Airflow Measuring Devices - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

AIRFLOW MEASURING DEVICES

Various devices for measuring airflow are available. Figure 19
shows a flow measuring station that consists of:
— An air straightener section to eliminate swirl type
airflow and help normalize the velocity profile
— Total pressure sensors aligned into the air stream and
spaced on an equal area basis
— Static pressure sensors with ports perpendicular to the
airflow direction and positioned on an equal area basis
— Separate manifolds for static and total pressure averaging
The flow measuring station in Figure 19 uses a tube as an air
straightener (Fig. 20) with total and static pressure sensors at the
downstream end. These tubes are arranged on an equal area basis.
AIR
STRAIGHTENER
TOTAL STATIC
PRESSURE PRESSURE
SENSOR
SENSOR
Fig. 19. Averaging Flow Measuring Station.
Fig. 20. Air Straighteners/Sensors and Manifolds.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
TOTAL PRESSURE
CONNECTION
W
L O
R F
A I
STATIC
PRESSURE
CONNECTION
MANIFOLD
C2659
ASSEMBLY
C2660
BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
Electronic flow stations use the tube type construction with
thermal velocity sensors instead of static and total pressure
sensors. Other flow sensing arrangements use holes in round
tubing (Fig. 21A and 21B) or airfoil designs (Fig. 21C). It is
important to consult the manufacturer of each type of flow
station for specifications, calibration data, and application
information (location limitations within ducts).
˚
AIRFLOW 39.25 39.25
TOTAL STATIC
PRESSURE PRESSURE
TUBE TUBE
A.
NOTE: STATIC PRESSURE SIGNAL WILL BE LESS THAN ACTUAL
STATIC PRESSURE AND CANNOT BE USED FOR STATIC
PRESSURE CONTROL. AS DEPRESSION VARIES WITH
AIR VELOCITY, VELOCITY PRESSURE (VP=TP - SP) WILL
BE AMPLIFIED.
TOTAL PRESSURE
PORT
TOTAL
FLOW
PRESSURE
Fig. 21. Miscellaneous Flow Sensing Arrangements.
Figure 22 illustrates illustrates an airflow pickup station
typically used in the primary air inlet to a VAV air terminal
unit. The pickup station consists of two tubes that measure
differential pressure. This measurement can be used in an
airflow calculation.
VAV BOX
INLET
SLEEVE
AIRFLOW
PICKUP
Fig. 22. Airflow Pickup Station
for VAV Box Applications.
279
˚
TOTAL
STATIC
PRESSURE
PRESSURE
TUBE
TUBE
(SEE NOTE)
B.
AIR PROFILE STATIC PRESSURE
PORTS
STATIC
PRESSURE
AIR PROFILE
C.
C2661
DAMPER
AXLE
M12214