Ductwork; Fans - Honeywell NOTIFIER SCS Series Operation And Installation Manual

Concepts of Smoke Control

Ductwork

Ductwork is constructed of a variety of materials including steel, aluminum, concrete, and masonry. Ductwork usually connects the fans with
the areas to be served. Air travels from the supply fan through the supply ducts into the building. Return air is often pulled through the ple-
num space above the ceiling as shown in Figure 2.11a. Ductwork, however, can be used for the return air as well, as shown in Figure 2.11b.
Both the supply and the return ductwork (when used) are usually located in the area above a suspended ceiling.
Figure 2.11 Supply and Return Ductwork with Plenum Return (a) and Duct Return (b)

Fans

There are two general types of fans: centrifugal and axial. An illustration of the basic parts of each is located in Figure 2.12. Airflow out of a
centrifugal fan is primarily in the direction perpendicular to the airflow in, whereas airflow out of an axial fan is primarily in a direction par-
allel to the airflow in.
Centrifugal Fans. Centrifugal fans are usually classified by impeller design. The three types are forward-curved, backward-curved, and air-
foil.
Forward-curved centrifugal fans rotate at a relatively low speed and are generally used to produce high flow rates and low static pressures.
They are commonly used for low-pressure HVAC applications including residential furnaces and packaged air-conditioning equipment.
Backward-curved fans rotate at approximately twice the speed of forward-curved fans and are used for general purpose HVAC applications.
These type fans are more efficient than forward-curved fans and are commonly more expensive. Both forward- and backward-curved fans
have single width blades. Airfoil fans are basically backward-curved fans with blades of varying thicknesses. Airfoil blades are designed
with the same technology that is used to design airplane wings. Airfoil fans are used for general purpose HVAC, but are reserved for large
system applications where high pressures are needed.
Backward rotation of a centrifugal fan results in reduced airflow in the normal direction. It does not result in reverse airflow, as is a common
misconception.
Axial Fans. The most common types of axial fans are propeller fans, tubeaxial fans, and vaneaxial fans.
The majority of propeller fans are designed with the goal of providing high flow rates and low pressures. Some common applications include
kitchen exhaust, rest room exhaust, stairwell pressurization, and space ventilation. Propeller fans are highly susceptible to adverse pressure
conditions such as the effects of wind. Tubeaxial fans are capable of operating at higher pressures than propeller fans. Blades of tubeaxial
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Figure 2.11a
Centrifugal Fan
Figure 2.12 Fan Types
General System Components
Figure 2.11b
Axial Fan
SCS Series Manual — P/N 15712:L 7/18/16