Honeywell AUTOMATIC CONTROL, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATION CONTROL
BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
279
An analysis of the accuracy of the pitot tube at 800 fpm
follows:
Velocity Pressure:
VP = (800 ÷ 4005)
2
= 0.0399 in. wc
Accuracy:
High
0.0399 in. wc
+0.0050 in. wc
0.0449 in. wc
or 848.6 fpm
Low
0.0399 in. wc
–0.0050 in. wc
0.0349 in. wc
or 748.2 fpm
Range
848.6 fpm
–748.2 fpm
100.4 fpm
or 100.4 ÷ 2 = ± 50.2 fpm
Percent Error:
( ± 50.2 ÷ 800) x 100 = ± 6.3%
In practical situations, the velocity of the air stream is not
uniform across the cross-section of a duct. Friction slows the
air moving close to the walls so the velocity is greater away
from the wall.
To determine the average velocity, a series of velocity pressure
readings at points of equal area is found. It is recommended to
use a formal pattern of sensing points across the duct cross-
section. These readings across the duct cross-section are known
as traverse readings. Figure 17 shows recommended pitot tube
locations for traversing round and rectangular ducts. In round
ducts, velocity pressure readings at the centers of the areas of
equal concentric areas are taken. Readings are taken along at
least two diameters perpendicular to each other. In rectangular
ducts, readings at the centers of equal rectangular areas are
taken. The velocities are then mathematically totaled and
averaged.
Fig. 17. Pitot Tube Locations for Traversing Round
and Rectangular Ducts.
The principle behind the manual pitot tube transverse is
simple and straightforward. However, care must be taken to
obtain accurate readings without inadvertent violation of the
formal traverse pattern. With the manual pitot tube traverse,
individual velocity readings must be calculated mathematically,
totaled, and divided by the number of readings to obtain average
velocity.
TOTAL AND STATIC PRESSURE SENSORS
Other arrangements are available to instantaneously average
sensed pressures and manifold these values to the exterior of
the duct. The Tchebycheff (Tcheb) tube method (Fig. 18) is
one such arrangement. This method separately manifolds the
total and static pressure sensors. Inside each manifold is a tube
with a single slot which receives an average pressure signal
from the manifold. The averaged signals from the total and
static pressure tubes may be used for indication and control
functions. This method assures an accurate reading of flow
conditions and a steady signal.
EQUAL
CONCENTRIC
AREAS
CENTERS
OF AREA OF
THE EQUAL
CONCENTRIC
AREAS
0.316 R
0.548 R
0.707 R
0.837 R
0.949 R
ROUND DUCT
16-64 EQUAL
RECTANGULAR AREAS
CENTERS
OF AREAS
RECTANGULAR DUCT
C2657 PITOT TUBE STATIONS INDICATED BY
AIRFLOW
C2658
Fig. 18. Tcheb Tube Sensors and Manifold.