain construction, as shown in Figure 2.13.
Fire Dampers.
Fire dampers in the United States are usually constructed and labeled in accordance with standard UL 555 (UL 1990a). Gen-
erally, fire dampers, whether multi-blade or curtain type, are held open by a fusible link that comes apart in a fire situation. Multi-blade
dampers are usually spring loaded so when the fusible link is broken the spring initiates the closing action. Curtain dampers can be closed by
spring activation or by gravity once the fusible link is broken.
Smoke Dampers.
Smoke dampers in the United States are usually constructed and classified for leakage in accordance with standard UL
555S (UL 1983). UL 555S contains requirements for leakage-rated dampers intended for use in HVAC systems. Smoke dampers are classi-
fied as 0, I, II, III, or IV based on the amount of leakage they allow. See Table 2.2 for the maximum leakage rates allowed for each classifi-
cation. Additionally, each classification is rated for ambient or elevated temperatures of 250°F or higher in increments of 100°F.
Class 0 smoke dampers (zero leakage) are usually used in nuclear power plants or similar situations where absolute containment is required.
Classes I, II, III, and IV are usually acceptable for smoke control applications in other types of buildings.
Table 2.2 Smoke Damper Classifications
Classification
Leakage (cfm/ft
2
)
at 1.0 in. H
2
O
at 4.0 in. H
2
O
at 8.0 in. H
2
O
at 23.0 in. H
2
O
0
0
0
0
0
I
4
8
11
14
II
10
20
28
35
III
40
80
112
140
IV
60
120
168
210
Curtain
Construction
Parallel Action Damper
Multi-blade Construction
Figure 2.13 Damper Types