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Service
the high pressure side of the gauge, the manometer will
read the difference between the two, or the velocity
pressure.
Velocity pressure and static pressure change in the duct
work with every change in the duct configuration, but
the total pressure, on the other hand, remains constant.
Hence, as the velocity pressure decreases, the static
pressure increases and vice versa, because the static
pressure is always the difference between total pres-
sure and the velocity pressure. It should be remem-
bered, however, that in an actual duct system, the inter-
nal friction will cause a loss of total pressure.
The static pressure in an exhaust system is always be-
low atmospheric pressure, and it is customary among
ventilation engineers to omit the minus sign affecting
the static (gage pressure). These men know, of course,
that the total pressure is higher than the static pressure
by the amount of the velocity pressure.
When the unit is designed for connection to a duct
system and the installing contractor assembles ducts,
elbows, registers, grilles, etc. to the outlet and/or inlet
of the unit, the static pressure drop through this exter-
nal duct work is called external static pressure. (See
Figure 67)
Fans selected must be capable of moving the desired
air flow through the entire air moving system includ-
ing the unit (internal SP) and also the duct system
(external SP).
At a given flow rate the internal pressure losses plus
the external static pressure losses equal the system static
pressure or the summation static pressure.
These pressures are of great importance when trouble-
shooting for causes of 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 velocity pressure also
62
decreases. Some of the velocity energy is lost as a re-
sult of the design of the duct where the area changes.
Some of the velocity energy is converted into static pres-
sure energy in the continuing duct work. This conver-
sion of velocity energy to static pressure is called static
regain.
When contacting YORK for assistance the following
information will be required:
1. YORK Order No.
2. Job name (not contractor)
3. Unit Model No.
4. Customer's Unit Identification
5. Design Data and Actual Data
A. Fan RPM
B. Unit SP (Across Fan)
C. Unit CFM
D. Pressure Drop of Water Across Cooling Coil
(PSIG)
E. Air Pressure Drop Across Cooling Coil
(inches of Water)
F. Temperature Differential Across Cooling Coil
G. Motor Voltage
H. Motor Amperes
6. For Fan or Motor RPM (Use a Tachometer, Stro-
boscope or Revolution Counter)
7. The Voltage and Amperes can be obtained by us-
ing the appropriate multimeter test device.
8. A sketch of the Duct Configuration would assist us
in trying to resolve the problem.
9. The most important item is to provide a detailed
explanation of the problem.
10. An orifice is the best method of measuring flow in
piping. This is also true in an air system and is the
reason for step 5D.
For further assistance in troubleshooting the air unit, re-
fer to the troubleshooting charts on the following pages.
YORK INTERNATIONAL
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