KAH Operation & Maintenance Guide
Page: 63 of 88
Krueger | 1401 N Plano Rd | Richardson, TX 75081 | 972.680.9136 | [email protected] | www.krueger-hvac.com
SECTION 4 - SERVICE & REPAIR
FIGURE 75 - STATIC PRESSURE AIR SYSTEM
EXTERNAL
STATIC
PRESSURE
INTERNAL STATIC
PRESSURE
The SP in an exhaust system is always below atmospheric
pressure, and it is customary among ventilation engineers
to omit the minus sign affecting the static (gauge pressure).
When the unit is designed for connection to a duct system,
and the installing contractor assembles the ducts, elbows,
registers, grilles, etc. to the outlet and/or inlet of the AHU,
the SP drop through this external ductwork is called
external SP as shown in Figure 75 (below).
The selected fans must be capable of moving the desired
airflow through the entire air moving system including the
AHU (internal SP), and the duct system (external SP).
At a given flow rate the internal pressure losses plus the
external SP losses equal the system SP or the summation
SP.
These pressures are important when troubleshooting for
issues such as reduced capacity, vibration and noise.
Changes in the cross sectional area of a duct (contractions
or enlargements) cause changes in the velocity of the air
flowing through the duct.
When the velocity decreases, the VP decreases. Some of
the velocity energy is lost because where the area changes
in the duct. Some of the velocity energy is converted into
SP energy in the continuing ductwork. This conversion of
velocity energy to SP is called static regain.
DUCT PRESSURES
Velocity
When air moves at a given velocity (V) in FPM in a duct, it
creates a pressure corresponding to the velocity, which is
a measure of the kinetic energy in the fluid and is known
as the velocity pressure (VP). VP is always exerted in the
direction of air flow. The relationship between the velocity
and the VP may be expressed by the following formulas:
Vp = (V / 4005)
2
V = 4005 √ Vp
4005 √ 0.250 = 2002 FPM
It is a simple matter to determine the velocity (FPM) of an
the air stream so the VP can be measured. For example,
if a Pitot Tube manometer hook-up reads 0.25” water, the
previous equation should be substituted.
Static Pressure
Independent of its velocity, air, when confined within
an enclosure such as a duct or tank, will exert itself
perpendicularly to the walls of the enclosure. This is the
compressive pressure existing in a fluid, and it is known
as the static pressure (SP). Unlike VP, which is always
positive, SP, when it is above atmospheric pressure will
be positive but when below atmospheric pressure it will be
negative. The discharge side of a fan in an air system will
read a positive pressure, the inlet side will read a negative
or minus pressure.
Total Pressure
Static pressure is exerted whether air is at rest or in motion.
The algebraic sum of static pressure and velocity pressure
gives the total pressure (TP).
Vp = TP – SP
The manometer does not sense the actual VP directly but
by using the pitot tube hook-up with the static opening
connected to the low pressure side of the gauge, and the
total pressure opening connected to the high pressure
side of the gauge, the manometer will read the difference
between the two, or the VP.
VP and SP change in the ductwork with every change in
the duct configuration, but the total pressure, on the other
hand, remains constant. As a result, as the VP decreases,
the SP increases and vice versa, because the SP is
always the difference between the total pressure and the
VP. However, in an actual duct system, the internal friction
would cause a loss of total pressure.