Honeywell AUTOMATIC CONTROL Engineering Manual page 276

BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
Static pressure: The pressure created by air (whether in motion
or not) confined in an enclosed area such as a duct or
building due to its potential energy. Static pressure,
denoted SP, is exerted perpendicularly on all interior
walls of the enclosure (duct or building) with respect
to a reference pressure outside the enclosure. When
static pressure is above atmospheric pressure it is
positive and when below atmospheric pressure it is
negative.
Static pressure sensor or tube: A sensing device with several
holes perpendicular to an airstream for measuring
static pressure.
Total pressure: The algebraic sum of Velocity Pressure (VP)
plus Static Pressure, denoted TP. Total pressure is
derived:
TP = VP + SP
Turndown: The relationship, in percent, between the maximum
minus the minimum airflow to the maximum airflow.
(
Max Flow – Min Flow
Turndown % =
For example, in a system with a maximum airflow of
2000 cfm and minimum airflow of 400 cfm, the
turndown is 80 percent.
Variable Air Volume (VAV) system: A central fan system in
which airflow in the duct varies depending on the
instantaneous load requirements of the connected VAV
terminal units.
Velocity: The speed or rate of flow of the air stream in a duct.
In this section, velocity is denoted V and is measured
in feet per minute (fpm). See General Engineering
Data section.
• Average Velocity—The sum of the air velocities from
equal area increments of a duct cross-section divided by
the number of increments. Average velocity, denoted
V , is derived:
AVG
V = ∑ (V + V
AVG 1
Where
N = Number of duct increments
ENGINEERING MANUAL OF AUTOMATION CONTROL
)
x 100
Max Flow
+ V +...... + V ) / N
2 3 N
• Peak Velocity—The greatest air velocity occurring in an
increment of a duct cross-section. Peak velocity is
denoted V .
PK
• Velocity Pressure: The pressure created by air moving
at a velocity due to its kinetic energy. Velocity pressure,
denoted VP, is always exerted in the direction of airflow
and is always a positive value. Velocity pressure and
velocity are related by the equation:
V =
Where:
V = Velocity in fpm
G = Gravitational acceleration in feet
VP = Velocity pressure in inches of
Dw = Density of water at a specified
Da = Density of the air flowing in the
1ft/12 in. = Conversion factor to convert
60 sec/1 min = Conversion factor to convert
The density of air (Da) is 0.075 lb/ft
29.92 in. wc atmospheric pressure, and 50 percent
relative humidity) and gravity (G) is = 32.2 ft/sec
70°F, the density of water (Dw) is 62.27 lb/ft
this data, the relationship of velocity to velocity
pressure is simplified:
32.2 ft

V = 2 x

sec 2
This equation reduces to:
V = 4005 VP
See General Engineering Data section for Velocity vs.
Velocity Pressure table.
266
VP Dw 1 f t
 
2 G x x
 
Da 12 in.
per second squared (ft/sec2)
water column (in. wc)
temperature measured in pounds
per cubic foot (lb/ft3)
duct measured in pounds per cubic
foot (lb/ft3)
inches to feet
seconds to minutes
3
(at 70°F,
62.27 lb/ft 3
1 f t
x V P i n . w c x x
0.075 lb/ft 3
12 in.
60 sec
1 m i n
2
. At
3
. With
60 sec


1 m i n