Air Flow Measurement Technologies

Transcription

1 Page 1/7 Typical Air Flow Measurement Applications Air Flow Control True Flow Feedback Theory of Vortex Shedding Air Flow Measurement Pitot Air Flow Measurement Thermal Air Flow Measurement Advantages of Vortex Shedding Air Flow Measurement

2 Page 2/7 Air Flow Measurement Technologies Typical Air Flow Measurement Applications Fan tracking Lab pressurization control Isolation room pressure control Outdoor air quantity monitoring Volumetric air flow control Air Flow Control True Flow Feedback If a parameter is important you should measure it directly Air flow measuring stations (AMS) applied to HVAC systems operating with variable volumes Air flow measuring stations (AMS) applied to HVAC systems operating with variable volumes Open loop Closed loop

3 Page 3/7 Theory of Vortex Shedding Air Flow Measurement Vortex Shedding Bluff body is a trapezoidal shaped strut A tube aligns air flow across bluff body Pressure pulses at sides of bluff body are sensed Multiple sensors across duct are averaged VorTek air flow measurement Vortex Shedding Air Flow Measurement Primary signal directly proportional to velocity (1 to 1 relationship) Does not require compensation for temperature, density and humidity changes Linear primary signal True velocity averaging Not affected by dust/dirt Self cleaning Recalibration not required VorTek air flow measurement Vortex shedding on the large scale: clouds passing an island in the sea The frequency of the vortices is directly proportional to the velocity of air

4 Page 4/7 Theory of Pitot Air Flow Measurement Differential Pressure Transducers for Velocity Pressure Measurement Convert velocity pressure to electronic signal Requires recalibration on a regular, ongoing basis Must measure very low pressure Must operate over temperature extremes Due to amplification of square root function, drift should be kept to a minimum Principle of operation, pitot tube Typical Pitot Air Flow Stations Simple, rugged construction Available in duct insertion or fan inlet configurations Limited turndown Airflow rated to square root of measured parameter Transmitter requires periodic recalibration Dirt and dust can cause plugging Differential pressure vs. air flow velocity in a pitot air flow measurement. Signal changes in proportion to the square of the flow Elements of a pitot type air flow measuring system

5 Page 5/7 Theory of Thermal Air Flow Measurement Very good for low velocity air flow Air must be clean and dry Highly non-linear Extensive signal conditioning required Humidity affects accuracy Electronics and software intensive Dirt insulates sensors and affects accuracy Glass sensors are fragile and cannot be repaired Each sensor is unique. Must be individually calibrated at the factory How thermal sensors measure air flow. Principle of operation, thermal anemometer A microprocessor measures the power needed to maintain a fixed temperature Typical thermal air flow stations configuration

6 Page 6/7 Advantages of Vortex Shedding Air Flow Measurement Linear sensor output True velocity averaging Drift free operation Probes imprivious to dirt and other materials in the airstream Integral calibration standard provided in the transmitter to allow for the calibration of the unit on site Diagnostic functions provided to allow online zero and sensor checking AC or DC power Jumper selected output signal Unique patented sensing principle Understand turndown limits of each device Good practice avoid areas where air is decompressing or turbulent Fan discharge Damper discharges Expanding transitions Elbows

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