Pitot Tube vs Annubar

The comparison is between a pitot tube and an averaging pitot tube (Annubar is the brand name of an averaging pitot tube)

Pitot Tube:

Averaging pitot tube:

You are correct. The averaging pitot tube has multiple vented ports as shown by the arrows above, the purpose of which is to get an average DP across the flow profile.

Different manufacturers have various shapes and numbers of ports. The upstream ports are blocked from the downstream ports by a barrier in the tube. There is no flow through the tube, only flowing forces are applied to it, developing upstream and down stream pressures.

Four reasons industrials use averaging pitot tubes.

1) It is problematic to find a single point in a flow stream that represents the average flow. It would take portable active flow instrumentation to get readings and a trained analyst to determine where that point is. Who can afford the time or effort?

2) The DP cell is the largest percentage of cost, not the tube. There's minimal savings in a plain pitot tube over an averaging pitot tube.

3) Flow forces. Flow forces can beat on invasive flow elements. The averaging pitot tube comes in models that extend the entire diameter of the pipe for support at both ends. Pitot tubes can be made tough, aircraft use them as speed sensors, but averaging styles perdominate in the industrial world.

4) Permanent pressure loss is a big factor in choice of a primary flow element. Pitot tubes develop relatively low DPs with a correspondingly low permanent pressure loss. Sometimes, the available pressure is marginal and minimizing the permanent pressure loss is a major factor.

The HVAC folks use pitot tubes for getting air flow measurements in low pressure ducts and lab hoods, where the flows are not severe.

Use an averaging pitot tube on your high pressure steam line.