Airflow Measuring Devices - Honeywell AUTOMATIC CONTROL Engineering Manual

BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS

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
STATIC
TOTAL
PRESSURE PRESSURE
SENSOR SENSOR
Fig. 19. Averaging Flow Measuring Station.
ENGINEERING MANUAL OF AUTOMATION CONTROL
TOTAL PRESSURE
CONNECTION
W
L O
R F
A I
STATIC
PRESSURE
CONNECTION
MANIFOLD
ASSEMBLY C2659
Fig. 20. Air Straighteners/Sensors and Manifolds.
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 AIR PROFILE
PORT
TOTAL
FLOW
PRESSURE
AIR PROFILE
Fig. 21. Miscellaneous Flow Sensing Arrangements.
280
C2660
˚
TOTAL
STATIC
PRESSURE
PRESSURE
TUBE
TUBE
(SEE NOTE)
B.
STATIC PRESSURE
PORTS
STATIC
PRESSURE
C.
C2661