JOHNSON CONTROLS
81
SECTION 4 - SERVICE AND REPAIR
FORM 102.20-OM2
ISSUE DATE: 6/01/2015
4
EXTERNAL
STATIC
PRESSURE
INTERNAL STATIC
PRESSURE
LD06365a
FIGURE 87 -
STATIC PRESSURE AIR SYSTEM
At a given flow rate the internal pressure losses plus
the external SP losses equal the system SP or the sum-
mation 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 (contrac-
tions 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.
REQUESTING ASSISTANCE
When contacting Johnson Controls for assistance, pro-
vide the following information:
•
Johnson Controls Order Number
•
Job Name (not contractor)
•
Unit Model Number
•
Customer’s Unit ID
•
Design and actual data
•
Fan RPM.
•
Unit SP (across fan).– Unit CFM
•
Pressure drop of water across cooling coil
(PSIG)
•
Air pressure drop across cooling coil
•
Temperature differential across cooling coil.
•
Motor voltage and amperes
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 mo-
tion. 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 open-
ing 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 de-
creases, 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 in-
ternal friction would cause a loss of total pressure.
The SP in an exhaust system is always below atmo-
spheric 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
The selected fans must be capable of moving the de-
sired airflow through the entire air moving system
including the AHU (internal SP), and the duct sys-
tem (external SP).
