Annubar

A pitot tube after Annubar principle ( also Annubar, Ellison Annubar or integrating pitot tube ) is a special form of the pitot tube, which is used in plant construction and process engineering for flow measurement.

Principle

This pitot tubes etc. can be used to measure the flow of gaseous or liquid fluids such as air, gas, steam, water in pipes and channels. Pitot tubes have a plurality of openings (usually holes) in and against the direction of flow. At the openings counter to the flow direction creates a dynamic pressure ( the so-called ram pressure ), at the openings in the flow direction results in a dynamic vacuum. Within the pitot tube, the voltage applied to the differential pressure openings pressures are averaged and measured outside the pitot tube. The measured outside the pitot tube differential pressure is the difference between the dynamic pressure and the dynamic vacuum:

• - Differential pressure
• - Total pressure of 1
• - Total pressure of 2
• - Dynamic pressure
• - Dynamic vacuum

By averaging function of this type pitot tube, the dynamic pressure probe disturbed flow profiles, as they are present in Rohleitungsbau behind obstructions or diversions, correct and better capture the flow more accurately than is the case at a single point.

Basis of calculation

The flow calculation using the dynamic pressure principle derives from the energy conservation law. In Anströmpunkt ( stagnation point ) of the pitot tube, the flow is slowed down and converts their kinetic energy (speed) into potential energy ( pressure ) around.

From the measured differential pressure, the flow of the fluid can be calculated:

This includes:

• - Mass flow ( or mass flow )
• - Dimensionless calibration constant of the dynamic pressure probe ( K-value )
• - Expansion factor
• - Inside diameter of the pipe
• - Differential pressure
• - Density of the fluid

The dimensionless calibration constant K is determined by the different manufacturers for their pitot tubes and communicated to the user. A typical value is from 0.62 to 0.68.

The expansion factor to correct the change in density of the fluid by the pressure loss of the pitot tube. For incompressible fluids (liquids) is; for compressible fluids is less than 1, but is typically in the range. The density of the fluid, the density immediately before the pitot tube.

Designs

Pitot tubes after Annubar principle differ in measurement profile and design of connection.

Measuring the profile of a pitot tube is located in the pipe member around which flow occurs. Visible from the outside of the connector, with the pitot tube is installed in the pipeline and measuring instruments such differential, pressure or temperature transmitters are grown.

Applications / operating limits

Pitot tubes after Annubarprinzip are mainly used in process plants in use, so for example in power plants, chemical and petrochemical plants, breweries, incineration plants and sewage treatment plants. Since dynamic pressure probes have dead volumes, they are not generally used for food or other sensitive products, but primarily for auxiliary media such as steam, compressed air, hot water, thermal oil, smoke gas etc.

Advantage over classical dp - measuring techniques such as aperture, venturi or nozzle, is the easier installation and low pressure loss ( or energy loss). An online calculator for a rough calculation of the pressure loss can be found here: / Online Calculator

Classical primary elements, however, have the advantage internationally standardized and standardized to have (ISO 5167 ). Inexpensive is the use of pitot tubes especially for large pipes, as the price of the pitot tubes is approximately linearly developed with the diameter of the square or cube of classic primary elements.