Flow Meters – Understanding Helical Rotor Performance

There are many reasons for selecting helical rotors over that of flat-bladed rotors. The helical blade transfers energy more efficiently from the flow stream to the rotor, producing a faster speed of response to flow changes (2-3 mS depending on the rotor mass and blade area).

Helical rotors also minimize rotor slip in flow startup or stop, representing more accurately the actual flow rate being measured. This is important if you have a fluid measurement requirement that is cyclic or involves short flow bursts, such as in hydraulic applications. Helical rotors have less fluid force on the bearing, which reduces friction and heat, extending the life of the bearing.

When using flat-bladed rotors, it is often a common practice to bend the tip of the support to “trim” the flowmeter. Trimming is a term meaning that the flow is directed to the rotor blade at an angle to achieve improved linearity in one direction. In addition, this will increase flow resistance and pressure drop across the flowmeter. The bent support will also want to rotate if not properly clamped.

Helical rotors do not require bending the supports to enhance the linearity.

Dual-rotor Turbine Concept:

• Unconditioned flow enters flowmeter.
• Straightening vanes smooth the flow as it enters the first rotor.
• Flow transfers momentum to the first rotor, making it spin counterclockwise. Flow then exits rotor, with a clockwise spin.
• Flow enters second rotor with a nearly perpendicular angle of attack, thereby transferring additional momentum to the second rotor. This additional momentum, results in greatly extended turndown.
• Flow exits flowmeter.
• Dual pickoffs transmit the rotor frequency signal to remote instrumentation.

Dual-rotor Performance Advantages:

• Wider flow ranges – up to 500:1
• No flow straighteners required
• High shock design
• Enhanced repeatability and overall accuracy

About the Author

Exact Flow, based in Scottsdale, Arizona USA, is the only supplier of innovative dual-rotor turbine flowmeter technology, delivering unique designs to meet the application requirements of the aerospace, industrial, automotive and OEM markets. Exact Flow’s primary focus is to enhance customer value by specializing in advanced embedded electronics, multiple process variable measurements and unique packaging, which minimizes the need for additional sensors and installation expense.

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