ASHRAE Journal - ACMA Supplement - Fall 2013 - 8

This is the
headline copy position
Cone Type
Variable
Inlet Vanes

This is the deck copy position. This is the deck copy position. This is the deck copy
position. This is the deck copy position.

Figure 2: Fan accessories for rating purposes. Image courtesy of AMCA International.
Total Pressure (Pt) is the difference in Total Pressure between
the fan outlet and inlet, Pt = Pt2-Pt1. Fan Velocity Pressure (Pv)
is the pressure corresponding to the average velocity at the fan
outlet. Fan Static Pressure (Ps) is the static pressure at the fan
outlet minus the total pressure at the fan inlet, Ps = Ps2-Pt1. Static
Pressure Rise (∆Ps) is the static pressure at the outlet minus the
static pressure at the inlet, ∆Ps = Ps2-Ps1. Therefore, the difference
between Fan Static Pressure and Static Pressure Rise is the
velocity pressure at the inlet. For ducted fans, Static Pressure
Rise is always greater than Fan Static Pressure, meaning a fan
selected for a given pressure difference is larger if the pressure
difference is assumed to be Fan Static Pressure and smaller if
assumed to be Static Pressure Rise.
Specifiers of fans need to understand these specific pressure
definitions and to be clear in their specifications relative to
whether the pressures they are specifying are Fan Total Pressure,
Fan Static Pressure, or Static Pressure Rise. They also must
clearly define the inlet and discharge boundaries of the fan,
which will determine which fan accessories are to be considered
internal losses of the fan and which are considered part of the
system losses that are included in the specified pressure values.
Figure 1 is a simplified diagram of and fan and ductwork system
that illustrates the above fan and system pressures for fan rating
8

Fa l l 2 0 1 3

and selection purposes.
The flow rate for fans can be expressed in various ways. Actual
Volume Flow Rate is the most common for industrial and powergeneration applications; it is the volume flow rate at the actual
inlet density of the gas entering the fan. Common units are actual
cubic feet per minute (ACFM) or cubic meters per hour (m3/hr).
Standard Volume Flow Rate is the volume flow rate corresponding to a standard temperature and pressure at the fan inlet.
Common units are SCFM or Nm3/hr. Mass Flow Rate is the
mass flow rate of the gas entering the fan inlet. Common units
are pounds per hour (lb/hr) or kilograms per hour (kg/hr). Note
that all flow rates are expressed at the inlet of a fan.
It is likewise important for specifiers to clearly define the density
of the gas at the inlet of the fan. This should include the effects
of temperature, elevation, suction pressure, and specific gravity
(including moisture content) of the gas. Remember that the molecular weight of air is 28.964 and the molecular weight of water
is 18, meaning that moist air is always less dense than dry air.
Specifiers also must identify all the expected operating points
for the fan and/or define the fan system line relative to flow and
pressure and any changes that are expected to occur to the gas
temperature or density over time, or at various operating points

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ASHRAE Journal - ACMA Supplement - Fall 2013

Table of Contents for the Digital Edition of ASHRAE Journal - ACMA Supplement - Fall 2013