Characteristics Of Fans And Fan Laws; General; Fan Types - Honeywell AUTOMATIC CONTROL Engineering Manual

BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
NORMAL STACK EFFECT
NOTE: ARROWS INDICATE DIRECTION OF AIR MOVEMENT
Fig. 3. Stack Effect in a Building.
CHARACTERISTICS OF FANS
AND FAN LAWS
NOTE: This text provides an overview of fan characteristics and
fan laws. For more information, see the Trane Air
Conditioning Manual listed in REFERENCE.

GENERAL

In airflow systems, a fan converts mechanical, rotative energy
into fluid energy. This is basically accomplished by a wheel or
propeller which imparts a forward motion to the air. For HVAC
applications, fans rarely exceed a total pressure of 12 in. wc.
Fans must be properly installed to achieve smooth control
and correct performance. In general, manufacturer
recommendations should be followed and the following noted
(from Engineering Fundamentals of Fans and Roof Ventilators,
Plant Engineering, Copyright 1982):
— Fans should be located so the discharge of one does not
enter the intake of another fan.
— Intake area should be at least 20 percent greater than
the fan wheel discharge area.
— Fans located opposite from each other should be
separated by at least six fan diameters.
— Elbows or other abrupt duct transformations on the
discharge side of the fan should not be closer than one
diameter from the fan wheel.
— Direction of fan discharge and rotation should be
selected to match duct or mounting requirements.

FAN TYPES

Two main types of fans are used in airflow systems centrifugal
and axial:
Centrifugal Fans: A centrifugal fan (Fig. 4) has airflow within
the wheel that is substantially radial to the shaft (or
away from the axis of the shaft). The air from an in-
line centrifugal fan does not have to turn before being
ENGINEERING MANUAL OF AUTOMATION CONTROL
REVERSE STACK EFFECT
NEUTRAL
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expelled from the fan housing. Some centrifugal fan
designs are differentiated by the inclination of the
blades. Each blade design has a peculiar advantage:
Photo Courtesy of the Trane Company
Fig. 4. Centrifugal Fan.
— Backward inclined blades are generally larger
and more quiet than forward inclined blades.
They are more suitable for larger sizes.
— Forward inclined blades are suitable in small
packaged units and operate at a lower static
pressure.
— Air foil blades are backward inclined, and are
efficient and quiet due to an air foil shaped blade.
Generally these are used on the largest fans.
Axial Fans: An axial fan has airflow through the wheel that is
substantially parallel to the shaft (or along the axis of
the shaft). Various designs of axial fans are available
(Fig. 5), mainly differentiated by the duty of the fan.
Each design has a peculiar advantage:
A.
PROPELLER FAN TUBEAXIAL FAN
Fig. 5. Types of Axial Fans.
— Propeller fans are low pressure, high airflow,
noisy fans. They work up to a maximum static
pressure of 0.75 in. wc.
— Tubeaxial fans are heavy-duty propeller fans
arranged for duct connection. They discharge air
with a motion that causes high friction loss and
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C.
B.
VANEAXIAL FAN
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